The Contemporary 
 
 Science Series 
 
 The Evolution 
 
 &f Sex 
 
 
 . 
 
 wmm 
 
5foui %azk 
 HaU ^allege of Agriculture 
 At OJarnEll UninerattB 
 
 3tliaca. N. 5. 
 
 library 
 
Cornell University Library 
 QH 471.G29 1908 
 
 The evolution of sex ■ / 
 
The original of this book is in 
 the Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://archive.org/details/cu31924003078890 
 
THE CONTEMPORAR Y SCIENCE SERIES. 
 Edited by HAVELOCK ELLIS. 
 
 THE EVOLUTION OF SEX. 
 
THE 
 
 Evolution of Sex. 
 
 BY 
 
 Professors PATRICK GEDDES 
 
 AND 
 
 J. ARTHUR THOMSON. 
 WITH 92 ILLUSTRATIONS. 
 
 REVISED EDITION. 
 
 THE WALTER SCOTT PUBLISHING CO., LTD., 
 
 PATERNOSTER SQUARE, LONDON, E.C. 
 
 CHARLES SCRIBNER'S SONS, 
 
 153-157 FIETH AVENUE, NEW YORK. 
 
 190"?. 
 
PREFACE 
 
 TO THE SECOND EDITION. 
 
 THE rapid exhaustion of the large first edition (1889) of 
 this book, and of subsequent reprints also, seems to 
 show the need of a general survey of the essential phenomena 
 of reproduction and sex. That this is in our case associated 
 with a particular theory of the nature of these, need not hinder 
 the reader from discriminating between facts and interpreta- 
 tions. Hence in this revised edition, though many alterations 
 and additions have been made, the original character of the 
 work has been retained, and that notwithstanding the difficulty 
 that the authors have in the past ten years been diverging 
 biologically — the one towards a " Neo-Lamarckian " position, 
 the other towards a " Neo-Darwinian '' one. Yet they remain 
 agreed on the main endeavour of the book, which is to set 
 forth the fundamental unity underlying the Protean phenomena 
 of sex and reproduction. 
 
 The biological interpretation worked out in this volume 
 ma| be summarised in three propositions : — 
 
 ■I A.) In all living creatures there are two great lines of 
 variation, primarily determined by the very nature of proto- 
 plasmic change (metabolism); for the ratio of the constructive 
 (anabolic) changes to the disruptive (katabolic) ones, that is of 
 income to outlay, of gains to losses, is a variable one. In one 
 
VI PREFACE. 
 
 sex, the female, the balance of debtor and creditor is the more 
 favourable one; the anabolic processes tend to preponderate, 
 and this profit may be at first devoted to growth, but later to- 
 wards offspring, of which she hence can afford to bear the larger 
 share. To put it more precisely, the life-ratio of anabolic to 
 
 katabolic changes, j~ in the female is normally greater than the 
 
 corresponding life-ratio, -r, in the male. This, for us, is the 
 
 fundamental, the physiological, the constitutional difference 
 between the sexes; and it becomes expressed from the very 
 outset in the contrast between their essential reproductive 
 elements, and may be traced on into the more superficial 
 secondary sexual characters. 
 
 (£.) There is much experimental evidence to show that 
 the determining stimuli which cause the balance of life to 
 swing to one side or other, which shunt the plastic organism 
 to this side or to that, are to be found, at least very largely, in 
 the external or environmental conditions — food, temperature, 
 light, chemical media, and so on ; so that the determination of 
 sex becomes to some extent practicable in an increasing 
 number of forms. 
 
 (C.) Yet, the sexual dimorphism, in the main and in detail, 
 has an adaptive significance also, securing the advantages of 
 cross-fertilisation and the like, and is therefore to some extent 
 the result of the continual action of natural selection, though 
 this may of course check variation in one form as well as 
 favour it in another. 
 
 So far our main theses. Yet these are obviously but the 
 preliminaries to others. That such an interpretation of the 
 phenomena, of sex and reproduction must have its bearing on 
 the whole treatment of the problem of the origin of species is 
 manifest, and it appears to us that these two main variational 
 
PREFACE. Vll 
 
 lines, upon which we seek to explain the divergent evolution of 
 the sexes, are to be detected also in the variety and the species, 
 in the group and beyond, it may be in the very contrast of 
 plant and animal. But each new mode of interpretation in- 
 evitably invites us towards the re-examination of the whole 
 evolutionary process in all its aspects, in all its products. 
 And if, as none now deny, the study of the simpler manifesta- 
 tions of life be a legitimate and even a necessary preliminary 
 to the understanding of human life itself, we must seek fully 
 to rise from our elementary interpretation of reproduction and 
 sex to the study of their vast complexity upon the human 
 plane — anthropological and social, psychological or ethical, 
 educational or practical. We have attempted to indicate some 
 of these considerations more fully in the concluding chapter, 
 as also in some of our separate papers and encyclopaedia 
 articles cited in the text, to which a single other reference 
 may be added.* Thus our volume has been from its incep- 
 tion a dozen years ago but the beginning of a larger scheme 
 which its general vastness on one hand, and the pressure of 
 individual duties and cares upon the other, have alike delayed. 
 But though this can never adequately be carried out, we still 
 hope we may be able to do something, together or separately. 
 
 The past reception of the book in biological circles has 
 been varied. It has been, severely let alone by some authors, 
 even when discussing the subject, and generously borrowed 
 from by others, with or without acknowledgment. It has 
 sometimes completely failed to be understood; we think 
 we may say chiefly by those too pure morphologists to whom 
 our starting-point, the ordinary physiological conception of 
 protoplasmic metabolism as laid down, by Claude Bernard 
 
 * Cf. the authors "On the Moral Evolution of Sex," The Ever- 
 green, Edinburgh, Summer, 1896. 
 
vui preface. 
 
 and his successors, had not been previously familiar. Some- 
 times too it has been thoughtfully and keenly criticised, and 
 from this we trust we have not wholly failed to profit. But 
 it has also provoked a certain amount of research, which is 
 better than all compliments and criticisms. We venture to 
 think that the general tendency of research has been sub- 
 stantially to confirm, not weaken our general theory : yet our 
 hope is that the growing strength of the still young school of 
 experimental evolutionists may before many years yield results 
 which will involve not merely a revision, but a recasting of our 
 book. 
 
 From a wide circle, beyond that of professed biologists, we 
 have also received criticisms suggestive as well as encouraging, 
 and we trust, therefore, that, with all its shortcomings, this 
 revised edition will be found freed of at least some errors, and 
 freshened by the incorporation of some new observations and 
 thoughts. 
 
 P. G. 
 
 J. A. T. 
 
 Edinburgh,"! 
 
 {December, igoo. 
 Aberdeen, J 
 
PREFACE 
 
 TO THE FIRST EDITION (1889). 
 
 IN course of the preparation of critical summaries, such as the 
 articles " Reproduction " or " Sex," contributed by one of 
 us to the " Encyclopaedia Britannica," or the account of recent 
 progress annually prepared for the Zoological Record by the 
 other, we have not only naturally accumulated considerable 
 material towards a general theory of the subject, but have come 
 to take up an altered and unconventional view upon the general 
 questions of biology, particularly upon that of the factors of 
 organic evolution. Hence this little book has the difficult task 
 of inviting the criticism of the biological student, although 
 primarily addressing itself to the general reader or beginner. 
 The specialist therefore must not expect exhaustiveness, despite 
 a good deal of small type and bibliography, over which other 
 readers (for whose sakes technicalities have also been kept 
 down as much as possible) may lightly skim. 
 
 Our central thesis has been, in the first place, to present an 
 outline of the main processes for the continuance of organic 
 life with such unity as our present knowledge renders possible ; 
 and in the second, to point the way towards the interpretation 
 of these processes in those ultimate biological terms which 
 physiologists are already reaching as regards the functions of 
 individual life, — those of the constructive and destructive 
 changes (anabolism and katabolism) of living matter or proto- 
 plasm. * 
 
X PREFACE. 
 
 But while Books I. and II. are thus the more important, 
 and such chapters as "Hermaphroditism," " Parthenogenesis," 
 "Alternation of Generations," have only a subordinate and 
 comparatively technical interest, it will be seen that our theme 
 raises nearly all the burning questions of biology. Hence, for 
 instance, a running discussion and criticism of the speculative 
 views of Professor Weismann, to which their very recent intro- 
 duction to English readers* has awakened so wide an interest. 
 At once of less technical difficulty, and in some respects even 
 wider issues, is the discussion of Mr Darwin's theory of sexual 
 selection, reopened by the other leading contribution to the 
 year's biological literature which we owe to Mr Alfred Russel 
 Wallace.! Besides entering this controversy at the outset of the 
 volume, we have in the sequel attempted to show that the view 
 taken of the processes concerned with the maintenance of the 
 species leads necessarily to a profound alteration of our views 
 regarding its origin, although the vast problems thus raised 
 necessarily remain open for fuller separate treatment. It is right, 
 however, to say that the restatement of the theory of organic 
 evolution, for which we here seek to prepare (that not of indefi- 
 nite but definite variation, with progress and survival essen- 
 tially through the subordination of individual struggle and 
 development to species-maintaining ends), leads us frankly to 
 face the responsibility of thus popularising a field of natural 
 knowledge from which there are so many superficial reasons to 
 shrink, and which knowledge and ignorance so commonly 
 conspire to veil. For if not only the utmost degeneracy be 
 manifestly connected with the continuance of organic species, 
 but also the highest progress and blossoming of life in all its 
 forms, of man or beast or flower, it becomes the first practical 
 
 ''"Heredity." Oxford, 1889. + " Darwinism." Lond. 1889. 
 
PREFACE. XI 
 
 application of biological science not only to investigate and 
 map out these two paths of organic progress, but to illuminate 
 them. Hence we have attempted to indicate the application of 
 the general organic survey, which has been our main theme, to 
 such questions as those of human population and progress, 
 although here, more even than elsewhere, our treatment can be 
 at best only suggestive, not exhaustive. While limits of space 
 have made it impossible to give the botanical side of our sub- 
 ject its proportionate share of attention, our illustrations of the 
 essential facts are sufficient to show the parallelism of the 
 reproductive processes throughout nature. 
 
 It remains to express our thanks to Professor F. Jeffrey 
 Bell for some valuable suggestions while the work was passing 
 through the press ; to Mr G. F. Scott-Elliot for assistance in 
 summarising certain portions of the literature ; and to our 
 engravers, Messrs Harry S. Percy, F. V. M'Combie, and G. 
 A. Morison, especially to the first-named, who has executed 
 the great majority of our illustrations with much care and skill. 
 
 PATRICK GEDDES. 
 
 J. ARTHUR THOMSON. 
 
CONTENTS. 
 
 BOOK I.— MALE AND FEMALE. 
 
 CHAPTER I. 
 
 The Sexes and .Sexual Selection 
 
 page 
 3-15 
 
 i 1. Primary and secondary sexual characters - 3 
 
 I 2. Illustrations from Darwin - 5 
 
 i 3, Darwin's explanation by sexual selection - - 8 
 
 i 4. Criticisms of Darwin's explanation - 10 
 
 (a.) Wallace - - - - -. 10 
 
 (b. ) Brooks - - - - - - 11 
 
 (c. ) St George Mivart - - - - 13 
 
 CHAPTER II. 
 
 The Sexes, and Criticism of Sexual Selection - 16-33 
 
 § I. Sex-differences - - - - - - 16 
 
 § 2. Differences in general habit. Males more active, 
 
 females more passive ----- 16 
 
 § 3. Differences in size. Males smaller, females larger. 
 
 Pigmies and exceptions - - - - 19 
 
 § 4. Secondary differences in colour, skin, &c. Males 
 relatively more katabolic, females relatively more 
 anabolic ------ 22 
 
 § 5. Sexual selection : Its limits as an explanation - - 27 
 
 Postulate of extreme aesthetic sensi- 
 tiveness - - . - 29 
 Darwin and Wallace combined and 
 
 supplemented - - - 31 
 
 Sexual selection a minor accelerant, 
 natural selection a check to ex- 
 tremes of variation - - 31 
 
CONTENTS. 
 CHAPTER III. 
 
 PAGE 
 
 The Determination of Sex (Hypotheses and Obser- 
 vations) 34-44 
 
 § I. The period at which sex is settled. Ploss, Sutton, 
 
 Laulanie, &c. . - . - - 34 
 
 § 2. Over five hundred theories suggested — 
 Theological. 
 Metaphysical. 
 
 Statistical and hypothetical. 
 Experimental. (Chap. IV.) 
 § 3. Theory of male and female ova is no solution - - 3° 
 
 § 4. Theory of " polyspermy," or multiple fertilisation, 
 
 dismissed ------ 3° 
 
 § 5. The theory of age of elements allowed. Thury, 
 
 Hensen, &c. ------ 3° 
 
 §6. Theory of parental age of secondary moment. Hofacker 
 
 and Sadler ■""""." 37 
 
 §7. Theories of "comparative vigour," &c, require analysis 38 
 § 8. Theory of Starkweather,— many factors combined under 
 
 "superiority" ----- 39 
 § 9. Darwin's position ----- 40 
 § 10. Dusing's synthetic treatment, and theory of self-regula- 
 tion of numbers ----- 40 
 |n. The sexes of twins - - - - - 4 1 
 
 CHAPTER IV. 
 The Determination of Sex (Constructive Treatment) 45~59 
 § 1. Nutrition as a factor determining sex. Favourable 
 
 nutrition tends to females - 45 
 
 (a.) Yung's tadpoles - 45 
 
 (b. ) Case of bees ... - 46 
 
 (c. ) Von Siebold's observations - 48 
 
 (d. ) Case of aphides - 49 
 
 (e. ) Butterflies and moths - 50 
 
 (f.) Crustaceans ----- 50 
 
 (g. ) Rotifers . - - - - - 51 
 
 (h.) Mammals - - - - - . 51 
 
 (z'.) Human species - 51 
 
 (j.) Plants ----- 52 
 
 §2. Temperature as a factor. Favourable conditions tend to 
 
 females ...... 53 
 
 § 3. Summary of factors : — 
 
 (a.) Nutrition, age, &c, of parents affecting — 
 (b.) Condition of sex-cells, followed by — 
 (c. ) Environment of embryo ... 54 
 §4. General conclusion: — Anabolic conditions favour pro- 
 duction of females, katabolic conditions males - 55 
 § 5. Hence corroboration of conclusion of Chap. II. , that fe- 
 males were preponderatingly anabolic, males katabolic 55 
 §6. Notes on Weismann's theory of heredity - - 55 
 
CONTENTS. 
 
 BOOK II.— ANALYSIS OF SEX- 
 CELLS. 
 
 CHAPTER v. 
 
 Sexual Organs and Tissues 
 
 § I. Essential sexual organs of animals 
 
 § 2. Associated ducts 
 
 § 3. Origin of yolk-glands, &c. 
 
 § 4. Organs auxiliary to impregnation 
 
 § 5- E ggl a y in g organs 
 
 § 6. Brood-pouches - 
 
 -ORGANS, TISSUES, 
 
 PAGE 
 63-7O 
 
 63 
 
 66 
 67 
 (■7 
 68 
 69 
 
 IIekm 
 
 § 
 § 
 
 § 
 § 
 
 § 1 
 
 A- 
 5- 
 6. 
 
 7- 
 
 CHAPTER VI. 
 
 APHRODITTSM ----.. 71-86 
 
 1. Definition of hermaphroditism ; its varied forms - 71 
 
 2. Embryonic hermaphroditism. Ploss, Laulanie, Sutton 71 
 
 3. Casual or abnormal hermaphroditism, from jelly-fish to 
 mammal ...... y 2 
 
 Partial hermaphroditism, from butterflies to birds - 72 
 
 Normal adult hermaphroditism, from sponges to toads 75 
 
 Degrees of normal hermaphroditism - - - 7S 
 
 Self- fertilisation and its preventives - - - 79 
 
 8. Complemental males — cirripedes and Myzostomata - 82 
 
 9. Conditions of hermaphroditism ; its association with 
 passivity and parasitism - S3 
 
 o. Origin of hermaphroditism ; a primitive or a secondary 
 
 condition -..-.. £4 
 
 CHAPTER VII. 
 
 The Sex-Elements (General and Historical) - - S7-103 
 
 § 1. The ovum-theory ..... s§ 
 
 g 2. The history of embryology, "evolution" and " epi- 
 
 genesis " ...... SS 
 
 (a.) Harvey's epigenesis and prevision of ovum- 
 theory - SS 
 (b. ) Malpighi and early observers - - S9 
 {c.) Preformation school; " evolution " accord- 
 ing to Haller, Bonnet, and Buffon ; 
 ovists and animalculists - - - 89 
 (J.) Wolffs demonstration of epigenesis - 91 
 {e. ) Wolff's successors - - - ' - 92 
 § 3. The cell-theory ..... g 2 
 
 § 4. The protoplasmic movement - 92 
 § 5- Protozoa contrasted with Metazoa : the making of the 
 
 " body ''.----- 95 
 
 § 6. General origin of the sex-cells in sponges - - - 97 
 
 ,, ,, ,, ,, ccelenterates - 97 
 
 ,, ,, ,, ,, other Metazoa - 97 
 
 § 7. Early separation of the sex-cells in a minority of cases 98 
 
XVI CONTENTS. 
 
 PAGE 
 
 § 8. "Body" versus reproductive cells, and the continuity 
 
 of the latter ------ 99 
 
 (a.) Owen ----- 99 
 
 (b.) Haeckel and Rauber . . - ioo 
 
 (c.) Brooks - - - - " I0 ° 
 
 (rf.)Jager I0 ° 
 
 («.) Gallon IO ° 
 
 (/)Nussbaum ----- ioo 
 
 \g.) Weismann ----- ioo 
 
 §9. Weismann's theory of the continuity of the germ plasma 101 
 
 CHAPTER VIII. 
 The "Egg-Cell or Ovum ----- 104-116 
 
 § 1. Structure of ovum — 
 
 Cell-substance and protoplasm - - - 104 
 
 Nucleus and chromatin - 105 
 
 § 2. Growth of ovum — 
 
 Transition from amoeboid to encysted phase - 106 
 
 § 3. The yolk- 
 Its threefold mode of origin - - - 107 
 Its diffuse, polar, or central disposition - - 108 
 Resulting influence on segmentation - - 109 
 
 § 4. Composite ova - - - - - - no 
 
 § 5- Egg-envelopes — 
 
 (a. ) From ovum itself - - - - 1 10 
 
 (b. ) From surrounding cells - - - no 
 
 (c. ) From special glands - - - no 
 
 § 6. Birds' eggs- 
 Concrete illustration of facts and problems - no 
 
 § 7. Chemistry of the ovum- 
 Its capital of anastates - - - - in 
 
 § 8. Maturation of ovum — 
 
 Occurrence, formation, history of polar globules ; 
 
 parthenogenetic ova - - - - 112 
 
 § 9. Theories of polar globules — 
 
 (a) Minot, Balfour, &c. - - - 113 
 
 (b.) Giard, Mark, Biitschli, &c. - - 114 
 
 (c.) Weismann, Hertwig, &c. - - - 114 
 
 CHAPTER IX. 
 
 The Male-Cell or Spermatozoon . - - - 117-125 
 
 § 1. General contrast between sperm and ovum — 
 
 An index to contrast between male and female - 117 
 
 § 2. History of Discovery- 
 fa. ) Hamm and Leeuwenhoek - - - 117 
 [b. ) Animalculists - - - - 117 
 (c. ) Classed as Entozoa or parasites - - 117 
 (d.) Kblliker's demonstration of cellular origin - 118 
 
CONTENTS. XVII 
 
 i 3. Structure of spermatozoon — 
 
 " Head," " tail," "middle portion," &c 118 
 
 1 4. Physiology of spermatozoon — 
 
 Locomotor energy and persistent vitality 120 
 
 i 5. Origin of spermatozoa — 
 
 Theory of spermatogenesis - 121 
 
 i 6. Further comparison of sperm and ovum — 
 
 Processes comparable with formation of polar 
 
 globules 123 
 
 i 7. Chemistry of the sperm - 123 
 
 CHAPTER X. 
 
 heory of Sex : Its Nature and Origin - - 126-142 
 
 § I. Suggested theories of male and female — 
 
 Rolph 126 
 
 Minot - 127 
 
 Brooks - - 127 
 
 §2. Nature of sex — seen in Sex-cells 127 
 
 The cell-cycle 127 
 
 Protoplasmic interpretation 131 
 
 §3. Problem of origin of sex - 135 
 
 § 4. Nature of sex as seen in its origin among plants 136 
 
 § 5- Nature of sex as seen in its origin among animals 137 
 
 § 6. General conclusions from foregoing chapters 1 39 
 
 BOOK III.— PROCESSES OF REPRODUCTION. 
 
 CHAPTER XL 
 
 sxdal Reproduction - 145-168 
 
 § 1. Different modes of reproduction 145 
 
 § 2. Facts involved in sexual reproduction 145 
 
 § 3. Fertilisation in plants — 
 
 From Sprengel to Strasburger - 146 
 
 § 4. Fertilisation in higher animals — 
 
 From Martin Barry and Biitschli to Van Bene- 
 
 den and Boveri 150 
 
 § 5. Fertilisation in Protozoa 157 
 
 §6. Origin of fertilisation — 
 
 (2.) Plasmodium - 159 
 
 (/>.) Multiple conjugation 159 
 
 (c.) Ordinary conjugation ■ - 160 
 
 (a?.) Union of incipiently dimorphic cells 161 
 
 \e.) Fertilisation by differentiated sex-cells 161 
 
 § 7. Hybridisation in animals and plants 162 
 
 § 8. Telegony - - , - 166 
 
XV1U CONTENTS. 
 
 CHAPTER XII, 
 
 PA 
 
 Theory of Fertilisation ..... 169-1 
 
 § 1. Old theories — 
 
 (a) Ovists, {!>) animalculists, (c) the "aura 
 
 seminalis" - - - - - 161 
 
 § 2. Modern morphological theories — 
 
 Nuclei all-important. Hertwig, Strasburger, &c. 17 
 Cell-substance also important. Nussbaum, 
 
 Boveri, &c. ----- 17 
 § 3. Modern physiological theories — 
 
 Sachs, De Bary, Marshall Ward, &c. - - 17; 
 
 Cienkowski and Rolph - - - - 1731 
 
 Weismann's view - - - - 173] 
 
 Critique and statement of present theory - 173 
 
 § 4. Use of fertilisation to the species — 
 
 Rejuvenescence — - 17 
 
 Van Beneden and Biitschli - - - 17? 
 
 Galton and Hensen - - - - 171, 
 
 Weismann's critique - - - 176 
 
 The observations of Maupas - - - 176 
 
 A source of variation. Brooks and Weismann - 179 
 
 CHAPTER XIII. 
 
 Degenerate Sexual Reproduction or Parthenogenesis- 183-199 
 
 § 1. History of discovery ----- jg., 
 
 § 2. Degrees of parthenogenesis — 
 
 Artificial, pathological, occasional, partial, sea- 
 sonal, juvenile, total - . . jgn 
 § 3. Occurrence in animals — 
 
 Rotifers, crustaceans, insects - - - !gg 
 
 § 4. Occurrence in plants — 
 
 Phanerogams and fungi - - . . j„ 
 
 § 5. The offspring of parthenogenesis - - . jqj 
 
 § 6. Effects on the species - : q 2 
 
 § 7. Peculiarities of parthenogenetic ova — 
 
 Weismann's discovery - „, 
 
 § 8. Theory of parthenogenesis — 
 
 (a.) Balfour - - . . . 
 
 (c) Rolph ■ - - ■ ■ \l\ 
 
 (d.) Strasburger - :?jj 
 
 (e. ) Weismann - j: J 
 
 The theory proposed - ^5 
 
 § 9. Origin of parthenogenesis - 
 
 197 
 
CONTENTS. XIX 
 CHAPTER XIV. 
 
 PAGE 
 
 ^sexual Reproduction 200-212 
 
 1 § I. Artificial division - 200 
 
 I § 2. Regeneration - . . 201 
 
 §3. Degrees of asexual reproduction - 203 
 
 § 4. Asexual reproduction in plants and animals - • 203 
 
 CHAPTER XV. 
 
 Alternation of Generations - - 213-229 
 § I. History of discovery - - 213 
 §2. Rhythm between sexual and asexual reproduction 214 
 § 3. Alternation between sexual and degenerate sexual repro- 
 duction 216 
 § 4. Combination of both these alternations 219 
 § 5- Alternation of juvenile parthenogenetic reproduction 
 
 with the adult sexual process 220 
 § 6. Alternation of parthenogenesis and ordinary sexual 
 
 reproduction - 220 
 
 §7- Alternation of different sexual generations 221 
 
 §8. Occurrence of these alternations in animals 221 
 
 § 9. Beard's hypothesis of alternation of generations in 
 
 vertebrates - - 223 
 
 §10. Occurrence of alternations in plants - 223 
 
 § 1 1. The problem of heredity in alternating generations 224 
 
 § 12. Hints as to the rationale of alternation 225 
 
 §13. Origin of alternation of generations 226 
 
 BOOK IV.— THEORY OF REPRODUCTION. 
 CHAPTER XVI. 
 
 Growth and Rlproduction 233-245 
 
 § I. Facts of growth 233 
 
 § 2. Spencer's theory of growth 234 
 
 § 3. Cell-division - 235 
 
 § 4. Protoplasmic restatement - - 237 
 
 § 5. Antithesis between growth and reproduction - 238 
 § 6. The contrast in the individual — • 
 
 (a.) In distribution of organs 239 
 
 (£.) In the periods of life • 240 
 
 §7. The contrast between asexual and sexual reproduction- 241 
 
 CHAPTER XVII. 
 
 Theory of Reproduction — continued - - 246-252 
 
 § 1. The essential fact in reproduction 246 
 
 § 2. The beginnings of reproduction - 246 
 
 § 3. Cell-division - - 247 
 
CONTENTS. 
 
 pa! 
 § 4. Gradations from asexual severance to liberation of sex- 
 cells - - - - - - - 24 
 
 § 5. The close connection between reproduction and death - 24, 
 
 § 6. Reproduction as influenced by the environment - 24: 
 
 § "]. General conclusion ----- 25' 
 
 CHAPTER XVIII. 
 
 Special Physiology of Sex and Reproduction - - 253-282, 1 
 
 § I. The continuity of the germ-plasm - - - 2531 
 
 § 2. Sexual maturation ----- 255 
 
 §3. Menstruation - - - -' - 250 
 
 § 4. Sexual union ------ 261. 
 
 § 5. Parturition ------ 26C 
 
 § 6. Early nutrition ------ 265' 
 
 § 7. Lactation -...-. 266^ 
 
 § 8. Other secretions ... 26; 
 
 § 9. Incubation ------ 26 
 
 §10. Nemesis of reproduction ... 272 
 
 § 11. Organic immortality ----- 275 
 
 CHAPTER XIX. 
 
 Psychological and Ethical Aspiccts - - - 283-298 
 
 §1. Common ground between animals and men - - 283 
 
 § 2. The love of mates - - - - - 283 
 
 § 3. Sexual attraction ----- 285 
 § 4. Intellectual and emotional differences between the 
 
 sexes - - - - - . 286 
 
 § 5- Love for offspring ----- 291 
 
 § 6. Egoism and altruism ----- 295 
 
 CHAPTER XX. 
 
 Laws of Multiplication - 
 
 § 1. Rate of reproduction and rate of increase 
 
 § 2. History of discussion - - - . 
 
 §3. Spencer's analysis; individuation and genesis - 
 
 § 4. Spencer's application to man - 
 
 § 5. General statement of the population question - 
 
 § 6. Sterility - 
 
 CHAPTER XXI. 
 
 The Reproductive Factor in Evolution - . 316-333 
 
 §1. General history of evolution - . . . - g 
 
 § 2. The reproductive factor so far as hitherto recognised - 32? 
 
 §3. Suggested lines of further construction - i 2 Q 
 
 299-315 
 299 
 300 
 300 
 3°4 
 305 
 314 
 
BOOK I. 
 
 THE SEXES and SEXUAL SELECTION. 
 1. 
 
THE EVOLUTION OF SEX. 
 
 t 
 
 CHAPTER I. 
 
 The Sexes and Sexual Selection. 
 
 TT^HAT all higher animals are represented by distinct male 
 JJ_ and female forms, is one of the most patent facts of 
 observation, striking enough in many a beast and bird to catch 
 any eye, and familiarly expressed in not a few popular names 
 which contrast the two sexes. In lower animals, the contrast, 
 and indeed the separateness, qf the sexes often disappears; yet 
 ven naturalists have sometimes mistaken for different species, 
 Cvhat were afterwards recognised to be but the male and female 
 of a single form. 
 
 § i. Primary and Secondary Characters. — When we pass 
 from this commonplace of observation and experience to inquire' 
 more precisely into the differences between the sexes, we speedily 
 recognise that these are of very different degrees. In some cases 
 no marked differences whatever are recognisable ; thus a male 
 star-fish or sea-urchin looks exactly like the female, and a care- 
 ful examination of the essential reproductive organs is requisite 
 to determine whether these respectively produce male elements 
 or eggs. In other cases, for instance in most reptiles, no 
 external differences are at all striking, but the aspect of the 
 internal organs, both essential and auxiliary to reproduction, at 
 once settles the question. In a great number of cases, again, 
 the sexes resemble one another closely, but each has certain 
 minor structural features at once decisive as to its respective 
 maleness or femaleness. Thus in the males there are 
 frequently prominent organs used in sexual union, while the 
 peculiar functions of the females are indicated in the special 
 egg-laying or young-feeding organs. All such characters, 
 
THE EVOLUTION OF SEX. 
 
 directly associated with the essential functions of the sexe 
 are included under the title oi primary sexual characters. 
 
 Of less real importance, though often much more strikin 
 are the numerous distinctions in size, colour, skin, skeleton, ar 
 
 Male and Female Bird of Paradise (Parodist* minorX-Yxom 
 Catalogue of Zoological Museum, Dresden. 
 
 the like, which often signalise either sex. These are termed 
 secondary sexual characters; for though they will be shown in 
 
THE SEXES AND SEXUAL SELECTION. 5 
 
 ae-ne cases at least to be truly part and parcel with the male or 
 i eunale constitution, they are only of secondary importance in 
 h-2 reproductive process. The beard of man and the mane 
 fethe lion, the antlers of stags and the tusks of elephants, the 
 c?irgeous plumage of the peacock or of the bird of paradise, 
 f e familiar examples of secondary sexual characters in males, 
 'ior are the females lacking in special characteristics, which 
 jirve as indices of their true nature. Large size is one of the 
 Ibmmonest of these ; while in some few cases the excellencies 
 n colour, and other adornments, are possessed by the females 
 Father than by their mates. 
 
 The whole subject of secondary sexual characters has found 
 its most extensive treatment in Darwin's " Descent of Man," 
 and to that work, therefore, the more so as its limits exceed 
 those of the present volume, the reader must be assumed to 
 make reference. All that can be here attempted is an illustra- 
 tion, by representative cases, of the main differences between 
 the sexes ; from which we shall pass to Darwin's interpretation, 
 land, after a fresh survey, to a re-statement from another point 
 of view. 
 
 , § 2. Illustrations from Darwin. — Among invertebrates, 
 prominent secondary sexual characters are rarely exhibited 
 outside the great division of jointed-footed animals or arthro- 
 pods. There, however, among crustaceans and spiders, but 
 especially among insects, beautiful illustrations abound. Thus 
 the great claws of crabs are frequently much larger in the 
 males; and male spiders often differ from their fiercely coy 
 mates, in smaller size, brighter colours, and sometimes in the 
 
 Winged Male and Wingless Female of a Moth 
 {Orgyia antique?). — From Leunis. 
 
 power of producing rasping sounds. Among insects, the males 
 are frequently distinguished by brighter colours attractively dis- 
 played, by weapons utilised in disposing of their rivals, and 
 by the exclusive possession of the power of noisy love-calling. 
 Thus, as the Greek observed, the cicadas "live happy, having 
 voiceless wives." Not a few male butterflies are pre-eminently 
 
6 THE EVOLUTION OF SEX. 
 
 more brilliant than the females ; and many male beetles fig 
 savagely for the possession of their mates. 
 
 Passing to backboned animals, we find that among fisr 
 the males are frequently distinguished by bright colours ai 
 ornamental appendages, as well as by structural adaptations f 
 combat. Thus the "gemmeous dragonet " (Callionymus lyra) 
 flushed with gorgeous colour, in great contrast to the " sordid 
 female, and is further adorned by a graceful elongation of tl 
 dorsal fin. In many cases, as in the sea-scorpion (Coth 
 scorpius), or in the stickleback (Gasterosteus), it is only at th< c 
 reproductive period that the males are thus transformed* 
 literally putting on a wedding-garment. Every one knows, on 
 the other hand, the hooked lower jaw of the male salmon, 
 which comes to be of use in the furious charges between rivals! 
 and this is but one illustration of many structures utilised in 
 the battle for mates. In regard to amphibians, it is enough to! 
 recall the notched crests and lurid colouring of our male newtsJ 
 and the indefatigable serenading powers of male frogs and 
 toads, to which the females are but weakly responsive. Among 
 reptiles, differences of this sort are comparatively rare, but male 
 snakes have often more strongly-pronounced tints, and the 
 scent-glands become more active during the breeding season.; 
 In this, as in many other cases, love has its noisy prayer re- 
 placed by the silent appeal of fragrant incense. Among lizards, 
 the males are often more brightly decorated, the splendour of 
 their colours being frequently exaggerated at pairing time. 
 They may be further distinguished by crests and wattle-like 
 pouches ; while horns, probably used in fighting, are borne by 
 some male chameleons. 
 
 It is among birds, however, that the organic apparatus of 
 courtship is most elaborate. The males very generally excel in, 
 brighter colours and ornaments. Beautiful plumes, elongated 
 feathery tresses, brightly-coloured combs and wattles, top-knots 
 and curious markings, occur with marvellous richness of variety. 
 These are frequently displayed by their possessors before the 
 eyes of their desired mates, with what seem to us like emotions 
 of love and vanity. Or it may be to the subtler charms of music 
 that the wooers mainly trust. During the breeding season, the 
 males are jealously excited and pugnacious, while some have 
 special weapons for dealing directly with their rivals. The differ- 
 ences between the magnificent male birds of paradise and their 
 soberly coloured mates, between the peacock with his hundred 
 
THE SEXES AND SEXUAL SELECTION. 
 
 s and the plain peahen, between the musical powers of 
 le and female songsters, are very familiar facts. Or again, 
 combs and "gills" of cocks, the "-wattles" of turkey-cocks, 
 i immense top-knot of the male umbrella-bird (Cephalopterus 
 uitus), the throat-pouch of the bustard, — illustrate another 
 
 Blackcock and Grey Hen (Male and Female). 
 
 series of secondary sexual characters. The spurs of cocks and 
 allied birds are the most familiar illustrations of weapons used 
 by the males in fighting with rivals. As in other animals, it is 
 important to notice that male birds often acquire their special 
 secondary characters, such as colour, markings, and special 
 forms of feathers, only as they approach sexual maturity, and 
 sometimes retain them in all their glory only during the 
 breeding season. 
 
 Among mammals, which stand in so many ways in marked 
 contrast to birds, the law of battle much more than the power 
 of charming decides the problem of courtship. Thus most 
 of the striking secondary characters of male mammals are 
 weapons. Yet there are crests and tufts of hair, and other 
 acknowledgments of the beauty test, while the incense of 
 odoriferous glands is a very frequent means of sexual attrac- 
 
8 THE EVOLUTION OF SEX. 
 
 tion. The colours too of the males are often more sha 
 contrasted, and there are minor differences, in voice and 
 like, which cannot be ignored. Of weapons, the larger ca 
 teeth of many male animals, such as boars ; the special t 
 of, for instance, the elephant and narwhal ; the antlers of si 
 
 The development of antlers in the successive years of a 
 stag's life, or in the general history of stags. — From 
 Carus Sterne. 
 
 all but exclusively restricted to the combative sex ; th 
 horns of antelopes, goats, etc., — which are usually mucl 
 stronger in the males, — are well-known illustrations. Th 
 manes of male lions, bisons, and baboons ; the beards ( 
 certain goats ; the crests along the backs of some antelopes)'; 
 the dewlaps of bulls, — illustrate another set of secondary 
 characters. The odoriferous glands of many mammals are mora 
 developed in the males, and become specially functional during 
 the breeding season. This is well illustrated in the case of 
 goats, deer, shrew-mice, elephants. The differences in colour 
 are slight compared with those seen between the sexes in birds, 
 but in not a few orders the distinction is marked enough, males 
 being, in the great majority of cases, the more strongly and 
 brilliantly coloured. Among monkeys the difference in colour 
 in the bare regions, and the subtler decorations in the arrange- 
 ment of the hair on the face, are often very conspicuous. 
 
 § 3. Danvin's Explanation — Sexual Selection. — Darwin 
 started from the occurrence of such variations, in structure and 
 habit, as might be useful either for attraction between the sexes 
 or in the direct contests of rival males. The possessors of 
 these variations succeeded better than their neighbours in the 
 art of courtship ; the factors which constituted success were 
 transmitted to the offspring ; and, gradually, the variations were 
 
THE SEXES AND SEXUAL SELECTION. 9 
 
 established and enhanced as secondary sexual characters of the 
 species. The process by which the possessors of the fortunate 
 excellencies of beauty and strength outbid or overcome their 
 less endowed competitors, he termed "sexual selection." It is 
 only fair, however, to state Mr Darwin's case by direct 
 quotation. 
 
 Sexual selection " depends on the advantage which certain 
 individuals have over others of the same sex and species solely 
 in respect of reproduction." ... In cases where " the males 
 have acquired their present structure, not from being better 
 fitted to survive in the struggle for existence, but from having 
 gained an advantage over other males, and from having trans- 
 mitted this advantage to their male offspring alone, sexual 
 selection must have come into action." ... "A slight degree 
 of variability, leading to some advantage, however slight, in 
 reiterated deadly contests, would suffice for the work of sexual 
 selection." ... So too, on the other hand, the females " have, 
 by a long selection of the more attractive males, added to their 
 beauty or other attractive qualities." ... "If any man can in 
 a short time give elegant carriage and beauty to his bantams, 
 according to his standard of beauty, I can see no reason to 
 dpubt that female birds, by selecting during thousands of 
 generations the most melodious or beautiful males, according 
 to their standard of beauty, might produce a marked effect." 
 ..." To sum up on the means through which, as far as we 
 can judge, sexual selection has led to the development of 
 secondary sexual characters. It has been shown that the 
 largest number of vigorous offspring will be reared from the 
 pairing of the strongest and best-armed males, victorious in 
 contests over other males, with the most vigorous and best- 
 nourished females, which are the first to breed in the spring. 
 If such females select the more attractive, and at the same 
 time vigorous males, they will rear a larger number of offspring 
 than the retarded females, which must pair with the less 
 vigorous and less attractive males. So it will be if the more 
 vigorous males select the more attractive, and at the same time 
 healthy and vigorous females; and this will especially hold 
 good if the male defends the female, and aids in providing 
 food for the young. The advantage thus gained by the more 
 vigorous pairs in rearing a larger number of offspring, has 
 apparently sufficed to render sexual selection efficient." 
 Another sentence from Darwin's first statement of his position 
 
10 THE EVOLUTION OF SEX. 
 
 must, however, be added. " I would not wish," he says in the 
 "Origin of Species," "to attribute all such sexual differences 
 to this agency; for we see peculiarities arising and becoming 
 attached to the male sex in our domestic animals, which we 
 cannot believe to be either useful to the males in battle or 
 attractive to the females." 
 
 § 4. Criticisms of Darwin's Explanation. — The above 
 explanation may be summed up in a single sentence, — a con- 
 genital variation, advantageous to its possessor (usually a male) 
 in courtship and reproduction, becomes established and per- 
 fected by the success it entails. Sexual selection is thus only 
 a special case of the more general process of natural selection, 
 with this difference, that the female for the most part takes the 
 place of the general environment in the picking and choosing 
 which is believed to work out the perfection of the species. 
 
 The more serious objections which have been hitherto 
 urged against this hypothesis, apart altogether from criticism 
 of special cases, are the following : — (a) Alfred Russel Wallavce 
 and others would explain the facts on the more general theovy 
 of natural selection, allowing comparatively little import to tbie 
 alleged sexual selection exercised by the female, (b) Some, 
 who allow great importance to both natural and sexual selec- 
 tion, are not satisfied with leaving variation a mere postulate. 
 The position occupied by Brooks will be sketched below. 
 (c) Different from either of the above is the position occupied 
 by St George Mivart and others, who attach comparatively 
 little importance to either natural or sexual selection, but seek 
 in terms of definite variation, constitutional tendencies, laws of 
 growth, — for the idea is very variously expressed, — to find the 
 primary and fundamental interpretation of sexual dimorphism. 
 
 (a) Wallace's Objection. — It is convenient to begin with 
 Wallace's early criticism, which precedes that of Brooks in 
 chronological order. This is the more helpful in clearing the 
 ground, since the two theories of Wallace and Darwin are 
 strikingly and, at first sight, irreconcilably opposed. Accord- 
 ing to Darwin, the gayness of male birds is due to selection on 
 the part of the females ; according to Wallace, the plainness of 
 female birds is due to natural selection, which has eliminated 
 those which persisted to the death in being gay. He points 
 out that conspicuousness during incubation would be dangerous 
 and fatal ; the more conspicuous have, he thinks, been picked 
 off their nests by hawks, foxes, and the like, and hence only 
 
THE SEXES AND SEXUAL SELECTION. II 
 
 the sober-coloured females now remain. Darwin starts from 
 inconspicuous forms, and derives gorgeous males by sexual 
 selection; Wallace starts from conspicuous forms, and derives 
 the sober females by natural selection ; the former trusts to the 
 preservation of beauty, the latter to its extinction. In 1773, 
 the Hon. Daines Barrington, a naturalist still remembered as 
 the correspondent of Gilbert White, suggested that singing- 
 birds were small, and hen-birds mute for safety's sake. This 
 suggestion Wallace has repeated and elaborated in reference 
 especially to birds and insects. The female butterfly, exposed 
 to danger during egg-laying, is frequently dull and inconspicuous 
 compared with her mate. The original brightness has been 
 forfeited by the sex as a ransom for life. Female birds in open 
 nests are similarly, in many cases, coloured like their surround- 
 ings ; while in those birds where the nests are domed or 
 covered, the plumage is gay in both sexes. 
 
 But in his book on " Darwinism " Wallace goes much 
 further in his destructive criticism of Darwin's sexual selection. 
 The phenomena of male ornament are discussed, and summed 
 up as being " due to the general laws of growth and develop- 
 ment," and such that it is " unnecessary to call to our aid so 
 hypothetical a cause as the cumulative action of female prefer- 
 ence." Or again, "if ornament is the natural product and 
 direct outcome of superabundant health and vigour, then no 
 other mode of selection is needed to account for the presence 
 of such ornament." This mode of criticism, however, belongs 
 to our third category. 
 
 (fr) Brooks has called attention to the sexual differences in 
 lizards, where the females do not incubate ; or in fishes, where 
 the females are even less exposed to danger than the males ; 
 or in domesticated birds, where, though all danger is removed, 
 the males are still the more conspicuous and diversified sex. 
 "The fact too that many structures, which are not at all con- 
 spicuous, are confined, like gay plumage, to male birds, also 
 indicates the existence of an explanation more fundamental 
 than the one proposed by Wallace, and the latter explanation 
 gives no reason why the females of allied species should often 
 be exactly alike when the males are very different." To the 
 explanation which Brooks proposes we must therefore pass. 
 
 According to Darwin, Brooks says, the greater modification 
 of the niales°is due to their struggling with rivals, and to their 
 selection by the females, but " I do not believe that this goes 
 
12 THE EVOLUTION OF SEX. 
 
 to the root of the matter." The study of domesticated pigeons, 
 for instance, shows that " something within the animal deter- 
 mines that the male should lead and the female follow in 
 the evolution of new breeds." The same is true in other 
 domesticated animals, where, from the nature of the circum- 
 stances, it is inadmissible to explain this with Darwin, by 
 supposing that the male is more exposed than the female to 
 the action of selection, whether natural or sexual. Darwin 
 concludes, indeed, that the male is more variable than the 
 female, but he gives no satisfactory reason why female varia- 
 tions should be less apt than male variations to become 
 hereditary, or, in other words, why the right of entail is so 
 much restricted to the male sex. Darwin merely attributes 
 this to the greater eagerness of the males, which "in almost all 
 animals have stronger passions than the females.'' The theory 
 which Brooks maintains, is bound up with an hypothesis of 
 heredity differing considerably from that held by Darwin. He 
 supposes that the cells of the body give off gemmules, chiefly 
 during change of function or of environment, and that " the 
 male reproductive cell has gradually acquired, as its especial 
 and distinctive function, a peculiar power to gather and store 
 up these gemmules." The female reproductive cells keep up 
 the general constancy of the species, the male cells transmit 
 variations. " A division of physiological labour has arisen 
 during the evolution of life, and the functions of the repro- 
 ductive elements have become specialised in different direc- 
 tions." " The male cell became adapted for storing up 
 gemmules" (the results of variations in the body), "and at the* 
 same time gradually lost its unnecessary and useless power to 
 transmit hereditary characteristics." "We thus look to the 
 cells of the male body for the origin of most of the variations 
 through which the species has attained to its present organisa- 
 tion." The males are the more variable, but more than that, 
 their variations are much more likely to be transmitted. "We 
 are thus able to understand the great difference in the males 
 of allied species, the difference between the adult male and the 
 female or young, and the great diversity and variability of 
 secondary male characters; and we should expect to find, what 
 actually is the case, that among the higher animals, when the 
 sexes have long been separated, the males are more variable 
 than the females." The contrast between Darwin and Brooks 
 may now be summed up again in a sentence. Darwin says, 
 
THE SEXES AND SEXUAL SELECTION. 1 3 
 
 the males are more diversified and richer in secondary sexual 
 characters, chiefly because of the sexual selection exercised 
 alike in courtship and in battle. Brooks admits sexual 
 selection, but believes that the males are naturally or con- 
 stitutionally more variable than the females, and that it is the 
 peculiar function of the male elements to transmit variations, 
 as opposed to the constant tradition of structure kept up by 
 the egg-cells or ova. In other words, the females may choose, 
 yet the males lead ; nay more, they must lead, for male varia- 
 tions have by hypothesis most likelihood of being transmitted. 
 
 Full consideration of this hypothesis would involve much 
 discussion of the problems of inheritance, but the general con- 
 clusion of the naturally greater variability of the males, will be 
 stated in a different light towards the close of the following 
 chapter. It will there be shown that the " something within 
 the animal," which determines the preponderance of male 
 variability, may be stated in simpler terms than are involved in 
 Brooks's theory. Moreover, the greater variability of the males, 
 which seems quite plain when we contrast, for instance, ruffs 
 and reeves, requires to be proved for each case. Karl Pearson 
 has disproved it for man. 
 
 Somewhat similar to Wallace's later position is that (c) 
 occupied by St George Mivart, who also looks for some deep 
 constitutional reason for sexual dimorphism. The entire 
 theory of sexual selection appears to him an unverified hypo- 
 thesis, only acquiring plausibility when supported by numerous 
 subsidiary suppositions. He submits a number of detailed 
 criticisms ; but his chief contention is, that the beauty of 
 males, and other secondary sexual characters, are not the 
 indirect results of a long process of external selection, but the 
 direct expressions of the internal differences of a progressively 
 varying internal constitution, i.e., of tendencies inherent in the 
 individual. 
 
 Mivart's position and the vague suggestions of Mantegazza 
 and others are of importance as indications of progress towards 
 a fundamental restatement. As we have seen, an obvious 
 objection to the theory of sexual selection is that, while it may 
 in part account for the persistence and progress of secondary 
 characters after they attained a certain degree of development, 
 it does not account for their preservation when weak or incon- 
 spicuous. In short, the theory may account for the perfecting, 
 but not for the origin of the characters. It may be enough to 
 
14 THE EVOLUTION OF SEX. 
 
 account for the length and the trimmings of the living garment, 
 but what we wish to know is the secret of the loom. Darwin's 
 account of the evolution of the eyes on the feathers of the Argus 
 pheasant is indeed ingenious and interesting; but, whatever its 
 probability, it is more important to ask what the predominant 
 brightness of males means as a general fact in physiology. It 
 is of interest, then, to notice the hints thrown out by Mante- 
 gazza, Wallace, and others, directly associating decorativeness 
 with superfluous reproductive material, and the putting on of 
 wedding-robes with the general excitement of the sexually 
 mature organism. From this record of the discussion, it is 
 time however to turn to a more constructive mode of treat- 
 ment. 
 
 In passing, however, it should be noted that the fact of 
 preferential or selective mating can be proved not only by 
 observation, as the Peckhams have done in the case of spiders, 
 but more conclusively by statistical enquiry, by investigating 
 "whether the type and variability of the mated and unmated 
 members of one or other sex are the same " (see Karl Pearson's 
 "Grammar of Science," 2nd ed., 1900, p. 425). Apart from 
 the particular problem of secondary sexual characters, it is of 
 the utmost importance whether the mating is selective or in- 
 discriminate. For if natural selection is at work its effect will 
 be checked by indiscriminate mating, and aided by preferential 
 or, to use the wider term, selective mating. 
 
THE SEXES AND SEXUAL SELECTION. 1 5 
 
 SUMMARY, 
 
 I, 2. The existence of male and female animals is a commonplace of 
 observation. They differ in primary and in secondary sexual characters, 
 of which illustrations are given, chiefly from Darwin. 
 
 3. Darwin's theory of sexual selection seeks to account for the preserva- 
 tion and perfection of variations, advantageous _in courtship or in battles 
 with rivals. 
 
 4. Wallace maintains that the females have been protectively retarded 
 by natural selection, and at a later date suggests that the greater decora- 
 tiveness, etc., of the males is an expression of superabundant vigour. 
 Brooks believes that the males are naturally the more variable, and pre- 
 dominate in power of transmitting variations. Mivart demands a deeper 
 analysis than is afforded by either sexual or natural selection. This physio- 
 logical rationale is hinted at. 
 
 LITERATURE. 
 
 BROOKS, W. K. — The Law of Heredity: A Study of the Cause of Varia- 
 tion and the Origin of Living Organisms. Baltimore, 1883. 
 
 Cunningham, J. T. — Sexual Dimorphism. London, 1900. 
 
 Le Dantec, F. — La Sexualite. Paris, 1899, 98 pp. 
 
 Darwin, C. — On the Origin of Species by Means of Natural Selection; 
 or, The Preservation of Favoured Races in the Struggle for Life. 
 London, 1859. 
 
 The Descent of Man, and Selection in Relation to Sex. London, 
 
 1871. 
 
 Delage, Y. — La structure du protoplasma, et les theories de l'heredite et 
 les grands problemes de la hiologie generate. Paris, 1895. 
 
 Ellis, Havelock. — Man and Woman (Contemporary Science Series). 
 
 Geddes, P., and Thomson, J. A.— L' evolution du sexe. Quelques 
 probabilites biologiques. Revue de Morale Sociale, I. 1899, pp. 95-m. 
 
 Mivart, St George. — Lessons from Nature. London, 1876. Cf. also 
 The Genesis of Species. London, 1871. 
 
 Morgan, C. Lloyd.— Animal Life and Intelligence. London, 1890-91. 
 
 Pearson, K— Grammar of Science (revised edition). London, 1900. 
 
 Wallace, A. R.— Contributions to the Theory of Natural Selection. 
 
 Darwinism': An Exposition of the Theory of Natural Selection, 
 
 with Some of its Applications. London, 1889. 
 
CHAPTER II. 
 
 The Sexes, and Criticism of Sexual Selection. 
 
 § i. Sex-Differences.— -To gain a firmer and broader founda- 
 tion on which to base a theory of the differences between the 
 sexes, it is necessary to take another review of the facts of the 
 case. Instead of considering the differences as they are ex- 
 pressed in the successive classes of animals, it will be more 
 convenient to arrange them for themselves, according as they 
 affect habit, size, length of life, and the like. The review must 
 again be merely representative, without any attempt at com- 
 pleteness. 
 
 Male and Female Coccus Insects, a, part of a cactus 
 plant with the excrescences due to coccus insects ; 
 i, male ; c t female. 
 
 § 2. General Habit. — Let us begin with an extreme yet well- 
 known case. The female cochineal insect, laden with carmine, 
 which some have interpreted as a reserve-product, spends 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 
 
 17 
 
 much of its life like a mere quiescent gall on the cactus plant. 
 The male, on the other hand, in his adult state is agile, 
 restless, and short-lived. Now this is no mere curiosity of the 
 entomologist, but in reality a vivid emblem of what is an aver- 
 age truth throughout the world of animals — the preponderating 
 passivity of the females, the predominant activity of the males. 
 These coccus insects are the martyrs of their respective sexes. 
 Take another illustration, again somewhat extreme. There is 
 a troublesome threadworm {Heterodera schachtii) infesting the 
 turnip plant, which parallels in more ways than one the contrast 
 of the coccus insects. The adult male is agile, and like many 
 another threadworm ; the adult female, however, is quiescent, 
 and bloated like a drawn-out lemon. It may be asked, how- 
 ever, is not this merely the natural nemesis 
 of parasitism? The life-history answers 
 this objection. The two sexes are at first 
 alike, — agile, and resembling most thread- 
 worms ; they become parasitic, and lose 
 both activity and nematode form ; but the 
 interesting fact is further, that the male 
 recovers himself, while the female remains a 
 victim. In other insect and worm types 
 the same story, in less accented characters, 
 may be distinctly read. In many crusta- 
 ceans, again, the females only are parasitic; 
 and while this is in part explained by their 
 habit of seeking shelter for egg-laying pur- 
 poses, it also expresses the constitutional 
 bias of the sex. The insect order of bee 
 parasites (Strepsiptera) is remarkable for the 
 completely passive and even larval character 
 of the blind parasitic females, while the 
 adult males are free, winged, and short-lived. 
 Throughout the class of insects there are 
 numerous illustrations of the excellence 
 of the males over the females, alike in 
 muscular power and sensory acuteness. 
 The diverse series of efforts by which the 
 males of so many different animals, from cicadas to birds, 
 sustain the love-chorus, affords another set of illustrations of 
 pre-eminent masculine activity. 
 
 Without multiplying instances, a review of the animal 
 
 2 
 
 Female Chondracanthus i a 
 
 parasitic Crustacean, 
 with pigmy male (a) 
 attached just above the 
 origin of the long egg- 
 sacs (/>) of the female. 
 — -From Claus. 
 
THE EVOLUTION OF SEX. 
 
 kingdom, or a perusal of Darwin's pages, will amply confirm the 
 conclusion that on an average the females incline to passivity, 
 the males to activity. In higher animals, it is true that the 
 contrast shows rather in many little ways than in any one 
 striking difference of habit, but even in the human species the 
 contrast is recognised. Every one will admit that strenuous 
 spasmodic bursts of activity characterise men, especially in 
 youth, and among the less civilised races ; while patient con- 
 tinuance, with less violent expenditure of energy, is as generally 
 associated with the work of women. 
 
 Both sexes of a Flea— the Jigger or Chigoe {Sarco^sylla penetrans)', the 
 female much swollen with eggs. — From Leuckart. 
 
 For completeness of argument, two other facts, which will 
 afterwards claim full discussion, may here be simply mentioned. 
 {a) At the very threshold of .sex-difference, we find that a little 
 active cell or spore, unable to develop of itself, unites in 
 fatigue with a larger more quiescent individual. Here, at the 
 very first, is the contrast between male and female, (b) The 
 same antithesis is seen, when we contrast, as we shall afterwards 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 1 9 
 
 do in detail, the actively motile, minute, male element of most 
 animals and many plants, with the larger passively quiescent 
 female-cell or ovum. 
 
 It is possible that the reader may urge as a difficulty against 
 the above contrast the exceedingly familiar case of the male 
 bees or " drones." It must be frankly allowed that exceptions 
 do indeed occur, though usually in conditions which afford a 
 key to the abnormality. Thus it will be allowed that the 
 "drones" are in a peculiar position as male members of a very 
 complex society, in which what is practically a third sex is 
 represented by the great body of "workers." They are no 
 more fair examples of the natural average of males, than the 
 hard-driven wives of the lazy Kaffir are of the normal functions 
 of women. Nor is the exception even here a real one, for the 
 drone, although passive as compared with the unsexed workers, 
 is active when compared with the extraordinarily passive queen. 
 
 To the above contrast of general habit, two other items 
 may be added, on which accurate observation is still unfortun- 
 ately very restricted. In some cases the body temperature, 
 which is an index to the pitch of the life, is distinctly lower in the 
 females, as has been noted in cases so widely separate as the 
 human species, insects, and plants. In many cases, further- 
 more, the longevity of the females is much greater. Such a 
 fact as that women pay lower insurance premiums than do 
 men, is often popularly accounted for by their greater immunity 
 from accident ; but the greater normal longevity on which the 
 actuary calculates, has, as we begin to see, a far deeper and 
 constitutional explanation. 
 
 § 3. Size. — Among the higher animals, there are curious 
 alternations in the preponderance of one sex over another in 
 size. Thus among mammals and birds the males are in most 
 cases the larger ; the same is true of lizards ; but in snakes the 
 females preponderate. In fishes, the males are on an average 
 smaller, sometimes very markedly so, even to the extent of not 
 being half as large as their mates. Below the line, among 
 backboneless animals, there is much greater constancy of 
 predominance in favour of the females. Thus among insects, 
 the more active males are generally smaller, and often very 
 markedly ; of spiders the same is true, and the males being 
 often very diminutive are forced to task their agility to the 
 utmost in making advances to their unamiable mates. So 
 again, crustacean males are often smaller than the females ; and 
 
THE EVOLUTION OF SEX. 
 
 in many parasitic species, what have been well called "pigmy" 
 males illustrate the contrast in an almost ludicrous degree. 
 
 Two cases from aberrant worm types exhibit very vividly this same anti- 
 thesis of size. Among the common rotifers, the males are almost always 
 very different from the females, and much smaller. Sometimes they seem 
 to have dwindled out of existence altogether, for only the females are known 
 (Philodina, Rotifer, Callidina, Adineta). In Polyarthra platyptera the 
 male is "hardly to be distinguished from a Vorticella which has become 
 detached from its stalk." In Hydalina senta (see fig.) the male is about 
 a third the size of the female, has no alimentary canal, and has only two 
 or three days of adult life. In the great majority the males are " little 
 more than perambulating bags of spermatozoa," though in a few cases, like 
 RMnops vilrea, degeneration seems hardly to have begun (Rousselet, 
 1897). Even when present, they are not indispensable, for partheno- 
 genesis is very general. 
 
 Relative sizes of a male and female Rotifer (Hydat/na senta). 
 — From Leunis. 
 
 In a remarkable marine worm, Bonellia, the male remains 
 like a remote ancestor of the female. It lives parasitically on 
 or within the latter, and is microscopic in size, measuring in fact 
 only about one hundredth part of the length of its host and mate. 
 Somewhat similar to the case of Bonellia is that of a vivi- 
 parous coccus insect (Lecanium hesperidum), where the males 
 are very degenerate, small, blind, and wingless. In spite of 
 this condition, perhaps indeed because of it, they are very 
 
THE SEXES, AXD CRITICISM OF SEXUAL SELECTION. 21 
 
 male, for even the larvae, while still within the mother, have 
 been shown to contain fully-developed spermatozoa. In a 
 little " bear animalcule " or Tardigrade, Macrobiotus macronyx, 
 the males are about half the size of the females and decidedly 
 more active. A particularly interesting case of sexual di- 
 morphism in molluscs has been described by Professor 
 E. G. Conklin ("Proc. Acad. Nat. Sci. Philadelphia," 1898, 
 pp. 435-444, 3 pis.). In Crepidula plana, which occurs in 
 
 Figure of tie female Bonellia (from Atlas of Naples Aquarium), 
 with its parasitic pigmy male enlarged. 
 
 shells tenanted by hermit-crabs {Eupagurus bemhardus), 
 the female is about fifteen times larger than the male, and 
 the latter- retains throughout life the power of locomotion 
 possessed by the females- in their young stages only. I he 
 females occurring along with the little hermit-crab, Eupa- 
 vurus longicarpus, are always dwarfed, having a body volume 
 only one-thirteenth of the more typical form. The cells of the 
 dwarfs are of the normal size, but there are fewer of them, and 
 
22 THE EVOLUTION OF SEX. 
 
 the eggs are also much less numerous. But this dwarfing is 
 a mere " modification," and not inherited; it probably depends 
 upon pressure or upon differences of nutrition and oxygena- 
 tion. That is to say, the dwarfs are not a race, but are 
 continually recruited from the young of the giants. It is 
 interesting, therefore, to note that environmental influences 
 may modify the female till in size it resembles the normally 
 dwarfish male; and that the small size of the male implies, as 
 in the case of the dwarfs, not smaller cells, but a smaller 
 number of cells. In both dwarfs and males, the process of 
 cell-division has been inhibited. 
 
 Dr T. W. Fulton, Naturalist to the Scottish Fishery Board, 
 has made valuable statistical observations on the size and 
 numerical proportions of male and female fishes, (i) The 
 females are usually considerably more numerous than the 
 males, and never less numerous except in the angler and the 
 cat-fish. The proportion of females to males among flat- 
 fishes ranges from about i : i in the flounder, to about 12:1 
 in the long rough dab. Among "round" fishes the same 
 proportion varies from about 3 : 2 in the cod, to 9 : 2 in the 
 common gurnard. (2) The female is longer and larger among 
 all the flat-fishes, sometimes by as much as 30 per cent. In 
 cod, haddock, angler, and cat-fish, the males are larger, while 
 in the whiting the females are slightly larger, and in the 
 common gurnard decidedly so. 
 
 One must not indeed base an argument on extreme cases, 
 but there is no doubt that up to the level of amphidians at 
 least the females are generally the larger. 
 
 Apparent exceptions occur, it is true, among the higher 
 animals. In birds and mammals the males are usually rather 
 larger than the females. This difference consists especially in 
 larger bones and muscles. The apparent exception is in part 
 the natural result of the increased stress of external activities 
 which are thrown upon the shoulders of the males when their 
 mates are incapacitated by incubation or pregnancy. Further- 
 more, we must recognise the strengthening influence of the 
 combats between males, and the effect produced on the accu- 
 mulative constitution of the females by the increased maternal 
 sacrifice characteristic of the highest animals. 
 
 § 4. Other Characters. — While it is easy to point to the 
 general physiological import of large size and the reverse, 
 physiology is not yet far enough advanced to afford firm foot- 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 23 
 
 hold in dealing with the details of secondary sexual characters. 
 It is only possible to point out the path which will eventually 
 lead us to their complete rationale. The point of view is 
 simple enough. The agility of males is not merely an adapta- 
 tion to enable that sex to exercise its functions with relation to 
 the other, but is a natural characteristic of the constitutional 
 activity of maleness; and the small size of many male fishes is 
 not an advantage at all, but simply again the result of the 
 contrast between the more vegetative growth of the female 
 and the costly activity of the male. So, brilliancy of colour, 
 exuberance of hair and feathers, activity of scent-glands, and 
 even the development of weapons, cannot be satisfactorily 
 explained by sexual selection alone, for this is merely a 
 secondary factor. In origin and continued development they 
 are outcrops of a male as opposed to a female constitution. 
 To sum up the position in a paradox, all secondary sexual 
 characters are at bottom primary, and are expressions of the 
 same general habit of body (or to use the medical term, 
 diathesis), as that which results in the production of male 
 elements in the one case, or female elements in the other. 
 This theory of the origin and primary meaning of those 
 variations which culminate in marked sexual dimorphism is 
 obviously similar to that adopted by Wallace in his book on 
 " Darwinism." 
 
 Three well-known facts must be recalled to the reader's 
 mind at this point; and firstly, that in a great number of cases 
 the secondary sexual characters make their appearance step by 
 step with sexual maturity itself. When the animal — be it a 
 bird or insect — becomes emphatically masculine, then it is that 
 these minor outcrops are exhibited. Thus the male bird of 
 paradise, eventually so resplendent, is usually in its youth 
 comparatively dull and female-like in its colouring and 
 plumage. Very often, too, whether in the wedding-robe of 
 male fishes or in the scent-glands of mammals, the character 
 rises and wanes in the same rhythm as that of the reproductive 
 periods. It is impossible not to regard at least many of the 
 secondary sexual characters as part and parcel of the sexual 
 diathesis, — as expressions for the most part of exuberant 
 maleness. 
 
 Secondly, when the reproductive organs are removed by 
 castration, the secondary sexual characters are often much 
 modified. Thus, as Darwin notes, stags never renew their 
 
£4 THE EVOLUTION OF SE5t. 
 
 antlers after castration, though normally, of course, they renew 
 them each breeding season. The reindeer, where the horns 
 occur on the females as well, is an interesting exception to the 
 rule, for after castration the male still renews the growth. This 
 however merely indicates that the originally sexual characters 
 have become organised into the general life of the body. In 
 sheep, antelopes, oxen, &c, castration modifies or reduces the 
 horns ; and the same is true of odoriferous glands. The 
 parasitic crustacean Sacadina has been shown by Delage to 
 effect a partial castration of the crabs to which it fixes itself, and 
 the same has been observed by Giard in other cases. In two 
 such cases ah approximation to the female form of appendage 
 has been observed. Rorig (1899) has shown that a diseased 
 state of the ovaries in deer is correlated with a development of 
 antlers, that atrophy of the testes is always followed by some 
 peculiarity in the antler-growth, that castration of a young 
 male always inhibits the development of antlers, and so on. 
 Sellheim (1898, 1899) finds that in many animals of both sexes 
 castration is followed by a prolongation of the period of bone- 
 growth. In the case of young cocks the effects of castration 
 are very variable, sometimes increasing, sometimes decreasing 
 the secondary sex characters. One result is clear, however, 
 that the whole body is affected; the larynx is intermediate in 
 size between that -of cock and hen, the syrinx is weakly de- 
 veloped and the capons seldom crow or do so abnormally, the 
 brain and heart are light in weight, fat accumulates in the sub- 
 cutaneous and subserous connective tissue, and the skeleton 
 shows many abnormalities. The experiments of J. Th. 
 Oudemans (1898) on castrating caterpillars — a difficult opera- 
 tion — led him to the conclusion, in marked contrast to the 
 above, that there was little result either on the external appear- 
 ance or on the habits of the adults. 
 
 Thirdly, it should also be noted that in aged females, which 
 have ceased to be functional in reproduction, the minor pecu- 
 liarities of their sex often disappear, and they become liker 
 males, both in structure and habits, — witness the familiar case 
 of "crowing hens." 
 
 From the presupposition, then, of the intimate connection 
 between the sexuality and the secondary characters (which is 
 indeed everywhere allowed), it is possible to advance a step 
 further. Thus in regard to colour, that the male is usually 
 brighter than the female is an acknowledged fact. But pig- 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 25 
 
 ments of many kinds are physiologically regarded as of the 
 nature of waste products. Such for instance is the guanin, so 
 abundant on the skin of fishes and some other animals. 
 Abundance of such pigments, and richness of variety in related 
 series, point to pre-eminent activity of chemical processes in the 
 animals which possess them. Technically expressed, abundant 
 pigments are expressions of intense metabolism. But pre- 
 dominant activity has been already seen to be characteristic of 
 the male sex; these bright colours, then, are often natural 
 to maleness. In a literal sense animals put on beauty for 
 ashes, and the males more so because they are males, and not 
 primarily for any other reason whatever. We are well aware 
 that, in spite of the researches of Krukenberg, Sorby, MacMunn,. 
 Newbigin, and others, our knowledge of the physiology of 
 pigments is still very scanty. Yet in many cases, alike among 
 plants and animals, pigments are expressions of disruptive 
 processes, and are of the nature of waste products; and this 
 general fact is at present sufficient for our contention, that bright 
 colouring or rich pigmenting is more characteristic of the male 
 than of the female constitution. 
 
 In the same way, the skin eruptions of male fishes at the 
 spawning season seem more pathological than decorative, and 
 may be directly connected with the sexual excitement. One 
 instance of the way in which the reproductive maturity is known 
 to effect a by no means obviously related result may be given. 
 Every field naturalist knows that the male stickleback builds a 
 nest among the weeds, and that he weaves the material together 
 by mucous threads secreted from the kidneys. The little animal 
 is also known to have strong passions; it is polygamous in 
 relation to its mates, and most pugnacious in relation to its 
 rivals. Professor Mobius has shown that the male reproductive 
 organs (or testes) become very large at the breeding season, and 
 that they press in an abnormal way upon the kidneys. This 
 encroachment produces a pathological condition in the kidneys, 
 and the result is the formation of a mucous secretion, somewhat 
 similar to what occurs in renal disease in higher forms. To 
 free itself from the irritant pressure of this secretion, the male 
 rubs itself against externalpbjects, most conveniently upon its 
 nest. Thus the curious \ aving instinct does not demand or 
 find rationale in the cumui itive action of natural selection upon 
 an inexplicable variation, but is traced back to a pathological 
 and mechanical origin in the emphatic maleness of the organism. 
 
26 
 
 THE EVOLUTION OF SEX. 
 
 The line of variation being thus given, it is of course conceiv- 
 able that natural selection may have accelerated it. 
 
 So too, though again the physiological details are scanty, the 
 superabundant growth of hair and feathers may be interpreted, 
 in some measure through getting rid of waste products, for we 
 shall see later how local katabolism favours cell multiplication. 
 Combs, wattles, and skin excrescences point to a predominance 
 of circulation in the skin of the feverish males, whose tempera- 
 tures are known in some cases to be decidedly higher than 
 those of the females. Even skeletal weapons like antlers may 
 be similarly interpreted; while the exaggerated activity of the 
 scent-glands is another expedient for excreting waste. 
 
 Male (c), Worker (6), and Queen (a) Ant.— From Chambers's Encyc, after Lubbock. 
 
 In regard to horns, feathers, and the like, in association with vigorous 
 circulation, two sentences from Rolph may be quoted: — " The exceedingly 
 abundant circulation, which periodically occurs in the at first soft frontal 
 protuberances of stags, admits and conditions the colossal development of 
 horn and delicate ensheathing velvet. ... In the same way, the rich 
 flow of blood in the feather papillae conditions the immense growth of the 
 feathers, . . . and the same is true of hairs, spines, and teeth." 
 
 Professor J. Kennel gives expression in an interesting essay to an entirely 
 different interpretation of such structures as antlers. It may be that they, 
 like the horns of Ruminants, were originally possessed by both sexes, and that 
 they have been lost by the females whose reproductive sacrifice leaves, as it 
 were, less to spare for such expensive structures as antlers are. .So it may 
 be that the female deer have ceased to develop antlers except where the 
 conditions of life rendered their retention indispensable, namely, in the 
 reindeer. In other words, this may be one of the many cases in which the 
 female is nearer not to the ancestral but to the youthful type. 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 27 
 
 Some of the even subtler differences between the sexes are 
 of interest in illustrating the general antithesis. Thus in the 
 love-lights of the Italian glow insect (Luciola), the colour is 
 said to be identical in the two sexes, and the intensity is much 
 the same. That of the female, however, who is in other respects 
 rather male-like in her amatory emotions, is more restricted. 
 It is interesting further to notice that the rhythm of the light 
 in the male is more rapid and the flashes are briefer, while that 
 of the female is longer and the flashes more distant and tremu- 
 lous. This illustration may thus serve as a literally illumined 
 index of the contrasted physiology of the sexes. 
 
 The case of the Psychidae- among Lepidoptera is particularly instructive. 
 Both males and females are normal caterpillars, but while the males become 
 winged, the female remains at the larval level, as regards absence of wings 
 and the usual adult appendages, oral as well as locomotor, but differing 
 from the larva in being reproductive. In short, the female degenerates to 
 the juvenile level except in productivity, while the male without doubt is 
 nearer the ancestral form. 
 
 § 5. Sexual Selection: its Limit as an Explanation. — We 
 are now in a better position to criticise Mr. Darwin's theory. 
 On his view, males are stronger, handsomer, or more emo- 
 tional, because ancestral forms happened to become so in a 
 slight degree. In other words, the reward of breeding-success 
 gradually perpetuated and perfected a casual advantage. 
 According to the present view, males are stronger, handsomer, 
 or more emotional, simply because they are males, — i.e., of 
 more active physiological habit than their mates. In phrase- 
 ology which will presently become more intelligible and 
 concrete, the males tend to live at a loss, are relatively more 
 katabolic. The females, on the other hand, tend to live at a 
 profit, are relatively more anabolic, — constructive processes 
 predominating in their life, whence indeed the capacity of 
 bearing offspring. 
 
 No one can dispute that the nutritive, vegetative, or self- 
 regarding processes within the plant or animal are opposed to 
 the reproductive, multiplying, or species-regarding processes, as 
 income to expenditure, or as building up to breaking down. 
 But within the ordinary nutritive or vegetative functions of the 
 body, there is necessarily a continuous antithesis between two 
 sets of processes, — constructive and destructive metabolism. 
 The contrast between these two processes is seen throughout 
 nature, whether in the alternating phases of cell life, or of 
 
28 THE EVOLUTION OF SEX. 
 
 activity and repose, or in the great antithesis between growth 
 and reproduction ; and it is this same contrast which we 
 recognise as the fundamental difference between male and 
 female. The proof of this will run through the work, but our 
 fundamental thesis may at once be roughly enunciated in a 
 diagrammatic expression (which in its present form we owe to 
 our friend Dr W. E. Fothergill):— 
 
 Here the sum-total of the functions are divided into 
 nutritive and reproductive, the former into anabolic and 
 katabolic processes, the latter into male and female activities, — 
 so far with all physiologists, without exception or dispute.* 
 Our special theory lies, however, in suggesting the parallelism 
 of the two sets of processes. Thus maleness is associated 
 with a life-ratio in which katabolism has a relatively greater 
 
 SUM OF FUNCTIONS. 
 
 Nutrition. Reproduction. 
 
 Anabolism. Katabolism. Female. Male. 
 
 predominance than in the female. In terms of this thesis, 
 therefore, both primary and secondary sexual characters 
 express the fundamental physiological bias characteristic of 
 either sex. Sexual selection resembles artificial selection, but 
 the female takes the place of the human breeder; it resembles 
 natural selection, but the selective females and the combative 
 
 * The reader whose physiological studies may not have familiarised him 
 with that conception (really dating from Claude Bernard) of all physiological 
 processes as finding their ultimate expression in the metabolism (anabolism 
 and katabolism) of protoplasm, will easily place himself in a position to 
 check our argument (often indeed, we trust to carry our interpretation of 
 sex into still further detail) by starting from the exposition of this doctrine 
 in Sir Michael Foster's article, " Physiology," in the Encyclopedia 
 Britamiica, or with Sir Burdon Sanderson's Presidential Address to 
 Section D, British Association, 1889. 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 20 
 
 males represent a role filled in the larger case by the fostering 
 or eliminating action of the environment. As a special case 
 of natural selection, Darwin's theory is open to the objection 
 of being teleological, i.e., of accounting for structures in terms 
 of a final advantage. It is also open to the logical critic to 
 urge that the structures to be explained have to be accounted 
 for before, as well as after, the stage when they were developed 
 enough to be useful. The origin, or in other words the funda- 
 mental physiological import, of the structures must be ex- 
 plained before we have a complete or adequate theory of 
 organic evolution. 
 
 Apart from this logical insufficiency, the theory of sexual 
 selection is open to many minor objections, with some of 
 which Darwin himself dealt. One detailed objection which 
 seems serious may be noticed. The evolution of coloured 
 markings by selective preference carries with it the postulate 
 of a certain level of sesthetic taste and critical power in the 
 female, and this not only very high and very scrupulous as to 
 details, but remaining permanent as a standard of fashion from 
 generation to generation, — large assumptions all, and scarcely 
 verifiable in human experience. Yet we cannot suppose that 
 Mr Darwin considered the human female as peculiarly un- 
 developed. It is true, doubtless, that both insects and birds 
 have so far and increasingly become educated in such sensi- 
 tiveness; but when we consider the complexity of the mark- 
 ings of the male bird or insect, and the slow gradations from 
 one stage of perfection to another, it seems difficult to credit 
 birds or butterflies with a degree of sesthetic development 
 exhibited by no human being without both special aesthetic 
 acuteness and special training. Moreover, the butterfly, which 
 is supposed to possess this extraordinary development of 
 psychological subtlety, will fly naively to a piece of white 
 paper on the ground, and is attracted by the primary sesthetic 
 stimulus of an old-fashioned wall-paper, not to speak of the 
 gaudy and monotonous brightness of some of our garden 
 flowers. Thus we have the further difficulty, that we must 
 suppose the female butterfly to have a double standard of 
 taste, one for the flowers which she and her mate both visit, 
 the other for the far more complex colouring and markings of 
 the males. And even among birds, if we take those unmis- 
 takable hints of real awakening of the sesthetic sense which 
 are exhibited by the Australian bowerbird or by the common 
 
30 THE EVOLUTION OF SEX. 
 
 jackdaw in its fondness for bright objects, how very rude is 
 this taste compared with the critical examination of in- 
 finitesimal variations of plumage on which Darwin relies. Is 
 not, therefore, his essential supposition too glaringly anthropo- 
 morphic ? 
 
 Again, the most beautiful males are often extremely com- 
 bative; and on the conventional view this is a mere coin- 
 cidence, yet a most unfortunate one for Mr Darwin's view. 
 Battle thus constantly decides the question of pairing, and in 
 cases where, by hypothesis, the female should have most 
 choice, she has simply to yield to the victor. On our view, 
 however, combative energy and sexual beauty rise pari passu 
 with male katabolism. 
 
 Or again, in the ALneas group of the genus Papilio, Darwin 
 notes how there are frequent gradations in the amount of 
 difference between the sexes. Sometimes the sexes are alike 
 dull, where we should have to suppose the aesthetic perception 
 must somehow have been lost or inhibited ; sometimes the 
 females are dull and the males splendid, — for Darwin, an 
 example of the result of sexual aesthetic perception, this of an 
 exquisitely subtle kind however, and without proportionate 
 cerebral enlargement In a third set of cases, both sexes are 
 splendid, which would suggest logically that the male in turn 
 had acquired a taste for splendour. But such cases, which 
 usually need more or less cumbrous additional hypothesis of 
 inheritance and so on to explain them, are intelligible enough 
 if we regard them as a relative increase of katabolism in the 
 life-ratio throughout a series of species. The third set may 
 be supposed to be relatively more katabolic than the first, 
 while the second set are midway; although, it may be freely 
 granted, a knowledge of the habits, size, &c, of the particular 
 species, would be necessary to verify the legitimacy of this 
 interpretation in each case.* 
 
 It is necessary once more to turn to the contrast between 
 the positions of Darwin and Wallace. According to Darwin, 
 sexual selection has accelerated the males into gay colouring; 
 
 * For a discussion of the progressive development of colouring and 
 markings, whether in butterflies or mammals, the reader may be referred 
 to the works of Professor Eimer, especially to his work on Lepidoptera. 
 Reference should also be made to Weismann's " Studies in the Theory of 
 Descent," for a discussion of the markings of caterpillars and butterflies. 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 3 1 
 
 according to Wallace's original view, natural selection has 
 retarded the females (birds or butterflies) and kept them in- 
 conspicuously plain. It is no longer difficult to establish a 
 compromise. Both sexes have differentiated towards their 
 respective goals, not along lines of indefinite variation, but on 
 paths determined by the characteristic constitutions. In other 
 words, the secondary dimorphism has a definite physiological 
 basis in the primary sex-difference. If this interpretation of 
 the origin of the variants be granted, it remains a matter of 
 observation, experiment, and statistics to determine how far 
 the limits are fixed by natural selection (in Wallace's cases), or 
 by sexual selection (in Darwin's). The present position allows 
 the efficacy of natural and sexual selection as limiting, eliminat- 
 ing, in a sense directive, factors, but regards gay colouring as 
 the expression of the relative predominance of katabolism in 
 the male sex, and quiet plainness as equally natural to the pre- 
 dominantly anabolic females. 
 
 At a later stage something more will be said of natural 
 selection, and its limits as an explanation of facts. But it 
 is here desirable to emphasise, that just as we admit the 
 importance of sexual selection as a minor accelerant in the 
 differentiation of the sexes, so we are bound to recognise that 
 natural selection is also continually in operation as a check to 
 a divergence of the sexes which would otherwise tend to 
 become extreme. If this retarding influence of natural selec- 
 tion on the evolutionary process were not continually present, 
 we should find cases like Bonellia and the rotifers much 
 commoner than they are among animals. But it is an error 
 to exaggerate this limiting action into an explanation of the 
 process itself. 
 
THE EVOLUTION OF SEX. 
 
 SUMMARY. 
 
 1-3. A broader basis must be sought from which to understand the 
 differences between the sexes. A general survey shows that the males are 
 more active in habit, the females more passive; that the males tend to be 
 smaller and to have a higher body-temperature, while the females tend to 
 be larger and to live longer. 
 
 4. The close association of secondary sexual characters with the re- 
 productive function is shown in the period or in the periodicity of their 
 development, in the effects of castration, in the peculiarities of aged 
 females, &c. Richer pigmentation, and other male characteristics, may 
 be interpreted as expressions of the relative katabolic predominance in the 
 constitution of males, as opposed to the relative anabolic preponderance of 
 the females. 
 
 5. Sexual selection, as an explanation of secondary sexual characters, 
 does not account for origins nor incipient stages, postulates subtle aesthetic 
 sensitiveness, and is beset by numerous minor difficulties. Yet the opposed 
 positions of Darwin and Wallace both emphasise indubitable facts; while 
 the criticisms of Mivart, the theory of Brooks, and the suggestions of 
 Rolph, Mantegazza, and others, lead on towards a deeper analysis. The 
 general conclusion reached, recognises sexual selection (so far with Darwin) 
 as a minor accelerant, natural selection (so far with Wallace) as a retarding 
 "brake," on the differentiation of sexual characters, which essentially find 
 a constitutional or organismal origin in the relatively katabolic or anabolic 
 diathesis which characterises males and females respectively. 
 
 LITERATURE. 
 
 Brooks, Darwin, Mivart, Wallace. — As before. 
 
 Cunningham, J. T. — Sexual Dimorphism. London, 1900. 
 
 Curatolo, G. E., and Taruli.i, L. — Influence of the Removal of the 
 
 Ovaries on Metabolism. Edinburgh Med. Journal, 1895, PP- 1 37 -I 39. 
 
 Arch. Ital. Biol., XXXIII., 1895, pp. 388-390. 
 Eimer, G. H. T. — Die Enstehung der Arten auf Grund von Vererben 
 
 erworbener Eigenschaften, nach den Gesetzen organischen Wachsens. 
 
 Jena, 1S88. 
 Fowler, G. H. — Effects of Castration. Proc. Zool. Soc. London, 
 
 1894, pp. 485-494. 
 Geddes, P. — Articles Reproduction, Sex, Variation and Selection. 
 
 Encycl. Brit. Also, On the Theory of Growth, Reproduction, Sex, 
 
 and Heredity. Proc. Roy. Soc. Edin. , 1885-86. 
 Hering, E. — Theory of the Functions in Living Matter (1888). Trans. 
 
 by F. A. Welby. Brain, 1S97, pp. 232-258. 
 Keller, C. A. — Evolution of the Colours of North American Land- Birds. 
 
 P. Calif. Acad., III., 1893, pp. 361, 19 pis. 
 
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 33 
 
 Kennel, J. — Studien iiber sexuellen Dimorphismus, Variation und ver- 
 wandte Erscheinungen. Schr. Nat. Ges. Dorpat, IX. (1896), p. 64. 
 
 Oudemans, J. Th.— Zool. Jahrb., XII., 1898, pp. 71-88, 3 pis., 2 figs. 
 
 Rolph, W. H. — Biologische Probleme. Leipzig, 1884. 
 
 Rousselet, C. F. — On the Male of Rhinops vitrea. J. R. Micr. Soc, 
 part 116, 1897, pp. 4-9, I pi. 
 
 Sellheim, O. — Beitrage zur Geburtshilfe und Gyn'akologie, I. (1898), 
 pp. 229-246; and II. (1899), pp. 236-259. 
 
 Weismann, A. — Studies in the Theory of Descent (Meldola's Transla- 
 tion). London, 1880-82. The Germ-Plasm. London, 1893. 
 
CHAPTER III. 
 
 The Determination of Sex (Hypotheses and Observations). 
 
 So far the differences between the sexes as observed in adult 
 forms. Attention must now be turned to the origin of sex itself 
 in the individual organism. The historic beginning of sex will 
 be discussed at a later stage; the present problem concerns 
 the factors which determine whether any given organism will 
 develop into a male or into a female. The question, in other 
 words, is that usually known as the determination of sex. 
 
 § i. The Period at which the Sex is Determined. — Every 
 organism, whether male or female, develops from a fertilised 
 egg-cell, apart of course from the occurrence of asexual and 
 parthenogenetic reproduction. This material, which in one 
 case develops into a male, in another into a female, is, so far 
 as our experience can go, always the same; andwhen the sex of 
 the organism is absolutely decided, is a question to which no 
 general answer can be given. In the higher animals (birds 
 and mammals) it is possible at quite an early date in embryonic 
 life to tell whether the young organism will turn into a male or 
 a female, though in the very earliest stages it is impossible to 
 determine whether the rudiment of the reproductive organs is 
 going to become a testis or an ovary. But in lower vertebrates, 
 such as frogs, the period of embryonic indifference is greatly 
 prolonged; and it seems certain that a hatched tadpole, even 
 after a tendency towards, say maleness, has actually arisen, 
 may in certain conditions have this altered in the opposite 
 direction. Among invertebrates, the sexual organs are often 
 late in acquiring definite predominance in favour of either sex, — 
 that is, the period of undecided indifference is, as one would 
 expect, usually much longer. 
 
 The factors which are influential in determining sex are 
 numerous, and come into play at different periods. The 
 constitution of the mother, the nutrition of the ova, the con- 
 stitution of the father, the state of the male element when 
 
THE DETERMINATION OF SEX. 35 
 
 fertilisation occurs, the embryonic nutrition, and even the 
 larval environment in some cases, these and yet other factors 
 have all to be considered. 
 
 Some observations by Laulanie as to the embryonic organs are of 
 interest in this connection. He distinguishes both in birds and mammals 
 three stages in the individual development of the reproductive organs. 
 These he calls (1) Germiparity, (2) Hermaphroditism, (3) Differentiated 
 Unisexuality; and regards them as parallel to the stages of historic evolu- 
 tion - . Even for the first stage, however, when the elements are still very 
 primitive, he would not allow the accuracy of the terms neutrality or 
 indifference. The elements in both sexes are almost similar, but yet their 
 future fate has been decided. 
 
 Sutton has also emphasised his conviction, that in the individual 
 development a state of embryonic hermaphroditism obtains, and main- 
 tains that one set of elements predominates over the other in the establish- 
 ment of the normal unisexual state. Ploss and others take up a similar 
 position in regard to an early hermaphrodite state. It can only be con- 
 cluded, that the higher the organism is in the series the earlier is its sexual 
 fate sealed; and that it is only in lower vertebrates, and among backbone- 
 less animals, that we can speak of prolonged neutrality of sex, or embryonic 
 hermaphroditism. 
 
 § 2. Ansivers to the Question: What Determines Sex ? — To 
 the question what determines whether an organism shall develop 
 into a male or into a female, many and varied answers have 
 been given. At the beginning of the last century, the theories 
 of sex were estimated at as many as five hundred, and they have 
 gone on increasing. It is evident that even an enumeration of 
 these is not possible, nor is it indeed desirable. As in so 
 many other cases, our ideas respecting the determination of sex 
 have been looked at in three different ways. For the theologian, 
 it was enough to say that " God made male and female." In 
 the period of academic metaphysics, still so far from ended, it 
 was natural to refer to " inherent properties of maleness and 
 femaleness;" and it is still a popular "explanation" to invoke 
 undefined "natural tendencies" to account for the production 
 of males or females. This mode of treatment, it need not be , 
 said, is being abandoned by biologists. It is recognised that 
 the problem is one for scientific analysis; thus the constitu- I. 
 tion, age, nutrition, and environment of the parents must be- 
 especially considered. These investigations, which are mainly 
 restricted to observation and statistics, will be first noticed; the 
 more experimental researches, and the general conclusions, will 
 be discussed in the next chapter. Finally, a physiological 
 
36 THE EVOLUTION OF SEX. 
 
 re-statement, in terms of protoplasmic metabolism, will be 
 suggested. 
 
 § 3. The theory that there are two kinds of ova, respectively 
 destined to develop into males or females, is more than a mere 
 begging of the question. The constitution of the ovum is 
 undoubtedly a fact of primal importance, but we must also 
 recognise the results of experiment, which show that later 
 influences may also be determinative. The hypothesis of two 
 kinds of ova was advanced, for example, by B. S. Schultze, but 
 as the grounds for his views are not admitted as correct, only 
 its existence need be noticed till more observations are forth- 
 coming. Even if two kinds of ova were demonstrable, the 
 question would remain what conditions determine the pre- 
 dominance of this or the other kind. What is the biological 
 meaning of a family with seven daughters and one son ? What 
 is the biological meaning of Shufeldt's case of alternating sex 
 in the five young birds which formed the brood of a sparrow- 
 hawk ? The oldest was male, the next female, and so on in 
 regular alternation. ("Amer. Naturalist," xxxii., 1898, pp. 567- 
 
 57°. 1 % 
 
 § 4. Numerous authors have attached great importance to 
 the process of fertilisation as a determinant of the sex. 
 
 One of the most crude positions has been that of Canestrini, who 
 ascribed the determination of sex to the number of sperms entering the 
 ovum : — The more sperms, the greater the tendency to male offspring. It 
 has, however, been shown by Fol, Pfluger, Hertwig, and others, that 
 " polyspermy," or the entrance of more than one sperm, is extremely rare, 
 is in fact generally impossible. In some of the cases where it is known to 
 occur, it indicates a pathological condition of the egg-shell, and tends to 
 produce abnormalities. Pfluger diluted the seminal fluid of male frogs, and 
 found that no change resulted in the normal numerical proportion of the 
 sexes. The case of drones, furthermore, where males are known to arise 
 from unfertilised ova, is a familiar example, exactly counter to Canestrini's 
 proposition, which may in fact be dismissed as wholly untenable. 
 
 § 5. Time of Fertilisation. — With greater weight various authorities 
 have insisted upon the time of fertilisation. Thus, according to Thury 
 (1863), followed by Dusing (1883), an ovum fertilised soon after liberation 
 tends to produce a female, while an older ovum will rather develop into a 
 male. As a practical breeder Thury claimed to determine the sex of cattle 
 upon this principle; Cornaz and Knight have both practically confirmed 
 this; while Girou has pointed out that female flowers fertilised as soon as 
 they were able to receive pollen tended to produce female offspring. 
 Hertwig has also shown that the internal phenomena of fertilisation vary 
 somewhat with the age of the ovum at the time. Hensen is inclined to 
 accept the general accuracy of Thury's conclusion, but extends it to the 
 male element as well. ' ' A very favourable condition in both ovum and 
 
THE DETERMINATION OF SEX. 
 
 37 
 
 sperm will probably lead to the formation of a female." "According to 
 its condition, a sperm may either insufficiently corroborate the favourable 
 state of the ovum, or constitutionally strengthen an ovum less satisfactorily 
 conditioned." A side-light is thrown on this by Vernon's experiments on 
 hybridising sea-urchins, which show that "the characteristics of the hybrid 
 offspring depend directly on the relative degrees of maturity of the sexual 
 products" ("Phil. Trans.," Series B, vol. cxc. (1898), pp. 465-529). The 
 
 Summary of Statistics bearing on Relative Number of Males and Females. 
 
 Observer. 
 
 No. of 
 Births. 
 
 Locality. 
 
 Father 
 
 older. 
 
 Proportion 
 
 of Males to 
 
 100 Females. 
 
 Father of 
 equal age. 
 Proportion 
 of Males to 
 100 Females. 
 
 Father 
 
 younger. 
 
 Proportion 
 
 of Males to 
 
 100 Females. 
 
 Average 
 Propor- 
 tion of 
 
 Males to 
 100 
 
 Females. 
 
 Remarks. 
 
 Hofacker 
 
 1,996 
 
 Tubingen 
 
 117.8 
 
 92.0 
 
 90-6 
 
 I07-5 
 
 
 Sadler 
 
 2,068 
 
 England 
 
 121. 4 
 
 94.8 
 
 86.5 
 
 114.7 
 
 
 Gohlert 
 
 4.584 
 
 
 108. 
 
 93-3 
 
 82.6 ■ 
 
 *°5-3 
 
 
 Legoyt 
 
 52)3" 
 
 Paris 
 
 104.49 
 
 102.14 
 
 97-5 
 
 102.97 
 
 
 Boulenger 
 
 6,006 
 
 Calais 
 
 109.98 
 
 107.92 
 
 101.63 
 
 107.9 
 
 
 Noirot 
 
 4,000 
 
 Dijon 
 
 99-7 
 
 
 116.0 
 
 103.5 
 
 
 Breslau 
 
 8,084 
 
 Zurich 
 
 103.9 
 
 103. 1 
 
 117.6 
 
 106.6 
 
 
 Stieda 
 
 100,590 
 
 Alsace- 
 Lorraine 
 
 105.03 
 
 
 108.39 
 
 106.27 
 
 Contradictory. 
 
 Berner 
 
 267,946 
 
 Sweden 
 
 104.61 
 
 106.23 
 
 107.45 
 
 106.0 
 
 Contradictory 
 (see text). 
 
 same observer has shown that the degree of staleness of the ova and sperms 
 of sea-urchins has an appreciable effect on the development ("P. Roy. 
 
 Soc," lxv. (1899), pp. 3SO-360)- 
 
 § 6. Age of Parents— -Hofacker (1823) and Sadler (1830) independently 
 published a body of statistics, each including about 2,000 births, in favour 
 of the generalisation that when the male parent is the older the offspring 
 are preponderatingly male ; while if the parents be of the same age, or 
 
38 THE EVOLUTION OF SEX. 
 
 a fortiori if the male parent be the younger, female offspring appear in 
 increasing majority. This conclusion, generally known as Hofacker's and 
 Sadler's law, has received both confirmation and perplexing contradiction. 
 It has been confirmed by Gohlert, Boulenger, Legoyt, and others, and by 
 some breeders of stock and birds, but is denied by other practical authori- 
 ties, and directly contradicted by the recent statistics of Stieda, from 
 Alsace-Lorraine, and of Berner, from Scandinavia. 
 
 The above table (in its upper part taken mainly from Hensen, after 
 CEsterlen) shows vividly how much the results of Stieda and Berner conflict 
 with the law of Hofacker and Sadler. In regard to Berner's statistics, it 
 ought to be further noted that the figures quoted refer to cases where the 
 father or mother is only from I to 10 years the older. If the father be 
 more than ten years older, the male majority is.103.54; if the mother 
 be more than ten years older, the proportion is 104.10, again against 
 Hofacker's and Sadler's conclusion. Compared with the above human 
 statistics, Schlechter's results in regard to horses also militate agaimt the 
 alleged law. 
 
 In regard to plants, various naturalists have drawn attention to the 
 influence of age upon sex. The following observations are quoted by 
 Heyer: — In Leontarus domestica, according to Rumpf, the female plant 
 may bear male blossoms before its proper female flowers. In Moms nigra, 
 and in other cases, according to Miller, male flowers may be borne first, 
 and afterwards fruit. Treviranus observed that the first flowers of beech, 
 chestnut, and other trees are male. Clausen gives similar examples; and 
 Hoffmann notes that in the horse-chestnut, and several other cases, male 
 flowers appear first, and afterwards hermaphrodites or females. 
 
 Most of the results in regard to the influence of age are, 
 however, extremely unsatisfactory and conflicting. This is 
 evident from the above statistics. The law of Hofacker and 
 Sadler cannot be regarded as in any sense established. In 
 fact, as Hensen remarks, unless statistics are enormously large 
 they prove very little. The number of other factors besides 
 parental age which may operate in any case is evidently great, 
 — health, nutrition, frequency of sexual intercourse, abstinence 
 after the birth of a male, and the like, all reduce the feasibility 
 of the statistical method. At present, at any rate, we are not 
 justified in ascribing much importance to the relative age of 
 the parent except as a secondary factor, influential doubtless 
 in relation to nutrition. 
 
 § 7. Comparative Vigour. — The best known, and probably 
 still most influential, theory is that of " comparative vigour." 
 As elaborated by Girou and others, this hypothesis connects 
 the sex of the' offspring with that of the more vigorous parent. 
 It cannot be said, however, that facts bear out the case. Thus 
 consumptive mothers produce a great excess of daughters, 
 while Girou's theory would lead us to expect the opposite. 
 
THE DETERMINATION OF SEX. 39 
 
 We require in fact to have " vigour " analysed out into its 
 component factors, and in so doing we shall afterwards find 
 not only facts but reasons in favour of the conclusion, in part 
 included in the above theory, that highly nourished females 
 tend to produce female offspring. That form of the hypo- 
 thesis which refers the determination of sex to " genital 
 superiority," or to "relative ardency," can hardly be seriously 
 considered. In this connection it has been maintained that in 
 " marriages of love," after a short betrothal, female offspring 
 predominate ; and a number of other interesting facts of a 
 like nature are suggested. Some scepticism as to the practi- 
 cability of such inductions is, however, inevitable. 
 
 § 8. Starkweather's Laiv of Sex. — Closely allied to the 
 theory of comparative vigour is that elaborately worked out by 
 Starkweather, which is suggestive enough to deserve separate 
 -summary. He starts from a discussion of the alleged superi- 
 ority of either sex. Few maintain that the sexes are essen- 
 tially equal, still fewer that the females excel ; the general 
 bias of authority has been in favour of the males. From the 
 earliest ages philosophers have contended that woman is but 
 an undeveloped man ; Darwin's theory of sexual selection 
 presupposes a superiority and an entail in the male line ; 
 for Spencer, the development of woman is early arrested by 
 procreative functions. In short, Darwin's man is as it were 
 an evolved woman, and Spencer's woman an arrested man. 
 
 This notion of the superiority of males has formed the 
 basis of many theories of sex. As a good illustration of this 
 opinion, a few sentences may be quoted from Richarz: — "The 
 sex is not a quality transmitted from the parents, but has its 
 basis in the degree of organisation attained by the offspring. 
 The male sex represents to a certain extent a higher grade of 
 organisation or development in the embryo. This is attained 
 when the reproductive efficiency of the mother is specially 
 well developed, and the resulting male offspring more or less 
 resembles the mother. But if the maternal reproductive 
 power be weak, the ovum does not attain to maleness, and the 
 resulting female offspring more or less resembles the father." 
 Thus Hough thinks males are born when the maternal system 
 is at its best ; more females at periods of growth, reparation, 
 or disease. Tiedman and others regard female offspring as 
 arrested in the original state ; while Velpau conversely regards 
 females as degenerate from primitive maleness. 
 
4° THE EVOLUTION OF SEX. 
 
 Reacting from such speculations as to superiority of either 
 sex, Starkweather firmly rriaintains that " neither sex is 
 physically the superior, but both are essentially equal in a 
 physiological sense." This is true in the average, but yet in 
 each pair a greater or less degree of superiority on one side or 
 other must usually be conceded. Granting this, Starkweather 
 states, as his chief conclusion, "that sex is determined by the 
 superior parent, also that the superior parent produces the 
 opposite sex." Referring the reader to the Ency. Brit. Article 
 "Sex," for some critical notes, it is enough here to notice, that 
 just like " comparative vigour," so " superiority " has little 
 more than verbal simplicity to recommend it, since it lumps a 
 great variety of factors under a common name. Yet, in justice 
 to its author, we may admit that it is the algebraic sum of 
 these which he aims at expressing. 
 
 § 9. Darwin 's Position. — Neither in regard to the origin of ' 
 sex, nor its determination in individual cases, did Darwin see 
 further than his contemporaries. He refers to the current 
 theories of the influence of age, period of impregnation, and 
 the like ; and further contributes a great body of statistics on 
 the numerical proportions of the sexes, and the supposed 
 influence of polygamy. "There is reason," he says, "to 
 suspect that in some cases man has by selection indirectly 
 influenced his own sex-producing powers." He falls back 
 upon the unanalysed " belief that the tendency to produce 
 either sex would be inherited like almost every other peculiarity, 
 for instance, that of producing twins." " In no case, as far as 
 we can see, would an inherited tendency to produce both sexes 
 in equal numbers, or to produce one sex in excess, be a direct 
 advantage or disadvantage to certain individuals more than to 
 others ; . . . and therefore a tendency of this kind could 
 not be gained through natural selection." " I formerly thought 
 that when a tendency to produce the two sexes in equal 
 numbers was advantageous to the species, it would follow from 
 natural selection, but I now see that the whole problem is so 
 intricate that it is safer to leave its solution for the future." 
 Any other hints that Darwin threw out, have been so well 
 elaborated by Diising's work on the advantageous self-regula- 
 tion of the sex-proportions, that reference to the latter is more 
 profitable. 
 
 § 10. Dihing on the Proportions of the Sexes, and the 
 Regulation of these. — In an important work, Diising has 
 
THE DETERMINATION OF SEX. 4 1 
 
 recently treated the whole subject with some synthetic result. 
 He recognises that the fates or factors determining the sex are 
 manifold, and operate at different periods. Much is determined 
 by the condition of the reproductive elements — i.e., by the con- 
 stitution and habits of the parents ; much depends also on the 
 period of fertilisation ; while again the nutrition of the embryo 
 may be of moment. Diising has collected a great body of 
 facts, from both plants and animals, in favour of his conclu- 
 sions; but the copious summary of his work, given in the article 
 "Sex" already referred to, need not here be repeated, while 
 some of his experimental results will be included in the next 
 chapter. 
 
 Diising's memoir is very important, however, for this special 
 reason, that he analyses what may be termed the mechanism 
 by which the proportion of the sexes is regulated. Instead of 
 vaguely referring the whole matter to natural selection, he 
 shows in detail how the numbers are in a sense self-regulating, 
 how there is always produced a majority of the sex that is 
 wanted. That is to say, if one sex be in the decided minority, 
 or under conditions which come to the same thing, then a 
 majority of that sex will be produced. If there be, for instance, 
 a great majority of males, there is the greater likelihood of the 
 ova being fertilised early, but that means a probable pre- 
 ponderance of female offspring, and thus the balance is 
 restored. It would be rash to say that in every case he 
 makes out his contention, but his general argument, that 
 disturbances in the proportion of the sexes bring about 
 their own compensation, is carefully and convincingly worked 
 out. ' 
 
 § ii. Sex of Twins. — It sometimes happens among many different 
 classes of animals that from one ovum two organisms develop. We have 
 then a case of "true" twins, as opposed to cases where multiple offspring 
 do not arise from one ovum. Such "true" twins are said to occur not' 
 uncommonly in the human species, and are either most markedly similar 
 to one another or strongly dissimilar. 
 
 From a very early date an exception to this rule has been known in 
 regard to cattle, and applies to some other organisms as well. From the 
 careful researches of Spiegelberg and others, it appears that in cattle (a) 
 the twins may be both female and then both normal, or (b) that the sexes 
 may be different and normal, or (c) that both may be males, in which case 
 one always exhibits the peculiar abnormality known as a. "free-martin." 
 The internal organs are male, but the external accessory organs are female, 
 and there are also rudimentary female ducts. No theory has yet explained 
 the facts of this case. 
 
42 THE EVOLUTION OF SEX. 
 
 It is now necessary, with Busing for transition, to pass from 
 the historical mode of treatment to something more con- 
 structive. Leaving mere hypotheses behind, as well as theories 
 based on insufficient statistics, an induction from experimental 
 evidence will be built up in the following chapter. 
 
THE DETERMINATION OF SEX. 43 
 
 SUMMARY. 
 
 1. The epoch at which the sex is finally determined is variable in 
 different animals, and diverse factors operate at successive epochs. 
 
 2. Theological and metaphysical theories of sex have preceded the 
 scientific; observation and statistics have been resorted to before experi- 
 ment; and over 500 theories in all have been set forth. 
 
 3-6^ That there are two kinds of ova is still for the most part an 
 assumption; that the entrance of more than one spermatozoon normally 
 occurs, and is a determining factor, is erroneous. Thury's emphasis on the 
 age of the ovum when fertilised is probably justified; while Hensen extends 
 this notion to the male element as well. The age of the parents is probably 
 only of secondary import, and the law of Hofacker and Sadler is not 
 confirmed. 
 
 7, 8. Theories of " comparative vigour " and the like must be dis- 
 missed; while Starkweather's theory of the relative superiority of either 
 sex, and of the influence of this on the sex of the offspring, requires further 
 analysis. 
 
 9, 10. Darwin's position contains nothing novel, and has been superseded 
 by Diising's synthetic treatment and explanation of the self-regulating 
 numerical proportion of the sexes. 
 
 11. From this point, after a note on the similar sex of "true'' twins, 
 we pass to the experimental data and constructive treatment. 
 
 LITERATURE. 
 
 Berner. — Hj. Om Kjonsdannelsens Aarsager, En biologisk Studie (with 
 
 numerous references). Christiania, 1883. 
 Born, G. — Experimented Untersuchungen iiber die Entstehung der 
 
 Geschlechtsunterschiede. Breslauer aerztliche Zeitschrift. 1881. 
 Ci.eisz, A. — Recherches des lois qui president a la creation des Sexes. 
 
 Paris, 1899, pp. 81 (with bibliography). 
 Cohn, L. — Die willkiirliche Bestimmung des Geschlechts. 2nd ed. 
 
 Wurzburg, 1898. 
 Darwin, C— The Descent of Man, Chap. VIII. London, 187 1. 
 
 The Variation of Animals and Plants under Domestication. Lond. 
 
 D USING, C. — Die Regulierung des Geschlechtsverhaltnisses bei der 
 
 Vermehrung der Menschen, Thiere, und Pflanzen. Jena, 1884; or, 
 
 Jen. Zeitsch. f. Naturw., XVII., 1883. 
 Geddes, P. — As before. 
 
 Girou DK Buzareingues. — De la generation. Paris, 1828.. 
 HiiNNEBERG, B. — Wodurch wird das Geschlechtsverhaltnis beim Men- 
 schen und den hoheren Tieren beeinflusst. Anatomische Ergebnisse 
 
 (Merkel and Bonnet), VII., 1897, pp. 697-721. 
 Hensen, V.— Physiologie der Zeugung. Hermann's Handbuch der 
 
 Physiologie, Bd. VI., pp. 304, with references to Ploss, Schultze, &c. 
 
 Leipzig, 1 88 1. 
 His, W. — Theorien der geschlechtlichen Zeugung. Arch. f. Anthropologic, 
 
 Bde.J-V.-VI. 
 
44 THE EVOLUTION OF SEX. 
 
 Hofacker.— Ueber die Eigenschaften, welche sich bei Menschen und 
 Thieren auf die Nachkommen vererben. Tubingen, 1828. 
 
 Janke, H.— Die willkiirliche Hervorbringung des Geschlechts. 2nd ed. 
 Berlin, 1888. 
 
 Laulanie\ F. — Comptes Rendus, CI., pp. 593-5. 1885. 
 
 R01.PH, W. H.— As before. 
 
 Roth, E.— Die Thatsachen der Vererbung (historical). Berlin, 1885. 
 
 Pfluger, E. — Ueber die das Geschlecht bestimmenden Ursachen und die 
 Geschlechts - verh'altnisse der Frosche. Arch. f. d. ges. Physiol., 
 XXIX., 1882. 
 
 Sadler. — The Law of Population. London, 1830. 
 
 Schenk, L. — Einfluss auf das Geschlechtsverh'altnis. Magdeburg, 1898. 
 
 Schlechter. — Ueber die Ursachen welche das Geschlecht bestimmen. 
 Rev. f. Tierheilkunde. Wien, 1884. Biol. Centralblt., IV., pp. 627-9. 
 
 Seligson, E. — Willkiirliche Zeugung van Knaben oder M'adchen. Miin- 
 chen, 1895. 
 
 Starkweather. — The Law of Sex. London, 1883. 
 
 Stieda. — Das Sexual Verhaltniss bei Geborenen. Strasburg, 1875. 
 
 Sutton, J. B. — General Pathology. London, 1886. 
 
 Thury. — Ueber des Gesetz der Erzeugung der Geschlechter. Leipzig, 
 1863. 
 
 Wappceus. — Allgemeine Bevolkerungs-Statistik. Leipzig, 1861. 
 
 Watase. — On the Phenomena of Sex-Differentiation. Journal of Mor- 
 phology, 1892. 
 
 Wilckens, M. — Untersuchungen iiber das Geschlechtsverhiiltnis. Berlin, 
 1886. 
 
CHAPTER IV. 
 
 The Determination of Sex. 
 (Experiment and Rationale.') 
 
 § i. Influence of Nutrition. — Throughout nature the influence 
 of food is undoubtedly one of the most important environ- 
 mental factors. To Claude Bernard, indeed, the whole problem 
 of evolution was very much a question of variations of nutrition. 
 " L'evolution, c'est l'ensemble constant de ces alternatives de 
 la nutrition ; c'est la nutrition considered dans sa realite, em- 
 brassee d'un coup d'ceil a. travers le temps." It is fitting that 
 we should begin our survey of the factors known to influence 
 sex with the fundamental function of nutrition. 
 
 (a) The Case of Tadpoles. — Not a few investigators who 
 have passed from statistics and hypothesis to experiment and 
 induction, have found their material in tadpoles, where the sex 
 seems to remain for a comparatively long period indeterminate. 
 If we take the verdict of Yung, who has had much experience 
 with these forms, tadpoles pass through a hermaphrodite stage, 
 in common, according to other authorities, with most animals. 
 During this phase external influences, and especially food, decide 
 their fate as regards sex, though the hermaphroditism, as we 
 shall afterwards see, sometimes persists in adult life. It is fair, 
 however, to notice that Pfliiger gives a somewhat different 
 account of the actual facts, distinguishing among tadpoles three 
 varieties — (a) distinct males, (b) distinct females, and (c) herma- 
 phrodites. In the last, testes, or male organs, develop round 
 primitive ovaries, and if the tadpoles are to become males the 
 enclosed female organs are absorbed. 
 
 Adopting the view stated by Yung, we shall simply state the 
 striking results of one series of observations. When the tadpoles 
 were left to themselves, the percentage of females was rather in 
 the majority. In three lots, the proportions of females to 
 males were as follows : — 54 : 46 ; 61 : 39 ; and 56 144. The 
 average number of females was thus about 5 7 in the hundred. 
 
4^ THE EVOLUTION OF SEX. 
 
 In the first brood, by feeding one set with beef, Yung raised the 
 percentage of females from 54 to 78; in the second, with fish, 
 the percentage rose from 61 to 81 ; while in the third set, when 
 the flesh of frogs was supplied, the percentage rose from 56 
 to 92. That is to say, in the last case the result of altered 
 diet was that there were 92 females to 8 males. From the 
 experience and carefulness of the observer, these striking 
 results are entitled to great weight. 
 
 (b) Case of Bees. — The three kinds of inmates in a beehive 
 are known to every one as queens, workers, and drones ; or, as 
 fertile females, imperfect females, and males. What are th<~ 
 factors determining the differences between these three forms ? 
 In the first place, it is believed that the eggs which give rise to 
 drones are not fertilised, while those that develop into queens 
 and workers have the normal history. But what fate rules the 
 destiny of the two latter, determining whether a given ovum 
 
 The Queen (a), Worker (c), and Drone (b) 
 of the Common Hive-Bee. 
 
 will develop into the possible mother of a new generation, or into 
 the better-brained but non-fertile working female? It seems 
 certain that the fate mainly lies in the quantity and quality of 
 the food. Royal diet, and plenty of it, develops the reproductive 
 organs of the future queens ; sparser and plainer food retards 
 the sexuality of the future workers, in which reproductive organs 
 do not develop. Up to a certain point, the nurse bees can 
 
THE DETERMINATION OF SEX. 
 
 A7 
 
 determine the future destiny of their charge by changing the 
 diet, and this in some cases is certainly done. If a larva on 
 the way to become a worker receive by chance some crumbs 
 from the royal superfluity, the reproductive function may develop, 
 and what are called "fertile workers," to a certain degree above 
 the average abortiveness, result ; or, by direct intention, a worker 
 grub may be reared into a queen bee. 
 
 The following table, after a recent analysis by A. von Planta, shows the 
 differences of diet as far as solids are concerned. For queens 69.38 per 
 cent., for drones 72.75 per cent., and for workers 71.63 per cent, is water. 
 
 SOLIDS. 
 
 Queens. 
 
 Drones. 
 i to 4 days. 
 
 Drones. 
 After 4 days. 
 
 Workers. 
 
 Nitrogenous 
 Fatty 
 Glucose 
 Ashes 
 
 45-14 
 1.1-55 
 20.39 
 4.06 
 
 55-9* 
 11.90 
 9-57 
 
 31.67 
 4-74 
 
 38.49 
 2.02 
 
 51.21 
 
 •6.84 
 27.65 
 
 From the above, it is seen that the queen larvje get a quantity of fatty 
 material double that given to the workers. The drones at first receive a 
 large percentage of nitrogenous material, but this soon falls below the 
 share which workers and queens obtain. The fatty material, at first 
 large, soon falls to about a third of that given to the queens. Hence the 
 percentage of glucose, except at first, is so much larger than in the other 
 two cases. 
 
 It is not necessary, however, to go into details to see the 
 importance of the main point, that differences of nutrition, in 
 great part at least, determine the all-important distinctions 
 between the development and retardation of femaleness. Nor 
 are there many facts more significant than this simple and well- 
 known one, that within the first eight days of larval life, the 
 addition of a little food will determine the striking structural 
 and functional differences between worker and queen, 
 
 Eimer has drawn attention to the interesting correlation ex- 
 hibited in the fact that a larva destined to become a worker, 
 but converted into a queen, attains with the increased sexuality 
 all the little structural and psychological differences which 
 .otherwise distinguish a queen. Regarding fertilisation as a sort 
 of nutrition, he considers drones, workers, and queens as three 
 terms of a series, and the same view is suggested by Rolph. 
 Eimer recalls some interesting corroborations from humble bees. 
 There the queen mother, awakened from her winter sleep by 
 the spring sun, makes a nest, collects food, and lays her first 
 
4 8 
 
 THE EVOLUTION OF SEX. 
 
 brood. These are not too abundantly supplied with nourish- 
 ment, the queen having much upon her shoulders ; they develop 
 into small females, workers in a sense, but yet fertile, though 
 only to the extent of producing drones. By-and-by a second 
 brood of workers is born; these have the advantage of the 
 existence of elder sisters, are more abundantly nourished, and 
 develop into large females. Still, like the first brood, they pro- 
 duce drones, though occasionally females. Finally, with the 
 advantage of two previous broods of small and large females, 
 the future queens are born. The above facts not only afford 
 an interesting corroboration of the influence of nutrition upon 
 sexuality, but are of importance as suggesting the origin of the 
 more highly specialised society of the hive bee. 
 
 (c) Von SiebohVs Experiments. — With a somewhat different purpose 
 than that at present pursued, Von Siebold made a series of careful observa- 
 tions on a species of wasp, Nematus ventricosus. These afford, as Rolph 
 has noted, some valuable results in regard to the determination of sex. In 
 this wasp, the fertilised ova, unlike those of hive bees, develop into males 
 as well as females ; while the unfertilised, or parthenogenetic eggs, may pro- 
 duce females in small percentage. From spring onwards, as warmth and 
 food both increased, Von Siebold estimated the percentages of males and 
 females in broods of larvte reared from fertilised ova. The results of a 
 series of observations may be condensed in a table : — 
 
 End of Larval Period 
 (Pupation). 
 
 No. of 
 
 Females 
 
 to 100 Males. 
 
 No. of 
 Females. 
 
 No. of 
 Males. 
 
 15th June 
 
 July 
 
 July 
 
 End of August 
 September 
 
 14 
 77 
 269 
 340 
 500 
 100 
 
 19 
 
 86 
 
 S79 
 
 136 
 
 66 
 
 215 
 
 As Rolph remarks, the results are not altogether satisfactory for the 
 present purpose, "but this much is clear, that the percentage of females in- 
 creases from spring to August, and then diminishes. We may conclude 
 without scruple, that the production of females from fertilised ova increases 
 with the temperature and with the food supply {Assimilationrteistung), 
 and decreases as these diminish." 
 
 From the work of Rolph, which is full of a suggestiveness which the 
 author unfortunately did not live to elaborate, we shall quote another 
 paragraph summing up further experiments of Von Siebold : — 
 
 "Not less instructive," he says, "are the experiments with unfertilised 
 ova (see Table). 
 
 "This table shows the same general result as before. The more" 
 abundant the metabolism (Stoffwec/isel) and the nutrition, the greater 
 
THE DETERMINATION OF SEX. 
 
 49 
 
 tendency to the production of females, which at the beginning and at the 
 end are wholly absent. In the above series of experiments, they only 
 appear when the metabolism and the nutrition were so abundant 
 that the entire development of the young wasps only occupied eighteen or 
 
 No. of 
 
 Duration of Embryonic 
 
 Sex. 
 
 Experiment ■ 
 
 and Larval State. 
 
 II 
 
 21 days 
 
 All Males. 
 
 12 
 
 19 ,. 
 
 All Males. 
 
 13 
 
 18 ., 
 
 493 Males. 2 Females. 
 
 14 
 
 17 .. 
 
 265 ,, 2 ,, 
 
 IS 
 
 17 ,> 
 
 374 ., 8 
 
 16 
 
 18 ,, 
 
 168 „ 1 
 
 17 
 
 24 ,. 
 
 1 j, 
 
 fewer days up to the period of pupation." The peculiarity in this last case, 
 if the experiments were correct, is that in parthenogenesis, where the 
 production of males is the normal condition, favourable environmental 
 influences anoear to introduce females. 
 
 Two Forms of a Common Plant-Louse or Aphis.— 
 This figure may serve to illustrate three different 
 things, — a winged male and a wingless female; a 
 winged and a wingless parthenogenetic female; a 
 winged sexual female, and an ordinary wingless 
 parthenogenetic female.— From Kessler. 
 
 (d) Case of Ap hides. —One of the most familiar illustrations 
 of the influence of nutrition upon sex, is found in the history 
 of the plant-lice or aphides, which is indeed full of other 
 suggestions in regard to the whole theory of sex and reproduc- 
 tion. Details in regard to these plant-lice, which multiply so 
 rapidly upon our rose-bushes, fruit-trees, and the like, differ 
 
 4 
 
SO THE EVOLUTION OF SEX. 
 
 somewhat in the various species, but the general facts are_ 
 recognised to be as follows. During the summer months, 
 with favourable temperature and abundant food, the aphides 
 produce parthenogenetically generation after generation of 
 females. The advent of autumn, however, with its attendant 
 cold and scarcity of food, brings about the birth of males, and 
 the consequent recurrence of strictly sexual reproduction. In 
 the artificial environment of a greenhouse, equivalent to a 
 perpetual summer of warmth and abundant food, the partheno- 
 genetic succession of females has been experimentally observed 
 for four years, — it seems in fact to continue until lowering of 
 the temperature and diminution of the food at once re-intro- 
 duce males and sexual reproduction. 
 
 (e) Butterflies and Moths. — Still keeping to insects, we may 
 note Mrs Treat's interesting experiment, that if caterpillars 
 were shut up and starved before entering the chrysalis state 
 the resultant butterflies or moths were males, while others of 
 the same brood highly nourished came out females. Gentry 
 too has shown for moths that innutritious or diseased food 
 produced males, and suggests this as a partial explanation of 
 the excess of male insects in autumn, although we suspect 
 that temperature is in this instance probably more important. 
 
 It should be noted, however, that Paulton's experiments on 
 the sexes of larvae of Smerinthus pofiuli give no support to the 
 conclusion that the sex can be determined by external con- 
 ditions during larval life. The larger female larvae require 
 more food, and when supplies are reduced they tend to starve 
 first ("Trans. Entomol. Soc. London," 1893, pp. 451-6). 
 
 {/) Crustaceans. — In support of the same contention, 
 Rolph has drawn attention to the following among other facts. 
 One of the brine shrimps {Artemia salina) resembles not a 
 few crustaceans in the local and periodic scarcity or absence of 
 males, associated of course with parthenogenesis. At Mar- 
 seilles, Rolph says, this artemia lives in especially favourable 
 conditions, as its large size plainly indicates ; there it produces 
 only females. Where the conditions of existence are less 
 prosperous, it produces males as well. "A certain maximum 
 of abundance and optimum of vital conditions in partheno- 
 genetic animals — daphnids and aphides, Apus, Branchipus, 
 Artemia, and numerous other crustaceans — produce females ; 
 while less favourable conditions are associated with the pro- 
 duction of males." In regard, however, to water-fleas 
 
THE DETERMINATION OF SEX. 5 1 
 
 (daphnids), it is fair to notice that Rolph's conclusions do 
 not quite consist with Weismann's, who, with unique experi- 
 ence in regard to these curious little animals, is disinclined to 
 allow the direct influence of temperature and nutrition in the 
 matter. 
 
 (g) In regard to Rotifers (Hydatina), Maupas maintains 
 that temperature is the sex-determining factor, and that the 
 sex of the offspring is determined two generations in advance ! 
 His experiments led him to conclude that when the ovum is 
 being differentiated in the ovum, the temperature determines 
 whether it shall develop into a male-producing or a female- 
 producing individual.. Nussbaum, on the other hand, disputes 
 the conclusiveness of this result, and maintains that nutrition 
 is the determining factor : females of Hydatina which have 
 been insufficiently fed during early life afterwards lay only male 
 eggs, while well-nourished forms produce female eggs. 
 
 (h) Mammals. — When we pass to higher animals, the diffi- 
 culties of proving the influence of nutrition upon sex are much 
 greater. Yet there are decisive observations which go to 
 increase the cumulative evidence. Thus an important experi- 
 ment was long ago made by Girou, who divided a flock of 
 three hundred ewes into equal parts, -of which the one-half 
 were extremely well fed and served by two young rams, while 
 the others were served by two mature rams and kept poorly 
 fed. The proportion of ewe lambs in the two cases was re- 
 spectively sixty and forty per cent. In spite of the combination 
 of two factors, the experiment is certainly a cogent one. 
 Diising brings forward further evidence in favour of the same 
 conclusion, noting, for instance, that it is usually the heavier 
 ewes which bring forth ewe lambs. He emphasises the fact 
 that the females having a more serious reproductive sacrifice, 
 are more dependent on variations of nutrition than males. 
 Even in birds, as Stolzmann points out, there is a much greater 
 flow of blood to the ovary than to the testes, — the demands are 
 greater, and the consequences therefore more serious if these 
 are not fulfilled. 
 
 (?) In the human species, lastly, the influence of nutrition, 
 though hard to estimate, is more than hinted at. Ploss may 
 be mentioned as an authority who has emphasised this factor 
 in homo. Statistics seem to show, that after an epidemic or a 
 war the male births are in a greater majority than is usually 
 the case. Diising also points out that females with small 
 
52 THE EVOLUTION OF SEX, 
 
 placenta and little menstruation bear more boys, and contends 
 that the number of males varies with the harvests and prices. 
 In towns, and in prosperous families, there seem to be more 
 females, while males are more numerous in the country and 
 among the poor. 
 
 Schenk has (1898) re-enunciated the view that nutrition is 
 the chief determining factor in deciding the sex of offspring. 
 But his evidence is quite insufficient ; indeed, when it is 
 critically examined it is seen to consist of three or four cases. 
 
 (_/) Determination of Sex in Plants. — It is at present ex- 
 tremely difficult to come to any very satisfactory conclusion in 
 regard to the influence of nutrition upon the sex of plants. 
 The whole subject, as far as its literature is concerned, has 
 been recently discussed by Heyer, but his survey is by no 
 means a sanguine one. His conclusions, in fact, seem to land 
 him in a scepticism as to all modification of the organism by 
 environmental influences, which we should of course be far 
 from sharing. It must be admitted that the experiments of 
 Girou (1823), Haberlandt (1869), and others, yielded no cer- 
 tain result ; while the conclusions of some others are conflict- 
 ing enough to justify not indeed Heyer's despair, but his 
 present caution. Still a few investigations, especially those of 
 Meehan (1878), which are essentially corroborated by Dusing 
 (1883), go to show, for some cases, that abundant moisture 
 and nourishment do tend to produce females. Some of 
 Meehan's points are extremely instructive. Thus old branches 
 of conifers, overgrown and shaded by younger ones, produce 
 only male inflorescence. In the American Cory/us rostrata, 
 and in many other instances, he is convinced that in early 
 stages the sex of a flower-bud is undetermined, and that its 
 determination as a male or female flower is mainly the result 
 of the nutritive conditions ("Proc. Acad. Nat. Sci. Phila- 
 delphia," 1899, pp. 84-86). Various botanists, quoted by 
 Heyer, confirm one another in the observation that prothallia 
 of ferns grown in unfavourable nutritive conditions produce 
 only antheridia (male organs), and no archegonia (female 
 organs). 
 
 The botanical evidence, though by no means strong, cor- 
 roborates the general result that good nourishment produces a 
 preponderance of females. The contrast of the sexes in some 
 of our common dioecious plants is here very instructive. 
 Taking for instance the dog-mercury (Mercuriaiis perennis) of 
 
THE DETERMINATION OF SEX. 53 
 
 any shady dell, or the day lychnis (L. diurna), hardly less 
 abundant on the sunnier slopes, experiments are still certainly 
 wanting with regard to given plants, as to what circumstances 
 originally determined their sexual differences ; but the fact of 
 superior constitutional vegetativeness in the females is here so 
 peculiarly obvious, that it can hardly fail to arouse a strong 
 impression that more or less advantageously nutritive con- 
 ditions, whether of the embryo or of the seedling, are sufficient 
 to account for the differences of sex. 
 
 § 2. Influence of Temperature. — In this connection not a 
 few writers have referred to an observation by Knight, which, 
 from its comparatively ancient date, perhaps deserves to be 
 recorded in his own words, if only to show the necessity of 
 caution in such matters. A water-melon was grown in a heated 
 glass-house, where the temperature sometimes rose on warm 
 days to 110° Fahr. "The plant grew with equal health and 
 luxuriance, and afforded a most abundant blossom; but all its 
 flowers were male. This result did not in any degree surprise 
 me, for I had many years previously succeeded, by long con- 
 tinued very low temperature, in making cucumber plants 
 produce female flowers only; and I entertain but little doubt 
 that the same fruit stalks might be made, in this and the 
 preceding species, to support either male or female flowers in 
 obedience to external causes." 
 
 This experiment was obviously more sanguine than satis- 
 factory. Heyer justly points out that of the water-melon only 
 a single plant was taken. Furthermore, he says, the water- 
 melon in nature usually bears only female flowers on the apices 
 of the older twigs, and may bear only a minimum number of 
 these. Knight's observations on cucumbers are also open to 
 serious objections, and were too scanty to prove anything. 
 
 Meehan finds that the male plants of hazel grow more 
 actively in heat than the female; and Ascherson has made the 
 interesting observation that the water-soldier (Stratiotes aloides) 
 bears only female flowers north of 52 lat., and from 50° south- 
 wards only male ones. On the other hand, Molliard maintains 
 ("Comptes Rendus," cxxvii., 1898, pp. 669-671) that in the 
 case of dog's mercury {Mercurialis annua) a high temperature 
 favours the production of female individuals, but whether the 
 heat simply promotes especially the development of the female 
 seeds, or has some direct effect on the nature of the seed, is 
 left undetermined. The same experimenter maintains in regard 
 
54 THE EVOLUTION OF SEX. 
 
 to the hop that the sex is not absolutely determined in the 
 seed, and that a transformation may be observed from male to 
 female inflorescences under conditions that are very unfavour- 
 able to the development of the vegetative organs, e.g., feeble 
 illumination. 
 
 In the human species, Dusing and others have noted that 
 more males are born during the colder months; and Schlechter 
 has reached the same results from observations upon horses. 
 The temperature of the time, not of birth but of sex determina- 
 tion, is however more important; nor must it be forgotten that 
 temperature may have many indirect and subtle influences. 
 
 § 3. Summary of Factors. — If we now sum up the case, it 
 must first be recognised that a number of factors co-operate in 
 the determination of sex ; but that the most important of these 
 may be more and more resolved into plus or minus nutrition, 
 operating upon parent, sex elements, embryo, and in some 
 cases larvae. 
 
 (a) Starting with the parent organisms themselves, we find 
 this general conclusion most probable, — that adverse circum- 
 stances, especially of nutrition, but also including age and the 
 like, tend to the production of males, the reverse conditions 
 favouring females. 
 
 (i) As to the reproductive elements, a highly nourished 
 ovum, compared with one less favourably conditioned, in every 
 probability will tend to a female rather than to a male develop- 
 ment. Fertilisation, when the ovum is fresh and vigorous, 
 before waste has begun to set in, will corroborate the same 
 tendency. 
 
 (c) Then if we accept Sutton's opinion as to a transitory 
 hermaphrodite period in most animals, from which the transition 
 to unisexuality is effected by the hypertrophy of the female side 
 or preponderance of the male in respective cases, the vast 
 importance of early environmental influences must be allowed. 
 The longer the period of sexual indifference (though this term 
 be an objectionable one) continues, the more important must 
 be those outside factors, whether directly operative or indirectly 
 through the parent. Here again, then, favourable conditions 
 of nutrition, temperature, and the like, tend towards the pro- 
 duction of females, the reverse increase the probability of male 
 preponderance. 
 
 The general conclusion, then, more or less clearly grasped 
 by numerous investigators, is that favourable nutritive con- 
 
THE bETERMINATION OF SEX. §§ 
 
 ditions tend to produce females, and unfavourable conditions 
 males. 
 
 § 4. Let us express this, however, in more precise language. 
 Such conditions as deficient or abnormal food, high tempera- 
 ture, deficient light, moisture, and the like, are such as tend to 
 induce a preponderance of waste over repair, — a relatively kata- 
 bolic habit of body, — and these conditions tend to result in the 
 production of males. Similarly, the opposed set of factors, 
 such as abundant and rich nutrition, abundant light and mois- 
 ture, favour constructive processes, i.e., make for a relatively 
 anabolic habit, and these conditions tend to result in the pro- 
 duction of females. With some element of uncertainty, we 
 may also include the influence of the age and physiological 
 prime of either sex, and of the period of fertilisation. ' But the 
 general conclusion is tolerably secure, — that in the determina- 
 tion of sex, influences inducing a relative predominance of 
 katabolism tend to result in production of males, as those 
 favouring a relative predominance of anabolism similarly in- 
 crease the probability of females. 
 
 § 5. This is not all, however; the above conclusion is in- 
 deed valuable, but it acquires a deeper significance when we 
 take it in connection with the result of a previous chapter. 
 There it was seen, as the conclusion of an independent induc- 
 tion, that the males were forms of smaller size, more active 
 habit, higher temperature, shorter life, &c; and that the females 
 were the larger, more passive, vegetative, and conservative forms. 
 Theories of "inherent" maleness or femaleness were rejected, 
 since practically merely verbal ; more accurately, however, they 
 have been interpreted and replaced by a more material con- 
 ception, which finds the bias of the whole life, the resultant of 
 its total activities, to be a predominance of the protoplasmic 
 processes either on the side of disruption or construction. 
 This conclusion has still to receive cumulative proof, but one 
 large piece of evidence is now forthcoming, that, namely, of 
 the present chapter. If influences favouring katabolism make 
 for the production of males, and if anabolic conditions favour 
 females, then we are strengthened in our previous conclusion, 
 that the male is the outcome of relatively predominant kata- 
 bolism, and the female of relatively predominant anabolism. 
 
 § 6. Weismann's Theory of Heredity. — In thinking of the 
 environment as a factor determining the sex, it is impossible to 
 ignore that such facts as we have noted above have some 
 
g6 THE EVOLUTION OP SEX, 
 
 bearing upon the problem of heredity. Much of the recent 
 progress in the elucidation of the facts of inheritance has been 
 due to Weismann, who, in his theory of the continuity of the 
 germ-plasm, has restated the very important and fundamental 
 conception of a continuity between the reproductive elements 
 of one generation and those of the next. To this restatement 
 we shall afterwards have to refer; it is with another position, 
 not peculiar to, but emphasised by the same authority, that we 
 have here to do, viz., with his denial of the inheritance of 
 individually acquired characters. Any new character exhibited 
 by an organism may arise in one of two ways, which it is easy 
 enough to distinguish theoretically; — it may be an outcrop of 
 some property inherent in the fertilised egg-cell, that is, it may 
 have a constitutional or germinal origin; but, on the other 
 hand, it may be impressed upon the individual organism by the 
 environment, or acquired in the course of its functioning, that 
 is, it may have a functional or environmental origin. But all 
 such functional and environmental modifications are, according 
 to Weismann, restricted to the individual organism; they are 
 not transmissible. 
 
 In this denial of the transmission of dints from without, 
 and of acquired habits other than constitutional, Weismann 
 expresses a scientific scepticism, based on the one hand on the 
 absence of data demonstrating what we may still call the 
 current belief, and on the other hand on the improbability of 
 modifications reacting from the " body " on the reproductive 
 cells in such a specific and representative way that the off- 
 spring inherit the modifications even in the slightest degree. 
 If such a reaction do not occur, Weismann's position is secure; 
 and though in a system saturated with alcohol, or transferred 
 to a new climate, the reproductive cells may vary along with 
 the body, no modification of nerve fix muscle can, as such, be 
 transmitted in inheritance. 
 
 The relative scarcity of experimental data, the divergence 
 of opinion as to the pathological evidence, and the difficulty 
 of applying our logical or anatomical distinctions to the 
 intricate facts of nature, make decisive statements impossible, 
 but it may be said that no clear case of the transmission of an 
 acquired modification has as yet been forthcoming. 
 
 Weismann's position — slightly modified to meet criticism — 
 must not be held to imply that the germ-cells lead a "charmed 
 life," insulated, as it were, from the general life of the body. 
 
THE DETERMINATION OP SEX. 57 
 
 That would indeed be a "physiological miracle,'' and it may 
 be safely said that no one believes in any such apartness. At 
 the same time, it may be useful to recall the facts of this 
 chapter in order to avoid exaggeration of the degree to which 
 the germ-cells are uninfluenced by modifications in the body. 
 For in such a case as Yung's tadpoles, influence of nutrition 
 saturated through the organism and did affect the reproductive 
 elements, not indeed to the degree of altering any structural 
 feature of the species, but yet to the extent of altering the 
 natural numerical proportions of the sexes. But it must be 
 clearly understood that this does not really touch the precise 
 question of the inheritance of acquired characters. 
 
5 8 the evolution op sex. 
 
 SUMMARY. 
 
 1. Nutrition is one of the most important factors in determining sex. 
 In illustration, note {a) the experiments of Yung, which raised the per- 
 centage of females from 56 to 92 by good feeding; [b) the case of bees, 
 where the differences between queen and worker well illustrate the enormous 
 results of a slight nutritive advantage; also the case of humble-bees, with 
 three successive broods increasing in nutritive prosperity and in femaleness; 
 (c) Von Siebold's experiments with a wasp, which showed most females in 
 favourable conditions ; (d) Aphides, in prosperity of summer, yield a 
 succession of parthenogenetic females, in cold and scarcity of autumn males 
 return; (e) among starved caterpillars of moths and butterflies more males 
 survive ; (f) Rolph's observations on crustaceans ; (g) experiments on 
 Rotifers; (h) also the facts noted by Girou, Diising, and others, on the 
 influence of good nourishment of mammalian mothers in favouring female 
 offspring; (z) the hints of the same results in the human species; (/) various 
 observations in regard to plants favouring the same general conclusion. 
 
 2. As to the influence of temperature, favourable conditions again tend 
 to femaleness of offspring, extremes to males. 
 
 3. These factors are now added up — (a) the nutrition, age, &c, of 
 parents; (b) the condition of the sex elements; (c) the environment of 
 embryo. 
 
 4. The generalisation is thus reached — anabolic conditions favour pre- 
 ponderance of females, katabolic conditions tend to produce males. 
 
 5. But females have been already seen to be relatively more anabolic, 
 and females relatively more katabolic. This view of sex is therefore 
 confirmed. 
 
 6. The determination of sex illustrates an outside influence penetrating 
 to the reproductive cells, but this does not touch the precise question as to 
 the inheritance of acquired characters. 
 
 LITERATURE. 
 
 See works mentioned in Chapter III., especially those of Dtising, 
 Geddes (article Sex, Ency. Brit.), Hensen, and Sutton; also those of 
 Eimer, Geddes, and Rolph in Chapter II. 
 Calman, J. T. — The Progress of Research on the Reproduction of the 
 
 Rotifera. Nat. Science, XIII., 1898, pp. 43-51 (with bibliography). 
 DiisiNG, C — As before; also, Die experimentelle Pruning der Theorie 
 
 von der Regulirung des Geschlechtsverhaltnisses. . Jen. Zeitschr. f. 
 
 Naturwiss., XIV., Supplement, 1885. 
 Heyer, F. — Untersuchungen iiber das Verhaltniss des Geschlechtes bei 
 
 einhausigen und zweih'ausigen Pflanzen, unter Berlicksichtigung des 
 
 Geschlechtsverhaltnisses bei den Thieren und den Menschen. Ber. 
 
 landwirthschaftl. Inst. Halle, V., 1884, pp. 1-152. 
 Kerherve, L. B. de. — De l'apparition provoquee des males chez les 
 
 Daphnies. Mem. Soc. Zool. France, VIII., 1895, PP- 200-211, 1 fig. 
 Maupas, E. — Sur le determinisme de la sexualite chez VHydatina senta. 
 
 C R. Ac. Sci. Paris, CXIII , 1891, pp. 388-390. 
 
THE DETERMINATION OP SEX. $C) 
 
 Meehan, T. — Relation of Heat to the Sexes of Flowers. Proc. Acad. 
 
 Nat. Science, Philadelphia (1884), pp. 111-117. 
 Nussbaum, M.— Die Entstehung des Geschlechts bei Hydatina scuta. 
 
 Arch. Mikr. Anat., XLIX., 1897, pp. 227-308. 
 Semper, C. — The Natural Conditions of Existence as they affect Animal 
 
 Life. Internat. Science Series, London, 1881. 
 Thomson, J. A. — Synthetic Summary of the Influence of the Environment 
 
 upon the Organism. Proc. Roy. Phys. Soc. Edin., IX. (1888), 
 
 pp. 446-499. (Supplementary to Semper's work, with bibliography. ) 
 The History and Theory of Heredity, rroc. Roy. Soc. Edin., 
 
 1889, pp. 91-116, with bibliography. 
 The Science of Life. London, 1899. 
 
 Treat. — Controlling Sex in Butterflies. Amer. Naturalist, VII., 1873. 
 
 Weismann, A. — Die Continuitat des Keimplasmas als Grundlage einer 
 Theorie der Vererbung, Jena, 1885; and numerous other papers, 
 now translated, in 2 vols. — Essays upon Heredity and Kindred 
 Biological Problems, authorised translation, edited by E. B. Poulton, 
 S. Schonland, and A. E. Shipley, 8vo. Oxford, 1899. But especially 
 " The Germ-Plasm : a Theory of Heredity," 1893. 
 
 Wilckens, M. — Untersuchungen iiber das Geschlechtsverhaltniss und die 
 Ursachen der Geschlechtsbildung in Haustieren. Biol. Centralblt., 
 VI. (1886), pp. 503-510; Landwirth. JB., XV., pp. 607-610. 
 
 Yung, E. — Contributions a l'Histoire de l'lnfluence des milieux Physiques 
 sur les Etres Vivants. Arch. Zool. Exper., VII. (1878), pp. 251-282; 
 (1883) pp. 31-55; Arch. Sci. Phys. Nat., XIV. (1885), pp. 502-522. 
 
 De l'influence de la nature des aliments sur la sexualite. Comptes 
 
 Rendus Ac. Sci. Paris, XCIII., 1881. 
 
 De l'influence des facteurs determinant le sexe. Revue de Morale 
 
 Sociale, II., No. 5, 1900, pp. 88-110. 
 
BOOK II. 
 
 ANALYSIS OF SEX-ORGANS, 
 TISSUES, CELLS. 
 
CHAPTER V. 
 Sexual Organs and Tissues. 
 
 IT is the object of this portion of the book to continue the 
 analysis of sexual characters, but now in a deeper way, 
 reviewing successively the organs, tissues, and cells concerned 
 in sexual reproduction. The essential and auxiliary organs of 
 the two sexes, the frequent combination of these in hermaphro- 
 dite plants and animals, the sex-cells both male and female, 
 will be discussed in order. This survey will be for the most 
 part structural or morphological; the physiological aspects 
 of sexual union and of fertilisation will be discussed at a later 
 stage. 
 
 § r. Essential Sexual Organs of Animals. — It is now a well- 
 established fact that among the ciliated infusorians, which 
 swarm especially in stagnant waters, a process occurs which 
 cannot but be described as in part sexual reproduction. Two 
 individuals, to all appearance alike, become temporarily asso- 
 ciated, exchange portions of their (micro-) nuclei, and then 
 separate.- This process of fertilisation is essential to the con- 
 tinued vigour of the species, and will be afterwards described 
 at length. Such a very simple form of sexual union differs 
 from what occurs in higher animals in two conspicuous 
 respects, — (a) the organisms are apparently quite similar in 
 form and structure ; (6) they are unicellular, and thus there is 
 no distinction between "body" and reproductive cells. What 
 is fertilised by the mutual exchange in those infusorians is, 
 roughly speaking, the entire animal, for the whole is but a 
 corpuscle of living matter. 
 
 Among the Protozoa, however, loose colonies of cells occur, 
 which bridge the gulf between unicellular and multicellular 
 animals. In these we find the first indications of the after- 
 wards conspicuous difference between " body " and repro- 
 ductive cells. From these loose colonies, certain of the units 
 are set adrift, and meeting with others more or less like 
 
64 
 
 THE EVOLUTION OF SEX. 
 
 themselves, fuse to form a double cell, virtually a fertilised 
 ovum, from which by continuous division a fresh colony is 
 then developed. In these transition forms there are thus 
 reproductive cells of slight distinctness, but as yet obviously 
 no sexual organs. 
 
 When we pass to the sponges, we find colonies consisting of 
 myriads of cells, among which there is a considerable division 
 of labour. An outer layer (or ectoderm) usually consisting of 
 much subordinated cells, an inner layer (or endoderm) of pre- 
 
 Volvox, a colony of cells, with some set apart for reproduction, after Klein. 
 
 dominantly active and well-nourished cells, a middle layer 
 of heterogeneous constituents, can be distinguished. Every 
 average infusorian is as good as its neighbours, so far as repro- 
 duction of new individuals by division is concerned; in the 
 colonial Protozoa, the units that are set adrift are very little 
 different from their fellows that remain behind; but this ceases 
 to be true when we pass to colonies where considerable division 
 of labour has been established. It is certainly true that even 
 a tiny fragment of sponge, cut off from the larger mass, may, 
 
SEXUAL ORGANS AND TISSUES. 65 
 
 if it contain sufficient samples of the body, and if the conditions 
 be favourable, reproduce a new individual. Cultivators of 
 bath sponges sometimes take advantage of this fact. But the 
 sponge starts its new colonies for itself usually in quite a 
 different way, namely, by the process of sexual reproduction. 
 Among the cells of the middle stratum of the sponge body 
 certain well-nourished passive cells appear. These are the ova, 
 at first very like, but eventually well marked from the other 
 constituent units of the layer. Besides these there are other 
 cells, either in the same sponge or in another, which exhibit 
 very different characters. Instead of growing large and rich in 
 reserve material like the egg-cells or ova, they divide repeatedly 
 into clusters of infinitesimal cells, and form in so doing the 
 male elements or spermatozoa. The male and female cells 
 meet one another, they form a fertilised ovum; the result is 
 continued division of the latter till a new sponge is built up. 
 Here then there are special reproductive cells, quite distinct 
 from those of the "body"; and here, furthermore, these repro- 
 ductive cells are markedly contrasted as male and female 
 elements. As yet, however, there are no sexual organs. 
 
 Passing to the next class, the stinging animals or ccelenter- 
 ates, we find in one of the simplest and most familiar of these, 
 the common fresh-water hydra, a good illustration of primitive 
 sexual organs. As in sponges, a cut-off fragment of the body, 
 if sufficient samples of the different component cells are in- 
 cluded, is able to reconstitute the whole. But no one body- 
 cell has of course any such power; this is possible for the 
 fertilised ovum alone. Now this ovum occurs, not anywhere 
 within a given layer as in sponges, but always near one spot 
 on the body. Towards the base of the tube a protuberance 
 of outer layer cells is developed. This forms a rudimentary 
 ovary, or female organ. It has this peculiarity, not however 
 unique, that while the organ consists of not a few cells, only 
 one of these becomes an ovum. A similar protrusion, or more 
 than one, often at the same time and on the same animal, may 
 be recognised further up the tube, nearer the tentacles of the 
 hydra. Smaller than the ovary, each protuberance consists of 
 numerous small cells, most of which, multiplying by division, 
 form male elements or spermatozoa. We have here the 
 simplest possible male organ or testis. 
 
 More elaborate organs occur in the other ccelenterates, 
 complicated however by two interesting facts, which will be 
 
 S 
 
66 THE EVOLUTION OF SEX. 
 
 afterwards discussed. (a) Many of the coelenterates are well 
 known to form elaborate colonies, — zoophytes, Portuguese 
 men-of-war, and the like. In these, division of labour fre- 
 quently goes further than the setting apart of special organs. 
 Entire individuals become reproductive " persons " (as they 
 are technically called), in contrast to the nutritive persons of 
 the colony. (b) In some of those reproductive individuals, 
 a curious phenomenon, known as migration of cells, has 
 been observed by Weismann and others. The reproductive 
 cells, arising in various parts of the body, have been shown to 
 migrate in some cases to another part, where they find final 
 lodgment in more or less definite organs. This occurrence is 
 intimately associated with " alternation of generations," and will 
 be afterwards discussed under that heading. 
 
 It is far from the purpose of the present work to describe 
 the details respecting the ovaries and testes, as they occur in 
 the various classes of animals. It is enough for our purpose to 
 have emphasised the fact of their gradual differentiation, and 
 to note that they are almost always developed in association 
 with the middle layer of the body, and usually occupy a pos- 
 terior position on the wall of the body-cavity. The details will 
 be found in any standard work on comparative anatomy, very 
 conveniently for example in Professor Jeffrey Bell's " Com- 
 parative Anatomy and Physiology," London, 1885. 
 
 § 2. Associated Duc/s. — It is only in a few animals, like hydra and its 
 allies, that the ovaries and testes are external organs, which have simply to 
 burst to liberate their contents. They are of course usually internal, and 
 thus arises the necessity of some means of communication with the outside 
 world. In the simplest cases, the male elements find their way out to the 
 surrounding medium without any specialised mode of exit. They there 
 meet, by chance combined with physical attraction at short range, with 
 the ova, which in the simplest cases again have found their way out in an 
 equally primitive fashion. Thus in the enigmatical parasitic Mesozoa 
 (Orthonectids, &c), liberation of the germs may occur by perforation or by 
 rupture of the excessively simple bodies. In some of the marine worms 
 (e.g. Polygordius), the liberation of the ova at least is accompanied by the 
 fatal rupture of the mother organism, a vivid instance of reproductive 
 sacrifice. Even in some of the common nereids, the same uneconomical 
 mode of liberation by rupture appears to occur. The forcible rupture may 
 be referred to pressure of the relatively large mass of growing cells which 
 the ovaries often present. 
 
 As high up as back-boned animals, the absence of ducts may be traced. 
 Thus among the sea-squirts or tunicates, the reproductive organs are fre- 
 quently ductless, and the same thing is true of some fishes. The sex-cells 
 burst into the body-cavity, and thence find their way to the exterior by 
 
SEXUAL ORGANS AND TISSUES. 67 
 
 apertures. In most cases, where ducts are absent, fertilisation of the 
 ova is external, but this is not necessarily so. In sponges, for instance, 
 fertilisation is almost always internal. Male elements are washed in by 
 the water-currents, find their way to the ova, and fertilise them in situ. 
 Almost without exception, embryo-sponges, not ova, make their way to the 
 exterior. In the higher animals, where definite ducts are present, alike for 
 the inward passage of spermatozoa and the exit of ova or embryos, it 
 ought further to be noticed that the ovaries can hardly ever be said to be 
 in direct connection with their ducts. The ova usually burst from the 
 ovary into the body-cavity, whence they are more or less immediately 
 caught up by, or forced into the canals, by which they pass outwards. 
 With the testes it is different, for if ducts be present, they are in direct 
 connection with the organs. 
 
 It is enough to state that in the great majority of cases ducts are 
 associated with the essential organs. Those of the male serve for the exit 
 of the spermatozoa, and may be terminally modified as intromittent organs. 
 Those of the females serve either solely for the emission of unfertilised eggs, 
 or for the reception of spermatozoa, and the subsequent exit of fertilised 
 ova or growing embryos. In some worm-types, and in all vertebrates, 
 from amphibians onwards, the reproductive ducts are also in various 
 degrees associated with excretory functions. For an account of the origin 
 of the ducts in higher animals, the reader must be referred to the embryo- 
 logical text-books of Balfour, Hertwig, Haddon, Marshall, and others. 
 Similarly for such modifications as that of the female duct into oviduct and 
 uterus, reference must be made to the larger anatomical works of Gegenbaur 
 and Wiedersheim, or for a briefer account to Parker's translation and 
 edition of Wiedersheim's smaller text-book, and to Professor Jeffrey Bell's 
 work already mentioned. 
 
 § 3. Yolk-Glands. — As we shall afterwards see, the ovum 
 is often furnished with a large quantity of nutrient material. 
 This serves as the food-capital for the growing embryo or young 
 larva. It is obtained in various ways, — from the vascular fluid, 
 from the sacrifice of adjacent cells, or from special organs 
 known as yolk-glands or vitellaria. The yolk-glands, as they 
 occur for instance in some of the lower worms (turbellarians, 
 flukes, tapeworms), are of some general interest. They repre- 
 sent, as Graff has shown, a degenerate portion of the ovary, in 
 which the cells have become even more highly anabolic than 
 ova. "The origin of the yolk-gland," Gegenbaur says, "is 
 probably to be found in the division of labour of a primitively 
 very large ovary." In more technical language, yolk-glands 
 are hypertrophied or hyper-anabolic portions of the ovary. 
 Apart from this nutritive capital, the egg is often equipped 
 with envelopes or shells of some sort, which may be furnished 
 by special organs, or by the sacrifice of surrounding cells, or 
 by the walls of the ducts as the eggs pass out. 
 
 § 4. Organs Auxiliary to hnpregnation. — In most animals 
 
68 THE EVOLUTION OF SEX. 
 
 in which internal fertilisation of the ova occurs, there are in 
 both sexes special structures auxiliary to the function of im- 
 pregnation. Thus the end of the male canal is commonly 
 modified into an intromittent tube or penis, through which the 
 male elements flow into the female duct. In the crustaceans 
 some of the external appendages are often modified, as in the 
 crayfish, to serve this purpose, and the same is the case with 
 minute structures on the posterior abdomen of many insects. 
 Sometimes, as in the snail {Helix), which may be taken as an 
 extreme type of reproductive specialisation, separate organs are 
 present, in which the spermatozoa are compacted into masses 
 or packets, known as spermatophores. In most cuttle-fishes, 
 these pass from the male ducts to one of the "arms," which 
 thus laden is occasionally set free bodily into the mantle-cavity 
 of the female, where it was of old mistaken for a worm, and 
 called Heclocotylus. So in some spiders, the palps near the 
 mouth receive the male elements, and transfer them to the 
 female. Special storing receptacles and secreting glands are 
 also very frequently in association with the male ducts, and 
 there is a long list of curious modifications utilised in the 
 process of copulation. Thus, male frogs have swollen first 
 fingers, and gristly fishes have " claspers," which are modified 
 parts of the hind limbs, and are inserted into the cloaca of the 
 female. The common snails eject a limy dart (spiculum amoris), 
 which appears to be a preliminary excitant to copulation. 
 
 So too, in the female sex, the terminations of the duct may 
 be modified for reception of the male intromittent organ, and 
 special receptacles may be present for storing the spermatozoa. 
 Where a single fertilisation occurs, as in the queen bee, previous 
 to a long-continued egg-laying period, the importance of a 
 storing organ is obvious. As the female is usually more or less 
 passive during copulation, the adaptations for this purpose are 
 less numerous than. in the males. It is interesting to notice, 
 that, among amphibians, where the male often takes upon him- 
 self distinctly maternal duties, one case is known where the 
 female seems more active than the male during copulation. 
 
 § 5- Egg~£- a yi n i Organs. — Cases where the ova simply pass 
 out into the water, or on to the land, are of course associated 
 with the absence of any special organs. In a great many 
 animals, however, more care is taken, and .auxiliary structures 
 are present. One of the simplest of useful developments is 
 exhibited by glands, the viscid secretion of which moors the 
 
SEXUAL ORGANS AND TISSUES. 69 
 
 ova, and keeps them from being set wholly adrift. In insects, 
 where it is specially important that the eggs should be well con- 
 cealed, or buried in conveniently nutritive material, hints of 
 the ancestral abdominal appendages remain as " ovipositors." 
 Throughout the series a great variety of structures occur in this 
 connection. 
 
 § 6. Brooding and Young-Feeding Organs. — From very 
 lowly animals onwards, structures are present which are utilised 
 in the protection of the young in their helpless stages. The 
 reproductive buds of some ccelenterates become true nurseries; 
 in one at least of the marine worms (Spirorbis spirillum), a 
 tentacle serves as a brood pouch; various adaptations, such as 
 tents of spines, or cavities in the skin, are utilised in echino- 
 derms. The young shelter under the hard cuticle, or among 
 the appendages of crustaceans, and in the gills of some bivalves. 
 The pockets of not a few fishes, the cavities on the back of the 
 Surinam toad, the pouches of marsupials, are only a few in- 
 stances amid a crowd. Sometimes, especially in fishes and 
 amphibians, — e.g., the sea-horse, with its breast-pouch, and 
 Rhinoderma darwinii, with its enlarged croaking sacs, — it is 
 the male which undertakes the brooding office. 
 
 When the young are born alive, the internal female ducts 
 become developed in this connection to form uteri. The ovary 
 appears to serve as a womb in the genus Girardinus among 
 fishes, but it is usually the median portion of the female duct 
 which has this function. In placental mammals, where the 
 young are born at an advanced stage, and where the maternal 
 sacrifice is at its maximum, the uterine adaptations become 
 more important and complex. The organs of lactation will be 
 afterwards discussed. 
 
 In illustration of the strange inter-relations between different 
 forms, we may refer to the fresh-water mussels. The larva? or 
 Glochidia are sheltered and nourished in the outer gill-plates 
 of the female, and are liberated when fishes come near. To 
 the skin of these the larvae fix themselves, become temporarily 
 parasitic, and undergo a metamorphosis, after which they fall off. 
 Without the presence of fishes the life-history cannot be com- 
 pleted. On the other hand, the young stages of the fresh- 
 water fish known as the bitterling {Rhodeus amarus) find 
 temporary shelter in the gills of the mussels. 
 
70 THE EVOLUTION OF SEX. 
 
 SUMMARY. 
 
 1. The gradual differentiation of essential sexual organs in animals, — 
 isolated cells, aggregated tissues, definite organs. 
 
 2. Associated male and female ducts for the liberation of male-elements, 
 fertilisation, exit of ova, or birth of embryos. 
 
 3. Yolk-glands, &c. , for nourishment and equipment of the ova. 
 Vitellaria have been interpreted as degenerate ovaries. 
 
 4. Illustrations of organs auxiliary to impregnation. In the male, — 
 penis, storing sacs, spermatophore-making organs, " claspers. " Curiosities, 
 such as the hectocotylus of cuttle-fishes, and the Cupid's dart of snails. 
 Adaptations in the female are less frequent, but storing receptacles for the 
 male-elements are common. 
 
 5. Egg-laying organs : — frequency of ovipositors. 
 
 6. Brood-pouches and the like are widely present in most classes of 
 animals. 
 
 LITERATURE. 
 
 Balfour, F. M. — A Treatise on Comparative Embryology. 2 vols. 
 
 London, 1881. 
 Bell, F. Jeffrey. — Comparative Anatomy and Physiology. London, 
 
 1885. 
 Claus, C. — Elementary Text-Book of Zoology, trans, by A. Sedgwick. 
 
 2 vols. London, 1885. 
 Geddes, P. — Op. cit. 
 Gegenbaur, C. — Elements of Comparative Anatomy, trans, by Prof. 
 
 Jeffrey Bell. London, 1878. 
 Haddon, A. C. — An Introduction to the Study of Embryology. London, 
 
 1887. 
 Hensen, Y. — Op. cit. 
 Hertwig, O. — Lehrbuch der Entwicklungsgeschichte des Menschen und 
 
 der Wirbelthiere. Jena, 1888. 
 Hatch ett Jackson's (W.) Edition of Rolleston's Forms of Animal Life. 
 
 Oxford, 1888. 
 Huxley, T. H. — Anatomy of Vertebrate and Invertebrate Animals. 
 
 London, 1871 and 1877. 
 Sachs, J. — Text-Book of Botany, edited by Prof. Vines. Second edition. 
 
 Oxford, 1882. And similar works. 
 Lectures on the Physiology of Plants, trans, by Prof. Marshall 
 
 Ward. Cambridge, 1887. 
 Vines, S. H. — Vegetable Reproduction (Ency. Brit.). Lectures on the 
 
 Physiology of Plants. Cambridge, 1886. 
 Wiedersheim, R. — Elements of the Comparative Anatomy of Verte- 
 brates, trans, by Prof. W. N. Parker. London, 1S97. Also un- 
 abridged work. 
 
CHAPTER VI. 
 
 HERMAPHRODITISM. 
 
 § i. When an organism combines within itself the production 
 of both male and female elements, it is said to be bisexual or 
 hermaphrodite. This is the case with many of the lower 
 animals, — such, for instance, as earthworms and snails. It is 
 not desirable to extend the term, as is sometimes done, to 
 cases like ciliated infusorians, where sex itself is only incipient. 
 In flowering plants, the stamens and carpels, which produce 
 microspores and macrospores respectively, are often, though 
 inaccurately, called the male and female organs, and when both 
 are present in the same flower the term hermaphrodite is often 
 used. But the flowering plant is a sporophyte with only a 
 vestige of the sexual generation left, and the term hermaphro- 
 dite should be kept for cases like the prothallus of a fern — a 
 sexual generation with male and female organs on the same 
 small expansion. While the general definition of hermaphro- 
 ditism, as the union of the two sexes in one organism, is plain 
 enough, the union is exhibited in a great variety of ways and 
 degrees. Of these it is necessary first to take account. 
 
 § 2. Embryonic Hermaphroditism. — Some animals are 
 hermaphrodite in their young stages, but unisexual in adult 
 life. Allusion has already been made to the case of tadpoles, 
 where the potential bisexuality occasionally lingers into adult 
 life. According to some, most higher animals pass through a 
 stage of embryonic hermaphroditism, but decisive proof of this 
 is wanting. 
 
 The research of Laulanie' may now be referred to at greater length. As 
 the result of observations on the development of the reproductive organs in 
 the higher vertebrates, and especially in birds, he seeks to establish a strict 
 parallelism between the individual, and what he believes to have been the 
 racial history. In the chick, he distinguishes three main stages in the 
 development — (i) germiparity, (2) hermaphroditism, (3) differentiated 
 unisexuality. These he regards as recapitulating the great steps of the 
 historic evolution. (1.) For the first period of "germiparity," — from the 
 
)2 THE EVOLUTION OF SEX. 
 
 fourth to the sixth day, — the designation, sexual neutrality, or indifference, 
 is inappropriate, since the "cortical ovules" of the germinal epithelium 
 have from the first the precise morphological significance of female ele- 
 ments or ova. In the female, they proceed by multiplication to form the 
 ovary; in the male, they degenerate. (2.) The period of hermaphroditism 
 begins with the seventh day. In the male, the male ovules, from which 
 the sperms are afterwards developed, appear in the central tissue ; but at 
 the same time cortical or female ovules may be seen persisting. Similarly, 
 in the developing ovary of the female, the central or medullary portion, 
 strictly separated by a partition of connective tissue from the egg-forming 
 layer, contains a large number of medullary or male ovules. (3.) This 
 hermaphroditism is of short duration. The cortical or female ovules 
 disappear from the testes by the eighth or ninth day; and the medullary 
 or male ovules have by the tenth day disappeared from the ovary. In 
 regard to mammals, Laulanie affirms, allowing some peculiarities, that the 
 same three stages of germiparity, hermaphroditism, and unisexuality occur. 
 
 Ploss has already been referred to as another investigator who main- 
 tains the existence of embryonic hermaphroditism. Such also is the view 
 held by Professor Sutton, who concludes that both sets of organs are 
 equally developed up to a definite period, and emphasises the consequent 
 necessity for the hypertrophy of one sexual rudiment over the other. 
 
 § 3. Casual or Abnormal Hermaphroditism. — In many species which 
 are normally unisexual, a casual hermaphrodite form occasionally presents 
 itself. The embryonic equilibrium or bisexuality — one of the two must in 
 a variable degree exist — is retained as an abnormality into adult life 
 Even as far up in the organic series as birds and mammals, such casual and 
 yet true hermaphrodites occur. In most cases at least the result is sterility. 
 Among amphibians, which abound in reproductive peculiarities, herma- 
 phroditism exceptionally occurs, and in some species of toad it seems to be 
 constant. The common frog, so much dissected in our laboratories, has 
 supplied several good illustrations. Thus Marshall notes that the testes 
 may be associated with genuine ova, or an ovary may occur on one side, 
 and a testis with an anterior ovarian portion upon the other. Bourne gives 
 a case of a frog with the ovary well developed on the right side, and 
 opposite this an ovary anteriorly replaced by testis. One of the toads 
 (Pelobates fuscus) seems to be frequently hermaphrodite, the male being 
 furnished with a rudimentary ovary in front of the testes. A similar 
 hermaphroditism is not at all infrequent in cod, herring, mackerel, and 
 many other fishes. Sometimes a fish is male on one side, female on the 
 other, or male anteriorly and female posteriorly. Sir J. Y. Simpson, in a 
 learned article on the subject, has distinguished cases of true hermaphro- 
 ditism, according to the position of the organs, into lateral, transverse, and 
 vertical or double. Among invertebrates the same has been occasionally 
 observed, especially among butterflies where striking differences in the 
 colouring of the wings on the two sides have in some cases been found 
 to correspond to an internal co-existence of ovary and testis. The same 
 has been observed in a lobster, and is probably commoner than the 
 recorded cases warrant one in asserting. As low down as ccelenterates, 
 casual hermaphroditism may occur, as F. E. Schulze showed in one of the 
 medusoids. 
 
 § 4. Partial Hermaphroditism. — An organism may be said to be truly 
 hermaphrodite when both male and female organs are present, or when, 
 
HERMAPHRODITISM. 73 
 
 without there being separate organs, both male and female elements are 
 produced. It is then both anatomically and physiologically hermaphro- 
 dite, and of this, as we shall see, there are abundant illustrations among 
 lower animals. Snail, earthworm, and leech are examples of this herma- 
 phroditism, in varying degrees of intimacy. 
 
 But, as we have just noticed, a species normally unisexual may occasion- 
 ally exhibit hermaphrodite individuals. In these only one of the double 
 essential organs may be functional, or both may be sterile. Whether 
 physiologically or not, such animals are anatomically hermaphrodite. Both 
 kinds of essential organs are at least present. 
 
 To those must now be added a further series of cases to which the term 
 partial hermaphroditism seems most applicable. Only one kind of sexual 
 organ, ovary or testis, is developed; but while one sex preponderates, there 
 are more or less emphatic hints of the other. As the reproductive organs 
 are to be regarded as the most important, but not by any means the sole 
 expression of the fundamental sex-differences, it is impossible to separate 
 partial hermaphroditism by any hard and fast line from the above, and 
 from the next set of cases (paragraphs 3 and 5). Almost all cases of partial 
 hermaphroditism occur as exceptions, though a few are constant. 
 
 In the higher animals, partial hermaphroditism is usually expressed in 
 the nature of the reproductive ducts. In this connection the structural 
 resemblance of the male and female organs must be once more emphasised. 
 Even the Greeks had their vague and fanciful theories of what we now call 
 the homology of the reproductive organs and ducts in the two sexes. 
 Through the labours of the anatomists of Cuvier's school, such as his 
 fellow-worker Geoffrey St Hilaire, and yet more through more recent 
 embryological discoveries, there is now both clearness and certainty as to 
 the main facts. The reproductive organs proper, the ducts, and the 
 external parts, are developed upon the same plan in male and female. 
 Thus, except in the lowest vertebrates, what serves as an oviduct in the 
 female, is equally present in the embryo male, and persists in the adult as 
 a more or less functionless rudiment. In the same way, what serves as 
 the duct for the sperms {vas deferens) in the male is equally present in the 
 embryo female, and persists in the adult as a rudiment, or is diverted to 
 some other purpose. This is a perfectly normal occurrence, dependent 
 upon the embryological history of the ducts in question. It is necessary, 
 however, to realise both the primitive resemblance and the fundamental 
 unity of the two sets of organs, in order to understand how partial herma- 
 phroditism is so frequent, and also to distinguish it from "spurious 
 hermaphroditism," where a merely superficial abnormality or even injury 
 of the ducts in one sex produces a resemblance to those of the other. 
 
 We have already mentioned that in the case of twin calves, two females 
 may occur, and both are then normal ; or two normal twin calves may be 
 born of opposite sexes ; but, in the third place, if both be males, one of 
 these very generally exhibits the peculiar phenomena of what is called a. 
 "free-martin." In the commonest form of this, partial hermaphroditism 
 is well illustrated. The essential organs are male, but there is a rudi- 
 mentary uterus and vagina, and the external organs are further those of a 
 female. 
 
 It is necessary to note that a simulation of even this partial hermaphro- 
 ditism may result from malformation or rudimentary development of the 
 external organs. On this subject we may quote an acknowledged autho- 
 
74 THE EVOLUTION OF SEX. 
 
 rity, alike in anatomical and embryological matters. " From the fact," 
 Prof. O. Hertwig remarks, " that the external sexual organs are originally 
 of uniform structure in the two sexes, we can understand the fact that, in 
 a disturbance of the normal development, forms arise in which it is ex- 
 tremely difficult to decide whether we have to deal with male or female 
 external organs. These cases, in earlier times, were falsely interpreted as 
 hermaphroditism. They may have a double origin. Either they are 
 referable to the fact that in the female sex the development may proceed 
 along the same path as in the male, or to this, that in the male the normal 
 development may come at an early age to a standstill, and lead to the 
 formation of structures which resemble the female parts." In the first 
 case, he goes on to say, there may be a simulation of a penis, and the 
 ovaries may even be shifted so as to produce an appearance like that of 
 the testes within their scrotal sac. In the second case, the processes of 
 coalescence which give rise to the penis may not occur, only a rudimentary 
 organ is formed, and there may even be an inhibition of the usual descent 
 of the testes into their sacs. 
 
 Of this superficial hermaphroditism, really not hermaphroditism at all, 
 there are numerous cases among mammals. But there remain a large 
 number of recorded instances, where the anatomy of the ducts was pre- 
 dominantly that of the sex opposite to that indicated by the essential 
 organs, and where the combination of the two sexes was also expressed in 
 external configuration and even in habit. Amphibians again furnish some 
 interesting examples. Attached to the anterior end of the testis in various 
 species of toad (Bufo), there is an organ known as " Bidder's," which has 
 contents like young ova. These do not, however, get past the early stages, 
 and the organ is quite different from the more than rudimentary ovary 
 which occurs constantly in the males of Bufo cinereus and some other 
 species. The two may in fact occur together. In the common frog, 
 dissectors have also recorded seveial cases of hermaphroditism expressed in 
 the ducts. Lastly, it is perhaps not going too far to include here some 
 reference to the curious " fatty bodies" which occur in all amphibians at 
 the apex of the reproductive organs in both sexes. These appear to 
 nourish the ovary and testis, especially during hybernation, and may 
 perhaps be associated with similar lymphoid structures in fishes and 
 reptiles. Prof. Milnes Marshall demonstrated that the fatty bodies result 
 from the degeneration of the anterior part of the reproductive organ while 
 still in an indifferent state. 
 
 Leaving the ducts out of account, we may arrange the 
 important phenomena of hermaphroditism in amphidians in a 
 series as follows : — • 
 
 (a) Embryonic hermaphroditism, demonstrated as of normal occurrence 
 in frog tadpoles. 
 
 (6) Casual hermaphroditism, demonstrated in frogs, e.g. in the occur- 
 rence of distinct ova in the seminiferous tubules of Rana viridis as reported 
 by F. Friedmann (Arch. Mikr. Anat., Hi. (1898), pp. 248-262, 1 pi. 
 
 expressed in Bidder's organ in male toads ; 
 
 (c) Partial hermaphroditism, - (also expressed in various states of the 
 
 , ducts). 
 
 (d) Normal adult hermaphroditism, in some species of Bufo. 
 
HERMAPHRODITISM. 75 
 
 The list need not be further followed; it is enough to note 
 the very wide occurrence of partial hermaphroditism. In many 
 cases, moreover, we find what may be called superficial her- 
 maphroditism, expressing itself in the external characters. 
 Forms occur in which the minor peculiarities of the two sexes- 
 colouring, decorations, weapons, and the like — appear blended 
 together, or in which the secondary sexual characters are at 
 variance with the internal organs. In most cases, one is safe 
 in saying that there is no true internal hermaphroditism in any 
 degree. Arrest of maturity or puberty, cessation of the repro- 
 ductive functions, removal or disease of the essential organs, 
 and the like, may alter the secondary sexual characters from 
 female towards male, or, less frequently, vice versa. A female 
 deer may develop a horn, or a hen a spur, and in such cases 
 the ovaries are generally found to be diseased. The prettiest 
 cases of superficial hermaphroditism occur among insects, 
 especially among moths and butterflies, where it often happens 
 that the wings on one side are those of the male, on the other 
 those of the female. Only the external features have been 
 observed in most cases; but it has been shown by dissection 
 that such superficial blending may exist along with internal 
 unisexuality, or, in a few cases, with genuine internal her- 
 maphroditism. A beautiful case of intimate blending of 
 superficial sex characters was lately shown to us by Mr W. de 
 V. Kane of Kingstown. A specimen of butterfly (Euchloe 
 euphenoides) showed the anterior half of the fore wings and 
 part of the hind wings with the characteristic white ground of 
 the female, while in the posterior half of the fore wings and on 
 most of the hind wings the characteristic sulphur of the male 
 prevailed. In other minor ways, the characteristics of the two 
 sexes, which are well marked, were intimately blended. In 
 all such cases we may suppose that the imperfection of the 
 normal unisexual differentiation removes the usual limits to 
 the appearance of this or that secondary sexual character. 
 
 § 5. Normal Adult Hermaphroditism— This is rare among 
 the higher animals, but common among the lower. On the 
 threshold of the vertebrate series, we find it indeed constant 
 among the Tunicata; but above these it is very rare. The 
 hag {Myxine) was shown by Cunningham, and afterwards by 
 Nansen, to be a protandrous hermaphrodite, but this conclusion 
 is contested by Bashford Dean ("Festschrift Kupffer," 1899). 
 "A testis is constantly found imbedded in the wall of the 
 
l6 THE EVOLUTION OF S£X. 
 
 ovary in Chrysophrys and Serranus, and the last-named fish 
 is said to be self-impregnating." In some species of male toad 
 {e.g., Bufo cinereus) a somewhat rudimentary ovary is always 
 present in front of the testes. All other cases among verte- 
 brates are either casual (par. 3) or partial (par. 4). Among 
 invertebrates, true hermaphroditism is frequent. 
 
 (1.) Sponges. — As already mentioned, the sex-cells of sponges arise 
 among the components of the middle layer (mesoglozd) of the body. It is 
 at least possible that in any sponge they may develop either into ova or 
 into sperms, or into both, within the same organism, according to nutritive 
 and other conditions. The facts, however, are these. Many sponges are 
 only known in a unisexual state, while others are genuinely hermaphrodite. 
 But among the latter it is not uncommon to find (e.g., in Sycandra 
 raphanus) that the production of one set of elements preponderates over 
 the other, and thus we have hermaphrodites with a distinctly male or 
 female bias. In other words, they are verging towards unisexuality. It 
 does happen in fact (e.g., in Oscarella lobularis) that a species normally 
 hermaphrodite may exhibit unisexual forms. 
 
 (2.) Coelenterates. — The members of this class are higher, in having the 
 production of the sex-cells more restricted, to definite regions, tissues, 
 organs, or even " persons." The highly active Ctenophores, like Berbe, 
 are all hermaphrodite, and that very closely. On one side of the meri- 
 dional branches of the alimentary canal ova arise, on the other side sperma- 
 tozoa. Among sea-anemones and corals the hermaphrodite condition 
 appears in a number of cases, but is sometimes obscured by the fact that 
 the two kinds of elements are produced at different times, corresponding to 
 different physiological rhythms in the life of the organism. The genus 
 Corallium (the red coral of commerce) is peculiarly instructive. The whole 
 colony may be unisexual, or one branch of the colony, or only certain 
 individuals on a branch, while genuine hermaphroditism of individual 
 polyps also occurs. Among hydrozoa (zoophytes, swimming-bells, jelly- 
 fish), hermaphroditism is a rare exception. The common hydra, which is 
 a somewhat degenerate type, is hermaphrodite, though at the same time 
 individuals may be found with only ovary or only testes. Eleittheria is 
 also hermaphrodite, and abortive ova occur in the male of Gonothyrea 
 loveni. Sometimes a colony is hermaphrodite (Dicoryne), but the stems 
 and individuals unisexual. Sometimes a stem is hermaphrodite, but the 
 individuals unisexual (certain sertularians). Among jelly-fishes the genus 
 Chrysaora is known to be hermaphrodite. 
 
 (3.) " Worms." — The condition of the sexual organs varies enormously 
 among the diverse types lumped together under the title of "worms "or 
 "Vermes." In the lowly turbellarians, all the genera are hermaphrodite 
 except two, but, as in many other cases, the organs do not reach maturity 
 at the same time, the male preceding. In the related trematodes or flukes, 
 hermaphroditism again obtains, with one exception, or perhaps two. The 
 certain exception is the curious parasite Bilharzia, where the male carries 
 the female about with him in a " gynsecophoric canal," formed of folds of 
 skin. In the adjacent class of cestodes or tapeworms, all the members are 
 hermaphrodite, with one alleged exception. The utility of the herma- 
 phrodite state, if the eggs of these parasitic animals are to be fertilised and 
 
HERMAPHRODITISM. 
 
 77 
 
 the species maintained, can hardly be doubted. It is important to notice 
 too, that self-fertilisation — that is, union of the eggs and sperms of the 
 same organism — has been proved to occur in several trematodes, and seems 
 to be almost universal in cestodes. This may be partly a cause and partly 
 a consequence of the degeneracy of these parasites, for frequent as herma- 
 phroditism is among plants and animals, self-fertilisation is extremely rare. 
 
 Hermaphroditism is rare among the free-living nemerteans, but con- 
 stant in the semi-parasitic leeches. An exception to separateness of the 
 sexes among threadworms or nematodes is found in the curious genus 
 Angiostomum. Here, in an organism which is anatomically a female, the 
 reproductive organ begins its activity by pro- 
 ducing spermatozoa, which fertilise the subse- 
 quent ova. The animal is thus physiologically 
 hermaphrodite, and at the same time self- 
 impregnating. Approaching the higher anne- 
 lid worms, we find the primitive Protodrilus 
 hermaphrodite; the earthworms are constantly 
 so, but all their marine relatives have the sexes 
 separate. The genus Sagitta, which stands by , 
 itself, is hermaphrodite ; the same condition is 
 known as a rarity among the ancient brachio- 
 pods (Lingula), but is frequent among the 
 colonial Polyzoa. Many, at least, of the My- 
 zostomata — aberrant parasites of Crinoids — are 
 hermaphrodite. 
 
 (4.) Echinoderma. — Almost all the mem- 
 bers of this class have separate sexes. Among 
 the few exceptions are the species of Synapta 
 (a divergent Holothuroid), a sand-star (Amphi- 
 ura squamata), and a starfish (Asterina gib- 
 bosa). The last is particularly interesting. At 
 RoscofT, the individuals are males for one or 
 two years, and then become females; at Ban- 
 yuls, the individuals are males for at least two 
 or three years, but eventually become females ; 
 at Naples some are wholly male, others 
 wholly female, others hermaphrodite imparti- 
 ally, others transitional. (See L. Cuenot, 
 Zool. Anzeiger, XXI., 1898, pp. 273-279, 3 
 
 (5.) Arthropods.— Among crustaceans, hermaphroditism is a rare ^ex- 
 ception, though it occurs in the majority of the fixed quiescent acorn-shells 
 and barnacles (Cirripedia). There it is associated with the presence of 
 small males, which Darwin called "complemental." The Cymothoida: 
 (Isopods) show a curious condition somewhat like that of Angiostomum 
 above noticed. The sexual organ of the young animal is male, of the 
 old, female in function. In such cases, one must remember the antithesis 
 between the body proper and the reproductive cells. In youth the demands 
 of the body during growth are greater; there is no anabolic surplus to 
 spare, all goes to increase the body. When mature size is reached and 
 growth and activities are lessened, there is more likelihood of anabolic 
 preponderance in the reproductive, as opposed to the vegetative, system. 
 
 Bilharzia, a parasitic trematode, 
 in which the male carries the 
 female in a special fold of skin 
 called the " gynascophorLc 
 canal." — After Leuckart. 
 
78 THE EVOLUTION OF SEX. 
 
 Myriopods and insects have always separate sexes, excluding of course 
 abnormal hermaphroditism among the latter. 
 
 (6.) Molluscs. — Most bivalves are of separate sexes, but exceptions 
 often occur — e.g., in common species of oyster, cockle, clam, &c. In the 
 case of the oyster, the familiar species (Ostrea edulis) is hermaphrodite, 
 and a neighbouring species apparently unisexual. In both cases the organs 
 are the same, but in 0. edulis the same intimate recesses of the reproduc- 
 tive organ produce at one time ova, at another time sperms. 
 
 The snails, or gasteropods, are divided into two great groups, according 
 to the twisting of their nerves. The one group (Streptoneura) have the 
 sexes separate; the members of the other series (Euthyneura) are her- 
 maphrodite. 
 
 The sea-butterflies, or pleropods, are hermaphrodite, but the elephant's 
 tooth shells (Scaphopods) are unisexual. So in cuttle-fishes (Cephalo- 
 pods), the sexes are separate. 
 
 In the limpet (Patella) hermaphroditism occurs as a casual variation, 
 3 out of 250. The low-level limpets, which are probably better fed, show 
 no preponderance of females, but it may be noted that the female repro- 
 ductive organ of the limpet is not larger than the male organ, and does not 
 therefore make special demands on nutrition. (See J. F. Gemmill, Anat. 
 Anzeiger, XII., 1896, pp. 393, 394.) 
 
 § 6. Degrees of Normal Hermaphroditism. — From what has 
 been already said, it is evident that hermaphroditism may be 
 more or less intimate. Thus the red coral is sometimes female 
 as regards one branch, and male as regards another; a leech 
 has the ovaries far forward, and independent of the long row 
 of testes ; in a tunicate the testes and ovary may form one 
 mass, the male cells spreading over the surface of the ovary. 
 In the same way, the organ of a scallop, which exhibits more 
 or less distinct male and female portions, is in a state of less 
 intimate anatomical hermaphroditism than the oyster, where 
 the same caeca of the same organ fulfil both functions at 
 different times. 
 
 This last caution must be kept in view, for there is through- 
 out, in varying degrees, a tendency to periodicity in the 
 production of male and female elements. Such a want of 
 "time-keeping" between the sexes is called dichogamy, and 
 is one of the conditions which render self-fertilisation rarely 
 possible. The male function has in the majority of cases the 
 precedence. Similarly in flowering plants, although it is not 
 quite accurate to call the stamens the male organs, or the 
 carpels the female organs, it may be said that " protandrous 
 dichogamy" (stamens taking the lead) is very much commoner 
 than "protogynous dichogamy," where the carpels are first 
 matured. This agrees with the curious cases of Angiostomum 
 
HERMAPHRODITISM. 79 
 
 and Cymothoidse already mentioned, where the organ was first 
 male and then female, and indeed with at least most cases 
 among closely hermaphrodite animals. Where the male organs 
 are situated in one part of the body, and the female in another, 
 there is less reason against the production of sperms going on 
 at the same time as the production of ova. In terms of our 
 general thesis, protogyny corresponds, like unisexual female- 
 ness, to a relative predominance of anabolism in the life-ratio, 
 and protandry with the reverse. 
 
 The common snail {Helix) is not only easily dissected, but 
 in the complexity of its arrangements is full of interest. Here, 
 not only are ova and sperms produced within the compass of 
 one small organ, but each little corner of the organ shows 
 female cells forming on the walls and male cells in the centre. 
 It has been suggested by Platner that the outer cells are the 
 better nourished; they therefore naturally become developed 
 into anabolic ova. In the large slug, Limax maximus, Babor 
 found a succession of sexual states, — female, hermaphrodite, 
 male, hermaphrodite, female; and suggests that the same 
 alternation may be observed elsewhere. (Verh. Zool.-bot. 
 Ges. Wien, xlviii., 1898, pp. 151-153-) 
 
 § 7. Self-Fertilisation. — We have noted above, that though 
 male and female organs be present in the same organism, they 
 tend to become mature at different times, and that the more 
 the closer the seats of formation of the two kinds of elements. 
 It is equally necessary to emphasise that, though both male 
 and female elements may be produced in the same plant or 
 animal, it is probably exceptional for the ovule to be penetrated 
 by a pollen cell from the same flower, and it is certainly rare 
 for an animal to fertilise its own ova. 
 
 It is believed by breeders of higher animals that "close- 
 breeding " beyond a certain point is dangerous to the welfare of 
 the stock. The offspring tend to be abnormal or unhealthy. 
 In view of this, the rarity of self-fertilisation among herma- 
 phrodites has been explained in terms of the disadvantage of 
 the process. In reality, however, this is putting the cart before 
 the horse. In hermaphrodites, we take it that the two kinds of 
 sexual elements mature and are liberated at different times, not 
 because of any reaction of the disadvantageousness of self- 
 fertilisation on the health of the species, but simply because the 
 simultaneous co-existence of opposite physiological processes is 
 in varying degrees prohibited. More technically, dichogamy is 
 
8o 
 
 THE EVOLUTION OF SEX. 
 
 not a secondary result of the disadvantage of self-fertilisation 
 nor of the advantage of cross-fertilisation, but increasing dicho- 
 gamy is the primary condition of cross-fertilisation. 
 
 Self-fertilisation does, however, occur as an exception among 
 animals, — thus in all probability in the interesting fish Ser- 
 ranus; certainly in many parasitic flukes or trematodes; com- 
 monly, if not almost always, in tape-worms or cestodes; also 
 in the curious thread-worm Angiostomum, and probably in 
 ctenophores, and in some other cases. In regard to some 
 cases, e.g., among hermaphrodite bivalves (where the sperms 
 are usually wafted in with the water), it is impossible as yet to 
 say whether self-impregnation does or does not occur. 
 
 Arguing from the bad effects of close breeding among 
 higher animals, Darwin and others have called attention to the 
 numerous contrivances among plants which are said to render 
 self-pollination impossible. In some cases the pollen of a 
 given flower is quite inoperative on the ovule of the same 
 flower, or has the result of producing weakly offspring. There 
 are also many mechanical devices, as the result of which 
 it is difficult or impossible for the pollen of the stamens 
 to reach the stigmas of the flower, or even to be dusted upon 
 them by the unconscious agency of the intruding insects. 
 Moreover, as among animals, so among plants, it is common 
 for the stamens to become mature before the carpels are ready, 
 or, in rarer cases, for the reverse to occur. 
 
 There is no doubt that cross-fertilisation very generally 
 occurs, and it is physiologically probable that this is a con- 
 siderable advantage, though probably less among plants than 
 among animals. But there is an increasing impression that 
 both the occurrence of cross-fertilisation, and the necessity of 
 it among higher plants, have been exaggerated. One of the 
 most thoughtful and observant of American botanists, Mr T. 
 Meehan, has raised a vigorous protest against the prevalent 
 view. In the Yucca, or Adam's needle, which is regarded as 
 cross-fertilised by insects, he showed by experiment that there 
 was in each flower " no abhorrence of its own pollen." " Even 
 when fertilised at all by insects, I am sure the fertilisation is 
 from the pollen of the same flower." 
 
 Then as to mechanical contrivances, he says, '' we are told 
 that iris, campanula, dandelion, ox-eye daisy, the garden pea, 
 lobelia, clover, and many others, are so arranged that they 
 cannot fertilise themselves without insect aid. I have enclosed 
 
HERMAPHRODITISM. 
 
 flowers of all those named in fine gauze bags, and they produced 
 seeds just as well as those exposed." 
 
 We cannot here enter into a full statement of Meehan's 
 careful observations, but his three main propositions well 
 deserve statement and due consideration: — 
 
 i. Cross-fertilisation by insect agency does not exist nearly 
 to the extent claimed for it. 
 
 Mvzostomata:-(i) A hermaphrodite bearing pigmy males (X); (2) a pigmy male.- 
 J ' From Nansen. 
 
 2 Where it does exist, there is no evidence that it is of any 
 material benefit to the race, but to the contrary. 
 
 ■x Difficulties in self-fertilisation result from physiological 
 disturbances that have no relation to the general welfare of 
 plants as species. , 
 
82 THE EVOLUTION OF SEX. 
 
 § 8. Complemental Males. — When Mr Darwin was inves- 
 tigating barnacles and acorn shells, in preparation for his 
 monograph on the group, he discovered the remarkable fact 
 that some of the hermaphrodite individuals carried minute 
 males concealed under their shells. These he regarded as 
 advantageous accessory forms, ensuring cross-fertilisation in the 
 hermaphrodites which harbour them. The great majority of 
 the cirripedes are hermaphrodite; but among the barnacles 
 proper, — the stalked forms, which are nearer the ancestral type, • 
 ■ — separate sexes sometimes occur. On the females of a few 
 of these, pigmy males, like those found upon hermaphrodites, 
 also occur. These pigmy males, whether on females or herma- 
 phrodites, are not only dwarfish, but are very often degenerate, 
 sometimes wanting (according to Darwin) both alimentary 
 canal and thoracic legs. Some of them, in fact, are little more 
 than parasitic testes. 
 
 The various steps in the evolution may be hypothetically 
 sketched: — 
 
 (a) The original state of affairs was probably the ordinary crustacean 
 condition of separate sexes, (b) A second stage may have been repre- 
 sented by a diminution in the size of the males, as in some of the "water- 
 fleas" or copepods, while the females became more and more sluggish, and 
 settled down, (c) In the genera Akippe and Cryptophiahis, in the species 
 Ibla cummingii and Scalpellum ornatum, there are true females, with 
 attached pigmy males, often several, leading a shabby existence as parasites. 
 {d) In other species of Scalpellin?i and Ibla the same pigmy males occur, but 
 attached, as we have noted, to hermaphrodites, which in these forms have 
 replaced the true females, (e) Lastly, in many genera, like Pollicipes, 
 only hermaphrodites occur. 
 
 In a description of the complementary male of Scalpellum vul^are, A. 
 Gruvel notes ("Archives de Biologie," xvi., 1899, pp. 27-47, ' P'-) tnat it 
 in many respects follows the hermaphrodite form, but is greatly simplified, 
 e.g. , in the absence of alimentary canal and of specialised vascular and 
 respiratory organs. He suggests that as the spermatozoa of the hermaphro- 
 dite form ripen before the eggs, some of the belated eggs may be fertilised 
 by the spermatozoa of the complementary male which is later in attaining 
 maturity. He supposes further that these belated eggs give rise to the 
 next brood of complementary males. But this, as is the case with so many 
 of these speculations, awaits experimental verification. 
 
 What Darwin did for the cirripedes, Graff and others have done for 
 another very curious set of animals, the Myzostomata. These are degene- 
 rate chtetopods or bristle-footed worms, which occur as outside parasites on 
 sea-lilies (crinoids), on the arms of which they make curious galls. There 
 is unfortunately lack of agreement among observers, and the life-history of 
 these curious forms requires further study. We can only indicate two 
 different sets of results. 
 
 According to Beard, "the various kinds of parasitism presented by the 
 
HERMAPHRODITISM. 83 
 
 numerous species of Myzostoma have led in some cases to the preservation 
 of males, in others to their extinction, in yet others to their conversion into 
 hermaphrodites." He distinguishes — 
 
 1. Purely dicecious forms with small males, e.g. M. pulvinar. 
 
 2. Hermaphrodite forms and true males, which remain as dwarf 
 
 " complemental males" on the back of the hermaphrodites, e.g. 
 M. glabrum. 
 
 3. Hermaphrodite forms and males, which, retaining their position 
 
 on the back of the others, afterwards become females, e.g. 
 M. alatum. 
 
 4. Hermaphrodite forms, in which the males have lost their dorsal 
 
 position, and have either become extinct or converted into pro- 
 tandric hermaphrodites, e.g. M. cirriferum. 
 
 According to Wheeler, Myzostoma glabrum is from the first herma- 
 phrodite and not dimorphic, but a functional male phase is succeeded by a 
 functional hermaphrodite phase, and that again by a functional female phase 
 during which the testes disappear. 
 
 " The cysticolous and endoparasitic species of the genus tend towards 
 a condition in which the functional male and female phases overlap but 
 little, thus exhibiting only a brief hermaphrodite phase (M. eremila), or 
 these phases no longer overlap and thus present two well-marked periods 
 of sexual maturity, one male and the other female (M. pulvinar)." 
 
 § 9. Conditions of Hermaphroditism. — A review of the 
 occurrence of normal hermaphroditism suggests few general 
 conclusions. Claus points out that hermaphroditism finds 
 most abundant expression in sluggish and fixed animals. 
 Flukes, tapeworms, leeches, even earthworms and land-snails, 
 may illustrate the sluggard hermaphrodites; among sponges, 
 sea-anemones, corals, Polyzoa, bivalves, &c, we find frequent 
 illustration of the association of fixedness and hermaphroditism. 
 Most of the tunicates are also fixed, and all are hermaphrodite. 
 But the pelagic tunicates are also hermaphrodite, and so are 
 the very active Ctenophora. Claus notes further that in flukes 
 and tapeworms hermaphroditism is associated with an isolated 
 habit of life. But there is often anything but isolation, for 
 flukes may occur near one another in great numbers; and as 
 many as ninety tapeworms (Bothriocephalus) have been known 
 to occur at one time in a single host. 
 
 Simon has gone further, in insisting on the physiological 
 connection between quiescent and parasitic habit and the 
 hermaphrodite condition. In flukes and tapeworms, leeches, 
 Myzostomata, and some cirripedes, we find the association of 
 hermaphroditism with a more or less intimate parasitic habit. 
 But what Simon points out is, that organisms on which great 
 demands are made, especially in the way of muscular exertion, 
 
84 THE EVOLUTION OF SEX. 
 
 cannot afford to be hermaphrodite; while a plethora of nutrition, 
 as in parasitism, tends to make the persistence of the double 
 state possible. He gives numerous illustrations in support of 
 this view. Others are content to interpret the hermaphroditism 
 in all these cases as an adaptation to ensure fertilisation, for 
 the possibilities of pairing between separate sexes are certainly 
 lessened if the animals are sluggish, sedentary, or parasitic. 
 
 § 10. Origin of Hermaphroditism. — (1) One view of the 
 matter is that hermaphroditism was the primitive state among 
 multicellular animals, at least after the differentiation of sex- 
 elements had been accomplished. In alternating rhythms, 
 eggs and sperms were produced. The organism was alternately 
 male and female. Of this primitive hermaphroditism, there 
 may be more or less of a recapitulation in the life-history of the 
 organism. Gegenbaur states the common opinion in the 
 following cautious and terse words : — " The hermaphrodite 
 stage is the lower, and the condition of distinct sexes has 
 been derived from it." Unisexual "differentiation, by the 
 reduction of one kind of sexual apparatus, takes place at very 
 different stages in the development of the organism, and often 
 when the sexual organs have attained a very high degree of 
 differentiation." The first structural stage in the separation 
 would probably be the restriction of areas, in which the forma- 
 tion of two kinds of cells still went on at different times in one 
 organism. In different individuals the opposite tendencies we 
 have already spoken of more and more predominated, till 
 unisexuality evolved out of hermaphroditism. 
 
 That environmental conditions are effective in changing 
 the hermaphrodite into the unisexual state is suggeste'd by 
 many experiments. And it has been shown in regard to some 
 flowering plants, e.g. butcher's broom (Ruscus aculeatus), that 
 the monoecious or dioecious condition may be evoked by alter- 
 ing the nutritive conditions. 
 
 (2) Quite different is the view which regards hermaphro- 
 ditism as a secondary condition, derived from primitive uni- 
 sexuality. Thus Pelseneer maintains that the " study of 
 Mollusca, Myzostomidag, Crustacea, and Pisces shows that 
 in these groups the separation of the sexes preceded herma- 
 phroditism; various cases in other, groups tend to show that 
 this is true universally; and the same conclusion applies to 
 plants. In Mollusca, Crustacea, and Pisces, at least, herma- 
 phroditism is grafted upon the female sex." 
 
HERMAPHRODITISM. 85 
 
 SUMMARY. 
 
 1. Hermaphroditism is the union of the two sexual functions in one 
 organism. This occurs, however, in varying degrees. 
 
 2. Embryonic hermaphroditism is probably a general fact with even 
 unisexual animals. It is certain in some cases. 
 
 3. Casual or abnormal hermaphroditism is not infrequent. 
 
 4. Partial hermaphroditism (not involving the essential organs) is 
 exceedingly common. 
 
 5. Normal adult hermaphroditism ; review of its occurrence. 
 
 6. True hermaphroditism occurs in many degrees of intimacy. 
 
 7. Self-fertilisation is a rare exception among animals; commoner in 
 plants. 
 
 8. " Complemental males" — pigmies attached to hermaphrodites — 
 occur in two groups. 
 
 9. The conditions of hermaphroditism. Commonest in sedentary, 
 sluggish, parasitic forms. 
 
 10. Hermaphroditism is primitive ; the unisexual state is a subsequent 
 differentiation. Or, Unisexuality is primitive; the hermaphrodite state 
 secondary. Possibly both suggestions may be true. 
 
 LITERATURE. 
 
 See already cited works of 
 Gegenbaur, Hensen, Hertwig, Hatchett Jackson and Rolleston, 
 
 passim. 
 Beard, J. — The Sexual Conditions of Myzostoma glabrum. MT. Zool. 
 
 Slat. Neapel., XIII., 1898, pp. 293-324, I pi. 
 Bourne. — On Certain Abnormalities in the Common Frog. 1. The 
 
 Occurrence of an Ovotestis. Quart. J. Micr. Sc, XXIV. 
 Brock. — Morph. Jahrb., IV. Beitrage zur Anatomie und Histologic der 
 
 Geschlechtsorgane der Knochenfische. 
 Giles. — Quart. Journ. Micr. Sci. 1888. 
 Haacke, W. — Die Bedeutung und die Folgen der Inzestzucht. Biol. 
 
 Centrlbl., XV., 1895, pp. 44-78. 
 Kennel, J.— Schrift. Nat. Ges. Jurjeff (Dorpat), IX., 1896, pp. 1-64. 
 Laulanik, F. — Comptes Rendus, CI. (1885), pp. 393-5. 
 Marshall, A. Milnes. — On Certain Abnormal Conditions of the 
 
 Reproductive Organs in the Frog. Journ. Anat. Physiol., XVIII., 
 
 pp. 121-44. 
 Meehan, T.— On Self-Fertilisation and Cross-Fertilisation in Flowers. 
 
 Penn. Monthly, VII. (1876), pp. 834-43. 
 Montgomery, T. H.— On Successive Protandric and Proterogynic Her- 
 maphroditism in Animals. Amer. Naturalist, XXIX., 1895, pp. 
 
 528-36. 
 Pelseneer, P.— L'hermaphroditisme chez les Mollusques. Arch. Biol., 
 
 1895, pp. 33-62, 2 pis. Quart. Journ. Micr. Sci., CXLV., 1894, 
 
 pp. 19-46, 2 pis. 
 
86 THE EVOLUTION OF SEX. 
 
 Pfluger, E. — Archiv. ges. Physiol., XXIX. 
 
 Prouho, H.— Dioicite et Hermaphroditisme chez les Myzostomes. Zool. 
 
 Anzeiger, XVIII., 1895, pp. 392-5. 
 Simtson, J. Y. — Todd's Cyclopaedia of Anatomy and Physiology. Art. 
 
 Hermaphroditism, pp. 684-738 (1836-9). 
 Spengel.— Arb. Wiirzburg, III., 1876. Ueber d. Urogenital System der 
 
 Amphibien. 
 
 Zwitterbildung bei Amphibien. Biol. Centrlbl., IV., 8. cf. 9. 
 
 Sutton, J. B. — Hypertrophy and its Value in Evolution. Proc. Zool. 
 
 Soc, London, 1888, pp. 432. 
 
 > General Pathology. London, 1886. 
 
 Wheeler, W. M.— The Sexual Phases of Myzostoma. MT. Zool. Stat. 
 
 Neapel., XII., 1896, pp. 227-302. Zool. Anzeig., XXII. (1S9?), 
 
 pp. 281-8. 
 
CHAPTER VII. 
 
 The Ultimate Sex-Elements {General and HisloricaT). 
 
 In our analysis of sex-characters we have followed the general 
 course of biological history. We first passed from the form 
 and habit of a male or female organism to the structure and 
 functions of the sexual organs. In discussing hermaphroditism, 
 we had occasion to refer to a third step of biological analysis, 
 that which involves an investigation of the properties of the 
 
 Mammalian ovum, showing nucleolus (a), nucleus (J) cytoplasm (c) 
 external porous zone or zona pellucida (J), and follicular cells W. 
 —From Hertwig, after Waldeyer. 
 
 tissues. Now it is necessary to penetrate deeper, namely, to 
 the sex-cells. After these have been considered we shall be 
 in a better position to re-ascend to some of the problems of 
 reproduction. 
 
88 THE EVOLUTION OF SEX. 
 
 § i. The Ovum Theory.— -It is now a commonplace of" 
 observation and established fact, that all organisms, reproduced 
 in the ordinary way, start in life as single cells. We see insects 
 laying their ova upon plants, or fishes shedding them in the 
 water, and may watch how these cells, provided they be 
 fertilised, give rise eventually to. the adult organisms. Con- 
 veniently in the ordinary frog-spawn from the ditch, we can 
 read, what was for so long a riddle, how development proceeds 
 by successive cell-divisions and by arrangement of the multiple 
 results. Readily seen in many instances, it is true of all cases 
 of ordinary sexual reproduction, that the organism starts from 
 the union of two sex-cells. In other words, it is with the division 
 of a fertilised ovum that development begins. 
 
 This profound fact, technically known as the "ovum 
 theory," has been not unjustly called by Agassiz " the greatest 
 discovery in the natural sciences of modern times." We shall 
 the better realise the magnitude of the difference which its 
 recognition has introduced into biology, if we briefly review 
 the history. 
 
 § 2. The History of Embryology: Evolution and Epigenesis. 
 — The development of the chick, so much studied in em- 
 bryological laboratories to-day, was the subject of inquiry two 
 thousand years ago in Greece. Some of the conspicuous marvels 
 of reproduction and development were the subjects of persistent 
 but fruitless speculation throughout many centuries. It was 
 only during the scientific renaissance of the seventeenth century 
 that the inquiry became more keen and precise, and began to 
 rely to some extent at least on genuine observation. 
 
 (a) Harvey (165 1), with the aid of magnifying glasses 
 {perspediia), demonstrated in the fowl's egg the connection 
 between the cicatricula of the yolk and the rudiments of 
 the chick, and also observed some of the stages of uterine 
 life in mammals. More important, however, were his far- 
 sighted general conclusions, — (1.) That every animal was pro- 
 duced from an ovum {ovum esse primordium commune omnibus 
 animalibus); and (2.) That the organs arose by new formation 
 {epigenesis), not from the mere expansion of some invisible pre- 
 formation. In this generalisation, without however any abandon- 
 ment of the hypothesis of spontaneous generation of germs, he 
 strove, as he said, to follow his master Aristotle, and was in so 
 doing as far ahead of his contemporaries as a strong genius 
 usually is. Before Harvey, the observational method had 
 
THE ULTIMATE SEX-ELEMENTS. 89 
 
 indeed begun. Thus, as Allen Thomson notes, Volcher Coiter 
 of Groningen (1573), along with Aldrovandus of Bologna, had 
 watched the incubated egg in its marvellous progress from day 
 to day. Fabricius ab Aquapendente (1621) had also studied 
 the changes in the incubated egg, and the stages of the 
 mammalian foetus. In keenness of vision, Harvey was far 
 ahead of these. 
 
 (b) Malpighi (1672), using a microscope with phenomenal 
 skill, traced the embryo back into the recesses of the cicatricula 
 or rudiment, but again missed a magnificent discover)', and 
 supposed the rudiments to have pre-existed in the egg. In 
 1677, Leeuwenhock was led by Hamm to the discovery of 
 the spermatozoa; and this was followed up, though not to 
 much profit, by Vallisneri and others. Steno, too, in 1664, 
 had given the ovary its present designation ; and De Graaf had 
 interpreted the vesicles of this organ, which now bear his name, 
 as for the most part equivalent to the ova which he had dis- 
 covered in the oviduct. Needham (1667), Swammerdam (1685), 
 and J. van Heme, also contributed items of information not 
 then appreciated in their real relations. 
 
 (c) The Theory of Preformation — Ovists and Animalculists. 
 — In the early part of the eighteenth century, the embryological 
 observations of investigators, e.g. Boerhaave, were summed up 
 in the conception that development was merely an expansion 
 or unfolding of a pre-existent or preformed rudiment within the 
 egg. Harvey had indeed striven for an opposite conclusion, 
 but his view was negatived, as we have seen, by Malpighi's 
 failure to trace the embryo beyond the rudiments of the 
 cicatricula. 
 
 The notion of a preformed rudiment, thus suggested by 
 Boerhaave, Malpighi, and others, rapidly became the prevalent 
 theory. In so far as it emphasises one side of the facts, it is 
 bound in modified form so to remain. Leibnitz, Malebranche, 
 and others found it to fit in better with their cosmic concep- 
 tions than the older view of Aristotle had done, and welcomed 
 it accordingly. 
 
 The positions occupied by the physiologist Haller well 
 illustrate the alterations of opinion. As Allen Thomson points 
 out in his article "Embryology," in the Encyclopedia 
 Britantiica, "Haller was originally educated as a believer in 
 the doctrine of ' preformation ' by his teacher Boerhaave, but 
 was soon led to abandon that view in favour of ' epigenesis ' or 
 
9° THE EVOLUTION OF SEX. 
 
 new formation. But some years later, and after having been 
 engaged in observing the phenomena of development in the 
 incubated egg, he again changed his views, and during the 
 remainder of his life was a keen opponent of the system of 
 epigenesis, and a defender and exponent of the theory of 
 ' evolution,' as it was then named." 
 
 The preformation theory found more and more definite 
 expression in the works of Bonnet, Buffon, and others. It is 
 now necessary to sum up its main propositions. 
 
 The germ, whether egg-cell or seed, was believed to be a 
 miniature model of the adult. "Preformed" in all trans 
 parency the organism lay within the egg, only requiring to 
 be unfolded. Many affirmed, that before fertilisation there 
 lay within the fowl's ovum an excessively minute but complete 
 chick. They compared the germ to a complex bud, which 
 hides within its hull the floral organs of the future. Harvey 
 had said, "the first concrement of the futute body grows, 
 gradually divides, and is distinguished into parts ; not all at 
 once, but some produced after the others, each emerging in its 
 order." Very different was Haller's first and last utterance, 
 " There is no becoming ; no part of the body is made from 
 another, all are created at once." This was obviously a short 
 and easy method with embryology, if the organism was literally 
 preformed in the germ, and its development simply a growth 
 and an unfolding. 
 
 But this was not all. The germ was more than a marvellous 
 bud-like miniature of the adult, it necessarily included in its 
 turn the next generation, and this the next — in short all future 
 generations. Germ within germ, in ever smaller miniature, after 
 the fashion of an infinite juggler's - box, was the corollary 
 logically appended to this theory of preformation and unfold- 
 ing, — of evolution, as it was then called, in a very different but 
 more literal sense from that in which we now use the word. 
 
 A side controversy of the time arose between two schools, 
 who called each other "ovists" and " animalculists." The 
 former maintained that the female germ element was the more 
 important, and only required to be as it were awakened by the 
 male element to begin the process of unfolding. The animal- 
 culists, on the other hand, asserted the claims of the sperm to be 
 the bearer of the miniature nest of organism within organism, 
 and supposed that it only required to be fed by the ovum to 
 enlarge and unfold the first of the models which it concealed. 
 
THE ULTIMATE SEX-ELEMENTS. 
 
 91 
 
 (d) Wolff's Reassertion of Epigenesis. — The above ingenious 
 construction was rudely shaken down, however, in 1759, when 
 Caspar Friedrich Wolff showed, in his doctorial dissertation, the 
 illegitimacy of the suppositions which lay at the root of the 
 
 The first stages of development in a number of animals. A , 
 Sponge Coral, Earthworm, or Starfish; B, Crayfish or 
 other Arthropod ; C, Tunicate, Lancelet, &c. ; D, Frog 
 or other Amphibian. 
 
 1 Fertilised ovum ; 2. Seemented ovum, a ball of cells, morula, 
 or blastosphere ; 3. The same, after further division or in 
 section ; 4. The gastrula stage. 
 
 preformation theory. He traced the chick back to a layer of 
 organised particles (the familiar cells of to-day), in which there 
 was no likeness of the future embryo, far less adult. Mors 
 
Q2 THE EVOLUTION OF SEX. 
 
 than that, he followed the disposition of these primitive elements 
 to the upbuilding of some of the important organs. He un- 
 doubtedly reacted too far in his emphasis on the entire sim- 
 plicity of the germ, and many of his details were mistaken ; but 
 none the less did he recall embryologists from speculation to take 
 the facts as they found them, and lay the foundation of modern 
 embryology in the fact that there is an observable process of de- 
 velopment from the apparently simple to the obviously complex. 
 
 (e) Wolff's Successors. — The important conclusion reached 
 by Wolff remained for about sixty years without effect. In 1817, 
 Christian Pander took up embryological research exactly where 
 AVolff had left it, and worked out the history of the chick in 
 more exact detail. In 1824, Prevost and Dumas noticed the 
 division of the ovum into masses ; and in the following year 
 Purkinje discovered the nucleus or "germinal vesicle." Von 
 Baer followed up his friend Pander's work, and in 1827 made 
 the memorable discovery of the mammalian ovum, which he 
 traced from uterus to oviduct, and then to its position in the 
 ovary itself. Thus, after a century and a half, De Graaf's 
 endeavour was at length fulfilled. Soon afterwards, Wagner, 
 von Siebold, and others, elucidated what was still hidden from 
 von Baer, — the real nature of the spermatozoa. Meanwhile, 
 Bichat's analysis (1801) of the organism into tissues, was with 
 improved appliances deepened in the casual description of 
 "cells"; and an important generalisation had its forecast in 
 1835, when Johannes Miiller pointed out in the vertebrate 
 notochord the existence of cells resembling those of plants. 
 
 §3. The Cell-Theory. — Without continuing the history 
 further, we must simply note that in 1838 Schleiden referred 
 all vegetable tissues to the cellular type, and traced back the 
 plant embryo to a single nucleated cell ; while, in the following 
 year, Schwann boldly extended this conception of plant struc- 
 ture and development to the animal world, and so fully consti- 
 tuted the "cell-theory." The ovum, recognised as a cell, 
 became a "primordium commune" in a deeper sense than 
 Harvey dreamt of; the masses described by Prevost and Dumas 
 were seen as the products of cell division; and Kolliker led the 
 way, now so well followed up, in tracing these cells to their 
 results in the tissues of the organism 
 
 § 4. Protoplast7iic Basis. — Only one step further is it possible 
 for biological analysis to penetrate, and that within the last few 
 years is being persistently essayed. It is impossible to rest at 
 
THE ULTIMATE SEX-ELEMENTS. 
 
 93 
 
 the cell-theory level. To recognise the ovum as a cell, and the 
 spermatozoon as another, to find the starting-point of the 
 organism in the double unity formed from these two, to demon- 
 strate the process of development as one of cell multiplication 
 and arrangement, express great but not final biological facts. 
 Thus it is'that of late years, what Michael Foster has called the 
 "protoplasmic movement" has made itself felt, not only in 
 study of the general functions of the body, but in the special 
 physiology of the reproductive cells and their history. Even 
 in morphological or structural studies, attention has shifted 
 from the shapes of cells to the structure of their living matter, 
 or from the different forms of ovum and spermatozoon to the 
 germinal protoplasm or Keimplasma which they contain. On 
 
 A 
 
 
 ^V" 
 
 
 
 /_ 
 
 — — 
 
 _\ 
 
 
 
 
 
 B 
 
 
 
 
 ■*- 
 
 ^l 
 
 
 
 *s 
 
 
 
 
 Diagram of Protoplasmic Changes. 
 
 this level, in fact where biology 
 
 B in plan, A in elevation. 
 
 m tact wnere oioiogy has touched the bottom, 
 morphology and physiology have become more than ever 
 inseparable. All the facts of structure on the one hand, and of 
 function on the other, have both to be interpreted in terms of 
 the constructive and disruptive changes in the living matter 
 itself. The general theory may be summarised in the accom- 
 panying diagram. Protoplasm is regarded as an exceedingly 
 complex and unstable compound, undergoing continual mole- 
 
94 
 
 THE EVOLUTION OF SEX. 
 
 cular change or metabolism. On the one hand, more or less 
 simple dead matter or food passes into life by a series of 
 assimilative ascending changes, with each of which it becomes 
 molecularly more complex and unstable. On the other hand, 
 the resulting protoplasm is continually breaking down into more 
 and more simple compounds, and finally into waste products. 
 The ascending, synthetic, constructive series of changes are 
 termed "anabolic"; and the descending, disruptive series, "kata- 
 bolic." Both processes may be manifold, and the predominance 
 of a particular series of anabolic or katabolic changes implies 
 the specialisation of the cell. 
 
 issB ' 
 
 \ 
 
 Proterospongia, a colonial infusorian, showing the difference between 
 outer and inner cells. — From Saville Kent. 
 
 The figure (a, on p. 93) represents the complex unstable protoplasm as if 
 occupying the summit of a double flight of steps ; it is formed up the 
 anabolic steps, it breaks up and descends by the katabolic. The lower 
 figure (b) is a projection of the other, its convergent and divergent lines 
 serving to represent the various special lines of anabolism and katabolism 
 respectively, and the definite component substances ("anastates" and 
 "katastates") which it is the task of the chemical physiologist to isolate 
 and interpret. 
 
 From a general physiological point of view it matters little if we make 
 a slightly different assumption, namely, that protoplasm does not exactly 
 share in the twofold process of metabolism, but is a substance per se, acting, 
 like a ferment, on the complex materials around it. Even if we suppose, 
 as some do, that there is no such substance as protoplasm, life being the 
 
THE ULTIMATE SEX-ELEMENTS. 95 
 
 expression of the inter-relations of many complex substances, still, the 
 two main aspects of metabolism — anabolic and katabolic — remain. 
 
 § 5. Protozoa and Metazoa. — It has been emphasised above 
 that every multicellular organism, reproduced in the ordinary 
 way, starts from a fertilised ovum, from what may be fairly 
 called a single cell. Sponge, butterfly, bird, and whale begin, 
 in a sense, at the level of the simplest animals or Protozoa, 
 which (with the exception of some which form colonies) remain 
 always unicellular. The simplest organisms leave off where 
 the higher plants and animals begin, i.e., as single corpuscles 
 of living matter. They correspond, in fact, to the reproduc- 
 tive cells of higher animals, and may be called, according to 
 
 Ophrydium, a colonial infusorian. — From Saville Kent. 
 
 their predominant character, protova and protosperms. A 
 fertilised ovum, as we have seen, proceeds by division to form 
 a "body"; the Protozoon remains, with few exceptions, a 
 single cell, in which there is obviously no distinction between 
 reproductive elements and the entire, organism. 
 
 Reference will have to be made to the Protozoa in three 
 connections, which may be here simply noted:— 
 
 (a) In their chief groups, and in the stages of their life- 
 histories, they express phases in the same cell cycle which 
 recurs in higher forms in the component elements of the body, 
 and in the reproductive cells. The contrast, in other words, 
 between an infusorian and an amoeba, between the ciliated 
 
9^ THE EVOLUTION OF SEX. 
 
 and amoeboid stage in the life-history of many forms, is a fore- 
 cast of the contrast between a ciliated cell and a white blood 
 corpuscle, between a mobile spermatozoon and a young ovum. 
 A predominance of the same protoplasmic processes is the 
 basal interpretation of the similarities of form. 
 
 (b) It is among the Protozoa that we must look, if we hope 
 to understand the origin and import either of "male and 
 female," or of fertilisation. 
 
 (c) Among the loose colonies which some Protozoa form, 
 and which bridge the gulf between the unicellular animals and 
 the Metazoa, there is seen the beginning not only of the for- 
 mation of a "body" (see figs, on pp. 94, 95), but also the 
 setting apart of special reproductive cells. On this poirt 
 more emphasis must be laid. The ordinary Protozoon is a 
 single cell, and forms no body. It divides indeed, and multi- 
 plies accordingly, but the products of division go asunder, 
 whereas in the segmentation of the ovum they remain con- 
 nected. In most Protozoa, there is continual self-recupera- 
 tion; in most, division occurs without any loss; in most, there 
 is no distinction between parent and offspring; in most, as 
 there is no body, there is no death. Thus it is that, with one 
 weighty caution to be afterwards noted, it seems justifiable to 
 speak with Weismann and others of the "immortality of the 
 Protozoa." In a certain sense too, as we shall see, it is justifi- 
 able to speak of the immortality of the reproductive cells in 
 higher animals. The body dies, but the reproductive cells 
 escape, before its death, to live on, as new organisms, enclosing 
 new sets of reproductive cells. Again there is similarity 
 between the Protozoa and the reproductive cells. 
 
 But in some of the loose colonies (e.g., Volvox), we see the 
 beginning of the change which introduced death as a constant 
 phenomenon (see fig. p. 139)- The cell, which starts one of 
 these colonies, divides ; the products of division, instead of 
 going apart as usual, remain connected; a loose body of many 
 cells is thus formed. In this cluster of cells, certain elements 
 are in turn set apart and eventually adrift, as reproductive 
 cells. They start new colonies, and thus we are introduced to 
 what is constant in higher animals. The only marked differ- 
 ences are — (a) that the body of the Metazoon is more than a 
 loose colony of cells; (b) that the reproductive elements are 
 usually liberated from some definite region or organ ; and (c) that 
 they are more markedly differentiated as male and female cells. 
 
THE ULTIMATE SEX-ELEMENTS. 97 
 
 § 6. General Origin of the Sex-Cells. — Except in the lowest 
 invertebrates, the sponges and ccelenterates, the reproductive 
 elements almost always arise in connection with the middle 
 layer (mesoderm or mesoblast) of the body. 
 
 . Neither in sponges nor in ccelenterates is there a middle layer exactly 
 comparable to the mesoderm of higher animals; the less definite middle 
 stratum is now frequently termed a mesoglsea. In sponges, we already 
 mentioned that the reproductive cells simply arise here and there among 
 the other elements of the stratum. The ova are highly nourished meso- 
 gheal cells; the primitive male cells, which divide into numerous minute 
 spermatozoa, are the reverse. 
 
 In ccelenterates the phenomena are of much interest; the origin of the 
 sex-cells is very diverse. Some time ago considerable emphasis was laid, 
 by E. van Beneden and others, on the fact that, in certain Hydrozoa, " the 
 ova are derived from the endoderm, and the sperms from the ectoderm." 
 Thus Gegenbaur, accepting this, remarks that in such cases "the 
 endoderm is the female, and the ectoderm the male germinal layer." 
 Such a generalisation, if established, would be plausible enough, seeing 
 that the inner or endoderm layer is the more nutritive or anabolic of the 
 two. A controversy, however, soon arose, the result of which was to over- 
 throw the generalisation. In hydra, we have already noticed that both 
 products arise from the ectoderm; the same was shown by Ciamician to be 
 true of Titbularia mesembryanthemum ; while in the EiUetidrium ramosunt 
 the ova appeared to arise from the ectoderm, and the male elements from 
 the endodenu, the very reverse of Van Beneden's conclusion. The matter 
 was settled, so far as the general facts are concerned, by Weismann, who 
 established the fact of active migration of the elements from one layer to 
 another. He has since been followed by other investigators, (a) The sex- 
 elements, both male and female, may appear first in the endoderm, whether 
 they originate there or not, and from this inner layer they migrate to the 
 ectoderm, where they ripen, (b) In rare cases they even ripen in the 
 endoderm. (c) Very commonly the sex-cells originate in the ectoderm and 
 ripen there, or they may pass thence into the endoderm and back again to 
 the ectoderm, (d) In the medusa of Obelia, the ova appear to ripen partly 
 in both layers. These facts, a convenient summary of which will be 
 found in Hatchett Jackson's erudite edition of Rolleston's " Forms of 
 Animal Life," show plainly enough how varied are the origin and history 
 of the sex-cells in these forms. 
 
 The colonial hydroids typically produce well marked reproductive 
 individuals or sexual zooids, set free as "swimming-bells" or meduioids 
 (in a process to be afterwards described under "Alternation of Genera- 
 tions "). In these the reproductive elements are typically developed. But 
 in varying degrees these medusoids have degenerated, and are frequently 
 not only not liberated, but lose their characteristic features, and become 
 mere reproductive buds. In these buds the sex-cells are normally 
 developed. But it very frequently happens that they arise more or less in 
 the body of the asexual vegetative hydroid. They ripen early, and sub- 
 sequently migrate to their proper place; the asexual stage incorporating 
 more and more of the originally separate sexual generation. Weismann has 
 emphasised the value of this early ripening as an advantage to the race, 
 
 7 
 
98 THE EVOLUTION OF SEX. 
 
 lessening the danger of its extinction ; and this has doubtless to be con- 
 sidered. But important as all such considerations are, they cannot dispense 
 with an enquiry into the physiology of the facts. 
 
 § 7. Early Separation of Sex-Cells. — Having noted the 
 general fact of mesodermic origin, and some of the interesting 
 phenomena observed in ccelenterates, we shall not further 
 pursue the subject except as regards one question, the period 
 at which the reproductive cells make their appearance. This 
 is sometimes early, sometimes late; and it is not yet decisively 
 known in how many cases early separation occurs, nor how far 
 the fact is of much significance. 
 
 In the case of a well-known fly, Chironomus, Professor Bal- 
 biani, unprejudiced by any theory of heredity, observed the 
 following facts: — Before the segmentation of the egg had at all 
 advanced, before what embryologists call the blastoderm was 
 more than incipient, two cells were observed to be set apart 
 externally. (These had nothing whatever to do with the polar 
 globules seen in most ova at maturation.) The development 
 proceeded apace, but the isolated cells took no share; they 
 may be presumed to have retained intact the characters which 
 they received when first divided off from the ovum. At a 
 certain stage, however, the isolated cells sank inwards, took up 
 an internal position, became the rudiments of the reproductive 
 organs. Here then, at an early stage, before differentiation is 
 marked, the reproductive cells are set apart. They must there- 
 fore preserve much of the character of the parent ovum, and 
 hand on the tradition intact by continuous cell-division to the 
 next generation. 
 
 In other words, in the preceding case, at a very early stage 
 in the embryo, the future reproductive cells are distinguishable 
 and separable from the body-forming cells. The latter develop 
 in manifold variety, into skin and nerve, muscle and blood, gut 
 and gland; they differentiate, and lose almost all protoplasmic 
 likeness to the mother ovum. But the reproductive cells are 
 set apart; they take no share in the differentiation, but remain 
 virtually unchanged, and continue unaltered the protoplasmic 
 tradition of the original ovum. After a while they, or their 
 division-products rather, will be liberated as reproductive cells. 
 These in a sense will be continuous with the parental germ. 
 Their protoplasm will be more or less identical. The original 
 ovum has certain characteristics, a b c x y z ; it divides, and all 
 its cells must at first more or less share these characteristics; 
 
THE ULTIMATE SEX-ELEMENTS. 99 
 
 the body-cells lose some of them, the insulated reproductive 
 cells retain them all. The ovum of the next generation has 
 thus also the characteristics a b c x y z, and must therefore 
 produce an organism essentially like the parent. 
 
 An early isolation of the reproductive cells, though rarely so 
 striking as in Chironomus, has been observed in many cases, — 
 e.g., in some other insects, in the aberrant worm-type Sagitta, 
 in leeches, in threadworms, in some Polyzoa, in some small 
 crustaceans known as Cladocera, in the water-flea Moina, in 
 the parasitic Hymenopteron Platygaster, in some arachnoids 
 (Phalangidse), in the bony fish Micromettus aggregatus. As 
 the series is ascended, the reproductive organs seem to be later 
 in making their appearance, at least they are only detected at 
 a later stage. It must also be pointed out that, in cases of alter- 
 nation of generations, an entire asexual generation, or more than 
 one, may intervene between one ovum and another. 
 
 Perhaps the most striking of all the cases of the early segre- 
 gation of the lineage of germ-cells is that described by Boveri 
 in Ascaris megalocephala, the threadworm of the horse. He 
 was able to trace back the germ-cells continuously to the two- 
 cell stage. At the very first cleavage the distinction between 
 somatic and reproductive is established. One of the first two 
 cells is the ancestor of all the cells of the body; the other is 
 the ancestor of all the germ-cells. " Moreover, from the out- 
 set the progenitor of the germ-cells differs from the somatic cells 
 not only in the greater size and richness of the chromatin of its 
 nucleus but also in its mode of mitosis, for in all those blasto- 
 meres destined to produce somatic cells a portion of the 
 chromatin is cast out into the cytoplasm, where it degenerates, 
 and only in the germ-cells is the sum-total of the chromatin 
 retained." — (E. B. Wilson, "The Cell in Development and 
 Inheritance," 1896, p. in.) 
 
 § 8. Body Cells and Reproductive Cells. — Various naturalists 
 have insisted on the contrast hinted at above, between the cells 
 of the embryo which go to form the body and those which are 
 set apart as reproductive organs. 
 
 (a) As early as 1849, Owen noted that, in the developing 
 germ, it was possible to distinguish between cells which became 
 much changed to form the body, and cells which remained 
 little changed and formed the reproductive organs. This view, 
 as Brooks points out, he unfortunately afterwards departed from 
 in his "Anatomy of the Vertebrates." 
 
100 THE EVOLUTION OF SEX. 
 
 (6) In 1866, Haeckel connected reproduction with discon- 
 tinuous growth, and insisted upon the material continuity 
 between parent and offspring. Somewhat later, both he and 
 Rauber drew a clear contrast between the somatic and repro- 
 ductive elements, between the "personal" and "germinal" 
 portions of the embryo, or between the body and the sex cells. 
 
 (c) W. K. Brooks, in 1876 and 1877, again drew attention 
 to this significant contrast. 
 
 (d) Yet more explicit, in 1877, was the ingenious Dr 
 Jager, now better known in a very different connection, and 
 a few of his sentences well deserve to be quoted. Referring 
 to a previous paper, he writes as follows: — "Through a great 
 series of generations, the germinal protoplasm retains its specific 
 properties, dividing in every reproduction into an ontogenetic 
 portion, out of which the individual is built up, and a phylo- 
 genetic portion, which is reserved to form the reproductive 
 material of the mature offspring. This reservation of the 
 phylogenetic material I described as the continuity of the germ- 
 protoplasm. Encapsuled in the ontogenetic material, the phylo- 
 genetic protoplasm is sheltered from external influences and 
 retains its specific and embryonic characters." 
 
 (<?) In an exceedingly clear manner, to which sufficient 
 attention seems hardly to have been accorded, Galton, in 1876 
 and at other dates, as again more indirectly in his work on 
 " Natural Inheritance," drew attention to the contrast between 
 the gemmules of the ovum (stirp) which go to form the body, 
 and those which, remaining undeveloped, form the sex-cells. 
 " The developed part of the stirp is almost sterile " (i.e., with- 
 out influence in heredity); " it is from the undeveloped residue 
 that the sexual elements are derived." 
 
 (/) Lastly, in 1880, Nussbaum, in an elaborate investiga- 
 tion on the differentiation of the reproductive cells, drew 
 emphatic attention to some cases of their early separation, and 
 reasserted Jager's conception of a continuity of germ-protoplasm. 
 In this survey, however, we do not pretend to decide the 
 difficult question of priority in the enunciation of this con- 
 ception. Like many other generalisations, it appears to have 
 arisen all but simultaneously in many minds. 
 
 (g) It is to Weismann, however, that biology is particu- 
 larly indebted for his vindication and elaboration of the 
 doctrine of genetic continuity. 
 
 § 9. Weismamis Theory of the Contimrty of the Germ- 
 
THE ULTIMATE SEX-ELEMENTS. 
 
 IOT 
 
 Protoplasm. — In some cases referred to in a foregoing paragraph, 
 it is possible to trace a direct cellular con- 
 tinuity, first of all, between the ovum and 
 early separated reproductive rudiments, 
 secondly, between the latter and the future 
 ova and sperms. There is not only cellular 
 continuity between the ovum which gives rise 
 to parent, and the ovum which gives rise to 
 offspring,— that the cell-theory demands, — 
 but there is a continuity in which the char- 
 acter of the original ovum is never lost by 
 differentiation. In fact, there is a continuous 
 chain of reproductive cells quite apart from 
 the body cells. It is in this sense that some 
 of the authors quoted have spoken of the 
 continuity of the germ-«//r. This is certainly 
 true for some cases. If it were true for all, 
 the problems of reproduction and heredity 
 would be much simpler than they at present 
 appear to be. 
 
 For in the present state of our knowledge 
 we can only speak of the continuity of the 
 reproductive cells, in exceptional or rather in 
 a small minority of cases. Alike in the higher 
 vertebrates and the lowly hydroids, the re- 
 productive cells may appear late. After the 
 differentiation of the vertebrate embryo has 
 progressed far, or the life of the polyps con- i ^ 
 tinued for long, the germ-cells make their (|| 
 appearance; and though we know of course J @ 
 that they are descendants of the original The reUio 
 
 i between re- 
 
 ovum, yet we must allow, with Weismann, g£^ c T t ^ 
 that in the form of special cells they are now 
 for the first time to be detected. Therefore, 
 'a continuity of germ-ce//s 
 
 Weismann says. 
 
 is now for the most part no longer demon 
 
 strable." . „_ . 
 
 Yet there is nothing that Weismann 
 more strongly insists upon, than the reality 
 of continuity between ovum and ovum. 
 In what does it consist, if a chain of ovum-like cells 
 is only true of a minority of organisms? It consists, 
 
 tinuous chain of basal 
 cells at first represents 
 a succession of Proto- 
 zoa ; further on, it re- 
 presents the ova from 
 which successive 
 ' ' bodies " are pro- 
 duced. At each gen- 
 eration, a spermato- 
 zoon fertilising the 
 liberated ovum is also 
 indicated. 
 
102 THE EVOLUTION OF SEX. 
 
 according to Weismann, in the " Keimplasma " or germ- 
 plasm. 
 
 The germ-plasm is the distinctive part of the nucleus of 
 the germ-cell. It has an extremely complex, and at the same 
 time persistent, structure. It is the substance which enables the 
 germ-cell to build up an organism ; it is the architectural living 
 matter, and the immortal bearer of all properties transmitted in 
 inheritance. " In every development," according to Weismann, 
 ■'a portion of this specific germ-plasm, which the parental 
 ovum contains, is unused in the upbuilding of the offspring's 
 body, and is reserved unchanged to form the germ-cells of the 
 next generation. . . . The germ-cells no longer appear as 
 products of the body, at least not in their most essential part — 
 the specific germ-plasm; they appear rather as something 
 opposed to the sum-total of body-cells; and the germ-cells of 
 successive generations are related to one another like genera- 
 tions of Protozoa." But the continuity is rarely kept up by a 
 chain of undifferentiated reproductive cells; it depends upon 
 the continuance and unchanged persistence of part of the 
 original germ-plasm. 
 
 It need hardly be pointed out that the conception of the 
 germ-plasm is theoretical. For just as we cannot point to any one 
 portion of the cell and say this is protoplasm — the living matter 
 — and nought else; so we cannot demonstrate the germ-plasm, 
 even if we accept the current view that it has its basis in the 
 chromatin of the nucleus. The theory is a conceptual inter- 
 pretation, and must be tested by its power of fitting facts. 
 
THE ULTIMATE SEX-ELEMENTS. 1 03 
 
 SUMMARY. 
 
 The progressive analysis through organism, organs, tissues, and cell-, 
 to the living matter itself. 
 
 1. The Ovum-theorj. — Every multicellular organism, reproduced in the 
 ordinary way, arises from a fertilised egg-cell, and development proceeds 
 by cell-division. 
 
 2. Epigenesis and Evolution. — History of the different views taken of 
 the development of the organism; ancient speculations. The scientific 
 renaissance, (a) Harvey's prevision of the ovum-theory, and emphasis 
 upon "epigenesis." (b) Observations of Malpighi and others, mostly 
 against Harvey's view, (c) The theory of preformation, — of a nest of 
 miniature models within the egg, only requiring to be unfolded in succes- 
 sive generations; Ovists versus Animalculists. (d) Wolff's reassertion of 
 " epigenesis," the foundation of modern embryology; his exaggeration of 
 the simplicity of the germ, (e) Wolffs successors. 
 
 3. The Cell-Theory. — All organisms have a cellular structure and a 
 cellular origin. 
 
 4. A protoplasmic basis now being laid. The "germ-plasm" more 
 important than the egg-cell. All to be interpreted in terms of protoplasmic 
 changes. 
 
 5. The contrast between Protozoa and Metazoa. — The making of a 
 " body " as distinct from reproductive cells. 
 
 6. General origin of the sex-cells, indefinite in sponges, variable in 
 ccelenterates, generally from the mesoderm in higher animals. 
 
 7. Early separation of the reproductive cells to be seen in a minority of 
 cases. 
 
 8. The contrast between somatic and reproductive cells, and the con- 
 tinuity of the latter; Owen, Haeckel, Rauber, Brooks, jager, Gallon, 
 Nussbaum, Weismann. 
 
 9. Weismann's theory of the continuity of the geim-piasm (a specific 
 nuclear matter), as opposed to continuity by a lineage of undifferentiated 
 germ-ceils, which is known to occur only in a minority of organisms. 
 
 LITERATURE. 
 
 For relevant literature and further details, consult the Text-books of 
 
 Balfour, Haddon, and Hertwig; also, 
 Delage. — Heredite, etc. 1895. 
 Geddes, P. — Encyclopcedia Britannica articles already referred to; also 
 
 Morphology, ibid. 
 Mensen, V. — Op. cit. 
 
 M'KENDRICK, J. G.— Text-book of Physiology. London, 1S88. 
 Thomson, J. A. —Arts. Cell and Embryology, new edition of 
 
 Chambers's Encyclopedia. 
 
 History and Theory of Heredity. Proc. Roy. Soc. Edin., 1888. 
 
 Waldeyer, W. — Die Karyokinese, &c. Arch. Mikr. Anat., 188S. 
 
 Wetsmann. — Opp. cit. 
 
 Wilson, E. B. — The Cell in Development and Inheritance. New edition, 
 
 1900. 
 Zoological Record, General Subjects: Cell, Oogenesis, &c., since 1S86. 
 
CHAPTER VIII. 
 
 The Egg-Cell or Ovum. 
 
 In the preceding chapter we sketched the history of the 
 "ovum-theory," which expresses the now familiar fact that 
 every organism, reproduced in the ordinary way, develops from 
 a fertilised egg-cell. The exceptions are the unicellular plants 
 and animals, all forms of asexual reproduction, and the special 
 
 Animal Cell, showing the chromatin elements of nucleus 
 (a) in a loDg coil, and the protoplasmic network (b) 
 round about, — From Carnoy. 
 
 case of parthenogenesis (where the egg-cell is not fertilised). 
 It is now necessary to attend more carefully to the essential 
 characters and history of this " primordium commune," the 
 starting-point of the individual life. 
 
 § i. Structuye of the Ovum.— -The ovum presents all the 
 
THE EGG-CELL OR OVUM. 
 
 1Q 5 
 
 essential features of any other animal cell. There is the cell- 
 substance, consisting in part of genuine living matter or proto- 
 plasm ; and there is the nucleus, or "germinal vesicle," which 
 plays such an important part in the ripening, fertilising, and 
 subsequent division of the cell. 
 
 Besides the living matter, there are simpler substances, 
 especially in many cases a reserve capital of yolk nutriment 
 for the future embryo. The modern masters of microscopic 
 technique have detected many marvels in the egg-cell, into 
 
 Ovum of a Threadworm (Ascaris), showing («) the chromatin 
 elements of the nucleus, and the appearance of the 
 surrounding yolk. — From Carnoy. 
 
 which we cannot at present enter, but it is important to 
 recognise clearly that although the ovum is in a sense simple, 
 being a single cell, it is not structureless like white of egg. 
 About details there is great diversity of opinion, but all are 
 agreed that the ovum has "organisation." 
 
 When Purkinie, in 1825, discovered the nucleus of the 
 fowl's egg, he probably had little idea that the minute 
 "vesicle" to which he directed the attention of investigators was 
 in reality an intricate microcosm. Little more than ten years 
 
Io6 THE EVOLUTION OF SEX. 
 
 elapsed before R. Wagner began to complicate matters by the 
 discovery of the nucleolus or germinal "spot" within the 
 "vesicle." We now know that the nucleus has not only a 
 very complex structure, but in a sense a strange internal life of 
 its own. 
 
 The nucleus, when quiescent, often lies in a little nest or 
 chamber within the cell substance, and is limited from the 
 latter by a more or less distinct nuclear membrane, which dis- 
 appears as the period of activity begins. Inside this mem- 
 brane, it is often possible to distinguish one or more of the 
 aforesaid nucleoli, lying in a more fluid material often called 
 the " nuclear sap." About these nucleoli and bodies more or 
 less like them, about the reasons for their variable number and 
 form, very little that is certain can be said. Much more 
 important is the most conspicuous constituent of the nucleus, 
 a system of strands, coils, or loops, which stain deeply with 
 various dyes, and are therefore known as the chromatin ele- 
 ments. In contrast thereto, the less stainable but perhaps 
 equally essential constituents of the nucleus are distinguished 
 as achromatin. 
 
 The chromatin elements in the resting nucleus are oftenest 
 arranged in a manifold coil, like a loosened ball of twine, while 
 in other cases they appear rather as a living network. Whether 
 the coil be continuous, as Van Beneden and others describe, 
 or interrupted, as Boveri and others maintain, is subsidiary to 
 the more striking fact, that in the state of activity the number 
 and disposition of the dislocated or loosened parts of the coil 
 remain definite and orderly, and that their behaviour is so like 
 that of minute independent individualities that any rough-and- 
 ready account of the mechanics of cell division must at once 
 be ruled out of court. It is within the chromatin substance 
 too that the germ-plasm, on which Weismann and others have 
 so much insisted, is believed to have its seat. There can be 
 no doubt that the nucleus is a very important factor in the life 
 of the cell. A non-nucleated fragment cut from a Protozoon 
 may show irritability and contractibility, but it cannot feed, 
 and soon dies. Boveri and Delage have shown in sea-urchins, 
 &c, that an ovum-fragment without a nucleus may be fertilised 
 and may develop into a larva, so that it seems as if the sperm- 
 nucleus may in certain cases suffice, but no instance is known 
 of development without any nucleus. 
 
 § 2. Growth of the Ovum. — When the ovum is very young, 
 
THE EGG-CELL OR OVUM. 107 
 
 it very frequently presents the features of an amoeboid cell. 
 In some cases this phase persists for a longer time, as in the 
 ovum of hydra, which in all essentials is comparable to an 
 amceba. Even in the simplest animals, however, the amoeboid 
 phase constantly shows a tendency to pass into greater quies- 
 cence, to become in fact more or less encysted. So is it with 
 ova, which though' at first often resembling various forms 
 of amceboid cells, tend more or less quickly to pass into 
 the encysted phase. The protoplasm no longer flows out in 
 irregular ever-changing processes, but is gathered up into a 
 sphere, rounded off, and surrounded by a more or less definite 
 envelope. This transition from a state of relative equilibrium 
 between activity and passivity, to one in which passivity un- 
 doubtedly preponderates, is associated with an increase of 
 nutriment and reserve-products. The ovum feeds, becomes 
 heavy with stored capital, becomes less active, and more en- 
 cysted in consequence. 
 
 § 3. Yolk. — The essential part of an egg-cell is always small, 
 though even in this there are great differences. The nucleus, 
 for instance, in the large eggs of amphibians, reptiles, and 
 birds, may be detected with the unaided eye; while in other 
 cases, such as sponges, the entire ovum is very minute. Yet 
 every one knows that eggs vary enormously in size. The egg 
 of a skate is very much larger than the egg of a salmon ; and 
 the egg-shell of the extinct giant bird of Madagascar (/&py- 
 ornis) is big enough to hold the contents of one hundred and 
 fifty hens' eggs. Similarly the contrast between the eggs of 
 ostrich and humming-bird is, as one would expect, extremely 
 striking. Yet the eggs of whales are "not larger than fern- 
 seed," and the same is true for most mammals, except the 
 very lowest. The differences in size, when very striking, are 
 due not so much to any marked disproportion in the essential 
 parts of the ova, but to certain extrinsic additions. The most 
 important of these is the yolk, which serves as nutritive 
 capital for the embryo or young animal. Besides the yolk, we 
 have also to take into account the frequent pigment, so familiar 
 in frog spawn, the albumen well seen in the white of birds' 
 eggs, various forms of protective and attaching viscid material, 
 and, lastly, more or less elaborate ' egg envelopes or shells. 
 The most important, however, is the yolk, and in regard to its 
 origin and disposition a little must be said. 
 
 The egg has its nutritive capital increased in three different 
 
IO» THE EVOLUTION OF SEX. 
 
 ways: — (a) Very generally it feeds on the nutritive substances 
 in the general lymph or vascular fluid of body. (/>) At the 
 same time, or in another case, it avails itself of the debris of 
 surrounding cells. In many instances, e.g., in the minute 
 ovary of hydra, in the ovary of Tubularia, or in the ovarian 
 tubes of insects, the ovum is but the surviving competitor 
 among a crowd of surrounding cells, which to start with were 
 all potential ova. This is an often forgotten chapter in the 
 struggle for existence, — the struggle between germ-cells. 
 There is a struggle between potential ova ; there is also 
 enormous elimination among the spermatozoa, even after they 
 come to close quarters with the ovum. Many are almost suc- 
 cessful, but in most cases only one fertilises, i.e., survives. 
 And even after the eggs begin to develop there is often 
 elimination apart from enemies, thus it is stated that only 
 about a third of the eggs of the New Zealand " lizard " 
 (Sphenodo?i or Hatlerid) ever hatch, (c) In the third place, 
 and this is the rarest form, the egg-cell acquires a store of 
 food-material from a special yolk gland, as in many of the 
 lower " worms." 
 
 The yolk, gained in one of the ways mentioned above, is 
 more or less readily distinguished from what is often called 
 the formative protoplasm. Out of the latter the embryo is 
 built up, while the yolk has for the most part only a secondary 
 and nutritive role. We cannot enter here into a discussion of 
 the difficult embryological question as to the extent in which 
 the yolk ever shares in directly contributing to embryonic 
 structures. The possibility of distinguishing between for- 
 mative protoplasm and the nutritive mateilal, depends on the 
 quantity of the latter that is present, and on the way in which 
 it is disposed, (a) When there is not much of it, as in the 
 small ova of mammals and many invertebrates, the yolk 
 material is diffusely distributed. Then the ovum undergoes 
 complete segmentation. (b) In the frog's ovum, on the other 
 hand, there is a large proportion of yolk, which has especially 
 accumulated in the lower hemisphere of the cell, while the 
 darker half includes the truly formative protoplasm. In this 
 case too the egg divides as a whole, but the divisions go on 
 much more rapidly in the upper hemisphere, and it is there 
 that the embryo is really formed, (c) A distinct mode of 
 yolk arrangement occurs in arthropods (crustaceans, insects, 
 &c), where the centre, not a pole, of the ovum is occupied by 
 
THE EGG-CELL OR OVUM. 
 
 109 
 
 the nutritive material. In this case the formative protoplasm 
 divides round about the nutrient qore. (d) In the majority of 
 nsnes, in reptiles, and in birds, the eggs show a much more 
 marked polar accumulation of yolk. On the top of a large 
 mass of nutritive material, the specifically lighter formative 
 protoplasm lies like a tiny drop, and in those cases the division 
 
 The relation between the disposition of the yolk and the mode 
 of segmentation: — A, diffuse yolk, e.g., sponge; B, polar, 
 e.g., frog; C, central yolk, e.g., crayfish; D, predomi- 
 nant, e.g., bird: — A', Total and equal segmentation; B', 
 total and unequal; C', partial and peripheral; D', partial 
 and discoidal segmentation. 
 
 of the ovum is very partial, — that is, it is mainly restricted to 
 the upper formative region. It is thus to be noted that the 
 quantity of yolk present, and its diffuse, polar, or central 
 arrangement, are associated with striking differences in the 
 degree and symmetry of the segmentation. 
 
110 THE EVOLUTION OF SEX. 
 
 § 4. Composite Ova. — We have emphasised the fact that the ovum must 
 be regarded as a single cell. To this a definite but pedantic objection has 
 been raised. In some parasitic flat worms there occur what have been 
 called compound ova. A minute single cell arises, as usual, in the ovary, 
 but in the course of its somewhat intricate history this becomes associated 
 with several nutrient cells derived from the yolk-gland. These surround 
 the original ovum, so that the whole now consists of several cells. But it 
 must be noticed that only the central cell — the ovum proper — is fertilised, 
 and that it contains all the formative protoplasm. Those that surround it 
 are wholly nutritive ; they eventually break up, and are absorbed. 
 
 In other cases, especially in insects, the ovum grows rich at the expense 
 of neighbouring cells, which are sacrificed to its nutritive equipment. But 
 it is evident enough that a cell remains a cell, however many of its neigh- 
 bours it may happen to absorb. 
 
 § 5' Eig Envelopes. — The ovum starts as a naked cell, but generally 
 becomes furnished with ensheathing envelopes. The exact history of the 
 egg-membranes and sheaths is a. very complex matter. Only the most 
 general facts can here be stated. The envelopes may be derived [a) from 
 the ovum itself, (b) from surrounding cells, {c) from the secretion of special 
 glands. 
 
 (a) Just as a protozoon often exhibits distinct outer and inner zones, 
 distinguished by minor physical and chemical peculiarities, so it is with the 
 ovum. What are called yolk or vitelline membranes are generally pro- 
 duced by the ovum itself. Furthermore, the outer protoplasm often forms 
 a distinct firm zone, known as the Zonapellucida. This may be traversed 
 by fine radiating pores establishing nutritive communication with the 
 exterior, and is then known as the Zona radiata. A special aperture or 
 micropyle is sometimes present, through which the sperm enters, or nutri- 
 tive supply is sustained. 
 
 (6) The ovum, in its young stages, is very frequently seen surrounded 
 by a circle of small cells, which form what is called a follicle. These may 
 produce a membrane or a glairy investment. 
 
 (c) As the ovum ripens, and passes from the ovary into the duct, it 
 often becomes surrounded by gelatinous, horny, limy, and other invest- 
 ments. In most cases, it necessarily follows that the egg has first been 
 fertilised. The investments are usually referable to the activity of the 
 walls of the oviduct or uterus, though sometimes there are special shell- 
 glands, and the like. The chitinous cases of some insect ova, the horny 
 mermaids' purses of many gristly fishes, the more or less limy egg- 
 envelopes of reptiles, the firm limy egg-shells of birds, so often stained with 
 pigments, afford good illustrations of these secondary investments. Quile 
 distinct are cocoons, such as those of earthworm and leech, which 
 surround several eggs, and are produced from the skin of the animal. 
 
 It may here be noted that the rather puzzling bodies known as " wind- 
 e gg s >" or more naively as cock's eggs, and regarded by some as the 
 producls of immature or exhausted females, are most probably not eggs at 
 all, but simply masses of albumen formed in the oviduct and coated with a 
 shell. (Raspail, "Bull. Soc. Zool. France," xxiii., 1898, pp. 94-97.) 
 
 § 6. Birds' Egos. — The student may be fitly directed to the 
 egg of the fowl, or of some other bird, for a convenient concrete 
 illustration of many facts. There he will see the great mass of 
 
THE EGG-CELL OR OVUM. Ill 
 
 yolk, of two kinds, yellow and white, and on the top of this the 
 minute area of formative protoplasm. It was on this, as it 
 gradually revealed the cloudy outlines of the embryo chick, 
 that the Greeks looked with naive unaided eyes. Here it was 
 that Aldrovandus, Harvey, Malpighi, Haller, and the early 
 embryologists, with clear vision, saw almost as much as their 
 appliances would permit. It was this which, in its primitive 
 simplicity, impressed Wolff with the reality of epigenesis; and 
 it is this that the observers of to-day look down upon through 
 their embryoscopes, or cut sections of with their microtomes. 
 Then round about all is the secondary investment of " white of 
 egg" or albumen; round this a shell membrane, between the 
 two layers of which the little air-chamber is formed at one end ; 
 and finally, the hard but porous limy shell. Mr Irvine, of 
 Granton, has shown that fowls kept with access to no carbonate 
 of lime, but only to other salts of lime, can still form a normal 
 shell. This still consists of carbonate of lime, and is as firm as 
 usual, demonstrating, like the same investigator's experiments 
 on crabs, that animals possess no little power of changing one 
 salt of lime into another. Then, in the eggs of other birds, 
 the import of the seven or more pigments which produce the 
 marvellous variety and beauty comes into question. Sorby has 
 shown that they are related to the pigments of blood and bile; 
 but what they exactly mean no one yet knows. Wider still, 
 the problem arises of how this coloration is so often pro- 
 tective. Or again, there is the curious fact that the size of 
 the egg is often much out of proportion to the size of the 
 bird, and the question arises as to how far this can be inter- 
 preted as the result of the more or less anabolic and sluggish 
 constitution. 
 
 § 7. Chemistry of the Egg. — Every one knows that the eggs of birds 
 form highly nutritious diet. As the egg contains nourishment for the young 
 bird for a considerable time, it must, like milk, contain all the essentials of 
 food. The results of a recent analysis of the fowl's egg may be taken as a 
 sample. 
 
 The germinal or formative disc consists chiefly of albuminoid bodies, 
 apparently of the globulin group, plus smaller quantities of lecithin and the 
 like. The subtle protoplasm itself, it need hardly be said, defies analysis. 
 
 In the yolk there are firm fats (tripalmitin, probably plus a little 
 stearine), and a fluid oil or glyceride. Fatty acids develop during hatching. 
 A relatively large quantity of lime is present, probably, for the most part, 
 as calcium albuminate. In the white of egg there are true albumins, also 
 globulins, and the quantity of peptones increases with the age of the egg. 
 
I I 2 THE EVOLUTION OF SEX, 
 
 During development the embryo becomes richer in mineral matters, fat, 
 and albumen, and the dry substance of the whole contents of the egg 
 diminishes considerably. 
 
 The yolk of many different kinds of ova has been analysed, and the 
 component substances distinguished as Ichthin (fishes), Emydin (tortoise), 
 and the like. More important were the discoveries of cholesterin, vitellin, 
 nucfoin, lecithin, and, in association with the latter, neiirin. As we can- 
 not here enter into the physiological import of such substances, it is enough 
 to say that the nutritive material in ova usually consists of a mixture of com- 
 plex, unstable, and highly nutritive substances. 
 
 § 8. Maturation of the Ovum. — When the egg-cell has 
 attained its mature size, a more or less enigmatical occurrence 
 takes place. The nucleus, hitherto generally central, moves to 
 the pole, alters considerably in its structure, and divides. A 
 minute cell, with half of the nucleus, and a small amount of 
 protoplasm, is given off. Not long after, the nucleus remaining 
 within the ovum repeats the process, and another tiny cell is 
 expelled. This process is known as the extrusion of the polar 
 globules. Of general, and probably of universal occurrence, 
 it has been most satisfactorily studied in invertebrate types. 
 There is considerable diversity as to the exact time at which 
 the extrusion occurs; generally, however, it precedes the 
 entrance of the fertilising sperm. The minute extruded cells 
 never have any history, though they occasionally linger for a 
 considerable time on the outskirts of the ovum. As an excep- 
 tion, they have been seen themselves to divide, and, with 
 equal rarity, a misguided spermatozoon has been observed to 
 penetrate them. Usually, however, they simply dwindle away. 
 The remaining female nucleus of the ovum is now ready to 
 unite with the male nucleus of the spermatozoon. At this 
 point, awaiting the essential moment of fertilisation, we shall 
 for the present leave it. 
 
 Weismann, assisted by C. Ischikawa, has demonstrated an 
 exceedingly interesting fact in regard to polar globule extrusion 
 in parthenogenetic ova. Instead of the two polar globules 
 which are usually extruded, parthenogenetic ova were shown to 
 form only one. This was demonstrated in a variety of cases, — 
 in water-fleas (daphnids and ostracods) and rotifers, — and is 
 believed to be a general fact. Blochmann, who has been suc- 
 cessful in demonstrating polar globules in several orders of 
 insects, has also observed that in the parthenogenetic ova of the 
 plant-louse or aphis, only one polar globule is formed, while 
 in other non-parthenogenetic aphid eggs, which only develop 
 
THE EGG-CELL OR OVUM. 113 
 
 after fertilisation, two occur as usual. To these facts we must 
 afterwards recur in connection with parthenogenesis. 
 
 What must be emphasised, however, is this: — the nuclei of 
 the mature ovum and spermatozoon contain half the number 
 of nuclear rods or chromosomes characteristic of the body- 
 cells of the animal in question. In their early immature stages 
 they contain the normal number. Therefore a reduction — a 
 halving — of the number must take place during the process of 
 maturation. The same is true in plants. 
 
 Similarly, before the nuclear fusion in two conjugating 
 individuals of the sun-animalcule (ActinoJ>hiys sol), there is, 
 as Schaudinn has shown, a "reduction-division," half of each 
 nucleus being got rid of, and there are several other pheno- 
 mena in Protozoa which appear to be analogous to the matura- 
 tion-processes in the ova of Metazoa. 
 
 § 9. 7 'heories of the Polar Globules. — The polar globules appear to 
 have been first observed in 1848 by Fr. Miiller and Lov^n, but it is only 
 within recent years that much has. been made of them. Thanks to the 
 masterly researches of Butschli and Hertwig, Giard, Fol, and others, it 
 became possible to interpret the extrusion as a case of cell-division or 
 budding. More recently, Van Beneden, whose monograph on the ovum of 
 the threadworm (Ascaris) will remain one of the classics in this department 
 of research, has raised a protest against regarding the extrusion as a 
 normal cell-division. The details of the process, as interpreted by him, 
 seemed to mark out the extrusion as something unique. Most authorities, 
 however, adhere to the older view, that the process is essentially one of 
 normal cell-division. 
 
 As to the meaning of the process, the chief opinions, only a mere out- 
 line of which can be given, are three, not including a number of suggestions 
 according to which the extrusion of the globules is a kind of " excretion" 
 of the ovum, or a "rejuvenescence" of the nucleus. 
 
 (a) According to some, the egg-cell is in a sense hermaphrodite, and 
 the polar-globule formation is an extrusion of the male element. Balfour 
 expressed his view in somewhat teleological language: — " I would suggest 
 that in the formation of the polar cells, part of the constituents of the 
 germinal vesicle, which are requisite for its functions as a complete and 
 independent nucleus, is removed to make room for the supply of the neces- 
 sary parts to it again by the spermatic nucleus. ... I will venture to 
 add the further suggestion, that the function of forming polar cells has 
 been acquired by the ovum for the express purpose of preventing partheno- 
 genesis." To this it must now be pointed out, that so far as one polar 
 globule is concerned, extrusion does not prevent parthenogenesis. This 
 view seems, according to Brooks, to have been first advanced by M'Crady. 
 It has been most carefully elaborated by Minot. According to Minot, 
 " in the cells proper, both sexes are potentially present ; to produce sexual 
 elements the cell divides into its parts ; in the case of the egg-cell, the 
 male polar globules are cast off, leaving the female ovum." In partheno- 
 genetic ova, he supposes that enough male element is retained, since only 
 
IT4 THE EVOLUTION OF SEX. 
 
 one polar globule appears to be formed. Van Beneden, whose opinion is 
 entitled to great weight, also inclines to regard the polar globules as male 
 extrusions. 
 
 Sabaticr distinguishes, besides true polar globules, other extrusions, and 
 believes the eliminated parts to be male elements. His views are connected 
 with an elaborate theory of polarities, according to which, for instance, the 
 peripheral extrusions are male, while central cores (in the development of 
 sperms) are female residues. 
 
 (l>) A very different view— morphological rather than physiological — 
 has been maintained by Giard (1876), Mark (i8Si), Biitschli, Whitman, 
 and others, that the polar bodies are equivalent to ova. The formation of 
 polar globules is an atavistic reminiscence of primitive parthenogenesis, 
 just as the mother sperm-cell or spermatogonium, which corresponds in the 
 male to the ovum in the female, divides up into what form spermatozoa, so 
 the ovum retains a slight power of division. In short, the polar globules 
 are " abortive ova." 
 
 (c) According to Weismann, Hertwig, and others, the gist of the matter 
 maybe expressed by the term "reducing-division." In different species 
 the cells of the body are characterised by the possession of a definite 
 number of chromatin elements. Thus, in one variety of the round-worm of 
 the horse there are four, in man there are two hundred. In the maturation 
 of ovum and spermatozoon the number is reduced preparatory to fertilisa- 
 tion, so that the fertilised ovum contains the normal quota, and not double 
 that as would be the case were there no reduction. The period and the 
 details of the reduction seem to differ greatly, but the general fact stands 
 out clearly that maturation implies a reduction of the number of chromo- 
 somes in the ultimate germ-cells to one-half the number characteristic of 
 the somatic cells of the species. The reduction phenomena occur in plants 
 as well as in animals, but the details remain somewhat uncertain. In the 
 higher plants the reduced number is seen in all the cells of the sexual or 
 gametophyte generation, beginning with the asexual mother-spore-cells 
 from v. hich this generation arises. 
 
THE EGG-CELL OR OVUM. 115 
 
 SUMMARY. 
 
 1. The ovum presents all the essential features of a cell — cytoplasm, 
 nucleoplasm, etc. 
 
 2. The ovum often passes from an amoeboid to an encysted phase, with 
 increase of nutrition and size. 
 
 3. The yolk is derived from the vascular fluid, or surrounding cells, or 
 special glands, and is present in varying quantity and disposition. If little, 
 it is diffuse; if much, it is polar or central ; and the different modes of egg- 
 division are associated with this. 
 
 4. In some cases the ovum is surrounded by a number of nutritive cells 
 (composite ova), and often becomes what it is by preying upon its neigh- 
 bours. This hardly affects its unicellular character. 
 
 5. Egg-envelopes are produced from the ovum itself (e.g., vitelline 
 membrane), or from surrounding cells (follicular sheath), or from special 
 glands (the outside shell). 
 
 6. Bird's egg noted as a concrete illustration of facts and problems. 
 
 7. The egg, so far as its nutritive material is concerned, includes a 
 mixture of complex, unstable, highly nutritive substances. 
 
 8. The maturation of the ovum is usually associated with a double cell- 
 division, known as the extrusion of polar globules. In parthenogenetic 
 ova only one occurs, with rare exceptions. 
 
 9. These polar globules have been interpreted variously: — (a) As 
 extrusions of male elements; or (b) as abortive ova; but (c) the whole 
 process implies a reduction of nuclear rods or chromosomes, preparatory to 
 fertilisation. 
 
 LITERATURE. 
 
 Balfour, F. M.— Op. cit. 
 
 Van Beneden, E. — Recherches sur la Fecondation. Arch, de Biologie, 
 IV., 1883. 
 
 Carnoy. — La Cellule, II., 1886, etc. 
 
 Geddes, P. — Op. cit. 
 
 Haddon, A. C.—Op. cit. 
 
 Haecker, V.— The Reduction of the Chromosomes in the Sexual Cells. 
 Annals of Botany, IX., 1895, pp. 95-101. 
 
 Hensen, V. — Op. cit. 
 
 Hertwig, O. — Op. cit. 
 
 Hatchett Jackson. — Introduction to his edition of Rolleston's Forms 
 of Animal Life. 
 
 M'Kendrick, J. G. — On the Modern Cell Theory, &c. Proc. Phil. Soc. 
 Glasgow, XIX., 1888. 
 
 Minot, C. S.— American Naturalist, XIV., 1880. 
 
 STRASBURGER. — The Periodic Reduction of the Number of the Chromo- 
 somes in the Life-history of Living Organisms. Annals of Botany, 
 VIII., 1894, pp. 281-316. 
 
Il6 THE EVOLUTION OF SEX. 
 
 Thomson, J. A.— Recent Researches on Oogenesis. Quarterly Journ. 
 
 Micr. Sci., XXVI., 1886. 
 
 Art. Embryology, Chambers's Encyclopedia. 
 
 Weismann, A. — Die Continuitat des Keimplasmas. Jena, 1885. 
 
 Die Eedeutung der sexuellen Fortpflanzung. Jena, 1886. 
 
 ■ And other papers translated, " Essays on Heredity," etc. Oxford, 
 
 1889. 
 
 — The Germ Plasm. London, 1893. 
 
 Wilson, E. B.— The Cell in Development and Inheritance. New York, 
 1896, 371 pp., 142 figs. 
 
CHAPTER IX. 
 
 The Male-Cell or Spermatozoon. 
 
 § i. The General Contrast between Ovum and Spermatozoon. 
 — Just as the ovum, large, well nourished, and passive, is as 
 such a cellular expression of female characteristics, so the 
 smaller size, less nutritive habit, and predominant activities of 
 the spermatozoa illustrate the qualities of maleness. As the 
 ovum is usually one of the largest, the sperm is one of the 
 smallest of cells, sometimes only T¥ ^.inns-th of the size of the 
 ovum, which is often microscopic. The yolk or food-capital, 
 and encysting membranes, which are often so prominent in the 
 former, are as conspicuously absent in the latter. The con- 
 trast, though less accented, is still quite discernible in plants, 
 In fact, the two kinds of cells are just as widely opposed in 
 their general features, as they are fundamentally complemen- 
 tary in their history. Before this opposition and complemen- 
 tariness can be fully understood, however, we must briefly sum 
 up the characters and history of the male elements. 
 
 § 2. History of Discovery. — In 1677, one of Leeuwenhoek's students, 
 Hamm by name, called his master's attention to the minute elements 
 actively moving in the male fluid. Leeuwenhoek, who some years pre- 
 viously for the first time observed what we now know as unicellular 
 organisms, was at once impressed by the import of the marvellously active 
 male units. Almost too much impressed, in fact, for he interpreted them 
 as minute preformed germs, which only required to be nourished by the 
 ovum to unfold into embryos. Thus the unfortunate aberration, already 
 noted as the doctrine of the animalculists, had its origin. For long no 
 progress whatever was made; some naturalists, like Vallisneri, depreciating 
 the import of the sperms altogether, and regarding them as worms which 
 hindered the coagulation of the seminal fluid; others going to the opposite 
 extreme, and regarding them as nests of germs. Thus Haller at first 
 considered them to be what Leeuwenhoek had suggested, but afterwards 
 admitted them merely as nativi hospites seminis. In 1835, even Von Baer 
 was inclined to interpret them as minute parasites peculiar to the male 
 fluid; and if the curious student will turn up the article Entozoa in Todd's 
 "Cyclopaedia of Anatomy and Physiology," of about the same date, he 
 will find that the veteran Owen includes the spermatozoa under that strange 
 
riS 
 
 THE EVOLUTION OF SEX. 
 
 heading. The very name spermatozoon recalls the view which so long 
 prevailed. 
 
 In 1837, R. Wagner emphasised their constancy in all the sexually 
 mature males which he examined, and their absence in infertile male 
 hybrids. Von Siebold demonstrated their presence in many of the lower 
 animals; and lastly, in 1841, Kolliker made one of his many important 
 contributions to biology, in proving that the sperms had a cellular origin in 
 the testes. 
 
 § 3. Structure of the Spe?-m. — The sperm, then, is a cell. 
 Though some, such as Kolliker, have inclined to regard it 
 rather as a nucleus, its truly cellular character has been proved 
 beyond dispute. As in the ovum, there is cell-substance and 
 nucleus, with this marked difference, that the cell-substance is 
 generally reduced to a minimum. 
 
 The sperm is almost always, moreover, a cell of a very 
 definite type or phase. It is like one of the highly motile 
 
 " Spermatic Animalculi " of the Rabbit and the Dog. 
 — From Buffon, after Leeuwenhoek. 
 
 Protozoa, like a flagellate infusorian. Usually it consists of a 
 minute "head," consisting almost entirely of nucleus, and of a 
 long contractile tail, which, working behind like a screw, propels 
 the essential "head" through the water or along the ducts. 
 Between the head and the tail there is an important middle 
 portion, which many observers agree in regarding as the bearer 
 of the centrosome — a now well-known component of a typical 
 animal cell. 
 
 Occasionally there is a departure from the usual flagellate 
 type. Thus in the threadworm Ascaris, the sperm has a blunt 
 pear-shaped form, and exhibits slight amceboid movements. 
 In some crustaceans and other arthropods, the cell is even more 
 quiescent, and may exhibit curious forms, such as that figured 
 for the crayfish. The relatively dormant activity may however 
 wake up, and the sperm exhibit active amceboid movements. 
 Zacharias has made some interesting experiments, showing the 
 
THE MALE-CELL OR SPERMATOZOON. 
 
 lie. 
 
 tnodifiability of sperms under reagents; thus, in a little crus- 
 tacean {Polyphemus pediculus), he first caused the cylindrical 
 sperm to form amoeboid processes, and afterwards to replace 
 these by what were to all intents and purposes cilia. This is 
 entirely congruent with other experiments and observations on 
 the passage of cells from one phase of the cell-cycle to another. 
 F. Silvestri records the interesting case of the millipede 
 Pachyiulus communis in which the spermatozoon is immobile 
 and cap-shaped, and is drawn into the ovum by a pseudo- 
 podium emitted by the latter through the micropyle. In other 
 words, the ovum here plays the active role, and the spermato- 
 zoon is passive. ("Atti Ace. Lincei (Rend.)," vii., 1898, pp 
 129133. 5 figs.) 
 
 Spermatozoa of crayfish (a), lobster (<5), crab (c), ascarid (d), 
 water-flea — Moina (e) s man (/"), ray (g), rat (A), guinea- 
 pig (2), a beetle — immature stage (k), sponge if). (Not 
 drawn to scale.) 
 
 The progress of microscopic technique has demonstrated many com- 
 plexities in the sperm as well as in the ovum. For a discussion of some of 
 the more important of these, the reader is referred to the " Encyclopaedia 
 Britannica," article Reproduction. A few points only need be noticed here. 
 Thus most spermatozoa exhibit not only a head (almost wholly from the 
 nucleus of the mother-cell) and a mobile tail (from the substance of the 
 mother-cell), but a median portion connecting these. The tail is not 
 unfrequently, as in salamander and man, furnished with a very delicate un- 
 dulating or vibratile band, and often shows, as in birds, an axial filament, 
 which like many other contractile structures is distinctly fibrillated. In a 
 few cases, as in the threadworm, the sperm is not left without any nutritive 
 capital, but furnished with this in the form of a cap, which falls off before 
 the essential moment of fertilisation arrives. It is very generally admitted 
 
i 20 THE EVOLUTION OF SEx:. 
 
 that the head consists almost wholly of chromatin, and that the tail is 
 mainly cytoplasmic, i.e., formed of cell-substance. The middle part con- 
 necting head and tail is believed by many to be formed by the centrosomes, 
 which play some part in the division of the egg. In the non-flowering 
 plants the male-cells or antherozoids often bear a close resemblance 10 those 
 of animals; in the flowering plants the male-elements are the reproductive 
 nuclei which issue from the pollen-tube; but the discoveries of Hirase and 
 Ikeno have shown that even in flowering plants, namely in Cycads, there 
 are motile spermatozoa. It is of interest to notice that a dimorphism of 
 spermatozoa has been recorded in various cases, e.g. , by Auerbach for 
 water-beetles, and even in man by Bardeleben. As has been already 
 noted, there is sometimes, as in Rotifers, a dimorphism of ova, and it is 
 probable that we have here again an illustration of the fundimental varia- 
 tional alternatives, — between relatively hatabolic and relatively anabolic 
 phases. Apart from this dimorphism, it should be noted that in the testis 
 of many higher animals, there seems to be, as in the ovary, a division of 
 labour between germ-cells proper and nutritive cells auxiliary to these. 
 
 § 4. Physiology of the Spermatozoon. — A few facts in regard 
 to the physiology of the sperm demand notice, {a) It is 
 specialised as a highly active cell; its minimal size, the usual 
 absence of any encumbering nutritive material, the contractility 
 of the tail, and the general shape, all fit it for characteristic 
 mobility. More than one histologist has likened it to a free 
 muscle-cell, or to a flagellate monad, (b) Furthermore, the 
 sperm has very considerable power of persistent vitality. 
 Not only does it often remain long unexpelled in the male 
 animal, without losing its functions, but it may retain its fertil- 
 ising power after remaining for weeks, or even months, in the 
 female organism. In the earthworm, the spermatozoa pass 
 from one worm to another, not directly to the ova nor to 
 female ducts, but to be stored up in special reservoirs or 
 spermatheca?. So it is with many animals. The spermatozoa 
 received by the queen bee during her single impregnation, are 
 for a considerable period — even for three years — used in fertil- 
 ising successive sets of worker and queen ova. . Quite unique, 
 however, is the case of one of Sir John Lubbock's queen ants, 
 which laid fertile eggs thirteen years after the last sexual union 
 with a male. The spermatozoa had apparently persisted all 
 that time. Hensen cites the facts that a hen will lay fertilised 
 eggs eighteen days, after the removal of the cock, and that in 
 bats, spermatozoa may remain alive a whole winter in the 
 uterus of the female. In most European bats, indeed, sexual 
 union occurs in autumn, but the sperms are simply stored in 
 the uterus, for ovulation and fertilisation do not take place till 
 
the male-Cell or spermatozoon. 
 
 121 
 
 spring. In exceptional cases, especially in young forms which 
 were not mature in autumn, pairing occurs in spring. An 
 exactly parallel condition is known in some snakes. Thus 
 Rollinat notes in regard to Tropidonotus viperinus that mature 
 females are inseminated in the autumn previous to the egg- 
 laying (in June or July), but in females laying for the first time, 
 copulation probably occurs in early spring (" Bull. Soc. Zool. 
 France, "xxiii., 1898, pp. 59-63). (c) Remarkable too, and again 
 suggestive of monads, is the power the sperms have of 
 resisting great deviations from the normal temperature. The 
 presence of acids has usually a paralysing influence, but alka- 
 line solutions have, on the whole, the opposite result. 
 
 Diagram of the Development of Spermatozoa (upper line), of the Maturation and 
 
 Fertilisation of the Ovum (lower line). 
 a, an amoeboid sex-cell ; A, ovum, with germinal vesicle, n ; B, ovum extruding first 
 
 polar body, j* 1 ; C, extrusion of second polar body, / 2 , nucleus n 2 , now reduced. 
 
 1, a mother-sperm-cell, dividing (2, 3) into immature and mature spermatozoa (s/.). 
 D, the entrance of a spermatozoon ; E, the male and female nuclei sp. 11 and « 2 approach 
 
 one another. 
 
 § 5. Origin of i lie Sperms. — For the last twenty years the 
 development of spermatozoa has-been the subject of almost 
 continuous research and controversy, and the all too-abundant 
 nomenclature affords a suggestive index to the confusion out 
 of which the subject is now emerging. In a general way, the 
 process is simply that of the varied segmentation of a mother- 
 sperm-cell, and the occurrence of a series of preparatory stages 
 before the sperm is finally matured. In detail, however, there 
 are many variations, and these are described in a maze of often 
 tautologous and ambiguous terms, such as spermatogonium, 
 
121 
 
 THE EVOLUTION OF SEX. 
 
 spermatoblast, spermatospore, spermatogemma, spermatomere, 
 spermosphere, and a dozen more. 
 
 One of the most defensible set of terms is that used by Voigt after 
 Semper, and also by Von la Valette St George. The sperm or spermatozoon 
 is differentiated from an immature cell or spermatide, this is modified from 
 or descended from a spermatocyte, the spermatocytes result from the 
 division of the mother-sperm-cell or spermatogonium, and this finally is a 
 modified form or a descendant of the primitive sex-cell or male ovule. 
 
 Comparison of Spermatogenesis and Ovum Segmentation. 
 
 Explanation. — The first line, A-E, exhibits types of ovum segmentation: — A, regular 
 morula; E, unequal segmentation, e.g., in some Molluscs; C, centrolecithal or 
 peripheral type, £.£"., in a shrimp Peneus; D, discoidal segmentation ; E, the same, 
 with the cells less markedly defined off from the yolk.- 
 
 In the next two lines various types of spermatogenesis are collated with the above 
 to illustrate the parallelism : — A' and A", morula type, as in Sponge, Turbellarian, 
 Spider, &c. ; B' and B", where the division is unequal, and one large nutritive cell is 
 seen (Plagiostome fishes, after Von la Valette St George); C and C", after Blomfield, 
 Jensen, &c, showing central cytophoral or blastophoral nutritive portion; D' and D", 
 sperm-blastoderm, with a few formative cells on large nutririve blastophore, after 
 Gilson, &c. ; E' and E", the same, with the sperm cells less definitely separated off, 
 after Von Ebner and his followers. 
 
 Difficulties become thick, however, when we inquire into the division of 
 the mother-sperm-cell or spermatogonium, and it is here that the observa- 
 tions of recognised authorities so much disagree. Accepting the results of 
 competent observers, we have elsewhere endeavoured to rationalise and 
 unify the conflicting observations, by comparing the different modes of 
 spermatogenesis with the different forms of ovum-segmentation. It has 
 
THE MALE-CELL OR SPERMATOZOON. 1 23 
 
 been already incidentally noticed" that the egg-cell may divide wholly and 
 equally, or unequally, or only very partially, or round a central core. 
 Just in the same way the mother-sperm-cells may divide into a uniform 
 ball of cells, or only at one pole, or only at the periphery round a central 
 residue. Balfour and others had hinted at this comparison in the use of 
 terms like sperm-morula; and Hermann had also concluded, "that the 
 division of the male ovule into a series of generations of daughter-cells, is 
 a phenomenon comparable to that exhibited by the ovum in the formal ion 
 of the blastoderm. ... It seems then more important to determine 
 exactly the mechanism of division, than to give a particular name to each 
 stage of segmentation." 
 
 Although this interpretation of spermatogenesis by collating it with 
 ovum segmentation appears to Minot "a fanciful comparison," in favour 
 of which he is "unable to recognise any evidence," neither the initial 
 homology between the mother-sperm-cell and ovum with which we start, 
 nor the striking parallelism between the modes of division of these 
 homologues, seem thereby even disputed, much less shaken. The widely 
 different conditions in which these two processes occur, and their very 
 different meaning to the organism, are of course obvious. 
 
 § 6. Further Comparison of Ovum and Sperm. — It is often said that 
 the sperm is the male cell which corresponds to the ovum. This is only 
 true in a certain sense. In function the two elements are indeed, in a 
 general way, of equal rank, and are obviously complementary. But even 
 in this respect, the two elements, which unite in equal proportions in the 
 essential act of fertilisation, are not exactly sperm and ovum, but (a) the 
 head or nucleus of the sperm and (b) the female nucleus doubly reduced by 
 the extrusion of two polar globules. The accurate structural resemblance 
 or homology seems to be between oogonium or primitive female germ-cell 
 and spermatogonium or primitive male germ-cell, or between the immature 
 ovarian ovum or oocyte and the spermatocyte. 
 
 The parallelism between the reduction of the number of chromosomes 
 in the maturation of the ovum and the similar reduction in the course of 
 spermatogenesis is of great interest and importance. Unfortunately the 
 results of different observers and in regard to different organisms remain 
 perplexingly discrepant. Professor E. B. Wilson gives the following state- 
 ment with special reference to the case of Ascaris megalocephala, the 
 threadworm of the horse: — 
 
 " Like the ova, the spermatozoa are descended from primordial germ- 
 cells which by mitotic division give rise to the spermatogonia from which 
 the spermatozoa are ultimately formed. Like the oogonia, the spermato- 
 gonia continue for a time to divide with the usual (somatic) number of 
 chromosomes, i.e., four in Ascaris megalocephala bivalens. Ceasing for a 
 time to divide, they now enlarge considerably to form spermatocytes, each 
 of which is morphologically equivalent to an unripe ovarian ovum, or 
 oocyte. Each spermatocyte finally divides twice in rapid succession, giving 
 rise first to two daughter-spermatocytes and then to four spermatids, each 
 of which is directly converted into a single spermatozoon. The history of 
 the chromatin in these two divisions is exactly parallel to that in the for- 
 mation of the polar bodies. . . . From each spermatocyte arise four 
 spermatozoa, and each sperm-nucleus receives half the usual number of 
 single chromosomes. The parallel with the egg-reduction is complete." 
 
 §7. Chemistry of the Sperm. — Comparatively little has been done in 
 
124 THE EVOLUTION OF SEX. 
 
 regard to the chemistry of the male elements in different animals. The 
 most important observations are those of Miescher, on the milt of salmon. 
 His analysis demonstrated the presence of lecithin, fat, and cholesterin, — 
 also component parts of the ovum. Besides these, after the heads of the 
 spermatozoa have been formed, Miescher detected the abundant presence 
 of a substance which he called protamine, a compound of nucleic acid. 
 Albuminoid material, and products of decomposition, such as sarkin and 
 guanin, were demonstrated, according to Hensen, by Picard. 
 
 More recently, Kossel has shown that the protamin in the salmon's 
 sperms and the analogous sturin in those of the sturgeon, act as basic 
 substances forming a salt-like combination with the nuclein substances. 
 ("SB. K. Preuss-Akad. Berlin," 1896, pp. 303-308.) 
 
 It is remarkable, however, as Dr T. Gregor Brodie points out 
 ("Science Progress," vii., 1898, pp. 131-149), that the spermatozoa 
 consist of substances which, relatively to proteids, are of simple con- 
 stitution. " If it be true that hereditary characteristics are transmitted by 
 the chromatin of the reproductive cells, we should have expected a most 
 complex chemical structure for these parts; and it therefore becomes the 
 more striking to note that the most complex protamine as yet obtained, 
 arbacin, is from an animal low in the scale (Arbacia, a sea-urchin), and 
 that in the higher vertebrates examined no protamine is present at all." 
 
 Zacharias has made a micro-chemical comparison of the male and 
 female elements in Characese, mosses, ferns, phanerogams, and amphibians. 
 He finds that the male cells are distinguished by their small or absent 
 nucleoli, and by their rich content of nuclein; while the female elements 
 exhibited a poverty of nuclein, an abundance of albumen, and one or more 
 nucleoli, more or less large in proportion. The male cells have, in 
 relation to their protoplasm, a larger nuclear mass than the female 
 elements. 
 
 It is interesting to notice that an analysis of two different kinds of 
 pollen showed a great analogy of composition between these male repro- 
 ductive cells and those of the salmon and ox. 
 
THE MALE-CELL OR SPERMATOZOON 1 2 5 
 
 SUMMARY. 
 
 1. The contrast between the elements is that between the sexes. The 
 large, passive, highly- nourished, relatively anabolic ovum; the small, 
 active, relatively katabolic sperm. 
 
 2. Hamm's discovery, 1677; Leeuwenhoek's interpretation ; the school 
 of animalculists ; Kolliker's demonstration of the cellular origin of the 
 sperm, 1841. 
 
 3. Structure of the sperm, — nuclear "head" of chromatin, protoplasmic 
 " tail," middle portion. The sperm comparable to a monad or flagellate 
 infusorian, only with less cell-substance. Its occasional degradation into 
 the amoeboid phase. 
 
 4. Physiology of the sperm ; its locomotor energy at a maximum, but 
 yet great power of endurance, like a monad or bacillus. 
 
 5. Origin of sperms from the division of a mother-sperm-cell homo- 
 logous with the ovum. The different modes of "spermatogenesis " may be 
 collated with the different modes of ovum-segmentation. 
 
 6. The occurrence in sperm-development of phenomena comparable 
 both structurally and functionally with polar-globule formation. 
 
 7. Chemistry of the sperm ; resemblance between pollen and sperma- 
 tozoa. 
 
 LITERATURE. 
 
 Baixowitz, E. — Structure of Spermatozoon. Archiv. Mikr. Anat., 
 XXXII., 1888, pp. 401-473, 5 pis. 
 
 Geddes, P., and Thomson, J. A. — History and Theory of Spermato- 
 genesis. Proc. Roy. Soc. Edin., 1886, pp. 803-823, 1 pi. See also 
 Zoological Record from 1886. 
 
 Hirase, S. — Journ. Coll. Sci. Tokyo, XII., 1898, pp. 102-149, 3 pis., 
 2 figs. 
 
 Ikeno, S. — Jahrb. wiss. Bot., XXXII., 1898, pp. 557-602, 3 pis., 2 figs. 
 Journ. Coll. Sci. Tokyo, XII., 1898, pp. 151-214, 8 pis. 
 
 Wilson, E. B. — The Cell in Development and Inheritance, 1896; new 
 ed., 1900. 
 
CHAPTER X. 
 
 Theory of Sex — its Nature and Origin. 
 
 Having got so far in our analysis, and before passing to the 
 study of the processes of reproduction, we must add up the 
 results in a general theory of the nature and origin of sex. 
 After this has been done, we shall be in a better position to 
 deal, in Book III., with fertilisation, parthenogenesis, and the 
 like. The number of speculations as to the nature of sex has 
 been well-nigh doubled since Drelincourt, in the last century, 
 brought together two hundred and sixty-two "groundless 
 hypotheses," and since Blumenbach quaintly remarked that 
 nothing was more certain than that Drelincourt's own theory 
 formed the two hundred and sixty-third. Subsequent in- 
 vestigators have, of course, long ago added Blumenbach's 
 " Bildungstrieb " to the list ; nor is it claimed that the 
 generalisation we have in our turn offered has yet received 
 "final form," if that phrase indeed be ever permissible in an 
 evolving science, except when applied to what is altogether 
 extinct. This much, however, is distinctly maintained, that 
 future developments of the theory of sex can only differ in 
 degree, not in kind, from that here suggested, inasmuch as the 
 present theory is, for the first time, an expression of the facts in 
 terms which are agreed to be fundamental in biology, those of 
 the anabolism and katabolism of protoplasm. 
 
 § i. Suggested Theories. — According to Rolph, — a fresh and 
 ingenious thinker, removed before attaining his mature strength, 
 — " the less nutritive, and therefore smaller, hungrier, and more 
 mobile organism [cells, he is speaking of] we call the male; 
 the more nutritive, and usually more quiescent organism is 
 the female." He goes on vividly to suggest why "the small 
 starving male cell seeks out the large well-nourished female cell 
 for the purposes of conjugation, to which. the latter, the larger 
 and better nourished it is, has on its own motive less 
 inclination," 
 
THEORY OF SEX.^ITS NATURE AND ORIGIN. I27 
 
 Minot, in his "theory of genoblasts,'' or sexual elements, 
 ventures little further than regarding male and female as 
 derivatives of primitive hermaphroditism in two opposite 
 directions. " As evolution continued, hermaphroditism was 
 replaced by a new differentiation, in consequence of which the 
 individuals of a species were — some capable of producing ova 
 only, others of producing spermatozoa only. Individuals of 
 the former kind we call females, of the latter males, and they 
 are said to have sex." " At present all we can say is, we do 
 not know why or how sexual individuals are produced." In 
 regard to the sex-elements, we have already noticed his opinion 
 that they are at first "hermaphroditic or asexual," and that 
 both differentiate by the extrusion or separation of the con- 
 tradictory elements, the ovum getting rid of male polar globules, 
 the sperms leaving behind a female mother-cell-remnant. 
 
 Brooks has emphasised rather a different aspect of the 
 question. " A division of physiological labour has arisen 
 during the evolution of life, the functions of the reproductive 
 elements have become specialised in different directions." 
 " The male cell became adapted for storing up gemmules, and, 
 at the same time, gradually lost its unnecessary and useless 
 power to transmit hereditary characteristics." " The males 
 are, as a rule, more variable than the females; the male leads, 
 and the female follows, in the evolution of new races." Brooks 
 does not exactly attack the problem of the nature and origin 
 of sex, but his hypothesis of the greater variability of males is 
 of interest. 
 
 To others, again, it seems sufficient to interpret sexual 
 dimorphism as an adaptation evolved in the course of natural 
 selection from variations of which no rationale can be given. 
 As to the adaptive character of sex there can be no doubt, 
 our problem is whether the variations which gave rise to the 
 dimorphism may not be interpreted in terms of the two main 
 alternatives of protoplasmic metabolism. 
 
 § 2. Nature of Sex as seen in the Sex-Elements — The Cell 
 Cycle. — As ova and sperms are the characteristic products of 
 female and male organisms, it is reasonable that an interpreta- 
 tion of sex should start at this level. Here, assuredly, the 
 difference between male and female has its fundamental and 
 most concentrated expression. For the bodies, after all, as 
 Weismann has so clearly emphasised, are but appendages to 
 this immortal chain of sex-cells. 
 
128 
 
 THE EVOLUTION OF SEX. 
 
 We have already pointed out that the sex-cells are more or 
 less on a level with the Protozoa. If we only knew, they 
 probably differ widely from them in those intricacies of nuclear 
 structure of which we only see the surface; yet as single cells 
 the sex-cells are comparable with the Protozoa. For the 
 moment, let _us study those simplest organisms. When we 
 consider an extended series of unicellular forms, amoebae, 
 foraminifers, sun-animalcules, infusorians, gregarines, and some 
 of the simplest alga? as well, we gradually begin to group these 
 in the mind under three divisions. First there are highly active 
 cells, — infusorians of all sorts; at the opposite extreme there 
 
 The divergence of male and female cells from 
 pi iinitive amoeboid indifference. 
 
 are quiescent forms, in which the life seems to sleep, and loco- 
 motion is almost absent, — the gregarines, and some unicellular 
 algas; and between these there are forms which in a via media 
 have effected a sort of compromise between activity and pas- 
 sivity, which are without the cilia of the one or the relative 
 stagnancy of the other, but possess outfiowings of their living 
 substance, — the familiar amceboid processes. We may thus 
 reach, almost by inspection, a rough and ready classification 
 of the Protozoa, into active, passive, and amceboid cells, — a 
 classification which, under varying titles, is more or less dis- 
 tinctly recognised by all the authorities on the subject. 
 
THEORY OF SEX — ITS NATURE AND ORIGIN. 
 
 129 
 
 But if we go further than casual inspection, and study the 
 life-history of some of the very simplest forms, such as some 
 
 
 The encysted Protomyxa, and its diviric.il into numerous individuals within the cyst. 
 — From Haeckel. 
 
 of the primitive Proteomyxa and Myxomycetcs, and follow, for 
 instance, Haeckel's account of the life-cycle in Protomyxa, we 
 soon gain new light on our classification. For in these life- 
 
 The cyst of Protomyxa bursting, the flagellate young stages becoming amoeboid, and 
 eventually uniting in a composite amoeboid mass, or " Plasmodium." — From Haeckel. 
 
 histories we find the cells now encysted, now active lashed spores, 
 and again sinking down into the compromise of equilibrium 
 
 9 
 
130 
 
 THE EVOLUTION OF SEX. 
 
 effected by amoebae. We are now in a position to recognise 
 that the chapters in the life-history of the simplest forms are, 
 as it were, prophecies of each of the three groups. In other 
 words, the most primitive organisms passed through a cycle 
 of three phases, one of which is accented by each of the three 
 main groups of the Protozoa. And while each main group is 
 characterised by one dominant phase of cell-life, — encysted, 
 amoeboid, or flagellate, — there are often transient hints of 
 
 V 
 
 A"" 
 
 H 
 
 \-^- 
 
 Tc 
 
 Diagram of the Cell-cycle,— of encysted (E), ciliated (C), and amoeboid 
 (A) phases. L, II., III., in Protozoa; IV., ovum and sperm 
 of fern prothallus ; V., encysted, ciliated, and amoeboid 
 animal cells; VI., ciliated animal cell pathologically 
 becoming amoeboid ; VII., typical and amoeboid sperm ; 
 VIII., amoeboid and encysted ovum. — From Geddes. 
 
 other phases. Thus an infusonan has its encysted chapter, 
 a gregarine its amoeboid stage, and a rhizopod may begin as 
 a mobile ciliated spore ; for each group, while accenting one 
 phase of the cycle, retains embryonic reminiscences of the 
 others. 
 
 We become more convinced that the triple division really 
 means much when we pass from the Protozoa to the cells which 
 compose higher animals. There we find active ciliated cells 
 
THEORY OF SEX — ITS NATURE AND ORIGIN. 131 
 
 in most of the classes, from the ciliated chambers which lash 
 the water through a sponge, to the cells lining the air passages 
 in man; passive encysted cells are illustrated in some forms of 
 connective, fatty, and skeletal tissue; while the white blood 
 corpuscles are obviously comparable to amcebas. Extended 
 observation here also shows us the cells passing from one phase 
 to another. Our rough classification of the Protozoa is verified 
 in the histology of higher animals, and reappears in the study 
 of their diseases. We are thus at length in a position to say, 
 that however these three phases were brought about, the forms 
 characteristic of them are of such wide occurrence through 
 nature as to justify our restatement of the familiar cell theory 
 in terms of a larger conception, that of the cell-cycle. That is 
 to say, from the conception of the cell as a corpuscle of living 
 protoplasm, amoeboid, encysted, or ciliated, as the case might 
 be, we come to regard these forms as the predominant phases 
 of a cycle, — primeval, certainly, in the history of the organic 
 world, and largely so even in the individual cell. 
 
 A final corroboration of the cell-cycle, and at the same time 
 a rationale of it, is obtainable on physiological lines, when we 
 inquire into the protoplasmic processes which lie behind the 
 changes in the form and habit. We have already spoken of 
 the modern physiologist's conception of living matter, or proto- 
 plasm, as an exceedingly complex and unstable substance or 
 mixture of substances, undergoing continual chemical change 
 or metabolism. On the one hand, there is a process of con- 
 struction ; the income of nutritive material, at first more or 
 less simple, is worked up by a series of chemical changes till it 
 reaches a climax of complexity and instability. These upbuild- 
 ing, constructive, synthetic processes are summed up in the 
 term anabolism. But, on the other hand, there is a process 
 of disruption ; the complex material breaks down into more 
 and more stable compounds, and finally into waste products. 
 This disruptive, descending series of chemical changes is known 
 as katabolism. Both constructive and disruptive changes occur 
 in manifold series. The same summit may be gained or left 
 by many different paths, but at the same time there is, as it 
 were, a distinct watershed, — any change in the cell must tend 
 to throw the preponderance towards one side or the other. In 
 a certain sense too the processes of income and expenditure 
 must balance, but only to the usual extent, that expenditure 
 must not altogether outrun income, else the cell's capital of 
 
132 THE EVOLUTION OF SEX. 
 
 living matter will be lost, — a fate which is often not successfully 
 avoided. The disruptive, or katabolic, or energy-expending 
 set of changes may be obviously greater in one cell than in 
 another, in proportion to the constructive or anabolic processes. 
 Then, we may shortly say that the one cell is relatively more 
 katabolic than the other, or vice vers& on the opposite supposi- 
 tion. Just as our expenditure and income should balance at 
 the year's end, but may vastly outstrip each other at particular 
 times, so it is with the cells of the body. Income may for a 
 time preponderate, and we increase in wealth, or similarly, in 
 weight, or in anabolism. Conversely, expenditure may pre- 
 dominate, and business may be prosecuted at a loss; just as 
 we may live on for a while with loss of weight, or with excessive 
 wear and tear. This losing game of life is what we call a 
 katabolic habit, tendency, or diathesis ; the converse gaining 
 one being the anabolic habit, tendency, or diathesis. The 
 words anabolic and katabolic are, of course, unfamiliar, and 
 undeniably ugly. Habit and temperament have very vague 
 associations, and tendency sounds metaphysical ; diathesis, 
 again, seems no better than the medical equivalent of this. 
 These things the reader must naturally feel ; yet the medical 
 man has some definite concrete meaning in his mind when he 
 speaks of gouty or neurotic diathesis, of bilious habit, strumous 
 tendency, or the like. There is more than metaphysical vague- 
 ness in his phraseology, and in ours. 
 
 We are now in a position proStably to return to the Pro- 
 tozoa, to the phases of cell-life, and to the sex-elements. After 
 what we have just said, it is evident that there are but three 
 main physiological possibilities, — preponderant anabolism, or 
 predominant katabolism, or an approximate (i.e., oscillating) 
 equilibrium between these tendencies. A growing surplus of 
 income, a lavish expenditure of energy, or a compromise in 
 which the cell lives neither far below nor quite up to its 
 income. Great passivity, great activity, or a safe average 
 between these ; conservative accumulation, spendthrift liberal- 
 ism, and a compromise between these. In many different 
 ways, more or less metaphorical, we may express the plain and 
 indubitable facts of anabolism and katabolism within the living 
 matter. The student may think of the processes, with some 
 degree of accuracy, under the metaphor of a ceaseless fountain, 
 which, while remaining approximately constant, is the expres- 
 sion of continual ascent and descent of drops. The protoplasm 
 
THEORY OF SEX — ITS NATURE AND ORIGIN. I33 
 
 itself must often be in as ceaseless change as the apex of the 
 jet. 
 
 In active, motile, ciliated, or flagellate cells, whether they be 
 constant forms or only phases, there is relatively predominant 
 katabolism, — predominant when compared with the life ex- 
 penditure of a passive, quiescent, enclosed, or encysted cell. 
 In amoeboid organisms these extremes are avoided ; there is 
 certainly great amplitude of variation still, but neither anabolism 
 nor katabolism gains the ascendant in any marked degree. 
 
 Suppose, then, in such an amoeboid cell, a continued 
 surplus of anabolism over katabolism, the result is necessarily 
 a growth in size, a reduction of kinetic energy and movement, 
 an increase in potential energy and reserve food-material. 
 Irregularities will tend to disappear, surface-tension too may 
 aid, and the cell acquires a spheroidal form. The result — 
 surely intelligible enough — is a large and quiescent ovum. 
 
 It will be remembered that young ova are very frequently 
 amoeboid ; that with a copious nutrition this disappears in 
 varying degrees of encystment; that ensheathing envelopes 
 arising from the ovum, sweated off like cysts round Protozoa, 
 are exceedingly common ; and that ova are the largest of all 
 animal cells. 
 
 Starting once more from an amoeboid cell, if katabolism 
 comes to be more and more marked, the increasing liberation 
 of kinetic energy thus implied must find its outward expression 
 in increased activity of movement and in diminished size ; the 
 more active cell becomes modified in form, in adaptation to 
 passage through its fluid environment, and the natural result is 
 a flagellate sperm. 
 
 In short, then, the respective morphological characters of 
 the sex-cells, female and male, find the same physiological 
 rationale as do the large passive encysted and smaller active 
 ciliated phases of the cell-cycle in general, and are alike the 
 outcome and expression of predominant anabolism and kata- 
 bolism respectively. Here again we reach the same formula 
 as before ; or, more cumbrously in words — the functions are 
 either self-maintaining or species -maintaining, individual or 
 reproductive; the former are divided into anabolic and kata- 
 bolic, the latter into male and female. But the second set of 
 products and processes, so far from being unrelated to the 
 other, as is commonly supposed, are in complete parallelism. 
 Femaleness is relative anabolic preponderance in reproduction, 
 
134 
 
 THE EVOLUTION OF SEX. 
 
 hence the ovum has necessarily the general character which 
 this "diathesis" produces in non -reproductive cells; and, 
 similarly, relative katabolic preponderance stamps its character 
 of active energy upon the spermatozoon as naturally as upon the 
 ciliated cell or the monad. 
 
 Here and throughout it must be remembered that we are 
 dealing with relative preponderance of anabolism or kata- 
 bolism, — with ratios in metabolism. The more exact expres- 
 
 sion of our point is that the ratio tf in the female is greater 
 than the ratio y? in the male. 
 
 Diagram showing the divergence of ovum and spermatozoon 
 from an undifferentiated amoeboid type of cell. 
 
 Rolph's characterisation of the male cells as hungry and starving (kata- 
 bolic) has been experimentally confirmed by their powerful attraction to 
 highly nutritive fluids, and is every day illustrated in their persistent 
 attraction to the ova. Platner has suggested, in the intimately herma- 
 phrodite gland of the snail, that the external cells which form the ova are 
 better nourished than the central cells which divide into sperms. Just as 
 an infusorian in dearth of food is known in some cases to divide into many 
 small individuals, so the mother-spenn-cell is perhaps the seat of similar 
 katabolic necessities. The long persistence of vitality seems at first sight a 
 difficulty, if the sperms are highly katabolic cells. It must be noticed, 
 however, {a) That there is often only retention, not continuance of activity, 
 e.g., when the sperms lie closely packed in the special storing reservoirs; 
 (b) That the secretions of the female ducts probably afford some nutriment 
 to the sperms, which expose an exceptionally large surface in proportion to 
 their mass; and (c) That to a certain extent we may think of them as 
 protoplasmic explosives, which may remain long inert, but on the presence 
 
"THEORY OF SEX — ITS NATURE AND ORIGIN. 1 35 
 
 of the required stimulus are able to start again into extraordinary activity. 
 That they have, like the exhausted infusorians in Maupas's experiment, 
 reached the limit of their dividing power, is also evident. We might 
 refer, for instance, to Von Bardeleben's observations of futile attempts at 
 division (cytokinesis without karyokinesis) in the final stages of spermato- 
 genesis in man ("Jena. Zeitschr. Nat.," xxxi., 189S, pp. 475-520, 3 pis., 
 5 figs.). 
 
 Professor Giard has discussed the question of the possible partheno- 
 genesis of the male elements or microgametes ("Comptes Rendus Soc. 
 Biol. Paris," 1899). He suggests that the phenomena observed by Delage, 
 and termed " merogonic," where a non-nucleated ovum-fragment, fertilised 
 by a spermatozoon, proceeds to develop normally, may be regarded as 
 cases where the microgamete or sperm-cell develops parthenogenetically. 
 Siedlecki has observed the parthenogenetic development of the micro- 
 gamete of the Sporozoon Adelea ovata; the same, as Klebs and others 
 have shown, may occur in the lower plants. 
 
 § 3. The Problem of the O/igin of Sex. — We must now 
 return once more to the standpoint of the empirical naturalist, 
 and set out towards the interpretation of sex from a different 
 side, that of its origin. 
 
 It has often been raised as a reproach against the now 
 fortunately dominant school of evolutionist naturalists, that 
 they could give no account of the origin of sex ; and it must 
 be admitted that there have not been many vigorous attempts 
 to tackle the problem. Apart from the simple fact, that evo- 
 lutionist biology is still young, there are three reasons for the 
 comparative silence in regard to the problem of sex. 
 
 (1.) The first of these is the prevalent opinion, that when 
 one has explained the utility or advantage of a fact, one has 
 sufficiently accounted for it. This opinion rests on an accept- 
 ance of the selection theory, and on a willingness to cover all 
 questions of origin with a frank "ignoramus," or with the 
 vague assumption of an indefinite variability which affords 
 selection sufficient material to work upon. Our attempt is to 
 push the postulate of indefinite variability as far back as 
 possible, and to suggest a physiological reason why the varia- 
 tions which have led to sex-dimorphism should have taken 
 the lines which they presumably did. It is evident that a pre- 
 occupation with the ulterior benefits of the existence of sex 
 may somewhat obscure the question of how male and female 
 have in reality come to be. 
 
 (2.) A second reason for the comparative silence may be 
 found in the fact that the problem remains insoluble until it is 
 analysed into its component problems. The question of the 
 
136 THE EVOLUTION OF SEk. 
 
 origin of sex to a mind unprepared for the consideration of 
 such a problem, suggests quite a number of difficulties, — -What 
 is the import and origin of sexual reproduction (the setting 
 apart of special cells) ? what is the meaning and beginning of 
 fertilisation (the interdependence and union of sex-cells)? what 
 is the reason of the individual, male or female, sex in any one 
 case (the determination of sex) ? and lastly, what is the nature 
 and origin of the difference between male and female? — the 
 question at present under discussion. For purposes of analysis, 
 these questions must be kept distinct. 
 
 (3.) A third reason why the problem of the origin of male 
 and female has been so much shirked, why naturalists have 
 beaten so much about the bush in seeking to solve it, is that in 
 ordinary life, for various reasons, mainly false, it is customary 
 to mark off the reproductive and sexual functions as facts 
 altogether per se. Modesty defeats itself in pruriency, and 
 good taste runs to the extreme of putting a premium upon 
 ignorance. Now this reflects itself in biology. Reproduction 
 and sex have been fenced off as facts by themselves ; they 
 have been disassociated from the general physiology of the 
 individual and the species. Hence the origin of sex has been 
 involved in special mystery and difficulty, because it has not 
 been recognised that the variation which first gave rise to 
 the difference between male and female, must have been a 
 variation only accenting in degree what might be traced 
 universally. 
 
 § 4. Nature of Sex as seen in its Origin among Plants. — 
 In tracing the origin of sex, we would wish to guard against 
 any impression of having consciously or unconsciously arranged 
 our facts in the light of the theory we hold. Hence we prefer 
 to follow some accessible account, taken essentially from the 
 morphological point of view. We shall follow Professor Vines 
 in his article "Reproduction — Vegetable," in the Encyclopedia 
 Brila?inica, at each stage, however, endeavouring to interpret 
 the facts, physiologically, in the light of protoplasmic pro- 
 cesses. 
 
 (1.) The simple alga, Protococcus, common on tree-stems, in pools, 
 wells, and the like— reproduces itself in a simple fashion. The cell divides 
 into a number of equal units or spores; these are set free, are mobile for a 
 while, eventually come to rest, and develop to the normal size. A hint, 
 however, of the beginning of a difference is seen when the cell occasionally 
 divides into a larger number of smaller spores. These, however, show no 
 difference in history. They settle down, and develop just like their more 
 
THEORY OF SEX— ITS NATURE AND ORIGIN, I37 
 
 richly dowered neighbours. We find here the occurrence of units of smaller 
 size, that is to say, less predominantly anabolic, but still these are able to 
 develop independently. 
 
 (2.) In a higher alga, Ulotkrix — one of the series known as Confervse — 
 both large and small reproductive cells are developed. The large ones 
 develop always of themselves, and so may the smaller forms. But the 
 smaller forms may also unite in pairs, and then start a new plant from the 
 double capital thus attained. When one of the smaller cells develops by 
 itself, the result, in some cases at least, is a weakly plant. They have what 
 Professor Vines calls an "imperfect sexuality," for while they are in part 
 dependent upon union with other cells, they are not wholly so. They are 
 anabolic enough, we may say, sometimes to develop independently, but 
 often they are individually too katabolic for anything but weak independent 
 development. In uniting, however, in mutual nutrition, they are strong. 
 The student will already see the relative femaleness of the large units, the 
 maleness of their smaller neighbours. 
 
 (3.) A third stage is reached in another alga, Ectocarpus, which is 
 peculiarly instructive. This may separate off large cells which develop by 
 themselves like parthenogenetic ova. From other parts of the plant 
 smaller units are liberated, which generally, though not yet invariably, 
 unite with one another before developing. But between these smaller 
 units a most important physiological difference has been observed by 
 Berthold. Some soon come to rest and settle down, and with these their 
 more energetic neighbours by-and-by unite. We have here a very distinct 
 beginning of the distinction between male and female elements. The 
 comparatively sluggish, more nutritive, preponderatingly anabolic cells, 
 which soon settle down — are female ; the more mobile, finally more 
 exhausted and emphatically katabolic cells — are male. As Vines says, 
 " the one is passive, the other active; the former is to be regarded as the 
 female, and the latter as the male reproductive cell." 
 
 (4.) Further, in another alga, Cutleria, the differentiation may be 
 traced. Two kinds of units result, which must unite with one another if 
 development is to take place, but these units arise from perfectly distinct 
 sources in the parent plant. The larger less mobile cells, which soon 
 come to rest, are fertilised by the smaller more active units. The more 
 anabolic or female cells are fertilised by the more katabolic or male cells, 
 which have now gone too far for the possibility of independent develop- 
 ment. 
 
 (5.) To complete the series, we may simply mention such a case as 
 that to which we shall presently return, — those forms of Volvox, where an 
 entire colony of cells produces either female or male elements, thus repre- 
 senting the beginning of an entirely unisexual many-celled organism. 
 
 While the above cases also involve the problem of the 
 origin of fertilisation, which is left over for the present, they 
 confirm our conclusion that relative predominance of kata- 
 bolism or anabolism is the characteristic of male or female 
 respectively. 
 
 § 5. Nature of Sex as seen in Origin among Animals. — 
 Among the Protozoa also, we can trace the beginnings of the 
 
13^ THE EVOLUTION Of SEX. 
 
 same "dimorphism" between male and female. A union 
 between similar cells is of course frequent, but that is not at 
 present to the point. What we refer to are the numerous 
 cases, especially among flagellate and Vorticella-like infusorians, 
 where the two individuals which unite are quite unlike one 
 another both in form and history. " There can be no doubt," 
 Hatchett Jackson remarks, "that the process is essentially a 
 sexual one; when the individuals are invariably different, there 
 is no reason why the terms male and female should not be 
 applied to them." In some cases we find as before that a 
 small active katabolic unit combines with a larger, more 
 passive, and anabolic individual. 
 
 In the bell-animalcule ( Vorticelld), so common on the 
 water-plants of our ponds, a minute free-swimming unit, formed 
 as one of the results of repeated division, unites with a stalked 
 individual of the normal size. In the related Epistylis, 
 Engelmann has described how an individual divides first of 
 all into two cells. One of these remains as such (like an 
 ovum), the other repeatedly divides (like a mother sperm-cell) 
 into numerous minute units. One of these subsequently 
 unites with the undivided cell, and Engelmann does not 
 hesitate to call the different elements male and female. In 
 some radiolarians (e.g., Collozouvi) dimorphic spores — large 
 and small — have been described, although their history has 
 not yet been fully traced. 
 
 As another illustration, it will be instructive to select the 
 case of Volvox. In this colonial organism, which is best 
 regarded as a multicellular protist, the component cells are at 
 first all alike. They are united by protoplasmic bridges, and 
 simply form a vegetative colony. In favourable environmental 
 conditions this state of affairs may persist, or be interrupted 
 only by parthenogenetic multiplication. When nutrition is 
 checked, however, sexual reproduction makes its appearance, 
 and that in a manner which illustrates most instructively the 
 differentiation of the two sets of elements. Some of the cells 
 are seen differentiating at the expense of others, accumulating 
 capital from their neighbours; and if their area of exploitation 
 be sufficiently large, emphatically anabolic cells or ova result ; 
 while if their area is reduced by the presence of numerous 
 competitors struggling to become ova, the result is the forma- 
 tion of smaller, less anabolic cells, which become ultimately 
 male, segment into antherozoids, meantime losing their vegeta- 
 
THEORY OF SEX — ITS NATURE AND ORIGIN. 
 
 139 
 
 tive greenness and becoming yellow. In some species, distinct 
 colonies may, in the same way, become predominantly anabolic 
 or katabolic, and be distinguished as completely female or 
 male colonies. Thus, again, we reach the conclusion, of a 
 predominant anabolism effecting the differentiation of female 
 elements, and of katabolism as characteristic of the male. 
 
 § 6. Conclusion. — In conclusion,, in defiance of Dr Minot's 
 dictum, that "such speculation passes far beyond the present 
 possibilities of science," we believe that the consideration (a) 
 
 ' u[ 'wmM^^ 
 
 Voluox glolator, a colonial Alga or Infusorian, showing the 
 ordinary cells (c) that make up the colony (or body), and 
 the special reproductive cells (a, &), both male and 
 female. — After Cohn. 
 
 of the characteristics of the sex-elements, alike in history, as 
 Minot himself emphasises, and in their finished form, (b) of 
 the incipient sex dimorphism seen among the simplest plants 
 and animals, (c) of phenomena, both normal and pathological, 
 in the sexual tissues and organs, (d) of the established facts 
 in regard to the determination of sex (chap. 4), and (e) of the 
 structural and functional, primary and secondary characteristics 
 of the sexes (chap. 2 and passim) — all lead to the conclusion, 
 that the female is the outcome and expression of relatively 
 
140. THE EVOLUTION OF SEX. 
 
 preponderant anabolism, and the male of relatively pre- 
 dominant katabolism. Corroborations will gradually appear 
 in the succeeding sections, as we discuss fertilisation, partheno- 
 genesis, or special facts like menstruation and lactation. 
 
 The late Mr G. J. Romanes criticised our thesis as a mere 
 re-statement of the facts of sexual dimorphism without any 
 explanation of them. But no other course is open to such 
 inquiries but that of re stating. Our whole point is that of 
 re-stating the idea of dimorphism in protoplasmic terms, at a 
 deeper level of analysis than heretofore, in kinetic terms instead 
 of static ones, and in such a way that the origin and evolution 
 of sex are seen to be not phenomena per se, specialised, as 
 naturalists have commonly thought, from the rest of the 
 organism, but congruent with the origin and evolution of 
 other organic differences. From another point of view, we 
 may say that we are seeking to re-state the phenomena of sex 
 by a deeper recognition of the unity of the organism, and of 
 organisms. 
 
THEORY OF SEX — ITS NATURE AND ORIGIN. 141 
 
 SUMMARY. 
 
 1. Suggested theories of the nature of male and female; their number 
 and vagueness. Three recent developments — (a) Rolph's penetrating sug- 
 gestion of more nutritive females, less nutritive males; {d) Minot's theory 
 of the differentiation of both kinds of sex cells from a primitive hermaphro- 
 ditism ; {c) the conclusion of Brooks, that the males are more variable, arid 
 alone transmit new variations. 
 
 2. Nature of sex seen in its essence in the sex-cells. The fundamental 
 protoplasmic antithesis illustrated in the Protozoa, in the cells of higher 
 animals, in life-histories. The conception of a cell-cycle. The physio- 
 logical import of this, — the protoplasmic possibilities, preponderant ana- 
 bolism, predominant katabolism, and a relative equilibrium. The anabolic 
 character of the ova. The katabolic character of the sperms. 
 
 3. The problem of the origin of sex, so little tackled, because of (a) the 
 logical sufficiency of the selection theory and pre-occupation with inquiries 
 as to the utilitarian justification of the facts, (b) the number of separate 
 problems involved, (it) the isolation of sex and reproduction from the 
 general life of the organism and species. 
 
 4. A series from simple plants, showing the gradual appearance of 
 dimorphic sex-cells, with the physiological interpretation thereof. The 
 dimorphism is the result of relatively preponderant katabolism and ana- 
 bolism, and this is the origin of male and female. 
 
 5. Illustrations of incipient dimorphism or sex among the Protozoa. 
 Special reference to the case of Volvox. 
 
 6. General conclusion, — (a) from the sex-cells, (b) from incipient sex, [c) 
 from organs and tissues, (d) from the determination of sex, (e) from the 
 characters of the sexes, — that male and female are the resulls and expres- 
 sions of relatively predominant katabolism and anabolism respectively. 
 
 LITERATURE. 
 
 Balbiani. — Lecons sur la generation des vertebres. Paris, 1879. 
 
 Brooks, W. K.— The Law of Heredity. Baltimore, 1883. 
 
 Dangeard, P. A. — Theorie de la Sexualite. Le Botaniste, Serie VI., 
 
 1898. 
 ElGENMANN, C. H. — Sex-differentiation in the viviparous Teleost Cyma- 
 
 togaster. Arch. Entwickelungsmechamk., IV., 1896, pp. 125-179, 
 
 6 pis., I fig. 
 Geddes, P. — Op. ciL, especially "Theory of Growth, Reproduction, Sex, 
 
 and Heredity," Proc. Roy. Soc. Edin., 1886; and Article "Sex," 
 
 Encyc. Brit., also "Restatement of Cell Theory," Proc. Roy. Soc. 
 
 Edin., 1883-84. 
 HARTOG, M. — Some Problems of Reproduction. Quart. Journ. Micr. 
 
 Sci., XXXIII., 1891, pp. 1-70. 
 Haycraft, J. B.— The R61e of Sex. Natural Science, VII., 1890. 
 Klebs, G. — Ueber einige Probleme der Physiologie der Eortpflanzung. 
 
 Jena, 1895, 26 pp. Die Bedingungen der Fortpflanzung bei einigen 
 
 Algen und Pilzen. Jena, 1896. 
 
I4 2 THE EVOLUTION OF SEX. 
 
 Lendl, A. — Hypothese iiber die Entstehung von Soma- und Propagation- 
 
 zellen. 8vo. Berlin, 1889, 78 pp., 16 figs. 
 Lignier, O. — Sur l'origine de la Generation et celle de la Sexualite. 
 
 Miscellanees Biologiques Dediees au Prof. A. Giard. Paris, 1889, 
 
 pp. 396-401. 
 Maupas, E. — La Rajeunissement Karyogamique chez le Cilies. Arch. 
 
 Zool. Expe"r., VII., 1889. 
 Minot, C. S.— Theorie der Genoblasten. Biol. Centralbl., II., p. 365. 
 
 Ueber Vererbung und Verjiingung. Biol. Centralbl., XV., 1895, 
 
 pp. 571-87. 
 Nussbaum, M. — Znr Differenzirung des Geschlechts im Thierreich. Arch. 
 
 Mikr. Anat., XVIII., 1880. 
 Rolph, W. H.— Biologische Probleme. Leipzig, 1884. 
 Romanes, G. J. — Darwin and after Darwin. 1895-98. 
 Ryder, J. A. — The Origin of Sex through Cumulative Integration, and 
 
 the Relation of Sexuality to the Genesis of Species. Amer. Philos. 
 
 Soc. , XXVIII., 1890, pp. 109-159. 
 SACHS, J — Text-book of Botany, edit, by Vines, second edition, 1882; 
 
 and Physiology of Plants, translated by Marshall Ward, 1887. 
 Vines, S. H. — Physiology of Plants, 1886; article "Reproduction — 
 
 Vegetable," Encyc. Brit. 
 Wkismann, A. — Op. cil. 
 
BOOK III. 
 
 PROCESSES OF REPRODUCTION. 
 
CHAPTER XI. 
 
 Sexual Reproduction. 
 
 § i. Different Modes of Reproduction. — It is well known that 
 a starfish deprived of an arm can replace this by a fresh growth; 
 that crabs can renew the great claws which they have lost in 
 fighting; and that, even as high up as the lizards, the loss of 
 part of a leg or a tail can be made good. In a great variety 
 of cases, but decreasingly as we ascend from lower to higher 
 organisms, there is reparation of external injuries. Generally 
 speaking, the facts bear out Lessona's generalisation that re- 
 generative processes are adaptive, occurring in those animals 
 and in those parts of animals in which mutilation tends in the 
 natural course of life to be frequent. Now this "regenera- 
 tion," as it is called, is in a certain degree a process of repro- 
 duction. By continuous growth the cells of a persistent stump 
 are able to reproduce the entire member. We know too that 
 a sponge, a hydra, or a sea-anemone may be cut into pieces, 
 with the result that each fragment grows into a new organism. 
 The same is done with many plants; and though these multi- 
 plications are artificial, they illustrate what Spencer and Haeckel 
 said long ago, that reproduction is but more or less discon- 
 tinuous growth. So again, we pass onwards insensibly from 
 cases of continuous budding, as in sponge or rose-bush, to 
 discontinuous budding in hydra, zoophyte, and tiger-lily, where 
 the offspring, vegetatively produced, are sooner or later set free. 
 Similarly in the Protozoa, an almost mechanical breakage begins 
 the series. This becomes more definite, in the production of 
 several buds at once, or of only one. Budding leads on to 
 ordinary division, both multiple and binary: while finally, in 
 colonial forms, the liberation of special reproductive units may 
 be observed. 
 
 § 2. Facts involved in Sexual Reproduction. — It is neces- 
 sary, at the outset, to be quite clear as to the occurrence of 
 several distinct facts in any ordinary case of sexual reproduction 
 
146 THE EVOLUTION OF SEX. 
 
 among many-celled organisms. (1.) There is, first of all, the 
 fact that special reproductive cells are present in more or less 
 marked contrast to the ordinary cells making up the body. 
 To this antithesis we have already given due prominence. (2.) 
 Then there is the further fact, that these special reproductive 
 cells are dimorphic; that they, and the organisms which pro- 
 duce them, are distinguishable as male and female. This has 
 been the main theme of the two preceding books. (3.) Lastly, 
 we have to recognise that these dimorphic sex-cells are mutually 
 dependent, — that if the egg-cell is to develop into an organism, 
 it must first be fertilised by a male element. On the facts 
 of fertilisation, therefore, as observed in plants and animals, 
 attention must now be concentrated. 
 
 § 3. Fertilisation in Plants. — "The Newly Discovered 
 Secret of Nature in the Structure and Fertilisation of Flowers," 
 so ran the title of a work published by Conrad Sprengel in 
 1793, embodying his pioneer investigations on a now familiar 
 field. Though not indeed the first to point out the importance 
 of insects in relation to fertilisation, — for that honour appears 
 to belong to Kolreuter (1761), — Sprengel laid sure foundations, 
 now somewhat hidden by the superstructure which Darwin and 
 others have built. To Sprengel's eyes, the many ways in which 
 the nectar is protected from rain seemed full of "intention." 
 He recognised in the markings of the petals illumined finger- 
 posts to lead insects to the hidden hoards; and he further 
 demonstrated that in some bisexual flowers it was physically 
 impossible for the pollen from the stamens to pass to the tips 
 of the carpels. His general conclusion, freely stated, was, that 
 " since a large number of flowers have the sexes separate, and 
 probably at least as many hermaphrodites have the stamens 
 and carpels ripening at different times, nature appears to have 
 designed that no flower shall be fertilised by its own pollen." 
 A few years later (1799), Andrew Knight maintained that no 
 hermaphrodite flower fertilises itself for a perpetuity of genera- 
 tions. 
 
 Sprengel's secret of nature had, however, to be set forth 
 afresh by Darwin, who, in his " Fertilisation of Orchids '"' 
 (1862), and "Effects of Cross- and Self-Fertilisation" (1876), 
 has not only shown, with great wealth of illustration, the mani- 
 fold devices for ensuring that insects unconsciously carry the 
 fertilising pollen from one flower to another, but has also 
 emphasised the advantage of cross-fertilisation for the health 
 
SEXUAL REPRODUCTION. 
 
 147 
 
 of the species "Nature tells us," he says, "in the most 
 emphatic manner that she abhors perpetual self-fertilisation." 
 Hildebrand, Hermann Miiller, Delpino, and others, have, with 
 consummate patience of observation, further traced out the 
 secrets of nature in this relation; and the student may be 
 referred to D'Arcy Thompson's valuable edition of Muller's 
 " Fertilisation of Flowers," Sir John Lubbock's " Flowers in 
 Relation to Insects," the classic works of Darwin, and P. 
 Knuth's "Handbuch der Bliitenbiologie," 2 vols., Leipzig, 
 1892. Reference must, however, also be made to Meehan's 
 protest (see pp. 8o, 81), that self-fertilisation is neither so rare 
 nor so "abhorrent " as is generally believed. 
 
 In a great number of cases, cross-fertilisation by means of 
 insects does occur; in many it must occur. In another by no 
 
 Bees visiting White Deadnettle (B), and Bioom (A). 
 
 means small set of flowering plants, — usually with inconspicuous 
 blossoms, — the fertilising gold dust is borne by the wind, and 
 falls, like the golden shower on Danae, upon adjacent flowers. 
 In many hermaphrodite flowers, again, self-fertilisation does 
 certainly take place; in some this is necessarily so. Indubi- 
 table self-fertilisation occurs in the small degenerate unopening 
 (cleistogamous) flowers of some plants, such as species of 
 balsam, deadnettle, pansy, &c. These occur along with 
 ordinary flowers, and, curiously enough, are sometimes more 
 fertile than they. 
 
 In most of the lower plants, the male elements are minute, 
 and actively mobile. They find their way through the water, 
 or along capillary spaces between the leaves, to the passive 
 female cells. In some cases there is a curvature of the male 
 
148 
 
 THE EVOLUTION OF SEX. 
 
 organ towards an adjacent female organ, apparently in obedi- 
 ence to chemical or physical attraction. Even here close 
 fertilisation seems exceptional, and is often impossible. 
 
 So far, however, only the external aspect of the process. 
 So long ago as 1694, Camerarius showed that if the male 
 flowers of hemp, maize, and other plants were removed, the 
 female flowers bore no seeds, or at least no fertile ones. In 
 1704, E. F. Geoffroy castrated certain plants by removing the 
 stamens, and noted that they remained barren. " Mirandum 
 sane," he wrote, "quam similem servet natura cunctis in 
 viventibus generandis harmoniam." Reasonable as this now 
 appears to us, the fundamental fact was not only slowly recog- 
 ■ nised, but on into the present century there were found 
 
 A, Enlarged section of ripe Anther (fi), liberating pollen {a). B, Diagrammatic 
 section of a Flower, showing pistil (c), — receiving stigma, conducting style, 
 ovary with seed id) ; the stamens (b) with pollen. C, The Pollen-tube 
 (a) growing down to the ovule (d) and female cell (/). The pollen-grain ia 
 here represented as distinctly two-celled. 
 
 naturalists who strongly opposed it, and denied the sexuality of 
 plants altogether. In 1830, however, Amici made a great step. 
 He traced the pollen-grain from its lighting on the carpel tip 
 down into the recesses of the ovule. Schleiden, whose name is 
 so closely associated with the founding of the "cell theory," 
 soon confirmed Amici's observation, but in doing so went 
 unfortunately much too far. Not only did the pollen-grain 
 send its tube into the ovule, but there, according to Schleiden, 
 it gave origin to the future embryo. This opinion, which, as 
 Heyer observes, made the male element really female, was 
 obviously parallel to that of the zoologists who found in the 
 "sperm -animalcule" the miniature embryo. The view of 
 
SEXUAL REPRODUCTION. 1 49 
 
 Camerarius and Amici of course prevailed ; and we now know 
 not only the fact that the pollen-grain is a male element which 
 unites in fertilisation with a female cell, but, thanks especially 
 to Strasburger, much about the intimate nature of the process. 
 In the last century, Millington emphasised the difference 
 between male and female flowers, and we can trace the 
 influence of this discovery in Erasmus Darwin's "Loves of the 
 Plants," 
 
 In the last few decennia, it has been shown, for many of the 
 lower plants, that fertilisation essentially involves the union of 
 the nuclei of male and female cells. By analogy the same was 
 believed to be true of higher plants, but direct demonstration 
 has only recently been forthcoming. Strasburger has followed 
 the whole history of the pollen-grain, from the anther of the 
 stamen to the embryo-sac of the carpel; and though some details 
 still remain obscure, his researches have undoubtedly succeeded 
 in elucidating the essential facts in the process. He shows 
 how the pollen-grain divides into a vegetative and a generative 
 cell, of which only the latter is directly important in fertilisation. 
 The generative cell, which consists like the sperm mostly 
 of nucleus with very little directly associated cell-substance, 
 itself divides to form two (or even more) generative nuclei. 
 One of these passes from the pollen-tube to enter into close 
 union with the nucleus of the female cell, with which it fuses 
 to form the double nucleus ruling the forthcoming develop- 
 ment. Exceptionally the other generative nucleus may also 
 unite with the nucleus of the egg-cell, but this is almost as rare 
 as "polyspermy" among animals. According to Strasburger, 
 the cell-substance of the pollen-grain or pollen-tube which sur- 
 rounds the nucleus has no direct influence in the essential act. 
 Fertilisation is a union of two nuclei, "the cell-substance of the 
 pollen-tube is only the vehicle." He confirms the observations 
 of Pfeffer, as to the reality of an osmotic attraction between at 
 least the surroundings of the two essential elements, in accord- 
 ance with which the pollen-tube bearing the generative nucleus 
 is marvellously guided to its destination. The differentiation 
 of the generative nucleus, in contrast to the more vegetative, 
 and the true nuclear union which forms the climax of fertilisa- 
 tion, are two very important facts, showing the unity of the 
 process not only in higher and lower plants but in all 
 organisms. 
 
 Although there are peculiarities distinguishing the processes 
 
150 THE EVOLUTION OF SEX. 
 
 of maturation and fertilisation in plants from those observed in 
 animals, it is impossible to deny the essential parallelism. A 
 discussion of this would lead us into technical details, which 
 cannot be profitably described without abundant figures, but 
 we may refer, for instance, to a comparative survey by Professor 
 V. Haecker ("Biol. Centralblatt," xvii., 1897, pp. 689-705, 
 721-745, 40 figs.). One of the most striking botanical dis- 
 coveries of recent years is the fact demonstrated by Hirase and 
 Ikeno, that in the Ginkho (Salisburia adiantifolia), and in 
 Cycas revohita, — gymnosperm flowering plants belonging to 
 the family of Cycads, — the male nuclei issue from the pollen- 
 tube as motile spermatozoa or antherozoids. Webber has also 
 announced a similar discovery in the case of Zamia integrifolia. 
 
 § 4. Fertilisation in Animals.— That the sperms are essential 
 to fertilisation was a conclusion by no means recognised when 
 those elements were first seen. Gradually, however, the fact 
 was demonstrated, both by experiment and observation. Jacobi 
 (1764) artificially fertilised the ova of salmon and trout with 
 the milt of these forms, and somewhat later the Abbe" Spallan- 
 zani extended these experiments to frogs and even higher 
 animals. Even he, however, believed that the seminal fluid 
 was the essential factor, not the contained spermatozoa. 
 Through the experiments of Prevost and Dumas (1824), 
 Leuckart (1849), and others, attention was directed to the 
 real import of the sperms, which Kolliker referred to their 
 cellular origin in the testes. The presence of the sperm within 
 the ovum was observed in the rabbit ovum by Martin Barry in 
 1843; by Warneck, in 1850, for the water-snail, a fact con- 
 firmed about ten years afterwards by Bischoff and Meissner; 
 in the frog ovum by Newport (1854); and in successive years 
 it was gradually recognised in a great variety of animals. 
 
 The adaptations which secure that the sperms shall reach 
 the ova are very varied. Sometimes it seems almost a matter 
 of chance, for the sperms from adjacent males may simply be 
 washed into the female, as in sponges and bivalves, with the 
 nutritive water-currents. In other cases, especially well seen 
 in most fishes, the female deposits her unfertilised ova in the 
 water; the male follows and covers them with spermatozoa. 
 Many may have watched from a bridge the female salmon 
 ploughing along the gravelly river bed depositing her ova, 
 careful to secure a suitable ground, yet not disturbing the 
 already laid eggs of her neighbours. Meanwhile she is attended 
 
SteXuAL repr6duction. i 5 J: 
 
 by her (frequently much smaller) mate, who deposits milt upon 
 the ova. In the frog, again, the eggs are fertilised externally 
 by the male just as they leave the body of his embraced mate. 
 Or it may be that the sperms are lodged in special packets, 
 which are taken up by the female in most of the newts, sur- 
 rounded with one of the male -arms in many cuttle-fishes, or 
 passed from one of the spider's palps to the female aperture. 
 In the majority of animals, e.g., insects and higher vertebrates, 
 copulation occurs, and the sperms pass from the male directly 
 to the female. Even then the history is very varied. They 
 may pass into special receptacles, as in insects, to be used 
 as occasion demands ; or, in higher animals, they may with 
 persistent locomotor energy work their way up the female 
 ducts. There they may soon meet and fertilise ova which 
 have been liberated from the ovary; or may persist, as we 
 noticed, for a prolonged period ; or may eventually perish. 
 
 When the sperms have come, in any of these varied ways, 
 into close proximity to the ovum, there is every reason to 
 believe that a strong osmotic attraction is set up between the 
 two kinds of elements. We have often suspected that the 
 approach of the conjugating cells of two Spirogyra filaments 
 might be directed along the line of an osmotic current; and 
 although we must confess that perhaps somewhat rough 
 evaporations, performed a few summers ago, gave no positive 
 confirmation to the idea that glucose or the like might be 
 present in appreciable quantity in the water, a recent observer, 
 we are glad to see, claims to have been more fortunate. 
 
 The spermatozoa, which seem so well to deserve Rolph's 
 epithet of "starved," appear to be powerfully drawn to the 
 well-nourished ovum, and the latter frequently rises to meet 
 the sperm in a small " attractive cone." Often, however, 
 there is an obstacle in the way of entrance in the form of the 
 egg-shell, which may be penetrable only at one spot, well 
 called the micropyle. Dewitz has made the interesting observa- 
 tion that round the egg-shells of the cockroach ova, the sperms 
 move in regular circles of ever-varying orbit; and points out 
 that thus, sooner or later, a sperm must hit upon the entrance. 
 He showed that this was a characteristic motion of these 
 elements on smooth spheres, for round empty egg-shells or on 
 similar vesicles they moved in an equally orderly and systematic 
 fashion. 
 
 The persistence with which the spermatozoa often force their 
 
152 THE EVOLUTION OF SEX. 
 
 way to the ova makes it impossible to doubt the reality of a 
 strong chemotactic attraction. One illustration may suffice. 
 According to Dr Sadone's account of the impregnation in 
 the Rotifer Hydatina senta ("Zool. Anzeiger," xx., 1897, 
 pp. 513-17), the spermatozoa of the male, which are injected 
 into the body-cavity of the female, reach the totally enclosed 
 eggs by boring through the thin membrane at a point where 
 the mature ova are situated — a process not known in any 
 other animals. The oval head of a spermatozoon was seen to 
 attach itself to the membrane of the ovary, the tail continued 
 to make lashing movements, the head was gradually forced 
 through the membrane, and the tail followed, the whole process 
 taking about ten minutes. 
 
 It was till recently believed that more than one sperm 
 might at least enter the ovum, but researches such as those 
 of Hertwig and Fol have shown that when one sperm has 
 found admittance, the way is usually barred against others. 
 The micropyle may be blocked, or the surrounding membrane 
 may be altered, or in other ways the ovum may exhibit what 
 Whitman calls "self-regulating receptivity," so as to be no 
 longer penetrable. We are safe in concluding, — that the ovum 
 is usually receptive only to one sperm; that in most cases the 
 entrance of more than one sperm is impossible; and that where 
 " polyspermy " does occur, pathological development is at 
 least often the result. 
 
 It may be well to note that there are, as in plants, various 
 steps in the process which is often roughly summed up in the 
 one word — fertilisation. 
 
 (1) There is the process by which the spermatozoa 
 are brought into general proximity to the ova. In higher 
 animals this is best termed insemination, and is accomplished 
 by copulation. 
 
 (2) There is the approach of the spermatozoon to the ovum, 
 but of this little is known. 
 
 (3) There is fertilisation in the strict sense, — the intimate 
 and orderly union of two sex -nuclei. 
 
 What takes place before fertilisation is, as we have just seen, very 
 varied indeed among animals ; what takes place after fertilisation is of 
 course cell division, but that, though referable to certain great types, must 
 necessarily be very diverse ; what takes place in the act of fertilisation, 
 however, is always essentially the same. The head of the spermatozoon 
 becomes the male nucleus (or pro-nucleus) of the fertilised ovum, entering 
 into close association with the female nucleus. The latter, as we have 
 
SEXUAL REPRODUCTION. 153 
 
 already noted, has had its own history; it is no longer the original ger- 
 minal vesicle, nor usually like it in appearance, it is the germinal vesicle 
 minus the quantity of nuclear substance given off in forming two polar 
 globules. This female nucleus (or pro-nucleus, as it is generally called) 
 comes into close association with the sperm or male-nucleus ; nor does it 
 remain quite passive in the process, though the greater activity in bringing 
 about the close association is certainly still exhibited by the male. Whit- 
 man has recently emphasised the reality of an attractive influence between 
 the pro-nuclei. Fusion of the pro-nuclei was observed so long ago as 
 1850 by Warneck in the pond-snail {Lymnaus). The result, however, 
 appears to have been overlooked, till the same fact was reobserved in 
 threadworm ova by Biitschli in 1874. Since that date the fact has been 
 continuously studied. Some observers still doubt whether what can be 
 accurately called fusion of nuclei ever occurs; and if fusion means inextric- 
 able confounding and mixing up of the male and female nuclear elements, 
 it is almost certain that such does not in any case happen. There is no 
 doubt, however, that the two nuclei become very closely associated, and 
 according to most observers a double unity is formed, in which the com- 
 ponent nuclear elements from the two origins so diverse are united in 
 perfectly orderly fashion. So exact, in fact, is this duality, that when the 
 first division of the egg takes place, each of the two daughter-cells has 
 in its nucleus half of the male and half of the female elements, and so on 
 perhaps in after-stages. 
 
 The object upon which the intimate phenomena of fertilisation have 
 been most studied is the ovum of the threadworm (Ascaris mega/ocephala) 
 which infests the horse. Since 1883 numerous important memoirs have 
 dealt with this subject, and with the same material. The general results 
 stand out clearly, though there remain not a few minor discrepancies. To 
 one of these, now explained, we may briefly refer. According to Van 
 Beneden, the normal ovum of the threadworm contained in its nucleus one 
 chromatin element, and was fertilised by a sperm also with one chromatin 
 element. Carnoy, however, described the normal ovum as containing two 
 chromatin elements, and as fertilised by a sperm also with two. In view 
 of the perfection with which both these investigators had unravelled the 
 structure and behaviour of the nuclei, the discrepancy seemed serious 
 enough. But Boveri has shown that both are right ; Van Beneden 's type 
 occurs; Carnoy's type also occurs. Nay more, an ovum with one chromatin 
 element seems to be always fertilised by a sperm with only one, while an 
 ovum with two chromatin elements is fertilised by a sperm likewise with two. 
 
 A few of the details may be summarised from the masterly researches of 
 Boveri. The extrusion of the two polar cells from the ovum is in reality a 
 double process of cell -division. The quantity of the nuclear substance in the 
 germinal vesicle is thereby reduced, and the number of nuclear elements is 
 also reduced to half the normal. Only one sperm penetrates the ovum, 
 unless the latter be unhealthy; and with the entrance of the sperm the ovum 
 undergoes a. simultaneous change, which excludes other male elements. 
 Only the head or nuclear portion of the sperm is of real importance in the 
 essential act of fertilisation ; the nutritive tail or cap simply dissolves away. 
 After the sperm-nucleus has penetrated to the centre of the ovum, and 
 after the extrusion of the polar bodies is quite completed, we have to deal 
 with two nuclei, not only closely approximate in structure, but alike in 
 further history. 
 
154 
 
 THE EVOLUTION OF SEX. 
 
 In Carnoy's type, both male and female nuclei contain two chromatin 
 elements, in the form of bent rods; and before union takes place, these 
 undergo a marked modification, the same in both cases. Round the chro- 
 matin rods vacuoles are formed, limiting them from the surrounding 
 protoplasm; into these the rods send out anastomosing processes, after 
 the fashion of little rhizopods ; gradually the rods thus resolve them- 
 selves into a network, in the meshes of which minute "nucleoli" are 
 also demonstrable. 
 
 Diagram of the Process of Fertilisation, after Boverl. — a, female pro-nucleus; 6, polar 
 bodies ; c, male nucleus ; d, sperm-cap ; ac, chromatin elements of united female 
 and male nuclei {a and c) ; e, centrosomes ; /, archoplasmic threads. 
 
 The two nuclei thus modified then unite, but that again so precisely, as 
 Van Beneden especially has shown, that each forms half of that spindle 
 figure which almost all nuclei take when about to divide. This double 
 spindle figure is the "segmentation nucleus," which will presently divide 
 into the two first daughter-nuclei of the ovum (see figs. AT. -X. ). 
 
 It is not possible here to discuss certain intricate changes which take 
 place meanwhile, not in the nuclei, but in the cell-substance of the ovum. 
 Doth Van Beneden and Boveri have recently agreed on the existence of two 
 
SEXUAL REPRODUCTION. 
 
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 ac- 
 
 
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 MK 
 
 MMmmmK 
 3m 111 
 
 ■ ■.. ■:-■ .- ; 
 
 ^1P 
 
 "Hg 
 
 K 
 
 ■r^HN, 
 
 
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1 56 THE EVOLUTION OF SEX. 
 
 "central corpuscles" (centrosomata) in the protoplasm. These serve as 
 " points of insertion" for protoplasmic threads, which exert a " muscular 
 action" upon the nuclear elements in the forthcoming division. Boveri 
 has traced with great care the history of a special kind of protoplasm (what 
 he calls the archoplasm), which has its centre in either "central corpuscle" 
 (e), and sends out fibrils (f), which moor themselves to the nuclear ele- 
 ments. The movements of the latter during the forthcoming first division 
 of the ovum are directly referable to the antagonistic action of these fibrils, 
 and thus we have hints of an intracellular muscularity. 
 
 In the spindle the nuclear elements, still distinguishable in their orderly 
 behaviour as male and female, eventually form what is known as the 
 " equatorial plate " (VI.), lying across the centre of the spindle. This is a 
 well-marked stage, and one characterised by apparent equilibrium. " It is 
 the resting-stage par excellence in the life of the cell. Movement is at an 
 end, a state of stability has set in, and this would continue ad infinitum, 
 did not a factor, which hitherto has played no part, assert itself and bring 
 about fresh movement. This new movement is the longitudinal division of 
 the chromatin elements, an independent expression of life — indeed, a re- 
 productive act — on the part of the nuclear elements." 
 
 Of each longitudinally split chromosome one half moves or is moved 
 towards the one centrosome, and the other half towards the other centro- 
 some. After this apparently equal partition a nuclear reconstruction is 
 gradually effected, and the ovum reaches the 2-cell stage. 
 
 One marvellous fact, showing the closeness of union in 
 fertilisation, may be briefly re -emphasised. In the double 
 nucleus formed from the union of male and female nuclei, Van 
 Beneden, Carnoy, and others, have shown that both constituents 
 have an equal share. The one half is paternal, the other 
 maternal, and this is true not only for Ascaris (Van Beneden) 
 and other threadworms (Carnoy), but for representatives of 
 other worm-types, ccelenterates, echinoderms, molluscs, and 
 tunicates. In the division which forms the first two daughter- 
 cells (IX., X.), half of each set of constituents goes to either 
 cell, and the dualism is kept up. Furthermore, it is probable 
 that of the chromatin loops observed in the division figure of a 
 daughter-cell, half are derived from the male parent, and half 
 from the female. The importance of this fact, in relation to 
 the influence of both parents upon the offspring, is very 
 obvious. 
 
 One of the clearest of modern exponents, Professor E. B. 
 Wilson, who has himself made important contributions to the 
 subject, sums up the present-day view of the matter in the 
 following sentences : — " From the mother comes in the main 
 the cytoplasm of the embryonic body, which is the principal 
 substratum of growth and differentiation. From both parents 
 comes the hereditary basis or chromatin by which these pro- 
 
SEXUAL REPRODUCTION. 1 57 
 
 cesses are controlled, and from which they receive the specific 
 stamp of the race. From the father comes the centrosome to 
 organise the machinery of mitotic division by which the egg 
 splits up into the elements of the tissues and by which each of 
 these elements receives its quota of the common heritage of 
 chromatin. Huxley hit the mark two score years ago when he 
 compared the organism to a web, of which the warp is derived 
 from the female and the woof from the male. What has since 
 been gained is the knowledge that this web is to be sought in 
 the chromatic substance of the nuclei, and that the centrosome 
 is the weaver at the loom." (See "The Cell in Development 
 and Inheritance," 1896, p. 171.) 
 
 The above short sketch will show how intricate, and yet at 
 the same time how orderly, are the intimate processes of fer- 
 tilisation. Variations do indeed occur, both in pathological 
 and in apparently normal cases ; but a general constancy is 
 now both clear and certain, not only for many different animals, 
 but also to a certain extent, as Strasburger and others have 
 shown, for plants. 
 
 § 5- Fertilisation in Protozoa. — In the nascent sexual union observed 
 in many Protozoa, considerable diversity obtains. The individuals which 
 unite may be to all appearance similar (to which cases the term conjugation 
 is generally applied), or they may be materially dimorphic, as in Vorlicella. 
 The union may be permanent, when the two units fuse into one ; or it may 
 only be temporary, during which an interchange of elements takes place. 
 The union may be complete, as in the conjugation of two Gregarines, or 
 partial, as in the slipper-animalcule, and between these may be placed the 
 state of affairs observed in various species of bell-animalcule ( Vorticella), 
 where the nuclei and the bulk of the smaller conjugate pass into the larger, 
 leaving, however, shrivelled remains which are cast off. This, as described 
 by Hs. Wallengren ("Biol. Centralblatt," xix., 1899, pp. 153-161, 
 3 figs.), shows that the distinction between total and partial conjugation is 
 only one of -degree. In many cases the nuclear elements are seen to play 
 an important part, disrupting and reconstructing during the process, while 
 a genuine fusion of the two nuclei has also been observed in permanent 
 conjugation. 
 
 In regard to the interchange of elements, there is considerable diver- 
 gence of observation. Joseph has noted what appears to be an interchange 
 of protoplasm; Schneider has observed the exchange of nuclear elements; 
 while Gruber and Maupas, and Joseph as well, have, in their studies on the 
 union of ciliated infusorians, laid emphasis on an accessory nuclear body, 
 generally known as the " micro-nucleus." This body lies by the side of the 
 larger nucleus, and while the latter simply disrupts and dissolves away, or 
 is extruded without playing any important part, the smaller micro-nucleus 
 divides in a regular way, and with the results there is micro-nuclear inter- 
 change between the two individuals. 
 
 According to Maupas, who has investigated the subject in most detail, 
 
158 THE EVOLUTION OF SEX. 
 
 the para- or micro-nucleus is a "hermaphrodite" sexual element, of sole 
 importance in conjugation. The stages in the process of fertilisation are as 
 follows : — 
 
 (1.) The micro-nucleus increases in size. 
 
 (2.) Division occurs until there are eight micro-nuclei. 
 
 (3.) Of these eight, seven disappear. 
 
 (4. ) The remaining one divides again, differentiating a male and female 
 pro-nucleus. 
 
 (5.) In the next stage, the male elements of the two individuals are 
 exchanged, and the new male nucleus fuses with the original 
 female portion. 
 
 (6, 7-) In two following stages, the nuclear dualism characteristic of the 
 ciliated infusorians is re-established. The old large nucleus 
 (macro-nucleus) has broken up and been eliminated meanwhile. 
 
 (8.) Finally, the individuals, separating from one another, reassume all 
 their original organisation before beginning again to divide in the 
 usual fashion. 
 
 The union of the male and female nuclear elements in ciliate infusorians 
 was admirably figured by Balbiani so long ago as 1858 ; and though he does 
 not seem rightly to have interpreted what he observed in this particular 
 case, he was right in his contention that sexual union and fertilisation really 
 occurred in the Protozoa. Balbiani's view has been for long scouted, and 
 yet, with renewed observation, naturalists have now come back to his con- 
 clusion. Maupas willingly allows that Balbiani figured beautifully what he 
 himself has since reobserved and interpreted. 
 
 The phenomena described by Maupas, as summarised above, have been 
 observed in towards a dozen ciliated infusorians, so that there is every 
 reason to believe in their general occurrence. In three species of the 
 slipper-animalcule (Paramecium), and in species of Stylonichia, Leu- 
 cophrys, Euplotes, Onychodromus, Spirostomum, &c, the facts are as above 
 stated. 
 
 It is of interest to cite the facts in regard to the common bell-animalcule 
 ( Vorlicella), because here the conjugating individuals are like ovum and 
 sperm in more ways than one. In some species — e.g., V. monilata — the 
 adult divides equally, to form two small individuals, which conjugate with 
 those of normal size. In V. microstoma there is again division into two, 
 but the products are of unequal size; one is much smaller than the other. 
 In the nearly allied Carchesium polypinum, the divisions are equal, but 
 they are repeated twice or thrice. The result in all cases is the production 
 of minute individuals, which eventually attach themselves to adults of the 
 normal size, first to the stalk, and then to the body. The accessory nuclear 
 bodies divide as usual ; the large individual ceases to feed, and hermetic- 
 ally closes its mouth, like an ovum when fertilised. The small individual 
 is gradually absorbed by the larger, as sperm by ovum ; and in an intricate 
 but orderly fashion a mixed nucleus results from the fusion of the micro- 
 nuclear elements of the two. The adult then begins to feed, to divide, 
 and so on, as usual. Here then there is (a) incipient dimorphism, (b) 
 absorption of smaller by larger, and (c) intimate nuclear union, — facts which 
 we have already emphasised in the fertilisation of multicellular animals. 
 
 § 6. Origin of Fertilisation. — To understand the origin of 
 the union of sex-cells, attention must still be concentrated on 
 
SEXUAL REPRODUCTION. 1 59 
 
 the Protozoa. That fertilisation really occurs at that low level 
 in a highly complex fashion, we have just seen. It is necessary, 
 however, to note the steps which lead up to what Maupas and 
 others have so patiently elucidated. 
 
 (a) In the primitive life-cycle exhibited by Prototnyxa (see 
 fig. at p. 129), the units which burst forth from the cyst sink 
 down into tiny amcebse, and unite together in numbers to form 
 a composite spreading mass of protoplasm, technically known 
 as a Plasmodium. This is undoubtedly a very primitive union 
 of cells, yet it occurs at very diverse levels in the organic series. 
 It is more or less familiar in the "flowers of tan," one of the 
 lowly Myxomycetes, where a nucleated mass of protoplasm, 
 of composite origin, spreads over the bark in the tan-yard. 
 The plasmodial union also occurs as a definite stage in the life- 
 history of the primitive neighbours of Prototnyxa, the Monera 
 of Haeckel. Pour the liquid contents or body-cavity fluid of a 
 freshly-dredged and still actively living sea-urchin into a bowl; 
 the cells which float in it, like blood-corpuscles in the blood, 
 draw together in clotted masses. Watch the process under a 
 microscope, and the formation of a plasmodium is seen. The 
 dying cells fuse into composite masses, just like the units of 
 Prototnyxa; and it is interesting to observe that, though they 
 are dying, the union provokes a brief but intense renewal of 
 amoeboid activity. To forestall our point, they as it were 
 fertilise each other in articulo mortis. In spite of the objection 
 of Michel and others, that such union, being pathological, is 
 not comparable to the multiple conjugation normal to the 
 myxomycete, we maintain the distinct analogy between the 
 Plasmodium formation in Myxomycetes and that exhibited by 
 the cells in the body-cavity fluid of many animals, and regard 
 this as so much additional evidence of the profound unity of 
 the normal and the pathological processes. Now it is from 
 this primitive union of cells, as illustrated in the lowest organ- 
 isms, that we start in explaining the origin of fertilisation. 
 Just as the very beginning of reproduction seems almost like 
 mechanical breakage, so the very beginning of fertilisation is 
 found in the almost mechanical flowing together of exhausted 
 cells. 
 
 (0) Between this and the process usually described as con- 
 jugation, there are some interesting links. Sometimes as many 
 as three or four spores of lowly Algae club together, as if to 
 gather .sufficient momentum to make a combined start in life. 
 
i6o 
 
 THE EVOLUTION OF SEX. 
 
 The young forms of the sun-animalcule (Adinosphcerium) 
 usually unite in twos, but Gabriel has observed in some cases 
 a multiple union. In another sun-animalcule (Adinophrys sol) 
 two to thirty individuals may unite loosely in what is often 
 called plastogamy, but close union (of nuclei) only occurs 
 between two individuals. So in gregarines (common parasites 
 in invertebrates), while the usual union is certainly dual, Gruber 
 has again observed what may be termed multiple conjugation. 
 Union of three has also been observed as an exception in 
 
 Diagrammatic representation of the stages 
 in the origin of fertilisation, — (I.) Plas- 
 modium ; (II.) multiple conjugation ; 
 (III.) ordinary conjugation ; (IV.) con- 
 jugation of dimorphic cells ; (V.) fertilisa- 
 tion of ovum by spermatozoon. 
 
 several infusorians. The union of more than two may thus be 
 interpreted as intermediate between the formation of plasmodia 
 and the normal dual conjugation. 
 
 (c) Conjugation of two similar unicellular organisms occurs, 
 as we have seen, very generally in the Protozoa, and is also a 
 common fact in the life-history of simple Algse. It is open to 
 every one possessed of a microscope to observe what conjuga- 
 tion means in such a common fresh-water alga as Spirogyra. 
 Opposite cells of adjacent filaments are attracted to one another, 
 and the contents of the one cell pass bodily over into the other. 
 
SEXUAL REPRODUCTION. 
 
 ibi 
 
 In the great majority of cases where conjugation occurs, the 
 uniting cells are to all appearance similar, but it must be 
 remembered that it does not follow from this that they are 
 physiologically alike. 
 
 (d) Both among plants and animals, all naturalists are 
 agreed that it is impossible to draw any line between the con- 
 jugation of similar and the union of more or less dimorphic 
 elements. " This differentiation presents," Sachs says, " espe- 
 cially in Algse, a most complete series of gradations between 
 the conjugation of similar cells and the fertilisation of oospheres 
 by antherozoids, any boundary line between these two processes 
 being unnatural and artificial." The gradual appearance of 
 
 Diagrammatic representation of the contrast 
 between conjugation (horizontal line) and 
 fertilisation (vertical line). 
 
 dimorphism has been already noted in discussing the origin 
 of sex, and need not be re-emphasised. 
 
 (e) Lastly, in fertilisation among higher plants and animals, 
 the two elements which unite are highly differentiated, alike in 
 contrast to one another and in opposition to the general cells 
 of the body. A consideration of the phenomena in loose 
 protist colonies like Volvox or Amputtina, which suggest the 
 bridge between unicellular and multicellular organisms, shows 
 how gradually this latter contrast also may have been brought 
 about. 
 
 To sum up, the steps in the development of the process of 
 fertilisation may be arranged in the following series :— 
 
 (a) The formation of plasmodia. 
 
 (b) Multiple conjugation. 
 
1 62 THE EVOLUTION OF SEX. 
 
 (c) Conjugation of two similar cells. 
 
 (d) Union of incipiently dimorphic cells. 
 
 (e) Fertilisation by differentiated sex-elements. 
 
 One difficulty must in fairness be allowed in connection 
 with the hypothesis of deriving conjugation from plasmodial 
 union. Some years ago, Sachs was inclined to regard the 
 Plasmodium formation of Myxomycetes as a process of multiple 
 conjugation, but he afterwards withdrew this mainly on the 
 ground that the nuclei have not been shown to coalesce. Now 
 there seems no result of studies on fertilisation more certain 
 than that the union of nuclei is an essential fact, but in Plasmo- 
 dium formation, such intimate association of nuclei cannot be 
 asserted. The difficulty of making this a starting-point is thus 
 at first sight considerable. 
 
 Yet it must be observed, (i) that our knowledge of the 
 nuclei in those lowly forms is still very inadequate j (2) that, 
 according to Gruber, the behaviour of the nucleus is sometimes 
 masked by the fact that, instead of existing as a discrete body 
 in the cell, it lies diffusely in the protoplasm ; but especially 
 (3) that it is quite consistent with the general evolutionary con- 
 ception to suppose that the primitive union was of very much 
 less definite character than that subsequently evolved. 
 
 Even in conjugation nuclear union is not always clear. It 
 is well known in the conjugation of Infusorians, but it has been 
 very rarely proved in other Protozoa. It has been observed, 
 however, in some cases, e.g., by Wolters, in the common 
 Monocysiis of the earthworm, and by Schaudinn, in the sun- 
 animalcule, Adinophrys sol. 
 
 Rhumbler has made an elaborate study of the possible 
 evolution of fertilisation-processes. He finds the first step 
 in cytotropism, in which chemotropic substances are secreted 
 between cells, and in this connection we must bear in 
 mind the experiments of Klebs, which show that the addi- 
 tion of various reagents to the culture-solution in which a 
 simple Alga or Mould is living will determine the occurrence 
 of sexual or asexual reproduction. The next step is plasto- 
 gamy, — two naked cells become apposed and fuse. At a 
 higher level, karyogamy is reached. ("Biol. Centralbl," 
 xviii., 1898, pp. 21-26, 33-38, 69-86, IT3-130, 14 figs.). 
 
 § 7. Hybridisation in Animals. — Many of the compound names of 
 animals, such as leopard, point back to a once prevalent belief that animals 
 of very different kinds might unite sexually and have fertile offspring. 
 
SEXUAL REPRODUCTION. 1 63 
 
 Only to a very limited extent is such a notion justified. Every one is aware 
 that by direct human control animals like horse and ass, dog and wolf, 
 lion and tiger, hare and rabbit, canary and finch, pheasant and hen, goose 
 and swan, have been successfully crossed. In nature, however, we know 
 relatively little of the occurrence and results of any such hybridisation. 
 It seems to occur in some fishes; different species of toad are often seen 
 in sexual union; it is said to be not uncommon between various species of 
 birds and insects. 
 
 M. Andre Suchetet, after many years' study of hybridism in birds and 
 mammals, stated the following provisional conclusions ( " Journ. de l'Anat. 
 Physiol.," xxxiii., 1897, pp. 326-355): — 
 
 (1) Cases of hybridism in mammals number about 93, of which 82 are 
 (a) crosses of species of the same genus, and 11 are (b) doubtful cases of 
 crosses between members of different genera. There is no instance" (c) of 
 crossing between members of distinct families or widely-separated genera. 
 Among birds, 262 cases are recorded, 178 of the first category (a), 68 of the 
 second (b), and 16 (some doubtful) of the third (c). 
 
 (2) Of the 82 crosses between mammals of distinct species but of the 
 same genus, the great majority (62) resulted in sterile offspring. In about 
 12 cases, the offspring have proved fertile with one of the parent species or 
 with a third species. In 7 or 8 cases the offspring have proved fertile inter 
 se, sometimes for three or four generations. 
 
 Among birds, in 178 crosses between members of distinct species but 
 of the same genus, only 22 resulted in fertile offspring, S inter se, the 
 others with the parent species, or with a third species, or with other 
 hybrids. 
 
 Of the 68 crosses between species of different genera but of the same 
 family, only one had offspring fertile with one of the parent species — the 
 male hybrid of Columba livia x Tnrtur tisorius was fertile with the female 
 of the latter species. The female hybrid resulting from the same cross 
 seemed sterile. In two other cases a hybrid of this category fertilised a 
 third species ; in another case it was fertilised by this third species. 
 
 (3) As to the causes of the sterility in the hybrid offspring, the repro- 
 ductive organs are sometimes atrophied, in other cases the ducts are 
 abnormal, but there remain many instances in regard to which we can only 
 shroud our ignorance with the word " constitutional." 
 
 In some cases hybridisation succeeds readily, in other cases it is very 
 difficult to bring it about. Thus Mr H. M. Vernon found that in some 
 Echinoderms — Sphtzrechinus and Strongylocentrolus lividus — hybridisation 
 occurred very readily and was highly successful. In some nearly related 
 frogs, on the other hand, it always fails. In certain cases the cause of the 
 difficulty is almost mechanical ; thus Pfliiger showed that the spermatozoa 
 of liana fusca, which have very pointed heads, thinner than those of related 
 forms, can fertilise the eggs of nearly all other species {R. arvalis, R. escu- 
 lenta, and Bufo communis), but the blunt, thick-headed spermatozoa of 
 R. arvalis and R. escuienta cannot fertilise the eggs of any other species. 
 On the other hand, Hertwig's experiments on sea-urchins point to the 
 conclusion that the state of the egg is very important in determining 
 whether the hybridising will succeed or not. Eggs in good condition 
 resist the entrance of spermatozoa to which the stale ova prove receptive. 
 It should also be noticed that fertilisation and segmentation may occur 
 without further development. This was Bom's experience in many cases 
 
164 THE EVOLUTION OF SEX. 
 
 with amphibians, and T. H. Morgan had the same result with the ova of 
 a starfish fertilised by the spermatozoa of * sea-urchin ; segmentation pro- 
 ceeded, a hybrid gastrula was formed, but no further progress was made. 
 
 There is no doubt that at least many species-hybrids tend to sterility, 
 but this is exhibited in varying degrees. The male mules are always 
 sterile, but some say that the females may be successfully impregnated by 
 horse or ass. In many cases hybrids are not fertile with one another, but 
 remain so with the parent form. In a few cases the reproductive functions 
 seem for a time at least to be exaggerated rather than diminished as the 
 result of crossing. 
 
 It seems also certain that while variety-hybrids are usually fertile, their 
 constitution is often unstable. They are often very variable, and apt to 
 die out, as has been repeatedly observed in the human species. The ill- 
 natured saying, " God made the white man, God made the black man, the 
 devil made the mulatto," refers to the frequently inconvenient variability 
 of those variety-hybrids. It is impossible, however, to generalise this. 
 All that can be said is that some cross-fertilisations are very disadvan- 
 tageous, while others seem to be as markedly the reverse. 
 
 Brooks has laid considerable emphasis on the variability of hybrids in 
 connection with his theory of heredity. "Hybrids and mongrels," he 
 says, " are highly variable, as we should expect from the fact that many 
 of the cells of their bodies must be placed under unnatural conditions, and 
 must therefore have a tendency to throw off gemmules. " " Hybrids, from 
 forms which have been long cultivated or domesticated, are more variable 
 than those from wild species or varieties, and the children of hybrids are 
 more variable than the hybrids themselves." " But domesticated animals 
 and plants live under unnatural conditions, and they are tfier eArg* more 
 prolific of gemmules than wild species ; and as the body of a male hybrid 
 is a new thing, the cells will be much more likely than those of th^p pure 
 parent to throw off gemmules. The fact that variation is due to the. male 
 influence, and that the action upon the male parentj>f unnatural or 
 changed conditions results in the variability of the chilcT, is well shown by 
 crossing the hybrid with the pure species ; for when the male hybrid is 
 crossed with a pure female, the children are much more variable than 
 those born from a hybrid mother by a pure father." It cannot be said, 
 however, that the evidence is as yet sufficient to warrant these general 
 conclusions. 
 
 In successful hybridisation, three results are common :— (a) a blending 
 of the parental characters, (£) more or less exclusive expression of the 
 characters of one parent, and (c ) a form quite unlike either parent. What 
 direction the new variation will take cannot be predicted, but in many 
 cases the result is a re-appearance of the characters of an ancestral form. 
 In some cases this may mean that latent characters which have for a time 
 been unexpressed are permitted to develop j in other cases it may mean 
 that a new permutation of qualities has independently reproduced an old 
 pattern or combination. Herr von Guaita found that if the Japanese 
 dancing mouse was crossed with an albino, the second generation consisted 
 of grey mice like the wild forms. (" Ber. Nat. Ges. Freiburg," x., 1898, 
 pp. 3I7-33 2 -) 
 
 Professor Cossar Ewart took a pure white fantail cock-pigeon, which in 
 colour had proved prepotent over a blue pouter, and paired it with a cross 
 previously made between an "owl" and an " archangel," and the result 
 
SEXUAL REPRODUCTION. 1 65 
 
 was a couple of fantail-owl-archangel crosses, one resembling the Shetland 
 rock-pigeon and the other the blue rock of India. Again, a smooth-coated 
 white rabbit, derived from an Angora and a smooth-coated white buck, 
 was mated with a smooth-coated, almost white doe (grand-daughter of a 
 Himalaya doe) ; and the result was that in a litter of three, one was the 
 image of the mother, a second was an Angora, like the paternal grand- 
 mother, and one was a Himalaya, like the maternal great-grandmother. 
 For further details "The Penycuik Experiments" (1899) should be con- 
 sulted. 
 
 The hybrid offspring often resembles one of its parents much more than 
 the other. Thus Standfuss found that in reciprocal crossing the male is able 
 to transmit the characters of its species in a higher degree than the female. 
 A reverse result is noted below. 
 
 The relative maturity of the two sex-elements has been shown by 
 Vernon to be of importance in the hybridisation of sea-urchins. The 
 characters of the offspring incline to be those of the species whose 
 elements were relatively the more mature when fertilisation occurred. 
 
 Standfuss has also noted that the freshly hatched larva often closely 
 resembles the female parent ; that with growth a resemblance to the male 
 parent gradually increases; and that the final extent of approximation 
 towards the male parent depends on the relative phylogenetic age of the 
 two species, the older being able to transmit its characters, whether of 
 structure or habit, better than the young. 
 
 In pairing normal forms with varieties and local races, Standfuss found 
 (1) that when the norm (" Grundart") is crossed with a gradually formed 
 local race of the same species, the result is u series of intermediate forms ; 
 but (2) that when the norm is crossed with a sporadic variety, the result in 
 many cases is that the issue agrees either with the norm or with the sport, 
 intermediate forms being absent. (" Handbuch der pal'aarktischen Gross- 
 Schmetterlinge," 2nd ed., Jena, 1896.) 
 
 In short, the result seems to depend on the issue of what may be called 
 the germinal struggle between hereditary characters of varying strength. 
 
 The results reached by Standfuss are not altogether corroborated by 
 other workers. Thus. J. W. Tutt gives an account ("Trans. Entomological 
 Society, London," 1898, pp. 17-42) of experiments made by Riding and 
 Bacot in hybridising two allied species of Tephrosia — T. bistortata Goeze 
 (crepuscularia auct.) and T. creptiscularia Hb. (biundularia auct.). The 
 hybrids show all degrees between full fertility and complete sterility ; they 
 may be fertile inter se and with the parent stock ; the phylogenetically older 
 species is more dominant in stamping its characters on the progeny and the 
 female parent more than the male ; a recently formed variety may be pre- 
 potent over the type from which it has sprung. The re-crossing of a hybrid 
 with one of the parent species produces offspring scarcely differing from that 
 parent species ; the inbreeding of the same cross produces a large percent- 
 age differing much from either parent form; the crossing of the hybrids 
 obtained from original reciprocal crosses tends to produce a mixed progeny, 
 some referable to known forms of the crossed species, others quite unlike 
 anything ever obtained in nature. 
 
 Henri Gadeau de Kerville calls attention to an interesting conclusion — 
 requiring, however, to be more carefully substantiated — that the results 
 of successful hybridisation are much oftener males than females, and 
 that male offspring are more numerous in proportion to the specific distance 
 
1 66 THE EVOLUTION OF SEX. 
 
 between the two parents ("Bull. Soc. Zool. France," xxiv., 1899, 
 PP- 49-51). 
 
 The early researches of Kolreuter (1761) gave a firm basis to the study 
 of hybridisation among plants. The comparative easiness of experiment 
 has advanced the botanical side of the subject to far greater certainty than 
 the zoological conclusions can pretend to. Among plants, as we should 
 expect from their greater vegetativeness, the fertility of hybrids seems 
 frequently established. Knight, Gartner, Herbert, Wichura, and others, 
 have brought together a great number of reliable observations, and 
 the whole subject has been admirably discussed by Nageli. For a 
 copious resume of the general results, for the most part after Nageli, the 
 student may be referred to chap. vi. of Sachs' Text-book of Botany, while 
 Wallace's "Darwinism" should be consulted for its rediscussion of hybri- 
 disation in animals. 
 
 § 8. Telegony. — The belief has been for long current among breeders that 
 the effective impregnation of a female may influence not only the immediate 
 offspring, but subsequent offspring by a different sire. It is said that a 
 pure-bred bitch lined by a mongrel dog is thereby spoiled for future breed- 
 ing. The supposed influence is technically called telegony, and the supposed 
 facts are usually explained by supposing that the surplus sperms in the first 
 impregnation exert an influence on the immature ova in the ovary, or by 
 supposing (in mammals) that the mother is affected by her first offspring 
 through the medium of the placenta. The first point, however, is to make 
 sure that there are facts to be explained, and this seems very doubtful. 
 What has been regarded as the influence of the first sire on the offspring of 
 the same mother by a second sire may receive some other explanation, may 
 be, for instance, an independent variation, or may be simply a reversion. 
 Much, if not most, of the alleged evidence is anecdotal; and careful 
 experiments made by Professor Cossar Ewart, on the lines of Lord Morton's 
 famous case, have yielded no evidence of the reality of telegony. 
 
 Dr Otto vom Rath has suggested (" Biol. Centralbl.," xviii., 1898, pp. 
 637-642) that the occurrence of badly-bred pups in a litter, which breeders 
 regard as the result of telegony, may be accounted for by co-infcetation, in 
 which case different sires are supposed to fertilise the ova which may be 
 shed into the oviduct at intervals of several days. But this, again, requires 
 further proof. 
 
SEXUAL REPRODUCTION. 167 
 
 SUMMARV. 
 
 1. Reproduction is but more or less discontinuous growth, 
 
 2. Sexual reproduction normally implies (a) special reproductive cells, 
 distinct from the body; (b) the dimorphism of these cells; (c) their 
 physiological dependence, — the ovum being unproductive without the 
 spermatozoon, and vice versA. 
 
 3. The discoveries of Camerarius, Amici, Kolreuter, Sprengel, and 
 others, laid the foundations of our knowledge of sexual reproduction 
 in plants. 
 
 4. The history of research on fertilisation in animals well illustrates the 
 gradually increasing precision of scientific inquiry. 
 
 5. The conjugation processes seen in Protozoa are of much importance 
 in suggesting the origin of differentiated fertilisation. 
 
 6. The origin of fertilisation may be traced through the following 
 grades : — [a) plasmodial union, i,b) multiple conjugation, {c) ordinary 
 conjugation, (d) union of dimorphic cells, (e) fertilisation of ovum by 
 spermatozoon. 
 
 7 Both in plants and animals hybridisation is often successful, but the 
 offspring frequently tend to be sterile. This, however, must not be 
 exaggerated. 
 
 8. Telegony has not been demonstrated. 
 
 LITERATURE. 
 
 See the already noted works of Balfour, Van Beneden, Carnoy, Geddes, 
 
 Haddon, Hensen, Hertwig, M'Kendrick, Sachs, Vines, and Wilson. 
 
 For recent papers see Zoological Record, from 1886; and Journal of 
 
 Royal Microscopical Society. 
 
 For bibliography of hybridism in plants see M. Abbado in Nuovo 
 
 Giorn. Bot. Ital., V., 1898, pp. 76-105, 265-303. 
 
 Ackermann, K. — Die Thierbastarde. Ber. Verein Kassel., 1897, pp. 
 103-121. 
 
 Born, G. — Beitrage zur Bastardirung zwischen den einheimischen Anuren- 
 arten. Pfliiger's Archiv., XXXII., 1883, and Archiv. Mikr. Anat., 
 XXVII., 1886. 
 
 Ewart, J. C. — The Penycuik Experiments. 1899. 
 
 Hartog, M. — Some Problems of Reproduction. Quart. Journ. Micr. 
 Sci., 1891. 
 
 JOHNSON. — The Plastogamy of Actinosphjerium. J. Morphol., IX., 189=;. 
 
 K.LEBS, G. — Zur Physiologie der Fortpflanzung einiger I'ilze. Jahrb. wi«. 
 Botanik, XXXIII., 1899. 
 
 Knuth, P Handbuch der Bliitenbiologie. 1898, 3 vols. See especially 
 
 Vol. I. for literature (from II. Midler to 1898), and for general discus- 
 sion of pollination, cleistogamy, parthenogenesis, &c. 
 
 Kohlbrugge, J. F. H. — Der Atavismus. Utrecht, 1897, 31 pp. 
 
 Kohlwey, H. — Arten- und Rassenbildung. Eine Einiiihrung in das 
 Gebiet der Tierzucht. Leipzig, 1897, 72 pp., 5 figs. 
 
1 68 THE EVOLUTION OF SEX. 
 
 Mobius, M. — Beitrage zur Lehre von der Fortpflanzung der Gewachse. 
 
 Jena, 1897, VI., and 212 pp., 36 figs. 
 Morgan, T. H. — The Development of the Frog's Egg. New York, 
 
 1897, 192 pp., 51 figs, (with bibliography). 
 
 Muller, H. — Fertilisation of Flowers. Translation by D'Arcy W. 
 
 Thompson. London, 1883. 
 Nussbaum, M. — Beitrage zur Lehre von der Fortpflanzung. Arch. Mikr. 
 
 Anat., XLI., 1893, pp. 1 19-145. 
 Pfluger, E. — Die Bastardzeugung bei den Batrachien. Pfliiger's 
 
 Archiv., XXIX., 1882; cf. Pfluger and Smith, op. cit., XXXII., 
 
 1883. 
 Reibmayer, Albert. — Inzucht und Vermischung beim Menschen. 
 
 Leipzig und Wien, 1897, 268 pp. 
 Reul. — Les Unions Consanguines ; Histoire de la Creation des Races 
 
 Celebres. Ann. Med. Ve'terinaire, XLVI. , 1897, pp. 1-8 et seq, 
 Rhumbler.— Beitrage zur Kenntniss der Rhizopoden. Zeitsch. f. wiss. 
 
 Zool., LXI., 1895. 
 Schaudinn, Fr. — Ueber die Copulation von Actinophrys sol. Sitzber. 
 
 Akad. Preuss. Berlin, 1896, pp. 83-89, 6 figs. 
 Standfuss, M. — Handbuch der palaearktischen Gross-Schmetterlinge 
 
 (2nd ed.), Jena, 1896; cf. F. A. Dixey, Science Progress, VII., 
 
 1898, pp. 185-202. 
 
 Suchetet, A. — Problemes hybridologiques. Journ. de l'Anat. Physiol., 
 
 XXXIII., 1897, pp. 326-355- 
 Des Hybrides a l'etat sauvage, Tome I. Oiseaux, 8 e . Paris, 
 
 CLII., 1001 pp., 1897. 
 Vernon, H. M. — P. Roy. Soc. London, LXIIL, 1898, pp. 228-231. 
 Verworn, M. — Biologische Protisten-Studien. Zeitschr. f. wiss. Zool., 
 
 L., 1890. 
 Wager, H. — The Sexuality of the Fungi. Annals of Botany, XIII., 
 
 1899, PP- 575-597- 
 
 Ward, M. — On the Sexuality of the Fungi. Quart. Journ. Micr. Science, 
 
 XXIV., 1884. 
 Wolters, M. — Die Conjugation und Sporenbildung bei Gregarinen. Arch. 
 
 Mikr. Anat., XXXVII., 1891. 
 
CHAPTER XII. 
 
 Theory of Fertilisation. 
 
 In his 49th Exercitation on the " efficient cause of the 
 chicken," Harvey thus quaintly expresses what has always 
 been, and still is, a baffling problem : — " Although it be a 
 known thing subscribed by all, that the fcetus assumes its 
 original and birth from the male and female, and consequently 
 that the egge is produced by the cock and henne, and the 
 chicken out of the egge, yet neither the schools of physicians 
 nor Aristotle's discerning brain have disclosed the manner 
 how the cock and its seed doth mint and coine the chicken 
 out of the egge." 
 
 § 1. Old Theories of Fertilisation. — (a) From Pythagoras 
 and Aristotle on to the " Ovists," of whom we have already 
 spoken, numerous naturalists have held the opinion that the 
 ovum was the all-important element, which only required to be 
 awakened to development by contact with the male fluid or 
 male elements. It must be allowed, that while ova may 
 exceptionally develop without sperms, the latter never come to 
 anything apart from ova. 
 
 (b) In contrast to the above opinion, we find ingenious 
 thinkers, so widely separate in time as Democritus and Para- 
 celsus, regarding the male fluid as very important, forestalling 
 Buffon and Darwin in fact in considering it in a sense an 
 extract or concentrated essence of the whole body. But it was 
 only after the spermatozoa were themselves detected that their 
 importance became unduly exaggerated, in the minds of those 
 who seem almost to have been nicknamed "animalculists." 
 It seems probable enough that Leeuwenhoek himself (1677) 
 saw the spermatozoon entering the ovum, — he at least said 
 that he did,— but that did not prevent him from ascribing to 
 the male elements all the credit of development. This became, 
 as we have seen, a favourite hypothesis, and imagination sup- 
 plied more than modern magnifiers to those observers who 
 
17° THE EVOLUTION OF SEX. 
 
 detected in the spermatozoon the members and lineaments of 
 the future organism. 
 
 (c) The third opinion, that both elements are of essential 
 and inseparable import, is obviously alone consistent with the 
 facts. This view also has had its gradual development, only 
 one phase of which need be noticed. Even after the nature of 
 the spermatozoa as male-cells was recognised, that is to say, 
 even since the middle of the nineteenth century, an old con- 
 ception of the male influence lingered persistently. This 
 namely, that contact was not essential, but that a " sort of 
 contagion," a "breath or miasma," "a plastical vertue," 
 " without touching at all, unless through the sides of many 
 mediums," was sufficient to effect what we call fertilisation. 
 The above expressions are used by Harvey, who further says, 
 " this is agreed upon by universal consent, that all animals 
 whatever, which arise from male and female, are generated by 
 the coition of both sexes, and so begotten as it were per con- 
 tagium aliquod." De Graaf attempted in vain to give more 
 precision to this " contagion " in his theory of an " aura 
 seminalis," or seminal breath which passed from the male fluid 
 to the ovum. But the conception of an " aura " was only a 
 verbal cloak for that absence of definite knowledge which the 
 slow progress of observation still necessitated. The theory 
 was partly strengthened by a number of erroneous observa- 
 tions, which seemed to show that successful fertilisation could 
 occur when the genital passages of the female were apparently 
 blocked by malformation or disease. Spallanzani gave a death- 
 blow to the theory of an " aura," by showing experimentally 
 that contact of the male fluid with the ovum was absolutely 
 necessary. Even he, however, went away from the true con- 
 clusion, by maintaining that the fertile male fluid of toads was 
 destitute of spermatozoa. That the above vague conceptions 
 have been replaced by the certain conclusion, that intimate 
 cellular union is the sine qua non of fertilisation, we have 
 already emphasised. 
 
 § 2. Modern Theories of Fertilisation — Morphological. — 
 Recent investigators of the facts of fertilisation have generalised 
 their results in different ways according to their dominant bias. 
 Some mainly restrict themselves to stating the morphological 
 facts, and to emphasising the relative importance of cell-sub- 
 stance and of nuclei in the union ; others attack the deeper 
 problem of the physiological import of the process, — a problem 
 
THEORY OF FERTILISATION. I71 
 
 the full solution of which is still remote; while others have 
 confined themselves rather to discussing the uses of fertilisation 
 in relation to the species. Some representative positions on 
 each of these planes must be sketched ; and, first of all, the 
 more morphological theories, and the very important question 
 whether the union of nuclei is everything, or whether the union 
 of cell-substance has also its import. 
 
 (a) Heriwig , s View. — Professor O. Ilertwig, who was one of the first 
 carefully to follow out the details of fertilisation in animals, thus sums up 
 his " Theorie der BefruchHmg" : — "In fertilisation, distinctly demon- 
 strable morphological processes occur. Of these the important and essen- 
 tial one is the union of two sexually differentiated cell-nuclei, the female 
 nucleus of the ovum and the male nucleus of the sperm. These contain 
 the fertilising nuclear substance, which is an organised substance, and acts 
 as such in the process. The female nuclear substance transmits the 
 characters of the mother, the male nucleus those of the father, to the 
 offspring." The nucleus is thus the essential element both in fertilisation 
 and in inheritance. 
 
 (b) Sirasbuiger's View. — What Hertwig maintains for animals, Stras- 
 burger does for plants. "The process of fertilisation depends upon the 
 union of the sperm nucleus with the nucleus of the egg-cell; the cell sub- 
 stance (cytoplasm) does not share in the process." " The cell-substance of 
 the pollen-grain is only the vehicle to conduct the generative-nucleus to its 
 destination." It may become nutritive, he allows however, to the germ- 
 rudiment. " Generally the uniting nuclei are almost perfectly alike," 
 though there may be slight differences in the size of the nucleoli. " The 
 two cell-nuclei do not differ in their nature, they are not sexually differen- 
 tiated in the ways that the individuals are from which they originate. All 
 sex-differentiations only serve to bring together the two nuclei essential to 
 the sexual process." 
 
 The opinions of these two authorities are certainly representative, and 
 they both agree in emphasising that the nuclei are all-important, and that it 
 does not matter much about the union of cell-substance. Some objections 
 to this view must be noticed, (a) It is permissible to doubt whether the 
 recent concentration of attention upon the nucleus has not led to some 
 under-appreciation of the general protoplasm. In the permanent conjuga- 
 tion of two cells, the entire contents of the two cells are obviously fused ; 
 and even when the union is temporary, Joseph has observed what looks 
 like an interchange of protoplasmic as well as of nuclear substance, (b) 
 There are a few observers still, such as Nussbaum, who maintain that in 
 fertilisation in animals the substance of the sperm is important as well as 
 its nucleus, [c) Strasburger notes the minimal quantity of cell-substance 
 so often present round the male nucleus, and urges that if it were im- 
 portant there would surely be more of it. But it is quite conceivable that 
 a minimal quantity of highly active protoplasm might have, like a ferment, 
 a momentous influence on a large quantity of a different character, (a?) It 
 is, moreover, a very probable view that cytoplasm and nucleus attain their 
 full significance only in inter-relation, forming what has been called a 
 "cell-firm." (e) Boveri made the delicate experiment of removing the 
 nucleus from a sea-urchin ovum which he afterwards fertilised. Although 
 
172 THE EVOLUTION OF SEX. 
 
 the ovum had therefore only paternal nuclear material it developed into a 
 larva. More recently Delage has extended the experiments and has reared 
 normal larvae of sea-urchin, worm (Lanice), and mollusc (Dentalium) from 
 non-nucleated ovum-fragments which were successfully fertilised. He 
 describes this remarkable phenomenon under the title of merogony. (/) 
 The researches of Boveri and others show that the sperm brings with it 
 into the ovum a protoplasmic centre — a " centrosome " — which appears 
 to be of much importance in the preparation for division. In this 
 preparation, according to Boveri, the "muscular fibrils " of a special kind 
 of protoplasm (or archoplasm) literally move the nuclear elements. 
 " The movement of the elements is wholly the result of the contraction of 
 the attached fibrils, and the final arrangement of these nuclear elements in 
 the ' equatorial plate ' is the result of the action of the archoplasmic 
 sphere exerted through the fibrils." Now this specially active protoplasm 
 has its centre in the two central corpuscles, each "ruling a sphere of 
 archoplasm." There seems general agreement as to the fact that the 
 sperm furnishes the centrosomes in at least the majority of cases, and there 
 is no doubt that these minute bodies play a prominent part in the division 
 which follows fertilisation. At this stage, then, it seems rash to deny that 
 even the minimal cell-substance of the spermatozoon may, as well as its 
 nucleus, have a momentous influence in fertilisation. 
 
 § 3. Physiological Theories of Fertilisation. — The morpho- 
 logical facts, established and verifiable by observation, form 
 the basis from which to attack the deeper problem of the 
 physiology of fertilisation. Here experiment is almost insuper- 
 ably difficult; only a few incidental results are as yet available; 
 the suggestions thrown out by various naturalists must there- 
 fore be appreciated according to their consistence with the 
 general principles of physiology, and with the general theory of 
 sex and reproduction. To some they may still appear a page 
 of probabilities. 
 
 Sachs compares the action of the male element upon the 
 egg-cell to that of a ferment. De Bary also suggests that pro- 
 found chemical differences exist between the two elements. 
 Very suggestive is the view of E.olph, who regarded the process 
 as essentially one of mutual digestion. His vivid words well 
 deserve quotation : — 
 
 "Conjugation occurs when nutrition is diminished, whether this be due 
 to want of light, or to the lowered temperature of autumn and winter, or 
 to a reduction of the organisms to minimal size. It is a necessity for 
 satisfaction, a gnawing hunger, which drives the animal to engulf its 
 neighbour, to 'isophagy.' The process of conjugation is only a special 
 form of nutrition, which occurs on a reduction of the nutritive income, or 
 an increase of the nutritive needs, in consequence of the above-mentioned 
 conditions. It is an 'isophagy,' which occurs in place of 'heterophagy.' 
 The less nutritive, and therefore smaller, hungrier, and more mobile 
 organism we call the male, — the more nutritive and usually relatively more 
 
THKORY OF FERTILISATION. 173 
 
 quiescent organism, the female. Therefore too is it, that the small starving 
 male seeks out the large well-nourished female for purposes of conjugation, 
 to which the latter, the larger and better nourished it is, is on its own 
 motive less inclined." Cienkowski has also inclined to a similar view, 
 regarding conjugation as equivalent to rapid assimilation. 
 
 In Holothuria tubulosa and some other Echinoderms, N. Iwanzoff has 
 observed ("Bull. Soc. Nat. Moscou," 1897, pp. 355-367, 1 pi.) that 
 immature ova, after a certain stage, show great sexual attraction for 
 spermatozoa, and emit many pseudopodia, while the mature ovum only 
 emits one. But the spermatozoa absorbed by the immature ova are 
 digested, which leads the author to the speculative suggestion that the 
 process of maturation weakens the nutritive vitality of the ovum so that it 
 is unable, fortunately, to digest the spermatozoon. 
 
 Simon also seeks to establish the following among other vague con- 
 clusions : — Sexuality has, he says, arisen twice (we should say much oftener), 
 once among plants, again among Protozoa. Two similar cells unite "in 
 order to reach the limit of their individuality." In both kingdoms the 
 union is at first protective, though in a different fashion in the two cases. 
 In the progressive differentiation, these two sex-cells are usually so con- 
 structed that the loss of substance in the union is reduced to a minimum, 
 hence the small mobile male and the large quiescent female cells. The 
 union brings about a chemico-physical process, which makes the female cell 
 capable of independent nutrition and growth, and evokes potential proper- 
 ties into actual life. 
 
 In marked contrast to Rolph's suggestion, and the view of 
 all those who believe that the sex-cells are profoundly different, 
 is the opinion maintained by Weismann. He denies that there 
 is a dynamical action in fertilisation. The momentous effect is 
 merely a restoration of the normal composition of the nucleus. 
 "The physiological values of sperm and egg-cell are equal; 
 they are as i : i. We can hardly ascribe to the body of the 
 ovum a higher import than that of being the common nutritive 
 basis for the two conjugating nuclei." The external differences 
 which are so obvious are only important as means towards the 
 conjugation of similar nuclei. " The germ-plasm in the male 
 and female reproductive cells is identical." Previous to the 
 essential moment of fertilisation, the germ-plasm is reduced in 
 maturation. Development will not take place unless the loss 
 be made good. This is what the sperm does in fertilisation. 
 In short, to Weismann the process is quantitative rather than 
 qualitative. 
 
 This supposition appears to us to be open to criticism. (1.) That the 
 nuclei are alone important in fertilisation, and that the cell substance is a 
 mere adjunct, cannot be said to be proved, and we have already noted some 
 of the facts which tell the other way. (2.) The structure of a cell is 
 recognised by all to be an expression of its dominant protoplasmic pro- 
 cesses. The sex-cells are usually highly dimorphic, and even Strasbnrger 
 
174 THE EVOLUTION OF SEX. 
 
 allows that there may be minor differences in their nuclei, as well as the 
 marked divergence in their cell -substance. The nucleus cannot be 
 regarded as an isolated element, but as one which shares in the general 
 life of the cell. We have already interpreted the differentiated male and 
 female cells as respectively katabolic and anabolic, and see no reason for 
 doubting, in spite of structural resemblance in the rough features of nuclei 
 (all that we know), that this difference saturates through the elements. 
 (3.) If the only important matter be the quantitative restoration of the 
 original amount of germ-plasma in the female nucleus, it seems difficult to 
 understand the phenomena of conjugation, whether permanent or transitory, 
 from which we believe fertilisation to have originated. (4.) The occasional 
 possibility of inducing division by replacing the sperms with other stimuli, 
 seems to point to a dynamical or chemical action. Thus Loeb induced 
 artificial parthenogenesis in sea-urchin ova by placing them for a couple of 
 hours in sea-water, to which some magnesium chloride had been added. 
 (S-) Delage's evidence that non-nucleated portions of ova may be readily 
 fertilised and form normal larvae, strongly suggests that the mingling of 
 heritable qualities, which is certainly one of the results of fertilisation, must 
 be distinguished from the physiological stimulus to division. 
 
 It is very desirable that the experiment which Fieri has begun of 
 extracting a ferment (ovulase) from seminal matter arid using it as a 
 fertilising stuff, should be confirmed or confiUed. 
 
 It has been already noted, in regard to the origin of fertilisa- 
 tion, that the almost mechanical flowing together of exhausted 
 cells is connected by the stages of multiple conjugation with 
 the ordinary form of the latter, while the respective differentia- 
 tion of the two elements effects the transition to fertilisation 
 proper. Historically, then, fertilisation may be compared to 
 mutual digestion, and, though bound up with reproduction, it 
 may have arisen from a nutritive want. With the differentiation 
 of the elements on anabolic and katabolic lines, the nature of 
 the fertilising act becomes more definite. The essentially 
 katabolic male cell, getting rid of all accessory nutritive material 
 contained in the sperm-cap and the like, brings to the ovum a 
 supply of characteristic products which stimulate the latter to 
 division. The profound chemical differences, surmised by 
 some, are intelligible as the outcome of the predominant 
 anabolism and katabolism in the two elements. The union 
 of the two sets of products restores the normal balance and 
 rhythm of cellular life. At the same time, it is of course 
 certain that the spermatozoon is the bearer of the inheritable 
 paternal characteristics. 
 
 § 4. Uses of fertilisation to the Species. — Not a few natu- 
 ralists have passed from the individual aspect of fertilisation to 
 its general import in relation to the life of the species. Why 
 
THEORY OF FERTILISATION. 175 
 
 should fertilisation occur at all, if parthenogenesis in some 
 cases works so well ? Part of this question is almost illegitimate, 
 if the existence of male and female be, as we think, simply the 
 expression of a more developed swing of "the organic see- 
 saw" between anabolism and katabolism. The answers have, 
 however, much interest, and are valuable, so long as they are 
 not magnified so as to hide the deeper physiological problems 
 lying below. The origin and physiological import of fertilisa- 
 tion can never be explained by any elucidation of its subsequent 
 advantagebusness. 
 
 The two naturalists who have recently reached the most 
 valuable results in regard to the results of fertilisation are 
 Maupas and Weismann. This they have done by very different 
 paths, — Maupas, in working out the details of conjugation in 
 infusorians ; Weismann, in his wider studies on the problems 
 of heredity and evolution. To Maupas, fertilisation is necessary 
 to prevent the death of the species ; to Weismann, fertilisation 
 is the ever-recurrent beginning of new vital changes, and the 
 continual preservation at the same time of the relative con- 
 stancy of the species. Several naturalists of the highest reputa- 
 tion have regarded fertilisation as a process which supplied a 
 fresh life-impulse to the species. Thus Galton has insisted, 
 with much clearness and force, on the liability of asexual, or 
 what he calls unisexual multiplication to end in degeneration 
 or extinction, and on the necessity of double parentage for 
 the preservation and progress of the species. Similarly, .Van 
 Beneden, Biitschli, and Hensen have all spoken of the process 
 as a rejuvenescence (rejeunissement, Verjiingung). The asexual 
 process of ' cell-multiplication is limited; conjugation in lower, 
 fertilisation in higher organisms supplies the recurrent impulse 
 which keeps the life of the species young. According to Van 
 Beneden, — " The faculty which cells possess of multiplying by 
 division is limited. There comes a time when they can divide 
 no further, unless they undergo rejuvenescence by fertilisation. 
 In animals and plants, the only cells capable of being re- 
 juvenesced are the eggs; the only cells capable of rejuvenescing 
 these are the sperms. All the other parts of the individual 
 are devoted to death. Fertilisation is the condition of the 
 continuity of life. Par elle le geneVateur echappe a la mort." 
 Hensen, in his admirable " Physiology of Reproduction," ex- 
 presses the same when he says: — "By normal fertilisation, 
 death is warded off (ferngehalten) from the germ and its 
 
176 THE EVOLUTION OF SEX. 
 
 products.'-' Biitschli has interpreted conjugation in similar 
 terms. 
 
 Weismann quotes the three opinions just mentioned, and 
 vigorously criticises them. He demands evidence for the 
 limitation of asexual reproduction assumed above, and speaks 
 of the "impossibility of proof." The whole "conception of 
 rejuvenescence has something indefinite and misty about it." 
 " How can one think that an infusorian, which by continued 
 division has at length exhausted its reproductive capacity, will 
 regain the same by uniting and fusing with another which has 
 also lost its power of further division ? Twice nothing cannot 
 give one; or if we assume that in each animal there persists 
 only half the reproductive capacity, so that the two together 
 would form one, this one can hardly call 'rejuvenescence.' It 
 would be simply an addition, as is under other circumstances 
 attained by simple growth, — that is, if we leave out of account 
 what in my eyes is the most important moment in conjugation, 
 viz., the mingling of two heredity-tendencies (Vererbungsten- 
 denzen)." He sarcastically compares the two exhausted 
 individuals to two exhausted rockets, which are supposed to 
 rejuvenesce in mutually affording the constituents of nitro- 
 glycerine. More forcibly he urges the difficulty suggested by 
 continued parthenogenesis, — a difficulty which we shall after- 
 wards have to discuss. "To the conception of rejuvenescence," 
 he says, in conclusion, " I could only agree, if it were proved 
 that multiplication by division can never, — not merely in certain 
 conditions,— but never continue unlimitedly. This cannot, 
 however, be proved, just as little as the reverse." But Weis- 
 mann must surely admit that the demonstration of even some 
 cases where species, normally reproducing asexually, come to 
 an absolute standstill if conjugation be prevented, would give 
 considerable strength to the interpretation of fertilisation as 
 rejuvenescence. Maupas has given examples of such cases. 
 
 The French observer has shown, as we have seen, that a 
 process of fertilisation occurs in ciliated infusorians. By an 
 elaborate process of nuclear division, disruption, elimination, 
 interchange, union, and reconstruction, two " slipper animal- 
 cules" fertilise one another. What is the meaning of all this? 
 
 Each infusorian, after conjugation, proceeds to divide, but 
 the results are to all appearance the same as it previously pro- 
 duced. There is no special sexually produced generation. 
 
 It has been often alleged that the subsequent dividing is 
 
THEORY OF FERTILISATION. 177 
 
 accelerated by conjugation; but Maupas finds that this is not 
 so. The reverse in fact is true, — it is a loss of time. While a 
 pair of infusorians (Onychod/omus grandis) were indulging in a 
 single conjugation, another had become, by ordinary asexual 
 division, the ancestor of from forty thousand to fifty thousand 
 individuals. 
 
 Moreover, the intense internal change preparatory to fer- 
 tilisation, and the general inertia during subsequent reconstruc- 
 tion, not only involve loss of time, but expose the infusorians 
 to great risk. It seems then like a condition of danger and 
 death rather than of multiplication and birth. 
 
 The riddle was, in part at least, solved by a long series of 
 careful observations. In November 1885, M. Maupas isolated 
 an infusorian (Stylonichia pustulate?), and observed its genera- 
 tions till March 1886. By that time there had been two 
 hundred and fifteen generations produced by ordinary division, 
 and since these lowly organisms do not conjugate with near 
 relatives, there had, of course, been no sexual union. 
 
 What was the result ? At the date referred to, the family 
 was observed to have exhausted itself. The members, though 
 not exactly old, were being born old. The asexual division 
 came to a standstill, and the powers of nutrition were also 
 lost. 
 
 Meanwhile, however, several of the individuals, before the 
 generations had exhausted themselves, had been removed to 
 another basin, where they conjugated with unrelated forms of 
 the same species. One of these was again isolated, and 
 watched for five months. The usual number of successive 
 generations occurred ; members removed at different stages 
 were again observed to conjugate successfully with unrelated 
 forms, and this was done on to the one hundred and thirtieth 
 generation. After that, however, the family being again near 
 its end, the removal was no longer any use. About the one 
 hundred and eightieth generation, the strange sight was seen 
 of individuals of the same family attempting to unite with 
 one another. The results were, however, nil, and the con- 
 jugates did not even recover from the effects of their forlorn 
 hope. 
 
 Without the normal sexual union, then, the family becomes 
 senile. Powers of nutrition, division, and conjugation with 
 unrelated forms, come to a standstill. The first symptom is 
 decrease in size, which may go on till the individuals only 
 
 12 
 
178 THE EVOLUTION OF SEX. 
 
 measure a quarter of their normal proportions. Various 
 internal structures then follow suit, " until at last we get form- 
 less abortions, incapable of living and reproducing them- 
 selves." The nuclear changes are no less momentous. The 
 important para- or micro-nucleus may partially or completely 
 atrophy, and conjugation is thus fatally sterile. The larger 
 nucleus may also become affected, " the chromatin gradually 
 disappearing altogether." Physiologically too, the organisms 
 become manifestly weaker, though there is what the author 
 calls a " surexcitation sexuelle." Such senile decay of the 
 individuals and of the isolated family inevitably ends in 
 death. 
 
 The general result is evident. Sexual union in those infu- 
 sorians, dangerous perhaps for the individual life, — a loss of 
 time so far as immediate multiplication is concerned, — is in a 
 new sense necessary for the species. The life runs in cycles 
 of asexual division, which are strictly limited. Conjugation 
 with unrelated forms must occur, else the whole life ebbs. 
 Without it, the Protozoa, which some have called "immortal," 
 die a natural death. Conjugation is the necessary condition 
 of their eternal youth and immortality. Even at this low level, 
 only through the fire of love can the phcenix of the species 
 renew its youth. 
 
 The need of extreme carefulness is shown, however, by the somewhat 
 discrepant results reached by D. Joukowsky ('* Verh. Nat. Med. Verein 
 Heidelberg," vi., 1898, pp. 17-42). After five months' culture his colony 
 of Paramecium caudalum showed no nuclear degeneration, but only a 
 marked reduction of cilia and a consequent sluggishness. In P. putrinum 
 effective conjugation between the descendants of one individual was ob- 
 served, but the author admits the probability that this has its limits. In 
 another infusorian, Pleurotricha lanceolata, over 458 generations were 
 observed without the occurrence of degeneration, and Joukowsky supposes 
 that degeneration depends not on the number of generations merely, but 
 on the rapidity of their successive occurrence. 
 
 At the beginning of this century, the too-much-forgotten 
 biologist Treviranus directed attention to fertilisation as a 
 source of variation, and his suggestion has been several times 
 independently revised. 
 
 Thus Brooks, to whose works we have repeatedly referred, 
 has emphasised not only the importance of fertilisation as a 
 source of progressive change, but further, that the male ele- 
 ment is much the more important in this connection. 
 
 Similarly, though on somewhat different lines, Weisrnann 
 
THEORY OF FERTILISATION. 1 79 
 
 suggested that the mingling of ni..'c and female germ-plasms 
 was a source of those variations on which natural selection 
 operates. " Sexual reproduction is well known to consist in 
 the fusion of two contrasted reproductive cells, or perhaps 
 even in the fusion of their nuclei alone. These reproductive 
 cells contain the germinal material or germ plasm, and this 
 again, in its specific molecular structure, is the bearer of the 
 hereditary tendencies of the organisms from which the repro- 
 ductive cells originate. Thus in sexual reproduction, two 
 hereditary tendencies are in a sense intermingled. In this 
 mingling, I see the cause of the hereditary individual char- 
 acteristics; and in the production of these characters, the task 
 of sexual reproduction. It has to supply the material for the 
 individual differences from which selection produces new 
 species." 
 
 Few will be inclined to oppose the thesis that sexual 
 union is productive of variation. To discuss the relations 
 of this view to other theories of variation is beyond our 
 present scope. Thus Weismann has suggested that germinal 
 variations may also arise because of the reducing divisions 
 which precede fertilisation or amphimixis, as he calls it, and 
 furthermore through nutritive and other stimuli which operate 
 through the body upon the germ-plasm. We should also 
 mention Hatschek's suggestion that sexual reproduction is a 
 remedy against the operation of injurious variations. We can 
 readily imagine that the excess of some particular line of 
 anabolic or katabolic differentiation may be neutralised through 
 fertilisation, and it may be in this way that the pairing of 
 diseased individuals is sometimes mercifully condoned by 
 nature. 
 
 Mobius may be cited here for his vigorous protest against 
 the prevalent idea that continuous vegetative multiplication 
 necessarily results in degeneration. He instances such cases 
 as the banana and date palm, which are never reproduced 
 sexually in cultivation, but it must be remembered that arti- 
 ficial selection is here in operation, to sustain the standard. 
 He admits, however, the advantages of sexual reproduction 
 both in conserving the specific characters and in prompting 
 new variations. ("Beitrage zur Lehre von der Fortpflanzung 
 der Gewachse." Jena, 1897.) 
 
 In regard to close in-breeding we have still much, need of 
 experiment, but it seems fairly certain that with a healthy 
 
ISO THE EVOLUTION OF SEX. 
 
 stock this may go far w J^ ; disadvantageous results. It 
 seems rather to fix and suotfgthen desirable characteristics. 
 On the other hand, if the stock be markedly tainted, the in- 
 breeding is continued at the risk of degeneracy. 
 
 Even when the stock is sound to start with, there seem to 
 be limits to close in-breeding, as Vom Rath found in experi- 
 ments with rats, and Von Guiata in experiments with mice. As 
 many breeders have recorded, there is a tendency to debility, 
 abnormality, and sterility. 
 
 According to Reibmayr, the success of a human race is in 
 part dependent on the alternation of periods of sustained in- 
 breeding, which serve to " fix " racial characters, and periods 
 of cross-breeding in which the advantages of "fresh blood" 
 are secured. 
 
THEORY OF FERTILISATION. 
 
 SUMMARY. 
 
 i. Old theories of "ovists," " animalculists,'' and of the "aura 
 seminalis. " 
 
 2. Modern morphological theories incline to lay the whole emphasis 
 upon the nuclei. The conclusions of Hertwig and Slrasburger are strongly 
 in favour of this view. The import of the centrosomes and general proto- 
 plasm must not, however, be overlooked. Several facts suggest that the 
 all-importance of the nuclei has been exaggerated. 
 
 3. Modern physiological theories of fertilisation are necessarily very 
 tentative. Sachs compares it to fermentation; Rolf to mutual digestion. 
 To Weismann, the process appears quantitative rather than qualitative. 
 Suggestion that the male cell brings to the ovum a supply of characteristic 
 chemical substances which act as a stimulus. 
 
 4. Uses of fertilisation to the species. Many regard fertilisation as a 
 necessary rejuvenescence of the life of the species. Weismann criticises 
 this view, but his criticism must be read in the light of the researches of 
 Maupas, who has shown that without conjugation the members of an 
 isolated family of infusorians eventually cease to feed and divide, passing 
 through stages of degeneration and senility to extinction. In this case, 
 conjugation is essential to the continued vitality of the species. According 
 to Brooks and Weismann, fertilisation is an important source of variation. 
 
 LITERATURE. 
 
 Baillet. — Quelques mots sur les croisements dits au premier sang. 
 
 Mem. Ac. Toulouse, V., 1893. 
 Bos, J. Ritzema. — Untersuchungen iiber die Folgen der Zucht in engster 
 
 Blutverwandtschaft. Biol. Centralbl., XIV., 1894, pp. 75-81. 
 BOVERI, Th. — Befruchtung. Anatomische Ergebnisse, I., 1693, pp. 386- 
 
 485 ; and subsequent reports in the same Record. 
 CoNKLiN, E. G. — The Fertilisation of the Ovum. Wood's Holl Bio- 
 logical Lectures, II., 1894, pp. 15-35, I0 fig s - 
 Hertwig, O. — Das Problem der Befruchtung, &c; Jenaische Zeitschrift 
 
 fur Naturwissenschaften, XVIII., 1885. 
 Klebs, G. — Ueber das Verhaltniss des mannlichen und weiblicheii 
 
 Geschlechts in der Natur. Jena, 1894, 30 pp. , 
 
 KCEHLER, R. — Pourquoi resemblons-nous a nos parents? Etude Physio- 
 
 logique sur la fecondation, sa nature et son origine. Revue Philoso- 
 
 phique, XXXV., 1893, pp. 337-386, 38 figs. 
 Maupas, E. — Comptes Rendus, 1886, 1887; and Archives de Zoologie 
 
 expdrimentale, 1888. 
 REGNAULT, F. — Des effets de la consanguinite'. Rev. Scient., LI., 1893, 
 
 pp. 232-6, 266-71. 
 Rhumbi.er, L. — Zellleib, — Schalen- und Kern-Verschmelzungen bei den 
 
 Rhizopoden und deren wahrscheinliche Beziehungen zu phylogene- 
 
 tischen Vorstufen der Metazoenbefruchtung. Biol. Centralblat. , 
 
 XVIII., 1898, pp. 21-26 el seq. 
 
1 82 THE EVOLUTION OF SEX. 
 
 Schimkewitsch, W. — Zur Frage iiber die Inzestzueht. Biol. Central- 
 
 blatt, XVI., 1896, pp. 177-181. 
 Standfuss, M. — Handbuch der paloearctischcn Gross-Schmetterlinge, 
 
 2nd ed. Jena, 1895, 392 pp., 8 pis. 
 Strasburger, E. — Neue Untersuchungen iiber den Befruchtungsvorgang 
 
 bei den Phanerogamen, als Grundlage fur eine Theorie der Zeugung. 
 
 Jena, 1884.. 
 Wilson, E. B.— The Cell in Development and Inheritance. New York 
 
 and London, 1896, 371 pp., 142 figs. 
 Weismann, A. — Opp. cit., especially Die Pecleutung dtr sexuellen Fort- 
 
 pflanzung fur die Seleklions-Theorie. Jena, 1886. 
 
CHAPTER XIII. 
 
 Degenerate Sexual Reproduction or Parthenogenesis. 
 
 § i. History of Discovery. — From very early times there appears 
 to have been an impression, that in exceptional circumstances 
 reproduction might occur without fertilisation. Even Aristotle 
 gave reasons for believing that, without sexual union, the un- 
 fertilised eggs of the honey-bee might give rise to perfect adults. 
 We now know that he was right, in his conclusion at least, so 
 far as the development of drones is concerned. In the early 
 belief in Lucina situ concubitu, much that was erroneous was 
 intermixed with a prevision of the truth; nor could we expect at 
 an early date that asexual multiplication (i.e., apart from ova alto- 
 gether) would be kept distinct from what we now mean by 
 parthenogenesis, or the development of ova without union with 
 sperms. In 1701, Albrecht observed that a female silkmoth, 
 which had been isolated in a glass case, laid fertile eggs ; and 
 though. this was for long discredited, the occasional partheno- 
 genesis of this insect has been repeatedly confirmed by com- 
 petent observers. 
 
 In 1745, the ingenious Bonnet drew attention to what is 
 now a very familiar fact, the successive generations of virgin 
 plant-lice or Aphides. Throughout the summer, he observed 
 the production of numerous generations of these little insects, 
 all females, necessarily therefore all virgins, and yet fertile. So 
 strange did the fact appear, that it was for long utterly dis- 
 credited. Reaumur eluded the difficulty, by affirming that the 
 Aphides were hermaphrodite ; but Dufour soon proved that this 
 was erroneous, though he could only confess his ignorance in 
 referring the phenomena to "spontaneous or equivocal gen- 
 eration," in which "the act of impregnation was in no degree 
 concerned." The facts, however, were repeatedly re-observed. 
 Kirby and Spence admitted them as incontestable, but could 
 regard them only as " one of the mysteries of the Creator, that 
 human intellect cannot fully penetrate." 
 
1 84 THE EVOLUTION OF SEX 
 
 Meanwhile Schaffer had observed the occurrence of par- 
 thenogenesis in minute aquatic crustaceans, the study of which 
 has since shed some vivid light on the whole subject. Pastor 
 Dzierzon had also clipped the wings of queen-bees, and in thus 
 preventing their nuptial flight and impregnation, observed 
 that the eggs they laid developed only into drones. The facts 
 soon began to be recognised, extended, and thought over by 
 naturalists of the standing of Owen (1843), Von Siebold (1856), 
 and Leuckart (1858), whose conclusions have afforded a firm 
 basis for the abundant subsequent observation and speculation 
 on this interesting subject. 
 
 § 2. Degiees of Parthenogenesis. — If we start then with Von 
 Siebold's definition of parthenogenesis, as the power possessed 
 by certain female animals of producing offspring without sexual 
 union with a male, it will clear the ground to notice, in the first 
 place, the numerous different degrees in which this develop- 
 ment without fertilisation may occur. 
 
 (a) Artificial Parthenogenesis. — There are a few curious 
 observations which go to show that in exceptional circum- 
 stances ova may develop when the male stimulus is replaced 
 by some artificial reagent. These observations must still be 
 taken cum grano satis, but they may be at least suggestive of 
 further experiment. Dewitz observed unfertilised frog ova to 
 undergo segmentation (sic) in corrosive sublimate solution. In 
 some cases one division occurred, in others several; in some 
 cases irregularly, in others normally. It happened both when 
 the ova were left in the reagent, and when they were merely 
 dipped and returned to water. The eggs experimented on 
 were those of the two common frogs Rana fusca and R. 
 escuknta, and of the tree-frog (Ilyla arborea). But it must be 
 noted that Leuckart long ago noted the occurrence of spon- 
 taneous division in frog ova. Similarly, Tichomiroff, experi- 
 menting with the unfertilised ova of the silkmoth, which are 
 occasionally parthenogenetic, was surprised to observe that ova, 
 which would not of themselves develop parthenogenetically, 
 might be induced to do so by certain stimuli. These con- 
 sisted in rubbing the unfertilised ova with a brush, or in 
 dipping them for two minutes in sulphuric acid and then 
 washing them. In both cases, he says, a percentage of the 
 ova thus artificially stimulated developed. It must be remem- 
 bered that occasional parthenogenesis occurs in this insect, 
 and all that Tichomiroff did was to incite this. J. Perez notes 
 
DEGENERATE SEXUAL REPRODUCTION. iSg 
 
 ("P. V. Soc. Sci. Bordeaux,' - 18967, pp. 9-10) that gentle 
 friction of silkmoth eggs stimulates parthenogenetic develop- 
 ment, but that it occurs apart from this, especially in the eggs 
 of very robust females. There is no doubt that reagents may 
 considerably modify ova; thus the brothers Hertwig showed 
 how it was in this way possible to overcome the non-receptivity 
 of the ovum to more than one sperm. Nor can one forget how 
 sexual reproduction in parasitic fungi tends to disappear, being 
 possibly replaced by the stimulus afforded from the waste 
 products of the host. In a similar way, the multiplication of 
 cells, so frequently associated in disease with the presence of 
 bacteria, has been referred by more than one pathologist to the 
 "spermatic influence" of these micro-organisms, or of the 
 katastates which they form. The most circumstantial account 
 of successful artificial parthenogenesis is that given by Professor 
 Jacques Loeb, who reared larvaa of a sea-urchin from unferti- 
 lised eggs which had been left for a couple of hours in sea- 
 water plus a solution of magnesium chloride, and then returned 
 to the normal medium. Balfour has also cited a remarkable 
 observation of Greefif, who saw unfertilised ova of the common 
 starfish developing in ordinary sea-water, in a perfectly normal 
 fashion, only more slowly than usual. 
 
 (b) Pathological Parthenogenesis. — It has very occasionally 
 been noticed in higher animals, where true parthenogenesis is 
 wholly unknown, that an unfertilised egg starts off on its own 
 resources without any male stimulus whatever. This is noted 
 by Leuckart for frog ova, by Oellacher for hens' eggs, and by 
 Bischoff and Hensen even in mammals. 
 
 Barfurth has re-investigated the question of parthenogenesis in the hen's 
 e gg> an d finds that the segments are non-nucleated, therefore not true cells. 
 He explains the process as wholly physico-chemical. (Versuche fiber die 
 parthenogenetische Furchung des Hiihnereies, "Arch. Entwickelungsme- 
 chanik.," vol. ii., 1896.) In the ovarian ova of various mammals, e.g. 
 guinea-pig, and rabbit, Janosik has observed (1} regular formation of polar 
 bodies, (2) division into numerous nucleated segments either equal or un- 
 equal, (3) fragmentation into many minu'e parts. (Die Atrophie der 
 Follikel und ein seltsames Verhalten der Eizelle, "Arch. f. Mikr. Ana- 
 tomie," xlviii., 1S96, pp. 169-181, 1 pi.) 
 
 Such cases must be regarded as rare abnormalities, com- 
 parable perhaps to pathological formations which not unfre- 
 quently take place in the ovary, and it is hardly necessary to 
 say that in no case did the development proceed far. 
 
1 86 THE EVOLUTION OF SEX. 
 
 (c) Occasional Parthenogenesis. — In some of the lower 
 animals, which are not themselves normally parthenogenetic, 
 but have relatives which are, occasional parthenogenesis has 
 been frequently observed. These differ from the above cases, 
 since the results are more successful, often in fact reaching 
 maturity, and also in this, that since related forms are partheno- 
 genetic, the " abnormality" is evidently of a much milder type. 
 The common silkmoth is a good example of this occasional 
 parthenogenesis, which certainly occurs, though rare both in 
 the genus and family. Out of 1,102 unfertilised eggs of the 
 silkmoth, observed by Nussbaum, only 22 developed, and that 
 only up to a certain point. " A whole series of insects," 
 Weismann says, "reproduce exceptionally by parthenogenesis, 
 for instance many butterflies, but that never to the extent that 
 all the eggs which an unfertilised female lays develop, but 
 only a fraction, and usually a very small fraction of the total 
 number, the rest perishing. Examples of successful occasional 
 parthenogenesis (to the extent at least of producing males) are 
 furnished by those worker bees, wasps, and ants which excep- 
 tionally become fertile." 
 
 (d) Partial Parthenogenesis. — The queen-bee, as has been 
 already mentioned, is impregnated by a drone in her nuptial 
 flight. The sperms thus received are stored up, and used to 
 fertilise the eggs as she lays them in the cells. Not all the eggs, 
 however, but only those which will produce future queens or 
 else workers. Other eggs, to all appearance similar, are un- 
 fertilised, and these, as Dzierzon first clearly showed, develop 
 solely into drones. It is a well-established fact that if ferti- 
 lisation be prevented by the imperfect development of the 
 wings, or by clipping them, the queen only lays drone eggs. 
 The same happens when she is old and her store of male 
 elements exhausted, or when the sperm receptacle has been 
 removed. Von Siebold carefully examined the eggs from drone- 
 cells, and found that they never contained spermatozoa. 
 Hensen notes an interesting side fact, obviously corroboratory, 
 that " German queen-bees, fertilised by Italian or Cyprian 
 drones, produced hybrid females but pure drones, a proof that 
 on the latter the sperm does not operate." Again, it some- 
 times happens that what are called "fertile workers" crop up, 
 which in consequence of some accident or misdirected inten- 
 tion in the nutrition, become less abortive than the host of 
 semi-females which make up the body of workers. They are 
 
DEGENERATE SEXUAL REPRODUCTION. 187 
 
 fertile enough to lay eggs, but their female organs do not seem 
 to admit of their being impregnated. Certain it is that they 
 only produce drones. What has just been said in regard to 
 beeSj is also true of some wasps and ants. 
 
 (e) Seasonal Parthenogenesis. — In some of the minute 
 aquatic crustaceans (Cladocera), popularly included under the 
 general title of water-fleas, parthenogenesis only occurs for a 
 season, and is periodically interrupted by the birth of males, 
 and the occurrence of the ordinary sexual reproduction. Males 
 generally reappear in the disadvantageous conditions of autumn, 
 but Weismann denies that there is a direct connection between 
 these facts. The common Aphides are parthenogenetic for 
 a succession of generations, sometimes as many as fourteen, 
 throughout the summer, but the cold and hard times of autumn 
 bring back the males and the sexual process. The fertilised 
 egg lives on through the winter, and develops with the warmth 
 of the next spring. By keeping up the temperature and 
 nutritive optimum for three or four years in the artificial 
 summer of a glass case, Reaumur and Kyber succeeded in 
 rearing as many as fifty continuous parthenogenetic genera- 
 tions. In the gall-wasps (Cynipidae) there is usually only one 
 parthenogenetic generation between the normal sexual repro- 
 ductions, but in many insects besides Aphides there are several. 
 It ought to be noted that the parthenogenetic Aphides are 
 hardly at the same structural level as the females which are 
 fertilised ; but as the differences mainly lie in the absence of 
 certain accessory genital organs, there is no reason for regarding 
 the parthenogenetic forms, as some have done, as larval. 
 
 (/) Juvenile Parthenogenesis. — Cases do occur, however, 
 where larval forms become precociously reproductive (as some- 
 times happens among higher organisms), and produce offspring 
 parthenogenetically. Such precocious production of partheno- 
 genetic ova must be distinguished from the entirely asexual 
 reproduction exhibited by many larvse. No very firm line can 
 indeed be drawn, but in the last cases no cells which can be 
 called ova are present. In 1865 Professor N. Wagner observed 
 what has been much studied since, that in the larvse of some 
 two-winged or dipterous midges (e.g., Miastor), the cells of the 
 reproductive rudiment develop into larvae within the mother- 
 larva's body. The mother falls a victim to her precocity, for the 
 brood of seven to ten larvae literally feed upon her to the death. 
 They finally leave the corpse and begin life for themselves, 
 
1 88 THE EVOLUTION OF SEX. 
 
 only, however, themselves to fall victims to a similar fate. The 
 process may thus go on for several generations, during which 
 the ova, or pseudova as some would insist upon calling them, 
 become smaller and smaller. Eventually the larvae become 
 too constitutionally poor to be precociously parthenogenetic, 
 and develop into adult midges — male and female, the latter 
 producing, however, only a few eggs. 
 
 In another dipterous insect known as Chironomus, the ova 
 begin to be produced at a very early stage, are laid just at the 
 time when the larval life ends, and develop parthenogenetically. 
 According to Jaworowski, by the rupture of the ovarian mem- 
 brane the ova fall into the body-cavity, where the abundant 
 nutritive stimulus takes the place of fertilisation. Juvenile 
 parthenogenesis is also said by Von Siebold to occur among 
 the Strepsiptera, little insects which infest bees. 
 
 (g) Total Parthe?iogenesis. — Lastly, in some of the minute 
 aquatic crustaceans and in many rotifers no males have ever 
 been found. There is every probability that the partheno- 
 genesis is thus total ; and as the numbers are abundant, it has 
 apparently been established without detriment to, at least, the 
 continuance of the species. 
 
 § 3. Occurrence of Parthenogenesis. — In three distinct sets 
 of animals — rotifers, crustaceans, and insects — parthenogenesis 
 has become a confirmed physiological habit. 
 
 (a) Take first the curious little rotifers, or wheel-animalcules, which 
 abound both in fresh and salt water. They are usually placed in the 
 chaotic alliance of worm-types, and have long been famous for their alleged 
 power of surviving prolonged desiccation. With one or two exceptions 
 the males are markedly different from the females, and are usually small 
 and degenerate. In one group (Philodinadse) the females have two ovaries, 
 while males have never been found. They have dwindled out of existence. 
 In the rest the females have one ovary, part of which has degenerated into 
 a yolk-gland, and small males occur. These are quite superfluous as mates, 
 however, for parthenogenesis prevails. Even when impregnation, which is 
 a peculiarly random process, occurs, the sperms appear to miss their mark, 
 and to perish in the body-cavity. The numbers keep up, notwithstanding, 
 so that we have here an entire class where parthenogenesis has firmly 
 established itself. 
 
 (t>) Among crustaceans, parthenogenesis is restricted to the lower orders, 
 viz., branchiopods and ostracods. In the former, it is exhibited by the 
 brine-shrimp Artemia and the common fresh-water Apus in one division; 
 by daphnids (e.g. , Daphnia and Moina, common "water-fleas") in the 
 other. In ostracods, some species of the common Cypi'is are partheno- 
 genetic. If a female water-flea, say Daphnia, be isolated from birth, 
 she becomes the mother of an abundant progeny of females. Males and 
 
DEGENERATE SEXUAL REPRODUCTION. 
 
 189 
 
 sexual reproduction do, however, eventually return, and the same is 
 probably true of the majority. Among three thousand specimens of the 
 brine-shrimp only one male occurred; while Von Siebold repeatedly in- 
 vestigated every member of a colony of Apus, once over five thousand in 
 number, without finding a single male. At other times he found one per 
 cent., while in certain unknown conditions 
 (probably when food is scarce and life generally 
 unfavourable) the males may be developed in 
 crowds. 
 
 In the daphnids, which have been so success- 
 fully studied by Weismann, the facts are more 
 complex. There are two kinds of eggs — winter 
 and summer ova. The former are large, thick 
 shelled, capable of resisting drought and the 
 like, and of remaining long latent. They only 
 develop if fertilised, and always produce females. 
 In every way they are highly anabolic ova. 
 The summer eggs, on the other hand, are 
 smaller, and thinner in the shell. They can 
 develop without fertilisation, and that is indeed 
 in some cases physically impossible. Males 
 are produced from summer eggs alone. They 
 usually appear in autumn, when life is becoming 
 harder, or the conditions more katabolic. 
 
 In the little cyprids the reproductive rela- 
 tions are very varied. Thus in Cypris ovum and 
 Notodromus monachus the males are abundant 
 all the year round, and parthenogenesis is 
 unknown. In other species, e.g., Candona 
 Candida, the males are still frequent, but 
 parthenogenesis nevertheless occurs. Lastly, 
 parthenogenesis prevails in some cases, like 
 Cypris fusca and C. pubera, and the males are 
 rare, appearing usually in spring. 
 
 We may find instances of permanent, 
 seasonal, or even merely local parthenogenesis, 
 — a variety of occurrence which strongly sug- 
 gests that environmental stimuli are of much 
 importance. 
 
 (c) In insects, as we have seen, the degrees 
 of parthenogenesis are very varied; so too is 
 the systematic position of the forms in which 
 normal parthenogenesis occurs. Two butter- 
 flies (Psyche helix and Solenobia, 2 sp. ) and a 
 beetle (Gastrophysa), some coccus-insects and 
 Aphides, certain saw-flies (Tenthredinidae) and 
 gall-wasps (Cynipidse), are normally parthenogenetic. In the butterflies 
 just noticed, the males seem to disappear for a stretch of years, and the 
 species gets on without them. The male of Psyche helix is very rare, and 
 was for long unknown. When the males are developed in Solenobia 
 trinquelrella, it is interesting to notice that they may predominate in 
 numbers over the females. A whole brood may be male ; they are brought 
 
 Owen's figure of the Genera- 
 lions of Aphides. At the 
 base an individual arises 
 from a fertilised egg-cell ; 
 this gives origin partheno- 
 genetically to a brood, and 
 so on through a succession 
 of generations. At the top 
 the male and female forms 
 reappear, and sexual re- 
 production returns. At the 
 side an earlier appear- 
 ance of sexual forms is 
 suggested. 
 
T90 THE EVOLUTION OF SEX. 
 
 back with a rush. About a score of mollis, including the silkmoth (Bombyx 
 mori) and death's-head (Sphinx atropos), have been known to exhibit casual 
 parthenogenesis; but the beetle above noticed stands alone. Bassett, Adler, 
 and others, have demonstrated an interesting alternation of parthenogenesis 
 and ordinary sexual reproduction in numerous gall-wasps. Forms which 
 had been legarded as quite distinct, and had received different generic 
 titles, have been shown in about a score of cases to be merery the partheno- 
 genetic and normal forms of the same insects. From a winter gall the 
 parthenogenetic form emerges which produces a summer gall. In this a 
 sexual form is produced, which eventually gives rise to the winter gall. 
 
 § 4. Parthenogenesis in Plants. — The passive bias is so strong in plants, 
 that it is easy to understand the rarity of parthenogenesis. The egg-cell 
 which develops of itself must retain the stimulus which the male element 
 in other cases supplies. It is natural, then, that what predominates in the 
 active rotifers should be uncommon in the sleeping plants. In some of the 
 flowering plants, what looked like parthenogenesis has repeatedly been 
 described, especially in regard to a native of New Holland, known as 
 Ccelebogyme. When cultivated in Europe, the male flowers degenerate, and 
 according to Braun and Hanstein disappear. Yet fertile seeds are pro- 
 duced. Karsten found, however, that stamens often persisted ; while 
 Strasburger has shown that what developed were not true egg-cells, but 
 adventitious growths from cells outside the embryo-sac. The same is true 
 of some other cases. Dr A, Ernst has recently described what he calls 
 true parthenogenesis in a Menisperm found by him in Caracas, and named 
 Disciphania Emstii. "Female plants, which bore no male flowers, and 
 which were grown perfectly isolated where there was no possibility of the 
 access of pollen from another plant, produced in three successive years an 
 increasing number of fertile fruits." 
 
 Kerner von Marilaun and H. O. Juel have shown that in Antennaria 
 alpina fertile seeds are produced without impregnation, the male plants 
 being very rare and producing no functional pollen - grains ("Bot. 
 Centralbl.," Ixxiv., 1898, pp. 369-372), and there are several similar cases 
 on record. 
 
 In the lower plants, however, cases of parthenogenesis abound, 
 apparently as one of the stages in the degeneration of sexual reproduction. 
 It has been casually observed of a species of the stonewort (Chara), that 
 when grown in certain waters the male organs disappear, yet the plants 
 continue multiplying. More interesting are the Fungi. To illustrate sexual 
 degeneration, De Bary gives a series from Fungi like those which kill the 
 salmon and potato (Saprolegnise and Peronospores:). What happens first, 
 is the degeneration of the male organs. The katabolic sex from beginning 
 to end is the more unstable. The male function goes first, but the form 
 remains after the reality has ceased. After a while, that is in related 
 species, the form goes too. Sometimes the function is changed, and the 
 male organs become protective sheaths. De Bary's series may be briefly 
 summed up. 
 
 (1.) In Pythium, the male organ discharges most of its protoplasm into 
 the female, — the usual process. 
 
 (2.) In Phytophthora, only a very small portion is thus given, and we 
 may almost say asked, for there are curious demand and supply 
 arrangements and compulsions between the male and female organs 
 in these Fungi. 
 
DEGENERATE SEXUAL REPRODUCTION. 19T 
 
 (3.) In Pcronospora, there is no perceptible passage of protoplasm from 
 male to female, though, without going back to the "aura 
 seminalis," we may allow the possibility of subtle osmosis. 
 
 (4.) In some Saprolegnue, there are indeed the usual antheridia or male 
 organs, which are directed towards the female organs, but do nut 
 open. The " explosive" character is diminishing. 
 
 (5.) In others, the male organs never get near the female. 
 
 (6.) In others, there are no male organs at all, but the female cells 
 develop as usual. 
 
 Parthenogenesis is thus reached, as the result plainly of a degenerative 
 process. We can follow the story further, however, forestalling for the 
 moment the subject of the next chapter. The male-organ has degenerated, 
 we have seen, while the female organ holds on its course. But this is 
 not always so ; in many cases it follows suit, and asexual reproduction 
 remains. 
 
 Now why should these Fungi among plants exhibit numerous instances 
 of parthenogenesis ? The more intimate the parasitism, the more degene- 
 rate the sexual reproduction, and all trace of it is often lost. It may be 
 that in the vital economy of the species the nutritive stimulus of the host 
 in some measure takes the place of fertilisation. Or it may be that the 
 stimuli which Klebs has shown to be operative in inducing the occurrence 
 of asexual or sexual reproduction in Alga? and Fungi which have both modes 
 of multiplication, are absent in the parasitic forms above referred to. 
 
 Male parthenogenesis, paradoxical as it sounds, is really exhibited 
 among lowly Algje. That is to say, a small spore (or male-cell) which 
 normally unites with a larger and more quiescent one (or female-cell), may 
 occasionally start developing on its own resources. The result, however, 
 is poor enough. As those spores are on the border line between asexuality 
 and differentiated sex-elements, the retention of a vegetative power of 
 division even by the incipient male-cell is not surprising. Nor must it be 
 forgotten that the mother-sperm-cell itself has a power of parthenogenetic 
 development. It divides, like its homologue the ovum, into a ball of cells, 
 but, having none of the conservative coherence of the latter, breaks up 
 into spermatozoa. It is exactly comparable to the interesting Protozoon 
 (Magosphsra) which Haeckel saw, which did its best to get beyond the 
 Protozoa, but failed as soon as it had succeeded. A single infusorian-like 
 cell divided into a ball of cells, but the ball had no coherence and broke up 
 into infusorians once more. 
 
 § 5. The Offspring of Parthenogenesis. — The fate of parthenogenetic 
 ova is very diverse. They may all perish, or all succeed ; they may turn 
 out wholly males or wholly females. Hensen notes the following suggestive 
 series, with decreasing reproductive, as opposed to constitutional, energy 
 at each level : — 
 
 (1.) Hermaphrodites, then only females. 
 
 (2.) Series of females, then mixed brood. 
 
 (3.) Several females, mixed brood, then only males. 
 
 (4.) Series of mixed broods, then males, or death of ova. 
 
 (5.) Mixed brood, with much mortality. 
 
 (6.) Males only. 
 
 (7.) Development only for a few stages. 
 Rolph has a different arrangement, but the same idea: — 
 
 (1.) Exceptional parthenogenesis with uncertain result [e.g\ t Silkmoth), 
 
192 
 
 THE EVOLUTION OF SEX. 
 
 (2.) Normal, producing males only (female solely from fertilised ova) 
 (e.g., Bees). 
 
 (3.) Mostly males, with occasional females (e.g., Nemalus). 
 
 (4.) Mostly females, with exceptional or periodic males (e.g., Apus, 
 Arlemia). 
 
 (5.) Only female, males unknown (e.g. , many Rotifers). 
 
 That parthenogenetic ova should develop with such diverse results is 
 not at all surprising. The absence of fertilisation removes one of the 
 factors determining sex ; but food, temperature, age of ovum, &c. , remain, 
 and produce bias now to one side, now to the other. To this we shall 
 presently return; meanwhile the facts of offspring may be more clearly 
 expressed thus : — 
 
 Example. 
 
 Most organism 1 ;. 
 Karities mentioned. 
 Many insects. 
 Hive-bee and some 
 
 other forms. 
 Ncmatus (allied to 
 
 bee). 
 Most gall-wasps. 
 Some saw-flies. 
 
 Some water-fleas. 
 Solenobia sometimes. 
 Aphides; some water- 
 fleas. 
 Many water-fleas. 
 Most rotifers. 
 Many rotifers. 
 
 Result. 
 
 [Nil 
 
 Partial and pathological development 
 Great mortality in a mixed brood. . 
 c5 's alone .... 
 
 i 's mostly, a few 9 's . . 
 
 (5 's and 9 's (one generation) 
 $ 's, and more than a few 9 's 
 9 9 9 (a succession), then a predomin- 
 ance of S 's 
 9 9 9, then equal numbers of S 's and 9 's 
 9 9 9, then a minority of S 's among 9 's 
 
 9 9 9 9, very rare $ 's 
 9 9 9 9, non-functional { 's among 9 's 
 i. 9 9 9 9 , ad infinitum, no 6 's 
 
 § 6. Effects of Parthenogenesis. — Since parthenogenesis is 
 dominant in rotifers, and well established among water-fleas 
 and plant-lice, it is very plain that whatever else it affects, it is 
 anything but prejudicial to numbers. An aphis will continue 
 for days producing a viviparous brood, at the rate of one per 
 hour; the offspring soon begin themselves to multiply; and 
 Huxley calculates that if this continued for a year without 
 mortality, a single aphis would be the ancestor of a progeny 
 which would weigh down five hundred millions of stout men ! 
 Not gardeners only have cause for gratitude that climate and 
 enemies prevent such untoward increase. But there are other 
 desiderata besides numbers. Can it be said that partheno- 
 genesis favours the general life and progress of the species? 
 More than one of the old naturalists, and in recent years 
 Brooks, Galton, Weismann, and others, have laid emphasis on 
 the value of fertilisation as a fountain of change. To Weismann 
 
DEGENERATE SEXUAL REPRODUCTION. 1 93 
 
 the intermingling of the male and female "germ-plasms" in 
 fertilisation is one of the possible sources of variation. If it be 
 removed therefore, as in rotifers, the species will be so much 
 the less likely to progress, the establishment of parthenogenesis 
 will be a handicapping of evolution. 
 
 But Weismann has shown that variation still occurs in 
 the parthenogenetic Cyprids, and variations of Rotifers, e.g., 
 Anurcea cochlearis, have been often recorded. In the well- 
 known case just mentioned, however, the varieties are cor- 
 related with the seasons, and it seems to us probable that they 
 are seasonal " modifications " rather than true constitutional 
 variations. 
 
 While recognising, then, the occurrence of variations in 
 forms where the reproduction is parthenogenetic, or even quite 
 asexual, we suggest the probability that the absence of fertilisa- 
 tion involves some diminution in the frequency and range of 
 variability. 
 
 § 7. Peculiar! fy of the Parthenogenetic Ova. — Before a theory 
 of parthenogenesis is sought, the natural question arises, Are 
 these eggs that develop of themselves in any way peculiar? 
 
 (a) For a while it was supposed (e.g., by Balfour) that 
 parthenogenetic ova did not form polar globules, and the 
 theory based upon this regarded the retention of these bodies 
 as taking the place of fertilisation. The demonstrated occur- 
 rence of one polar globule in several parthenogenetic eggs 
 partially demolished this theory, and it is only within the last 
 two or three years that it has been restated in accurate form. 
 
 (b) Simon shrewdly points out, that in some of the most 
 marked cases of parthenogenesis the sex-cells are insulated 
 from the body at a very early stage. This is notably so in 
 those midges which reproduce parthenogenetically even before 
 maturity. It is certainly striking that these forms should 
 unite an extreme earliness in the embryonic separation of 
 the germ-cells with a most precocious reproduction. These 
 germ-cells are ova which have a much less circuitous history 
 than in most cases ; they have far fewer cell-divisions behind 
 them, they have thus a reserve power of division which other 
 ova have not ; they are able, in fact, to develop of themselves. 
 Although this is not known to be true of all instances of 
 normal parthenogenesis (e.g., rotifers), it is true of some, and 
 that to a greater extent than was known when Simon wrote. 
 On the other hand, some forms where parthenogenesis is 
 
 J 3 
 
194 THE EVOLUTION OF SEX. 
 
 unknown (e.g., leeches and Sagittd), also exhibit the same early 
 differentiation of germ-cells, so that we can only look upon 
 the fact as one of the auxiliaries of parthenogenesis 
 
 (c) The peculiarity of parthenogenetic ova, which has of 
 late attracted much attention, is that they extrude only one 
 polar cell, — not two, like other eggs. Weismann, with the 
 assistance of Ischikawa, proved this for the parthenogenetic ova 
 of Rotifers and Ostracods, such as Polyphemus, Leplodora hya- 
 lina, Sida crystallina, Cypris reptans. Blochmann has also 
 corroborated Weismann's discovery by observations on aphides, 
 but he found two polar bodies in those unfertilised eggs of 
 bees which give rise to drones. The occurrence of two polar 
 bodies was also noted by Tlatner in the parthenogenetic ova 
 of a butterfly (Liparis). The apparent contradiction has been 
 in part explained by the discovery of Brauer that in the par- 
 thenogenetic ova of the brine-shrimp Artemia, two modes of 
 maturation occur. In most cases only one polar body is 
 formed, in other cases two are formed, but the second does not 
 leave the egg and behaves just like a sperm-nucleus. A further 
 enquiry into the exact history of the nuclear rods or chromo- 
 somes shows that the two cases can be plausibly reconciled. 
 
 § 8. Theory of Parthetwgenesis. — (a) We may begin with 
 Balfour's view of the case, though that of Minot has the priority. 
 " The function of forming polar cells has been acquired by the 
 ovum for the express purpose of preventing parthenogenesis." 
 If they were not formed, parthenogenesis would normally occur. 
 This is expressed in curiously teleological language, but the 
 main idea is clear enough, — the retained polar cells replace the 
 sperm nucleus. 
 
 (d) "In accordance with Minot's hypothesis of sexuality, it 
 might be assumed that in parthenogenetic ova the male element 
 was retained, and that the cell remained a true asexual cell, and 
 did not become a sexual element." " Blochmann and Weis- 
 mann have shown that this is the case, by their discovery that 
 in parthenogenetic ova only one polar globule is formed, while 
 there are always two in ova which are impregnated; hence it 
 is probable that one polar globule (by hypothesis, male) is 
 retained." 
 
 Minot's words are not beyond criticism either, though they 
 are not teleological. An ovum which retains a male element 
 is not happily described as remaining asexual; it would be 
 better to call it a case of intracellular hermaphroditism. It is 
 
DEGENERATE SEXUAL REPRODUCTION. 195 
 
 more important, however, to notice how Minot cleverly adapts 
 his theory to increased knowledge of the facts. The partheno- 
 genetic ovum only retains one polar globule, — one male element 
 is enough; two would be " polyspermy," which is abhorred. 
 
 (c) There was no fear that Rolph would indulge in teleology, 
 rigid necessitarian as he was. Parthenogenesis of ova was to 
 him the more natural process, the sperm a subsequent impor- 
 tation.. " There is for the ovum a certain minimal mass, which 
 must be surpassed if it is to develop at all; and a second mini- 
 mum, which the ovum must attain, if a female is to be produced." 
 Abundant nutrition of the ovum tends to parthenogenesis, pro- 
 ducing male offspring, as the lower stage; but if the second 
 limit be attained, resulting in females. In the opposite direc- 
 tion, if the ovum has fewer resources, it requires to be fertilised. 
 Females or males will again result according to the state of the 
 elements. If no fertilisation occur, the dependent ovum must 
 of course die. Rolph is always suggestive, but he erred in 
 regarding the sex-elements too quantitatively, in missing the 
 qualitative antithesis of sex, and the opposition observed in 
 cell-division. 
 
 (d) Strasburger also lays emphasis, in a subtler and more 
 technical way, on nutritive conditions. " In the rare cases of 
 parthenogenesis, specially favourable nutritive conditions may 
 counteract the lack of nuclear plasma." He notes three 
 different ways in which this may happen, and also inclines to 
 believe that retention of polar globules would favour partheno- 
 genetic development. It is important to notice how two 
 naturalists, so very different in their manner of attacking a 
 subject as Rolph and Strasburger are, come to this conclusion 
 at least in common, that favourable nutritive conditions favour 
 parthenogenesis. All the cells in the body tend to multiply, 
 the ova retaining this power develop embryos. 
 
 (e) Weismann has a peculiar right to be heard on the nature 
 of parthenogenesis; for not only has he been for many years 
 an investigator of the tiny daphnids or water-fleas, but he made 
 the important discovery, already noticed, that parthenogenetic 
 ova extrude only one polar globule. There has not been time 
 yet to prove that this is always true, but the probabilities are 
 strong that it is. Weismann's first supposition was that in 
 maturation of ordinary ova the first polar division got rid of 
 "oogenetic" nuclear plasm, and that the second removed part 
 of the essential germ-plasm; and that in the maturation of par- 
 
196 THE EVOLUTION OF SEX.- 
 
 thenogenetic ova only the first process occurred. This view is- 
 expressed in the following paragraphs. 
 
 " Parthenogenesis occurs when the entire sum of the ancestral elements 
 persists in the nucleus of the ovum. Development by fertilisation demands, 
 however, that half of these ancestral elements must first be extruded from 
 the ovum, whereupon the remaining half, in uniting with the sperm nucleus, 
 regains the original number. 
 
 " In both cases the beginning of development depends upon the presence 
 of a definite, and indeed similar mass of germ-plasma. In the ovum which 
 requires fertilisation, this is afforded by the importation of the sperm- 
 nucleus, and development follows on the heels of fertilisation. The par- 
 thenogenetic ovum already contains the necessary mass of germ-plasma, and 
 this becomes active as soon as the single polar body has freed the ovum 
 from the oogenetic nuclear-plasma." 
 
 He has now given up the hypothesis that the first polar body removes 
 " oogenetic idioplasm," and the second germ-plasm; he admits that germ- 
 plasm is got rid of in both cases. In parthenogenetic ova, however, where 
 only one polar body is extruded, a sufficient quantity of germ-plasm is 
 retained to make development without fertilisation possible. 
 
 The view which Weismann now holds (vide " The Germ-Plasm," 1893) 
 seems to amount to this, that the reduction of germ-plasm in the matura- 
 tion of ordinary ova is too great to admit of independent development, but 
 that in parthenogenetic ova the reduction is less and development without 
 fertilisation occurs. It maybe urged that we have not as yet sufficiently secure 
 data in regard to the reducing processes in parthenogenetic ova, and it must 
 be noted that there are some authorities who refuse to grant that the em- 
 phasis which Weismann and others have laid on the reducing divisions is 
 justified. Thus Professor Hartog maintains (" Natural Science," xiii., 1898, 
 pp. 1 1 5- 1 20) that the process of nuclear reduction does not affect the 
 quantity of nuclear material, but only the number of segments into which 
 it is divided. He suggests that it is the achromatic plasma or linin which 
 bears the hereditary characters, and that the chromatin has only a mechanical 
 function. 
 
 Weismann's pre-occupation with questions of inheritance has given a 
 bias to his theory, making it morphological rather than physiological. A 
 given quantum of germ-plasma, he says, fits the ovum to develop. The 
 parthenogenetic ovum keeps enough. The ordinary ovum extrudes so 
 much that it has to get it back again from another source. It appears to us 
 more in accordance with the facts to suppose that the entrance of the sperm 
 has a twofold importance, — (a) It bears with it certain hereditary charac- 
 teristics, doubtless in the nucleus for the most part ; (b) it brings with it a 
 stimulus to division of a qualitative character, doubtless in some part in its 
 small cell-substance. This last function — the dynamic function — Weismann 
 wholly denies. The sperm has to him only a quantitative function. 
 
 (/) Our theory of parthenogenesis may now be stated. Just 
 as the spores which illustrate the beginnings of sex may some- 
 times dispense with conjugation and germinate independently, 
 so may ova develop parthenogenetically. These are to be 
 regarded as incompletely differentiated female cells, which 
 
DEGENERATE SEXUAL REPRODUCTION. I97 
 
 retain a measure of katabolic (relatively male) products, and 
 thus do not need fertilisation. Such a successful balance 
 between anabolism and katabolism is indeed the ideal of all 
 organic life. In parasitic fungi, sexual reproduction disappears, 
 and surrounding waste products presumably help the purpose 
 otherwise effected by sexual organs, so peculiarities in the con- 
 ditions of parthenogenetic ova may explain the retention of the 
 normal balance which makes division possible without the 
 usual stimulus of fertilisation. Abundant and at the same 
 time stimulating nutrition (Rolph), early differentiation of the 
 sex-cells (Simon), the general preponderance of reproductive 
 over vegetative constitution (Hensen), their liberation before 
 the anabolic bias has carried them too far, are among these 
 
 f parthenogenesis (p). 
 Female < sex (s). 
 
 ^disease (D). 
 
 f parthenogenesis (p) 
 sex (s). 
 disease (D). 
 
 t: 
 
 Diagram illustrating the theory of parthenogenesis. 
 
 favouring conditions. Artificial parthenogenesis can be in- 
 duced in some cases by chemical reagents; thus in Loeb's 
 experiments with sea-urchin ova magnesium chloride seemed 
 to have (perhaps very indirectly) the same physiological role as 
 spermatozoa. The incipient segmentation observed in a few 
 ova is an independent effort to save themselves from being too 
 big to live, since they are not passive enough to remain dor- 
 mant. Waste has set in, self-digestion begins, the cell is forced 
 into the expedient of division. In higher animals this is all in 
 vain : in lower animals such imperfectly differentiated female 
 cells are commoner; they form the parthenogenetic ova. 
 
 § 9. Origin of Parthenogenesis. — From the occurrence of 
 parthenogenesis in the animal series, it is certain that it has 
 
198 THE EVOLUTION OF SEX. 
 
 originated as a degeneration from the ordinary sexual process. 
 It is no direct persistence of a primitive ideal state, though in 
 a sense a return to it. 
 
 It seems to us misleading to interpret the occurrence of 
 parthenogenesis as due to " motives " and " important ad- 
 vantages." These are afterthoughts of our importation. It is 
 not easy indeed to keep from metaphorical language which 
 suggests that polar globule-formation is a " contrivance," and 
 parthenogenesis a " device." Such casual words are of little 
 account ; but it is going far to say, as Weismann does, " that 
 sexual reproduction has here been given up, not by any 
 chance nor from internal conditions, but from quite definite 
 external grounds of utility (Zweckmassigkeitsgrunden)." Our 
 position is the converse, that parthenogenesis arises because of 
 necessary internal conditions, and may be perpetuated because 
 of certain advantages. 
 
DEGENERATE SEXUAL REPRODUCTION. I99 
 
 SUMMARY. 
 
 t. Parthenogenesis was formerly believed to be of wider occurrence 
 than it really is, but it is definitely known to be not uncommon in lower 
 animals. 
 
 2. Artificial, pathological, occasional, partial, seasonal, juvenile, and 
 total parthenogenesis must be distinguished. 
 
 3. The occurrence of parthenogenesis is especially well seen in rotifers, 
 crustaceans, and insects. 
 
 4. It is rare among plants, but certainly occurs in some. 
 5- The offspring of parthenogenetic ova are very diverse. 
 
 6. The effects of parthenogenesis on the species deserve consideration, 
 especially by those who find in sexual intermingling an important source of 
 specific variation. 
 
 7. So far as we know, parthenogenetic ova (with two or three excep- 
 tions) form only one polar body. 
 
 8. Parthenogenetic ova are here regarded as imperfectly differentiated 
 female cells, retaining certain characteristics which compensate for the 
 absence of fertilisation. 
 
 9. In origin parthenogenesis is regarded as a degeneration from the 
 ordinary sexual process. 
 
 LITERATURE. 
 
 See especially the already cited works of Balfour, Brooks, Hensen, 
 Minot, Rolph, Sachs, Weismann; also — 
 Blochmann — Ueber die Richtungskorper bei Insekteneiern. Biolog. Cen- 
 
 tralblatt, VII., and Morpholog. Jahrbuch, XII. 
 Brooks, W. K. — Law of Heredity. Baltimore, 1883. 
 Gerstvecker. — Bronn's Klassen und Ordnungen des Thierreich, Vol. V., 
 
 Arthropod a. 
 Giard, A. — Sur le developpement parthenogenetique de la microgamete 
 
 des metazoaires. C. R. Soc. Biol. Paris, 1899, 4 PP- 
 Hudson and Gosse. — The Rotifera. London, 1886. 
 Karsten, H. — Parthenogenesis und Generatibns-Wechsel im Thier und 
 
 Pflanzenreiche. Berlin, 1888. 
 Leuckart. — Art. "Zeugung" in Wagner's Handworterbuch d. Physiol., 
 
 Bd. IV, 1853. 
 Owen. — Parthenogenesis; or, The Successive Production of Procreating 
 
 Individuals from a Single Ovum. London, 1849. 
 Plate. — Beitrage zur Naturgeschichte der Rotatorien. Jenaische Zeitschft. 
 
 f. Naturwiss, XIX., 1886. 
 Seidlitz, G. — Die Parthenogenesis. Leipzig, 1872. 
 Von SlEBOLD. — Beitrage zur Parthenogenesis. Leipzig, 1871. 
 Simon, F. — Die Sexualitat, &c, Inaug. Dissertation. Breslau, 1883. 
 Weismann, A. — Beitr. zur Naturgeschichte der Daphnoiden. Leipzig, 
 
 1876-79. Ueber die Zahl der Richtungskorper und uber ihre Be- 
 
 deutung fur die Vererbung. Jena, 1887. The Germ-Plasm, 1893. 
 Weismann, A., and Ischikawa, C. — Ber.,naturforsch. Ges., Freiburg, 
 
 III., 1887. 
 Wilson, E. B. — The Cell in Development and Inheritance, 1896. 
 
CHAPTER XIV. 
 Asexual Reproduction. 
 
 § i. Artificial Division. — Weeping willows are by no means 
 scarce trees in Britain, yet, as they never flower, they must all 
 have grown from slips. In other words, their multiplication 
 is asexual and artificial. So too, only more naturally, the 
 Canadian pond-weed (Anacharis) has spread prodigiously in 
 
 A group of Sea-Anemones 
 
 our lochs, canals, and rivers, never bearing male flowers, but 
 owing its increase wholly to the asexual process. Every one 
 knows how the gardener increases his stock by slips and 
 cutilngs, thus taking advantage of the power a part has to 
 reproduce the whole. Bananas, potatoes, and yarns are in the 
 same way propagated asexually, and this is also the usual 
 
ASEXUAL REPRODUCTION. 201 
 
 method in the case of olives, figs, and date palms. Quite in 
 the same way, cultivators of bath sponges are able to bed out 
 little fragments. In the last century," the Abbe Trembiey 
 delighted himself and others by the now familiar observation, 
 that to get many hydra polypes, the simplest and quickest way 
 was to cut one in pieces. A fragment will reproduce the 
 whole, provided always that it have to start with fair samples 
 of the different kinds of cells in the body, and that it be not 
 too minute. The same may be done with the much larger 
 sea-anemones. So the earthworm, curtailed by the spade, does 
 not necessarily suffer loss, though it ma)' suffer pain. The head 
 portion grows a new tail, and even a decapitated portion may 
 
 The Formation of a Sponge Colony {OlynUiiis) by 
 budding. — After Haeckel. 
 
 reproduce a head and brain, not that this is saying much for 
 these. 
 
 § 2. Regeneration. — Spades and knives are not exactly 
 instruments of nature, but they have their counterparts. Fight- 
 ing with a rival a crab may lose its claw, or the same may 
 happen in the frequently fatal moulting. Slowly, however, the 
 loss is made good; the cells of the stump multiply, and arrange 
 themselves in obedience to the same necessities as before, and 
 a limb is regenerated. Many an appendage among the lower 
 animals is from time to time nipped off, only to be grown 
 again. A snail has been known to regenerate an amputated 
 eye-bearing horn twenty times. A starfish readily surrenders 
 an arm, and a lizard its tail. Indeed, many animals may be 
 said to have learnt organically, though not consciously, that 
 it is better for one member to perish than for the whole life to 
 
202 THE EVOLUTION OF SEX. 
 
 be lost. For animals, like men, are often wiser than they wot 
 of. In the panic of capture, strong convulsions may occur; 
 thus the sea-cucumber may surprise and perhaps shock its 
 persecutor by the ejection of its viscera; or a tetanic contrac- 
 tion of the muscles makes the slow-worm brittle in the hands 
 of its captor. The power of regeneration is most marked in 
 echinoderms, but persists as high up as reptiles. The re-growth 
 of part of a lizard's leg is the ckef-d'ceiivre in this line. Beyond 
 that, regeneration is restricted to little things. We constantly 
 regenerate the skin of our lips, but we cannot naturally re- 
 place an amputated limb. 
 
 The regenerative capacity depends on primary properties of 
 the living matter and of the organism — which we are far from 
 understanding, — but it seems probable, as Reaumur, Lessona, 
 Darwin, Weismann, and others have pointed out, that its dis- 
 tribution and mode of occurrence are adaptive, being related 
 to the normal risks of life. According to Lessona's law, which 
 Weismann has elaborated, regeneration occurs in those organ- 
 isms and in those parts of organisms which are in the course 
 of nature most liable to injury. (See Weismann, " Natural 
 Science," xiv., 1899, pp. 305-328; " Anat. Anzeiger," xv., 1899, 
 
 PP- 445-474- ) _ 
 
 This position may be argued for in two ways. One may 
 try to show that all parts known to be markedly capable of 
 regeneration are liable to be lost or injured under natural 
 conditions ; and one may show that parts not very liable to be 
 injured are not regenerated, although they are often of great 
 importance. 
 
 It must be admitted that there are cases of regeneration — 
 e.g., of a stork's bill or a newt's eye — where the natural liability 
 to injury is not at first sight evident. But inquiry shows that 
 male storks fight savagely, and that the head, and possibly the 
 eye of the newt, are often attacked by the larvae of the water- 
 beetle (Dyliscus viarginalis). But an explanation of the re- 
 generative capacity of the protected abdominal limbs of 
 hermit-crabs, which T. H. Morgan has demonstrated, is more 
 difficult, unless one supposes that the power has been inherited 
 from ancestral Crustaceans whose abdomen was unprotected, 
 or that the liability to injury to which the supposed adaptation 
 is related is associated with the process of moulting. That 
 the latter supposition should be tested is made clear by the 
 observations of Bordage on Phasmids and other Orthoptera. 
 
ASEXUAL REPRODUCTION. 2O3 
 
 But Morgan's criticism of Weismann's position has not yet 
 been adequately met. 
 
 As to the second point, it will be admitted by most that 
 injuries to internal organs are seldom made good. 
 
 § 3. Degrees of Asexual Reproduction. — The keynote of the 
 subject was struck by Spencer and Haeckel, when they denned 
 asexual reproduction as discontinuous growth. All growth is a 
 reproduction of the protoplasm and its nuclear elements, or, in 
 short, of the cells ; all reproduction (excluding the important 
 event of fertilisation) is growth. The ovum, asexually produced 
 from the parent ovum or its lineally descendant cells, grows and 
 reproduces itself in turn, building up the embryo. The embryo 
 grows into an adult organism, and the surplus of continued 
 growing energy results in the asexual production of buds, or 
 the sexual discharge of differentiated reproductive elements. 
 We may start from the ordinary processes of cell-multiplication 
 and regeneration exhibited in the normal organism. Then 
 come the processes by which lost members are regenerated, 
 involving more or less serious extra growth. From these we 
 may pass to the rarer and yet not rare cases, where the artificial 
 halves or fractions of an organism can grow into wholes. 
 Normal and frequent, however, are the very abundant cases of 
 budding, where a sponge or hydra, zoophyte or coral, has 
 surplus enough to grow off new individuals, which remain con- 
 tinuous with itself. The parent organism, whether zoophyte 
 or strawberry-plant, has an asexually produced progeny round 
 about, and in asexual continuity with itself. But they do not 
 always remain continuous ; the hydra produces buds, but 
 eventually sets them adrift. This is still better seen in many 
 of the hydroids, where individuals are separated off as swim- 
 ming-bells or medusoids. The multiplication has become 
 discontinuous. Continue the process, and we find the libera- 
 tion of special cells, clinging often for a time to the parent, 
 generally dependent for development on union with similar 
 cells of complementary constitution; we find, in fact, the sexual 
 reproduction which, in the higher organisms, so thoroughly 
 replaces the asexual process. 
 
 § 4. Occurrence of Asexual Reproduction in Plants and 
 Animals. — In plants, as one would expect from their vegetative 
 constitution, the asexual process is common, particularly among 
 the lower forms. The common liverworts (Marchanlia and 
 JLunularia), through the formation of asexual buds or gemmag 
 
204 
 
 THE EVOLUTION OF SEX. 
 
 in the cups upon their thallus, rapidly overrun our flower-pots, 
 and become a pest of the greenhouse. Many ferns too, notably 
 among the Aspleniums, reproduce by bulbils, arising upon the 
 frond ; and the bulbils which arise in the axils of the leaves of 
 the tiger-lily are familiar missiles for every child accustomed 
 to a flower-garden (see figs. pp. 211 and 240). The Alliums, 
 and some of our common grasses also, furnish us with examples 
 of the replacement of flowers by separable buds. Asexual re- 
 production, or multiplication by more or less discontinuous 
 
 Asexual Propagation of a Grass — (<z) the bulbils 
 rooting on the ground ; (b) their appear- 
 ance in the inflorescence ; (c) a small 
 portion enlarged. — From nature. 
 
 growth, without the differentiation of special and mutually 
 dependent sex-cells, occurs from the simplest animals on to the 
 tunicates or sea-squirts, from the base to just over the line 
 which separates backboneless and backboned animals. It is 
 necessary, however, to review the groups. 
 
 Protozoa. — Fertilisation began in almost mechanical fusion. Reproduc- 
 tion begins with almost mechanical rupture. The unit mass of protoplasm, 
 becoming too big for control, breaks. Thus it saves itself, and at the same 
 time multiplies. Such breakage may be seen in some primitive amoeboid 
 
ASEXUAL REPRODUCTION. 205 
 
 forms, but it also occurs in a few of the relatively high infusorians. That 
 the breakage sometimes means dissolution is certain; nor is reproduction 
 ever so very far removed from death. 
 
 The rupture becomes orderly and systematic in budding. This may be 
 multiple, as in the common Arcella, where a number of small buds are 
 constricted off all round. But the process is oftener concentrated in one 
 extrusion or overflow. In budding, the separated daughter-cell is in vary- 
 ing degree smaller than the parent, and the process resembles an overflow. 
 When the bud is approximately equal to the parent, and the process is of 
 the nature of a constriction, it is, of course, division. 
 
 The division may also be multiple, taking place in rapid succession and 
 in limited space, e.g. , within a cyst. Then we speak of spore-formation. 
 The last three modes of multiplication are exceedingly common among 
 Protozoa. 
 
 These buddings and divisions are not, of course, rough and ready pro- 
 cesses. The nucleus almost always shares in them in an orderly and com- 
 plicated fashion. There are variations in its behaviour as in higher 
 animals, but there is no doubt that cell-division, with a gradient of pro- 
 gress like everything else, is essentially one and the same in the vast 
 majority of cases. Gruber has been especially successful in proving that 
 fragments of Protozoa, artificially separated without nuclear elements, 
 cannot live long. Though they may retain for a time contractility and 
 irritability, they cannot feed or grow. The nucleus is essential to life, 
 though sometimes it seems to disappear, and become as it were a diffuse 
 precipitate in the protoplasm. 
 
 Sponges. — In sponges no one can fail to recognise the impossibility of 
 drawing any rigid line between growth and asexual reproduction. Between 
 simple extension of the parent mass, and the budding off of new indi- 
 viduals, no sure distinction can in many cases be made out. Sponges do 
 not divide, though they may be cut up, yet some give off discontinuous buds. 
 An outgrown tube may lose connection with the parent, or a great tumour- 
 like mass may be slowly extruded, or tiny brood-buds may be set adrift to 
 shift for themselves. In disadvantageous conditions the surface of a sponge 
 sometimes gathers into minute superficial buds, by means of which it is 
 possible that the life is saved. 
 
 In the fresh-water sponges, in disadvantageous circumstances, — of cold 
 in some countries, heat and drought in others, — some of the cells club 
 together to form gemmules, which often save the life of the otherwise 
 dying sponge. They are complex enough, with sheaths and spicules, and 
 sometimes even with a float, but in principle they simply do by a multiple 
 union what is otherwise attained by ovum and sperm. Best known in this 
 respect is the freshwater sponge (Spongilla); but gemmules have also been 
 described in other common sponges, e.g., in Clzona, the borer in oyster 
 shells. 
 
 CcelinieraUs. — In such names as zoophytes, sea-firs, sea-roses, there 
 is a suggestion of the undoubtedly plant-like character of many of the 
 ccelenterates. A sessile habit is very general, though often only a phase 
 in the life-history, and asexual reproduction runs riot. A well-fed hydra 
 is prolific in bud-bearing; and numerous gradations connect this with the 
 myriad colonies exhibited by many hydroids. The individuals forming a 
 united family share in the common life and nutriment. As the colony 
 becomes complex, it is often physically impossible for all the members to 
 
2 06 
 
 THE EVOLUTION OF SEX. 
 
 remain on terms of even approximate equality of internal and external 
 conditions. One becomes relatively overfed, another starved. Slight 
 differences of function gradually become emphasised and exaggerated, till 
 division of labour is established. The structural aspect of this is differen- 
 tiation or polymorphism among the members of the colony, and results in 
 the establishment of nutritive and reproductive, sensitive and protective, 
 "persons." Thus in the common Hydractinia, the open-mouthed nutri- 
 tive individuals are markedly contrasted with the dependent reproductive 
 persons; and again, in different form, the rhythm repeats itself in the 
 contrast between active, offensive, and sensitive elongated members, and 
 entirely passive and abortive spines, which form a chevaux-de-frise under 
 shelter of which the others cower. It is usually supposed that the sessile 
 hydroids are in a sense degenerate from more active ancestral types. The 
 free-swimming embryo becomes exhausted,. settles down, and exhibits pre- 
 dominant vegetativeness with postponed sexuality. In many cases, how- 
 
 One of the acarids or lice {Glyciphagus cursor) forming a life-saving cyst, while the 
 individual itself dies. 
 
 ever, there is a recovery of the ancestral liberty of action, for modified 
 "persons" are set adrift as active, free-swimming, sexual medusoids. 
 
 There are, however, active forms of the true medusoid type (Trachy- 
 meduSEe) which never descend to the sessile nadir of existence, but yet 
 exhibit the asexual tendency of the class in forming temporary clusters of 
 pendent buds. Lang has described a remarkable compound medusoid 
 (Gastroblasla raffaelii), which has sometimes as many as nine stomachs, 
 and may be assumed to be highly nutritive. The remarkable point, how- 
 ever, is that the compound adult is the result not only of continued bud- 
 ding, but of a process of rectangular incomplete division. Along with 
 some others it leads on towards the Portuguese man-of-war, or siphonophore 
 series. Here the larva develops at first into a simple medusa-like indi- 
 vidual, but this buds off a manifold series of "persons," which, by 
 dislocation or even migration, become arranged in all the beauty of the 
 siphonophore colonies, which surpass even Hydractinia in their division of 
 labour. It is difficult enough in some cases to distinguish between true 
 
ASEXUAL REPRODUCTION. 
 
 207 
 
 mere organs lik 
 
 "persons" — which Haeckel calls " Medusomes " — and 
 protective bracts, which are also budded off. 
 
 In another direction, viz., among the true jelly-fishes (Acraspeda), 
 where an active habit greatly preponderates, we still find the occurrence of 
 asexual multiplication. Some forms [e.g., Pelagia) are entirely free; at 
 the opposite extreme a few [Luceiyiaj ia) may be described as sedentary; 
 between these we find the common Aurelia, which settles clown in its 
 youth, and gives rise by division to what afterwards become the large 
 sexual jelly-fishes (see fig. p. 215). 
 
 Sipbonophore Colony, showing the float (a), the 
 swimming-bells (/'), and the nutritive, repro- 
 ductive, and other "persons" beneath. — 
 From Lang, after Haeckel. 
 
 There remain two classes of ccelentcrates, — the Ctenophora, like Bene, 
 which represent a climax of activity, and never divide; and the Actinozoa 
 (sea-anemones and corals), which lead to a passive terminus again, and 
 exhibit profuse asexual multiplication. A few sea-anemones divide nor- 
 mally, just as they may be multiplied by artificial cutting. Fragments may 
 also be given off in an arbitrary sort of fashion, reminding one of the over- 
 flow buds of sponges. In Metridhtm fimhrialiim. studied by II. B. Torrey 
 ( 41 Proc. California Acad. Sci.," 3rd Scries, i., I09S, pp. 345-360, 1 pi.), 
 
208 
 
 THE EVOLUTION OF SEX. 
 
 there is a great variety of asexual multiplication, which may occur by longi- 
 tudinal fission, laceration, and budding. The budding of corals takes many 
 forms, resulting in the quaint complexity of brain-corals and the like. In 
 one sea-anemone {Gonactinia prolifera), where transverse division occurs, 
 it is interesting to notice that this has only been observed in young forms 
 with undeveloped sexual organs. It recalled, in fact, the asexual multipli- 
 cation of a young jelly-fish. In another of the corals (one of the Antipa- 
 tharia) Brook observed that a nutritive "person" may, by constriction, 
 form a reproductive individual on either side. 
 
 Worms. — The lower worm-types are roughly distinguishable from most 
 of the higher by the broad fact that they are all of a piece, without rings 
 or segments. A physiological link, however, between 
 worms of only one segment and those with many, is 
 found in the asexual chains which some of the former 
 occasionally develop. Thus the little turbellarian 
 Microstomum lineare may bud off a temporary chain 
 of sixteen individual links. The budding begins at 
 the posterior end, and what is partly separated off is a 
 portion in excess of the normal size. The second link 
 grows till it attains the usual adult size, and as it ex- 
 ceeds this forms a third link. At the same time the 
 original individual may also be doing the same, and 
 thus a chain of four is formed. Two more buddings 
 by each of the links bring the asexual process to a 
 climax, and then the individuals separate from one 
 another and become sexual in freedom. It is important 
 to notice that the asexual reproduction takes place in 
 favourable nutritive conditions, and as each individual 
 exceeds its normal limit of growth. In some allied 
 planarians the asexual multiplication is effected not by 
 budding but by division. Zacharias observed that when 
 nutrition was checked the vegetative increase ceased, 
 and sexual reproduction set in. Not quite parallel with 
 the above, but quite asexual, is the prolific multiplica- 
 tion characteristic of the flukes and tapeworms. The 
 common liver - fluke has several asexual generations 
 Diagrammatic repre- before it finds its final host in the sheep, and is sur- 
 sentationofthefor- passe d j n this respect by some of its relatives. A 
 mation of a chain f, -,, . ' . 3 . , , . - 
 
 ofbdividualsinthe bladder-worm, in passive ease, with a plethora of 
 Turbellarian worm nutrition, may form asexually many "heads," each of 
 Microstomum tin- which, inside a future host,' grows out into the long 
 series of joints which compose the tapeworm. In their 
 profuse asexual multiplication these parasites are like 
 parasitic fungi, but unlike them in the retention of the sexual process 
 as well. 
 
 In their asexual reproduction, the Polyzoa recall sponges, for not only 
 do they all multiply by budding, and that abundantly, but they form 
 peculiar winter-buds like sponge-gemmules, by which, on the death of the 
 parent, the continuity of life is nevertheless sustained. The winter-buds or 
 statoblasts may further resemble sponge-gemmules in elaborateness of ex- 
 ternal equipment, a common characteristic of passive resting structures. 
 In the higher bristle-footed worm-types (Chaetopoda), asexual multiplica- 
 
 ea.7-e.- 
 nis. 
 
ASEXUAL REPRODUCTION. 209 
 
 tion occurs in great variety of expression. Some, when alarmed, break up 
 into pieces, and others are known to do this in apparently normal life. 
 Each part in favourable conditions may reproduce the whole. Thus, at 
 a comparatively high level among animals, reproduclion may be literally 
 rupture. Oftener, however, budding precedes the division, and curious 
 chains of ringed worms are thus produced. Nor do the budded individuals 
 always keep in a straight line, but, as in the freshwater naids, may abut at 
 angles, and form a quaint living branch. To what degrees this irregularity 
 of budding may attain is well seen in the accompanying cut of a portion of 
 a worm {Syl/is ramosa), found on the " Challenger" voyage. The buds 
 occur laterally, terminally, or on any broken surface, and the result is an 
 almost bush-like compound organism rivalling even the hydroids in the 
 freedom of its branching. Some of the branches become males or females, 
 and go separate, or are sent adrift. In other syllids the separation of a 
 series of joints as a sexual individual has been repeatedly observed, or this 
 may be reduced till only one joint, laden with reproductive elements, is set 
 
 A Sea-worm {Myrianlddj which has budded off a chain of individuals. — After 
 Milne-Edwards. 
 
 free. In many of the Chsetopods the budding begins when the normal 
 size of the individual has been stopped by unfavourable conditions, which 
 bring about separation, and the subsequent sexuality of the liberated indi- 
 viduals. When the entire individual is modified at the sexual period the 
 term epigamy is used; this is illustrated among Nereida?, Syllidea?, and 
 Hesioniae. When only a part is modified sexually and is discharged as a 
 mouthless free-swimming form, the term schizogamy is used. This is best 
 known in various species of Syllis. In some cases, however, Exogone gem- 
 mifera and Autolytus longeferiens, both epigamy and schizogamy are illus- 
 trated in one life-cycle. (See A. Malaquin, " Epigamie et Schizogamie 
 chez les Annelides," Zool. Anzeig., xix. (1896), pp. 420-423.) The facts in 
 regard to reproductive processes in these Folychaet worms are indeed very 
 complex; we cannot do more than note three representative modes. 
 
 (a) In Nereis, for instance, certain of the segments become sexually 
 mature (epitokous) and differ markedly from the non-reproductive (atokous) 
 segments ; but reproduction (usually fatal) occurs without any separation of 
 the epitokous and atokous parts. 
 
 14 
 
THE EVOLUTION OF SEX. 
 
 (/■) In the palolo-worm, Eunice viridis, portions of which swarm in 
 extraordinary numbers at certain definite times off Samoa, about two-thirds 
 
 
 f %*il 
 
 
 
 *|#4r 
 
 #Sl-' v 
 
 
 
 %<%// 
 
 
 PT 
 
 Syllis rainosa, a ringed marine worm, in which asexual multiplication has pro- 
 duced a branched appearance.— From M'lntosh, "Challenger" Rep. on 
 Annelida. 
 
 of the body is reproductive or epitokous, and differs in its narrowness, dark 
 colour, and structural details from the anterior atokous region. The epito- 
 
 Comet form of a Starfish, showing how one arm "regenerates " or reproduces 
 other four. — From Carus Sterne, after Haeckel. 
 
 kous region is set free, breaks up into fragments, liberates the germ-cells, 
 and dies. All the swarming portions wlitch make the water for a short 
 
ASEXUAL REPRODUCTION. 
 
 211 
 
 time "like vermicelli soup" are headless. The intact worm seems to live 
 in the crevices of the coral reef. 
 
 (c ) In Syllidese the epitokous segments separate and bud out a head ; the 
 atokous segments also survive, and have the power of forming new segments. 
 
 Starfishes and the like surrender their "arms" so readily, that some 
 have supposed that they might, in this way, normally multiply. A volun- 
 tary surrender of parts as a mode of multiplication is, however, in this case 
 difficult to prove. It is said to occur in Cucumaria planet, one of the sea- 
 cucumbers. While crustaceans, insects, spiders, and molluscs may lose and 
 regrow certain parts, no asexual multiplication occurs. 
 
 Adventitious buds forming at the sides of a leaf of Bryojihylhmt 
 calycinum. — From nature. 
 
 In the tunicates the asexual process has again full play. It is not con- 
 fined to the passive sessile forms, where one might expect it, but occurs in 
 some of the free-swimmers as well. From a creeping stem buds may arise, 
 like plants from a rhizome; or a parent form may bud off all round, and 
 finally die away, leaving the offspring in a circle round a cavity. Both by 
 budding and division chains may be formed, as in the salpas. In these 
 lowly vertebrates asexual multiplication terminates. How the process 
 often alternates in regular rhythm with ordinary sexual reproduction will 
 be discussed in the next chapter. 
 
212 THE EVOLUTION OF SEX. 
 
 SUMMARY. 
 
 1. Artificial division maybe readily utilised as a means of multiplica- 
 tion in plants and in lower animals. 
 
 2. Regeneration of lost parts is very common both in plants and 
 animals. 
 
 3. Asexual reproduction from continuous budding to discontinuous 
 multiplication has many degrees. 
 
 4. It occurs in many types from Protozoa to Tunicata. 
 
 LITERATURE. 
 
 General Works cited : the ordinary Zoological and Botanical Text-books. 
 Cauli.ery, M., and Mesnil, F. — Les formes epitoques et revolution des 
 
 Cirratuliens. Ann. Univ. Lyon., XXXIX., 1898, pp. 1-200, 6 pis. 
 Fredericq. — La Lutte pour l'Existence chez les Animaux Marins. Paris, 
 
 1 889. ( For " Regeneration of Parts, " &c. ) 
 Haeckel.— Generelle Morphologic Berlin, 1866. 
 Lang, A. — Ueber den Einfluss des Festsitzen auf den Thieren, und neber 
 
 den Ursprung der ungeschlechtlichen Fortpflanzung. Jena, 1888. 
 Spencer. — Principles of Biology. London, 1S66. 
 
CHAPTER XV. 
 
 Alternation of Generations. 
 
 § i. History of Discovery. — Early in the century the poet 
 Chamisso, accompanying Kotzebue on his circumnavigation of 
 the globe, observed in one of the locomotor tunicates (Salpa) 
 that a solitary form gave birth to embryos of a different char- 
 acter, connected together in chains, and that each link of the 
 chain again produced a solitary form. Chamisso's observation 
 does not seem to have been quite accurate, but there is no 
 doubt that he first called attention to what is by no means an 
 uncommon fact, that an organism produces an offspring very 
 unlike itself, which by and by gives origin to a form like the 
 parent. The progress of marine zoology and the study of 
 parasitic worms gave naturalists like Sars, Dalyell, Loven, Von 
 Siebold, and Leuckart, early glimpses of many alternations in 
 life-history, but Steenstrup was the first to generalise the results. 
 This he did (1842) some twenty years after Chamisso, in a 
 work entitled " On the Alternation of Generations ; or, The 
 Propagation and Development of Animals through Alternate 
 Generations, a peculiar form of fostering the young in the 
 lower classes of animals." From hydroids and flukes, he gave 
 illustrations of the "natural phenomena of an animal producing 
 an offspring, which at no time resembles its parent, but which 
 itself brings forth a progeny that returns in its form and nature 
 to the parent." The interpolated generation he distinguished 
 by the name of "Amme" or "wet-nurse." In 1849, Owen 
 submitted Steenstrup's essay to stern criticism, rejecting especi- 
 ally the metaphorical name "nurse" as but a verbal explanation, 
 and proposing to explain what he also called "alternation 
 of generations," along with parthenogenesis and other pheno- 
 mena, by the supposition of a residual germ force or spermatic 
 power in the cells of the apparently asexual offspring. In 
 this he partially prophesied the modern conception of a 
 residual persistent germ-plasma. Soon afterwards Leuckart 
 
214 THE EVOLUTION OF SEX. 
 
 attempted to treat all as cases of metamorphosis, thereby greatly 
 extending the meaning of that term. The labours of some of 
 the foremost naturalists have both extended Steenstrup's obser- 
 vations and rendered them more precise. We now know that 
 the phenomenon is of wider occurrence than was at first sup- 
 posed, and also that the title has been unduly extended to cover 
 
 Diagrammatic representation of alternation of 
 generations, as, asexual generation ; s, 
 sexual generation. 
 
 II. Shows alternation of asexual (as) 
 and sexual (s) generations. 
 
 In I. the sexual is becoming increas- 
 ingly subordinated to the asexual (as in 
 flowering plants). 
 
 In III. the asexual is increasingly 
 subordinated to the sexual (in mosses). 
 
 several entirely different sets of facts. It is necessary, therefore, 
 to notice the various forms which the rhythm of reproduction 
 
 may take. 
 
 § 
 
 1 he clearest case to start with is that of many hydroids. 
 A sessile, plant-like zoophyte, which buds off numerous nutritive 
 persons, produces in the warm months modified individuals 
 which are set adrift as medusoid persons. Unlike the hydroid 
 which bore them, these become sexual; and from their fertilised 
 
 S 2. The Rhythm between Sexual and Asexual Reproduction.- 
 
ALTERNATION OF GENERATIONS. 
 
 "5 
 
 ova an embryo develops, which eventually settles clown to start 
 a new sessile colony. And thus through the seasons we have 
 hydroid asexually producing sexual medusoids, and these again 
 producing hydroids. The life-history for two complete rhythms 
 may be written in the formula, in which M, F, and A stand for 
 male, female, and asexual forms respectively,— 
 
 M _ . __ M _ ' _ M 
 
 F F F 
 
 A, asexual hydroid ; S, sexual medusoid ; fertilised ova at base. 
 
 Or take, in slight contrast, the life-story of the common jelly- 
 fish Aurelia. Large free-swimming sexual animals produce ova 
 which are fertilised by sperms ; the embryo develops, not how- 
 
 The alternation of generations in the common jelly-fish Aure2iet\ i, the free-swimming embryo, 
 or planula ; -j., the embryo settled down ; 3, 4, 5, 6, the developing asexual stage, or hydra- 
 tuba \ 7, 8, the formation of a pile of individuals ; 9, the liberation of these ; 10, 11, the 
 acquisition of the free-living sexual medusa form. — From Hatckel. 
 
2l6 THE EVOLUTION OF SEX. 
 
 ever into a jelly-fish, but into a sessile hydroid-like organism or 
 "hydra-tuba." By growth and division this asexually produces 
 the jelly-fish in turn. Here the sexual generation is more stable 
 and conspicuous, the reverse of the former case, but the same 
 formula applies. 
 
 Or take a case from another class of animals, the marine 
 worms. Some of the syllids have the following life-history. A 
 worm remains asexual, never attaining either the external 
 characteristics or the internal organs of the sexual individuals. 
 It gives rise to these, however, by an asexual process of chain- 
 making. Sexual individuals are budded off from the asexual, 
 into which their fertilised ova in turn develop. This must, of 
 course, be distinguished from cases where asexual multiplica- 
 tion is only a phase preceding the acquisition of sexuality 
 The above cases are again expressible in the simplest formula. 
 
 Now take a more complex case, from among the tuni- 
 cates, the highest point at which the genuine alternation can be 
 said to occur. From the single ovum of a sexual Salpa an 
 asexual individual develops which remains for a time connected 
 with its parent. It eventually escapes and becomes free-swim- 
 ming, and after a time forms a process or stolon on the surface 
 of which buds are formed. The buds develop and form a 
 chain of individuals. Then the stolon with its chain is set 
 free from the asexual parent, and the links of the chain become 
 sexual Salpas. 
 
 In the allied Doliohim the life-history is more complicated. 
 A sexual form gives rise sexually to tailed asexual larvae; the 
 larva loses its tail and forms two processes or dorsal and ven- 
 tral stolons; buds from the ventral stolon migrate to the dorsal 
 stolon and develop there into a second set of asexual forms; 
 the dorsal stolon also forms buds on its own account; some of 
 these individuals on the dorsal stolon are set adrift, and each 
 develops a ventral stolon from which the young sexual forms 
 arise. There are thus several asexual generations between one 
 sexual stage and the next. 
 
 § 3. Alternation between Sexual and Degener a' e Sexual Reproduction. — 
 The cases we have just noticed are both easier to state and easier to explain 
 than others which are sometimes also included under the vague title of 
 " alternation of generations." The above alternations were between sexual 
 and asexual reproduction, and must-be distinguished, vague as the boundary 
 must be, from alternation between the ordinary sexual process and a 
 degenerate form of the same. 
 
 The history of some of the flukes (Trematoda) may be taken as a first 
 
ALTERNATION OF GENERATIONS. 2l^ 
 
 illustration. The common liver-fluke (Distomum or Fasciola hepalica) 
 which causes the disastrous "rot" in sheep has a life of vicissitudes. The 
 fertilised ovum gives rise to an embryo ; this passes from the sheep, which 
 its sexual parent infested, to the water by the field side. There it leads for 
 a few hours an active life, knocking against many things, but finally 
 attaching itself to a minute water-snail {Limnmus truncalulus). Into this 
 it bores, losing its covering of active cilia with change of habit, and be- 
 coming much altered into a passive vegetative form known as a sporocyst. 
 Now this sporocyst sometimes divides ; and if this were all, and the results 
 grew up into liver-flukes, we should have the old formula and less loss of 
 sheep. But direct development never occurs, and we may leave the casual 
 division at present out of account. Certain cells within the sporocyst 
 develop like parthenogenetic ova. They produce within the body of the 
 sporocyst another brood of what are called Redia. There may be several 
 generations of redise, but the final result is a brood of minute tailed organisms 
 (Cercarice), which leave the water-snails, leave the water even, creep up 
 grass stems, and encyst themselves. There most wait for death, a few 
 for the attainment of adult sexual life if they chance to be eaten by a 
 sheep. 
 
 This cannot be accurately ranked as parallel to what occurs among the 
 above-mentioned tunicates, for the redia? arise from precocious reproductive 
 cells. These cannot be called ova, and there is no fertilisation, but yet the 
 process is not one of division, or of budding. It is a degenerate process of 
 parthenogenetic reproduction in early life. The facts may be again summed 
 up in a formula, which does not take account of the occasional division 
 of the "sporocyst." 
 
 A, asexual larvae ; S, sexual fluke ; the upper circles represent 
 the special reproductive cells ; fertilised ova at the base. 
 
 This alternation between sexual reproduction with the usual fertilisation, 
 and reproduction by means of special cells which yet require no fertilisation, 
 prevails in many plants, e.g., ferns and mosses. From a fertilised egg-cell 
 the ordinary fern-plant, with which every one is familiar, develops. But 
 this is quite asexual, if we mean by that that it is neither male nor female, 
 and that it produces neither male nor female elements At the same time 
 it produces special reproductive cells, — not egg-cells exactly, any more than 
 those within the sporocyst were, but yet able to develop of themselves into 
 a new organism. This is not another fern-plant, however, but an incon- 
 spicuous green organism, much less vegetative, and sexual. The so-called 
 "spore " formed on the leaves of the sexless fern-plant falls to the ground, 
 develops a "prothallus," which bears male or female organs, or both. An 
 egg-cell is fertilised by a male element, and the conspicuous vegetative fern- 
 plant once more arises. The formula is therefore as follows:— 
 
2l8 
 
 THE EVOLUTION OF SEX. 
 
 Where A=sexless vegetative fern-plant; 
 
 sp. — the parthenogenetic special reproductive cell or spore ; 
 S = the sexual inconspicuous "prothallus," with male and female organs. 
 
 Now take the history of a moss. Unlike the fern, the more conspicuous 
 "moss-plant" is sexual. It bears male and female organs, and an egg-cell 
 is fertilised by a male element. The fertilised egg-cell, however, does not 
 lose its hold of the mother plant, but grows like an encumbering parasite 
 upon it. Obviously, then, it does not give rise to another "moss-plant." 
 The result of the fertilised egg-cell is a tiny sexless stalk, which bears on its 
 apex the special reproductive cells or spores with which we are now familiar. 
 In other words, the fertilised egg-cell develops into a parasitic spore-bearing 
 generation. The "spores " fall into the ground, as they did in the fern, 
 and there grow into a usually thread-like structure, from which the sexual 
 moss-plants are budded off. If we do not emphasise the transitional thread- 
 like stage, — the protonema as it is called, — the formula is as follows: — 
 
 Where A — inconspicuous sexless parasitic generation upon the "moss-plant" ; 
 
 sp. = the special parthenogenetic reproductive cell or spore produced hy A ; 
 S = the conspicuous sexual " moss-plant," budded from the threads developed 
 from the spore. 
 
 If we do emphasise the " protonema " stage (/), and regard the moss- 
 plants as asexually budded from it, the formula runs: — 
 
 In the fern, the vegetative sexless generation was ihe more conspicuous; 
 in mosses, the sexual generation. In a way this recalls the contrast 
 between the life-history of many a zoophyte, and that of the common 
 jelly-fish Aurelia. The asexual hydroid colony is more conspicuous than 
 
ALTERNATION OF GENERATIONS. 
 
 219 
 
 the usually small swimming-bell, but the sexual jelly-fish is much more 
 conspicuous than the minute asexual "hydra-tuba." The common com- 
 parison between medusoid and hydroid on the one hand, and prothallus 
 and fern-plant on the other, is rather misleading, simply because the 
 hydroid merely buds off the medusoid, while the fern-plant produces the 
 prothallus from a special reproductive cell or spore. In some ferns and 
 mosses, however, a more exact parallel is occasionally exhibited. The 
 production of "spores" may be suppressed, and from the place where they 
 should he formed a (sexual) fern-prothallus or a new (sexual) moss-plant is 
 vegetatively developed, just as medusoid from hydroid. This exceptional 
 occurrence is technically called apospory. The very opposite of this also 
 occurs, the suppression not of the spore-bearing, but of the sexual genera- 
 tions. The fern-plant then arises vegetatively from the prothallus; and 
 this would be paralleled if we supposed the sporocyst of the fluke to bud 
 off redise (as it sometimes does), and these to continue the species without 
 ever becoming really sexual, solely by means of the special cells above 
 described. This is called apogamy. 
 
 Apogamy has also been described by Treub in the remarkable flowering 
 plant Balanophora, where the abortion and disappearance of the egg- 
 apparatus is closely parallel to what occurs in one of the brackens (Pteris 
 cretica). ("Ann. Jard. Bot. Buitenzorg," xv., 1898, pp. 1-25, 8 plates.) 
 
 § 4. Combination of both these Alternations. — The asexual hydroid buds 
 off a medusoid, the fertilised ovum of which develops into a hydroid. 
 Here there is simple alternation between sexual and asexual reproduction. 
 
 s~* 
 
 ^~* 
 
 A sexless fern-plant forms special reproductive cells (spores), which 
 develop parthenogenetically into a sexual prothallus, from the fertilised 
 egg-cell of which the fern-plant arises. 
 
 The difference between these two alternations has been as often pointed 
 out as it has been ignored. The former is called true alternation of 
 generations (or metagenesis); the latter is called by zoologists, in reference 
 to flukes for instance, heterogamy. Comparisons between the alternations 
 in plants and animals have seldom recognised the distinction. 
 
220 
 
 THE EVOLUTION OF SEX. 
 
 Let it be recognised, however, and we can readily proceed to more 
 complicated cases where the two are combined. Returning to the liver- 
 fluke and others like it, we find that the sporocyst sometimes multiplies in 
 a genuinely asexual fashion — without the intervention of precocious ova, 
 special reproductive cells, germs, or spores, as they may be called — 
 by direct division or budding. For such cases the formula must be 
 modified as follows: — 
 
 The complication is not serious. It is simply that, before the/ multipli- 
 cation by special cells sets in, there may be more than one (A', A") entirely 
 asexual (and not merely sexless) generation. 
 
 § 5. Alternation of Juvenile Parthetiogenetic Reproduction with the 
 Adult Sexual Process. — We have already noted the curious precocity of 
 some midge larvae, which reproduce while still young. Cells within the 
 body, apparently precocious ova, develop parthenogenetically into larva, 
 which prey upon the mother larva, eventually kill her and leave her, only 
 themselves to become in turn similar victims of precocity. This may con- 
 tinue for a series of generations, with continuous decrease in the size of 
 the reproductive cells, till finally true sexuality and adult life is attained. 
 The reproductive cells here are rather more differentiated than those in the 
 young flukes, but the close parallelism is indubitable. Except that there is 
 for a while no fertilisation, the process can hardly be called asexual. The 
 formula may be expressed in a gentle curve: — 
 
 Where the starting-point is as before a fertilised ovum ; 
 L=prematurely reproductive larva ; 
 ps= precocious parthenogenetic "pseudova"; 
 S=adult sexual male or female organism. 
 
 Somewhat different is the curious case of Gyrodactylus, a trematode 
 parasitic on fresh-water fishes, where three generations are found enclosed, 
 one within the other, in a fashion which recalls the fancies of the preforma- 
 tionists. In this case, however, it seems likely that internal fertilisation 
 really occurs. 
 
 § 6. Alternation of Parthenogenesis and Ordinary Sexual Reproduction. 
 — In our gradual ascent, jve now reach the frequent alternation of partheno- 
 genesis and ordinary sexual reproduction. The special cells which develop 
 without fertilisation are now genuine parthenogenetic ova, and the organ- 
 isms which produce them are adults, not juveniles. The formulte will 
 
ALTERNATION OF GENERATIONS. 221 
 
 differ mainly in the number of generations through which the partheno- 
 genesis may be continued. 
 
 Where the starting-point is a fertilised ovum ; 
 
 P = parthenogenetic female, producing a parthenogenetic 
 
 ovum, from which arise other parthenogenetic forms, 
 
 or eventually ; 
 S = male and female. 
 
 § 7. Alternation of Different Sexual Generations. — The rhythm may 
 be followed in yet a higher scale. In a. very few cases there is an alter- 
 nation between two different sexual generations. Thus one of the thread- 
 worms (Leplodera appendiculata) found in the snail gives rise, by the 
 ordinary sexual process, to a different form, which leads a free life, and 
 subsequently gives origin to the parasite. In both generations the sexes 
 are distinct. More remarkable still is the history of another nematode 
 (Angioslomum nigrovenosum), found in the lung of the frog. It is physio- 
 logically hermaphrodite, though its organ is ovary-like ; its eggs are 
 fertilised by its own sperms, which mature first; the progeny become sexual 
 — males and females — in the earth, and their offspring return to the frog, 
 where they become hermaphrodites. Another example of alternation of 
 sexual generations is found in one of the threadworms which occur in man 
 (Rhabdonema strongyloides). 
 
 § 8. Occurrence of these Alternations in Animals. — From sponges to 
 tunicates such alternations occur. Beyond the latter, unless we wish to be 
 very subtle, they cease. It is necessary to be clear about the fact that 
 asexual and sexual reproduction may occur together in the same form. 
 The common hydra gives off buds in an entirely asexual way, but it is also 
 a sexual animal, with male and female organs. There may be periods of 
 vegetative growth and climacterics of sexuality in the same organism, with- 
 out any alternation of generations 
 
 It is possible that the term alternation of generations may be applied 
 to some of the phenomena observed in the Protozoa. Thus Brandt main- 
 tained that all the colonial radiolarians, known as Sphaerozoa, form on the 
 one hand isospores, which are all equal and apparently parthenogenetic, 
 and on the other hand anisospores, which are large and small, — in fact, 
 sexually dimorphic. He believes — though the fact cannot be called 
 demonstrated — that two unequal anisospores unite to form a double cell, a 
 fertilised unit, which will produce isospores again, and these the normal 
 colony. The generation of these Sphaerozoa is further complicated (a) by 
 division of the colonies, (6) by division of the individuals of young vegetative 
 
2 22 THE EVOLUTION OF SEX. 
 
 colonies, and (c) by the formation of special " extra-capsular " reproductive 
 bodies in young colonies. 
 
 The history of the common fresh-water sponge (Spongilla), as told by 
 Marshall, is one of many vicissitudes. In autumn the sponge begins to 
 suffer from the cold and scarcity of food. It dies away ; but some of the 
 units save themselves, and, in a sense, the parent, by forming the 
 "gemmules" we have already noticed. These winter in a quiescent state 
 within the parental corpse, but in spring they get out of the debris, and 
 start male or female sponges. The males are short-lived, but their male 
 elements fertilise the ova of the females. The fertilised ovum develops 
 into a ciliated embryo, and this into an asexual sponge, which produces the 
 gemmules. 
 
 The starting-point a fertilised ovum, which develops into 
 A = asexual sponge, which forms only 
 G=gemmules, which develop into 
 S = male and female sponges. 
 
 Besides the hydroid and medusoid, the hydra-tuba and jelly-fish alterna- 
 tions, which we have already noticed, there are many complications of 
 degree among ccelenterates. The medusoid stage degenerates by subtle 
 gradations, ceasing to be free, and eventually becoming what, if its history 
 were not known, would be called an organ rather than a " person" of the 
 colony. Furthermore, it may itself take to budding, and continue the 
 asexual habit of the hydroid from which it springs. Outside the Hydrozoa, 
 genuine alternation of generations does not occur, unless that described by 
 Semper for Fungia corals be accepted as such. 
 
 A very interesting alternation has been described by W. K. Brooks in 
 a remarkable medusa (Epenthesis macradyi). On the reproductive organs 
 of this swim-bell there grow, like parasites, what are exactly comparable 
 to the reproductive buds (blastostyles) of a hydroid, and these form medu- 
 soids by budding. The result is a compound colony, which approaches the 
 Siphonophora. The process recalls and surpasses the apogamy of a few ferns. 
 
 Among worm-types, the strict alternation of generations in some of the 
 marine cheetopods (syllids), the more complicated phenomena of so many 
 trematodes, the sexual rhythms of that peculiar threadworm Angiostomum, 
 have been already discussed. It is necessary, however, to state the case for 
 tapeworms, which are usually included among the examples of alternation 
 of generations. The usual view is, that the embryo of a tapeworm develops 
 into an asexual bladder-worm, which asexually buds off a " head," or more 
 than one. Such a "head," passing to another host, buds off asexually 
 the chain of reproductive joints or sexual individuals which constitute a 
 tapeworm. Asexual bladder-worm, asexual "head," and sexual joints, 
 form the series. That there is a genuine alternation of generation is 
 believed by some authorities, but there are emphatic difficulties against 
 this supposition, except in the occasional occurrence of a bladder-worm 
 with several " heads," each of which may develop into a tapeworm. The 
 
ALTERNATION OF GENERATIONS. 223 
 
 case is well stated by Hatchett Jackson in his monumental edition of 
 Rolleston's " Forms of Animal Life," and we accept his verdict that there 
 is really one individual throughout, except when asexual multiplication of 
 heads occurs. The tapeworm, on this view, is an adult sexual bladder- 
 worm, and the joints are only highly individualised segments. 
 
 Of the parthenogenetic cycles in crustaceans and insects, the juvenile 
 reproduction of some of the latter, and the true alternation of generations 
 in sometunicates, enough has already been said. 
 
 Von Jhering is responsible for starting the paradox that in higher 
 animals a mother may bring forth her grandchildren. He refers to the 
 case of the hyaena-like edentate rraopus, where a single ovum gives rise to 
 eight embryos, which are thus in a pedantic sense grandchildren ! The 
 frequent occurrence of twins in all groups, the remarkable case of an earth- 
 worm (Lumbricus trapezoides) in which a double embryo is constant, and 
 the morphological resemblance of polar globules to abortive germs, led Von 
 Jhering to maintain that the origin of multiple embryos from a single 
 ovum is the primitive and normal condition, and that the development of 
 only one is secondary and adaptive. The data are hardly sufficient for such 
 a striking conclusion. 
 
 Paul Marchal has described in regard to Encyrtus fuscicollis, one of the 
 parasitic Hymenoptera, which lays its eggs in those of another insect, 
 Hyponomeutes, the remarkable peculiarity that the ovum gives rise not to 
 one embryo, but to "a legion of small morulae," forming a chain of 50-100 
 embryos. The amniotic envelope loses its vesicular form and becomes a 
 long flexible tube, within which lie the embryos surrounded by granular 
 material apparently derived from a disruption of part of the original egg. 
 As the author says, this is a mode of reproduction unique among Arthro- 
 pods, if not among animals. (" Comptes Rendus," cxxvi., 1898, pp. 662-4.) 
 
 § 9. Beard's Hypothesis of Alternation in Vertebrates. — Beard has 
 propounded a somewhat subtle theory which suggests that there is a 
 disguised alternation of generations in vertebrate animals, just as there 
 is in flowering plants. In several invertebrate types, e.g. Echinoderms 
 and Phoronis, the egg gives rise to a larva which does not directly develop 
 into the definitive organism, but serves as the foundation on which the 
 development recommences as it were de novo. Similarly, Beard believes 
 that in Vertebrates, whether skate or chick, there are traces of an asexual 
 larval stage on the lop of which the embryo proper develops. At the 
 " critical stage," when the embryo begins to put on its generic and specific 
 characters, it also sets about the task of suppressing the larval foundation. 
 ("On Certain Problems of Vertebrate Embryology," Jena, 1896, vi. and 
 77PP-) 
 
 § 10. Occurrence of Alternations in Plants.- — In the lower 
 plants, algse and fungi, an alternation between spore-producing 
 and truly sexual generations is frequent. In mosses and ferns it 
 is almost constant, and yet more marked. Occasionally either 
 spore-formation or sex-cell formation may be suppressed, and 
 the life-history thus simplified. In a few of the higher plants 
 both are exceptionally suppressed, and we have thus a reversion 
 to a purely vegetative process, just as if a hydra went on giving 
 off daughter-buds without ever becoming sexual. In the 
 
2 24 THE EVOLUTION OF SEX. 
 
 flowering plants, what corresponds to the sexual generation of 
 a fern is much reduced; it has come to remain continuous with 
 the vegetative asexual generation, on which it has reacted in 
 subtle physiological influence. Just as in the higher animals, 
 alternation of generations finds at most only a rudimentary 
 expression. 
 
 §11. Heredity in Alternating Generations. — The problem 
 of the relative constancy of inheritance is now in part solved by 
 the theory of germinal continuity. The ovum which develops 
 into an offspring is virtually continuous with the ovum which 
 gave rise to the parent, A chain of ovum-like cells is only 
 demonstrable in a few cases; but Weismann overcomes this 
 difficulty, by supposing that what really keeps up the proto- 
 plasmic tradition or continuity between the parental ovum and 
 the next generation, is a specific and stable portion of the 
 nucleus, — the "germ plasm." When a medusoid goes off from 
 a hydroid, it carries with it a legacy of this germ-plasm, con- 
 tinuous with that which gave rise to the hydroid. This legacy 
 forms the reproductive elements of the medusoid, which in 
 turn give rise to hydroids. The medusoid itself is a modified 
 asexual growth, into which some of the germ-plasm of the 
 hydroid has migrated; it is literally only the bearer or trustee of 
 the hydroid germ-plasm. Weismann's. classic researches on 
 hydroids have shown that the reproductive cells, which, by 
 hypothesis, bear the germ-plasm, often arise far down in the 
 hydroid body, and actually migrate to their final seat in the 
 bearer. Where the alternation is not between sexual and 
 asexual, but between the ordinary sexual process and multipli- 
 cation by special parthenogenetic cells, as is the case in many 
 flukes, we may in the same way suppose that the cells within a 
 sporocyst which give rise to rediae are, like ova, charged with 
 this reproductive germ-plasm. It is very interesting to notice 
 that, as far back as 1849, Owen had a distinct prevision, not 
 only of the distinction between body-forming cells and repro- 
 ductive-cells, of which so much is now made, but of the essential 
 idea of the " germ-plasm." Speaking of the recurrence of a 
 parental form after numerous interpolated generations, he says, 
 " the essential condition is the retention of certain of the 
 progeny of the primary impregnated germ-cell, or, in other 
 words, of the germ-mass unchanged in the body of the first 
 individual developed from that germ-mass, with so much of 
 the spermatic force inherited by the retained germ-cells from 
 
ALTERNATION OF GENERATIONS. 225 
 
 the parent cell or germ-vesicle as suffices to set on foot and 
 maintain the same series of formative actions as those which 
 constituted the individual containing them." In this some- 
 what over-weighted sentence, if we read " germ-plasm " instead 
 of "spermatic force," we have a close approximation to the 
 modern conception of Weismann. Again, Owen says, " an 
 impregnated germ-cell imparts its spermatic power to its cell- 
 offspring; but when these perish, or when the power is 
 exhausted by a long descent, it must be renewed by fresh 
 impregnation. But nature is economical, and so long as 
 sufficient power is retained by the progeny of the primary 
 impregnated vesicle (the essential part of an ovum), individuals 
 are developed from that progeny without the recurrence of the 
 impregnating act." 
 
 § 12. Hints as to the Rationale of Alternation. — The puzzle 
 of alternation may be lessened if we consider the physiological 
 aspect of the facts. A fixed hydroid contrasted with a swim- 
 ming-bell or medusoid, a sessile hydra-tuba contrasted with an 
 actively locomotor jelly-fish, illustrate not a peculiar antithesis, 
 but a most general and fundamental rhythm of organic life, — ■ 
 that between nutrition and reproduction. The hydroid has a 
 relatively passive habit and a copious nutrition ; it is prepon- 
 derating^ vegetative and asexual. The reverse habit, the 
 physiological rebound, finds expression in the medusoid. In 
 the same way, though the alternation is less strictly between 
 asexual and sexual, the contrast between leafy spore-bearing 
 fern-plant and inconspicuous sexual prothallus is again funda- 
 mentally parallel. The notation adopted must have already 
 suggested our fundamental diagram, the different forms of which 
 may be separated out or superposed : — 
 
 SUM OF FUNCTIONS. 
 
 Anabo'iism. Katabolism. Female. Male. 
 
 is 
 
2 26 THE EVOLUTION OF SEX. 
 
 Although it has just been shown that the process of alter- 
 nation demands a much more thorough analysis and discrimi- 
 nation of the different cases than has hitherto been customary, 
 and this on the physiological as well as merely on the morpho- 
 logical side, the general aspect of the process, in which an 
 asexual form alternates with one or more dimorphic sexual 
 generations, makes it evident that we have here to do in two 
 generations with what is often so obvious in one, — the familiar 
 antithesis between nutrition and reproduction. A consideration 
 of the physiological distinctions between the asexual and sexual 
 generations, shows that the former is the expression of favour- 
 able nutritive conditions resulting in vegetative growth, or at 
 most in asexual multiplication, while the latter is conditioned 
 by less propitious circumstances. Just as a well-nourished 
 plant may continue propagating itself by shoots and runners, 
 and just as an aphis in artificial summer may for years repro- 
 duce parthenogenetically, so a hydroid with abundant food and 
 otherwise favourable environment may be retained for a pro- 
 longed period vegetative and asexual, while dearth of food and 
 otherwise altered conditions evoke the appearance of the sexual 
 generation. The contrast between the deeply-rooted well- 
 expanded fern-plant and the weakly-rooted slightly-exposed 
 prothallus, is obviously that between an Organism in conditions 
 favourable to the continuance and preponderance of anabolic 
 processes, and an organism in an environment where katabolism 
 is, at an early stage, likely to gain relative ascendency. The 
 former is thus naturally asexual, the latter sexual. A survey, 
 in fact, of the conditions and characteristics of the two sets of 
 forms, inevitably leads us to regard the asexual generation as 
 the expression of relatively predominant anabolism, and the 
 sexual as equally emphatically katabolic. Alternation of genera- 
 tions in its less complex forms may thus be described as a 
 rhythm between a relatively anabolic and katabolic prepon- 
 derance. 
 
 § 13. Origin of Alternation of Generations. — Even in an individual 
 plant or animal there are vegetative and reproductive periods ; alternation 
 of generations involves the separation of these to different individuals, by 
 the interpolation of more or less asexual reproduction. In most hydroids, 
 the asexual vegetative tendency preponderates; in most medusoids, great 
 activity is dominant. But the origin in each particular case is involved 
 in the pedigree of the organism. Thus Haeckel distinguishes a progressive 
 from a retrogressive origin; in the former, the organisms are in transition 
 from preponderant asexual to sexual reproduction; in the latter, the 
 
ALTERNATION OF GENERATIONS. 227 
 
 organisms are returning or degenerating from dominant sexuality to an 
 asexual process. It is safe to say that the latter is more frequently the 
 right interpretation of the facts. So far as reproduction is concerned, one 
 of those medusoids (Trachymedusa?) which have no corresponding hydroid 
 parent, or a jelly-fish like Pelagia which has no fixed asexual hydra-tuba 
 stage, is nearer the ancestral habit than those members of both divisions 
 which exhibit alternation of generations. In regard to alternating series of 
 similar forms with different degrees of sexuality, e.g. , parthenogenesis and 
 true sexual reproduction in aphides, Weismann suggested that the alterna- 
 tion may be associated with the periodic action of external influences 
 ("Studies in the Theory of Descent," chap. v.). But in contrast to such 
 cases he distinguished, {a) an origin from metamorphosis, where one stage in 
 the life-history becomes precociously reproductive, e.g. , in the midge 
 Cecidomyia; (d) the case of the Hydromedusa;, where sexuality is post- 
 poned in early life, and asexual reproduction dominates ; and {c) an origin 
 from division of labour within a colony. Without entering upon a discus- 
 sion of each case in relation to its history and environment, it is not possible 
 to do more than reassert that in many different degrees the continuous alter- 
 nation between growth and multiplication, nutrition and reproduction, 
 asexuality and sexuality, anabolism and kataholism, may express itself in 
 the life-history of the organism. 
 
2 28 THE EVOLUTION OF SEX. 
 
 SUMMARY. 
 
 1. The fact that successive generations may be markedly different was 
 observed by the poet Chamisso, and first made precise by the zoologist 
 Steenstrup. 
 
 2. A fixed asexual hydroid buds off and liberates locomotor sexual 
 medusoids, whose fertilised ova give rise again to hydroids. Asexual and 
 sexual generations alternate. 
 
 3. The offspring of the liver-fluke forms from certain cells in its body 
 a numerous progeny; these repeat the same process several times; the last 
 generation grow into the sexual liver-flukes. Reproduction by special 
 cells, like precocious undifferentiated ova, alternates with reproduction by 
 ordinary fertilised ova. So too the vegetative sexless " fern-plant " gives 
 rise to spores, which develop into an inconspicuous sexual prothallus. 
 From the fertilised egg-cell of the latter the "fern-plant" arises. 
 
 4. These two different kinds of alternations (§ 2 and § 3) may be com- 
 bined in a more complicated manner. 
 
 5- In some flies precocious parthenogenetic reproduction alternates with 
 the normal sexual reproduction of the adults. 
 
 6. In many insects and crustaceans, parthenogenetic reproduction alter- 
 nates with the normal sexual process. There may be one or many inter- 
 vening parthenogenetic generations. 
 
 7. A hermaphrodite threadworm parasitic in the frog fertilises its own 
 eggs, which develop into free-living males and females, from the fertilised 
 ova of which the hermaphrodite parasites again arise. Here there is an 
 alternation of sexual generations. 
 
 8. The occurrence of these various alternations is widespread, from 
 sponges to tunicates. 
 
 9. Beard's hypothesis of alternation in vertebrates. 
 
 10. In plants they occur in algae and fungi, are almost constant in ferns 
 and mosses, but are inconspicuous in higher plants. 
 
 11. The problem of heredity is somewhat complicated by such alter- 
 nations. 
 
 12. Alternation of generations may be described as a rhythm between 
 a relatively anabolic and katabolic preponderance. 
 
 13. Alternation of generations has probably arisen in various quite 
 distinct ways. 
 
 LITERATURE. 
 
 See the general works already cited; also, Steenstrup " On the Alternation 
 of Generations," transl. Ray Soc, 1845 ; Owen's " Parthenogenesis," 
 &c, 1849; Haeckel's " Generelle Morphologie," 1866; Weismann, 
 A., Die Entstehung der Sexualzellen bei den Hydromedusen, Jena, 
 1883, and Papers on Heredity (Translation), Oxford, 1889; Vines' 
 article "Reproduction — Vegetable," Ency. Brit. 
 
ALTERNATION OF GENERATIONS. 229 
 
 Beard, J. — On the Phenomena of Reproduction in Animals and Plants, 
 on Antithetic Alternation of Generations, and on the Conjugation of 
 the Infusoria. Anat. Anzeiger, XI., 1895, pp. 234-255, 5 figs. 
 
 Bower, F. O. — On Apospory and Allied Phenomena. Trans. Linn. Soc. 
 Botany, 1887. 
 
 Brooks, W. K. — Life-History of Epenthesis McCradyi n. sp. Stud. Biol. 
 Lab. John Hopkins Univ., IV., 1888, pp. 147-62, 3 pis. 
 
 Chamisso, A. de. — De Animalibus quibusdam e classe vermium Lin- 
 naeana in circumnavigatione terrse auspicanti comite N. Romanyoff 
 duce Ottone de Kotzebue annis 1815, 1816, 1817, 1818 peracta. 
 Fasc. I. De Salpa, Beroline, 1819. 
 
BOOK IV. 
 
 THEORY OF REPRODUCTION. 
 
CHAPTER XVI. 
 
 Growth and Reproduction. 
 
 § i. Facts of Groivtk. — In a well-known aphorism Linnaeus 
 noted that living organisms are not alone in their power of 
 growth. Crystals become centres for other crystals, till a large 
 mass results; and the product, as every case of minerals shows, 
 is often both orderly and complex. But there are great 
 differences between organic and inorganic growth, one of the 
 most obvious being that the organism is able to grow larger at 
 the expense of material different from itself, while the crystal 
 can only increase out of material similar to itself. The grass 
 grows at the expense of water, soil, and air; and the foal grows 
 at the expense of the grass, but a dead thing cannot grow in 
 this way. Probably of much less importance is the old dis- 
 tinction that the growth of organisms has a peculiar method of 
 its own, that of intussusception as distinguished from mere 
 accretion. The new particles which are taken in, more than 
 replacing previous expenditure, are not deposited upon the 
 surface of already established material, as is the case with a 
 crystal, but are intercalated in the interstices of previous 
 particles. We cannot enter here upon the long-continued 
 controversy, whether such structures as the cell-wall and starch- 
 grains of plants grow thicker or larger by accretion in crystal- 
 like fashion, or by intercalation which is supposed to be 
 characteristically organic. It is worth noticing, however, as 
 Biitschli points out, that if the living matter has the form of an 
 intricate network, the fresh material of replacement or growth 
 may be added to the surfaces of the threads which make the 
 web. Thus what is roughly called intercalation may be more 
 literally an internal accretion. 
 
 Hunger is a dominant characteristic of living matter. When 
 a unit mass or cell has been giving off energy in doing any kind 
 of work, its substance is chemically impaired, — less capable of 
 doing further work until new energy has been supplied by 
 
234 THE EVOLUTION OF SEX. 
 
 nutrition. The supply which the lifelong hunger of the proto- 
 plasm demands, is frequently afforded in greater abundance 
 than the actual necessities require. There is a surplus for 
 further upbuilding after mere reparation has been made. This 
 surplus is the condition of growth. In other words, growth or 
 addition to the capital of the organism occurs when income is 
 in excess of expenditure, when construction preponderates 
 over disruption. 
 
 But beside this familiar fact, it is necessary to place another 
 certainty, that of the limit of growth. There are among the 
 Protozoa a few giants, such as the large amoeboid Pelomyxa, 
 some of the gregarines, and even more markedly the extinct 
 nummulites, which were sometimes as large as half-crowns. So 
 an occasional alga, like Botrydium, may swell out into a large 
 single cell, and the ova of animals, e.g. birds, are often greatly 
 expanded by the accumulation of yolk. Yet cells generally 
 remain very small. They have their maximum size, approxi- 
 mately constant for each species. Up to this point they grow, 
 but no further. The same, as every one knows, is true of most 
 multicellular animals. The size fluctuates slightly according to 
 the conditions of individual life, but the average is strikingly 
 constant. 
 
 § 2. Spencer's Theory of Growth. — The first adequate dis- 
 cussion of growth is due to Spencer. He pointed out, that in 
 the growth of similarly shaped bodies the increase of volume 
 continually tends to outrun that of the surface. The mass of 
 living matter must grow more rapidly than the surface through 
 which it is kept alive. In spherical and all other regular units 
 the mass increases as the cube of the diameter, the surface only 
 as the square. Thus the cell, as it grows, must get into physio- 
 logical difficulties, for the nutritive necessities of the increasing 
 mass are ever less adequately supplied by the less rapidly 
 increasing absorbent surface. The early excess of repair over 
 waste secures the growth of the cell. Then a nemesis of grow- 
 ing wealth begins. The increase of surface is necessarily 
 disproportionate to that of contents, and so there is less 
 opportunity for nutrition, respiration, and excretion. Waste 
 thus gains upon, overtakes, balances, and threatens to exceed 
 repair. Suppose a cell to have become as big as it can well be, 
 a number of alternatives are possible. Growth may cease, and 
 a balance be struck ; or the form of the unit may be altered, 
 and surface gained by flattening out, or very frequently by 
 
GROWTH AND REPRODUCTION. 
 
 235 
 
 outflowing processes. On the other hand, waste may continue 
 on the increase, and bring about dissolution or death ; while 
 closely akin to this, there is the most frequent alternative, that 
 the cell divide, halve its mass, gain new surface, and restore the 
 balance. Independent suggestions, similar to Spencer's, have 
 been made by Professor Leuckart and Dr. Alexander James. 
 
 § 3. Cell-Division. — What usually occurs, then, at the maxi- 
 mum or limit of growth, is that the cell divides. This, in its 
 simplest forms, is rough enough to suggest rupture or overflow; 
 
 Diagram of the changes in the nucleus during cell-division : — coil stage 
 (a), the formation of a double star (£, c, d\ and the recession of the 
 divided chromatin elements to opposite poles (e) to form the daughter- 
 nuclei (_/") of the two daughter-cells. The centrosomes are not 
 shown. — From Hatschek, after Flemming. 
 
 but in the vast majority of cases it is an orderly and definite 
 process, in which the nucleus plays an important part, and the 
 centrosomes behave as if they were intimately concerned with 
 the mechanism of division. By a complicated series of changes, 
 both in form and position, the essential nuclear elements group 
 themselves so as to form the daughter-nuclei of each product 
 of division. That attractions and repulsions do exist within 
 the cells is certain ; an analysis of their precise nature— the 
 final problem of histology — is still far in the distance. We 
 
236 
 
 THE EVOLUTION OF SEX. 
 
 cannot get within miles of it. The problem has always loomed 
 before embryologists and histologists, — the historians and 
 mechanicians of the organism. Pander, in the first quarter of 
 this century, was inquiring into the mechanics of development, 
 
 Illustrating the Mechanism ol Cell-Division, — (a) the 
 chromatin elements of the nucleus forming an 
 "equatorial plate" lu the lower figure, drawn 
 towards the poles to form two daughter-nuclei in 
 the upper ; (Jf) part of the intricate system of fine 
 plasmic threads ; (c) the centrosomes from which 
 these radiate. — From Boveri. 
 
 and Lotze followed him with some luminous suggestions. The 
 task has been continued by numerous modern workers; 
 various "laws of cell-division" have been formulated; many 
 attempts to elucidate the " mechanism " have been made ; 
 
GROWTH AND REPRODUCTION. 237 
 
 but the inquiry is still tentative. The reader may be referred 
 to treatises on the cell by Wilson, Hertwig, and others. 
 
 § 4. Protoplasmic Restatement. — In the above helpful sug- 
 gestion, Spencer has emphasised the reasonableness and general 
 necessity of cell-division at the limit of growth, refraining from 
 the deeper question of the actual mechanism involved. In 
 truth such cautious reserve must still be maintained, but 
 Spencer's analysis perhaps admits of being expressed in lower 
 terms. The early growth of the cell, the increasing bulk of 
 contained protoplasm, the accumulation of nutritive material, 
 correspond to a predominance of protoplasmic processes 
 which are constructive or anabolic. The growing disproportion 
 between mass and surface must however imply a relative 
 decrease of anabolism. Yet the life, or general metabolism, 
 continues, and this entails a gradually increasing preponder- 
 ance of destructive processes, or katabolism. As long as 
 growth continues, the algebraic sum of the protoplasmic pro- 
 cesses must of course be plus on the side of anabolism, and 
 growth may be now more precisely denned as the outcome of 
 the preponderance of an anabolic tendency, rhythm, or bias. 
 The limit of growth, when waste has overtaken and is begin- 
 ning to exceed the income or repair, corresponds in the same 
 way to the maximum of katabolic preponderance consistent 
 with life. The limit of growth is the end of the race between 
 anabolism and katabolism, the latter being the winner. Thus 
 cell-division in plants occurs especially at night, when nutrition 
 is at a standstill, and when there is therefore a relative kata- 
 bolic preponderance. 
 
 What is true for the cell, is true for cell-aggregates. 
 Organisms in their entirety have very definite limits of growth. 
 Increase beyond that takes place at a risk, hence giant varia- 
 tions are peculiarly unstable and short-lived. Or again, just as 
 the single cell has found, probably somewhat pathologically, a 
 surface-gaining expedient in the emission of mobile processes, 
 so many organs, notably leaves, have struck a balance between 
 mass and surface by becoming split up into lobes and more or 
 less discontinuous expansions. 
 
 Spencer has laid great stress on the importance of the 
 physiological capital with which the organism begins ; this 
 represents, in active animals at least, the start which their 
 anabolism gets at the outset. Other things equal, growth 
 varies — (a) directly as nutrition ; (b) directly as the surplus of 
 
238 THE EVOLUTION OF SEX. 
 
 nutrition over expenditure; (c) directly as the rate at which 
 this surplus increases or decreases ; (</) directly (in organisms 
 of large expenditure) as the initial bulk; and (e) directly as 
 the degree of organisation, — the whole series of variables 
 being finally in close relation to the doctrines of the persist- 
 ence of matter and conservation of energy. Some apparent 
 exceptions are readily explained. Thus, many plants seem to 
 grow indefinitely, but they expend very little energy, and have 
 often enormous surface area in proportion to mass. The 
 crocodile goes on slowly growing, though at a gradually di- 
 minishing rate, but it again expends relatively little energy in 
 proportion to its high nutrition. In many fishes, however, of 
 great activity, the limit of growth seems to be very indefinite. 
 Birds which expend most energy have their size most sharply 
 defined. 
 
 § 5. The Antithesis between Growth and Multiplication, 
 between Nutrition and Reproduction. — The life of organisms is 
 conspicuously rhythmic. Plants have their long period of 
 vegetative growth, and then suddenly burst into flower. Ani- 
 mals in their young stages grow rapidly, and as the growth 
 ceases reproduction normally begins. Or again, just as peren- 
 nial plants are strictly vegetative throughout a great part of 
 the year, but have their stated recurrence of flowers and fruit, 
 so many animals for prolonged periods are virtually asexual, 
 but exhibit periodic returns of a reproductive or sexual tide. 
 Foliage and fruiting, periods of nutrition and crises of repro- 
 duction, hunger and love, must be interpreted as life-tides, 
 which will be seen to be but special expressions of the 
 fundamental organic rhythm between sleep and waking, rest 
 and work, upbuilding and expenditure, which are expressed on 
 the protoplasmic plane as anabolism and katabolism. 
 
 The common hydra, in abundant nutritive conditions, pro- 
 duces numerous buds, and even these sometimes begin them- 
 selves to bear another generation. In other words, we may 
 almost say, with plenty of food the polype grows abundantly, 
 so obviously is this asexual reproduction continuous with 
 growth. A check to the nutritive conditions, however, brings 
 on the development of the sexual organs and the occurrence 
 of sexual reproduction. In planarian worms, the asexual 
 multiplication of which we have already noted, Zacharias ob- 
 served that favourable nutritive conditions were associated 
 with the formation of asexual chains, while a check to the 
 
GROWTH AND REPRODUCTION. 239 
 
 nutrition brought about both the separation and the sexual 
 maturity of the links. Rywosch corroborates this, noting in 
 Microstomum lineare that the generative organs do not become 
 completely matured till the individuals cease to be links in a 
 chain, and that the sexuality is hastened by outside influences 
 such as checked nutrition. The gardener root-prunes his 
 apple-tree, thereby checking nutrition to improve the yield of 
 fruit, in other words, to augment reproduction. Reversely, the 
 removal of reproductive organs may increase the development 
 of the general "body" both in plant and animal, — witness 
 the castrated ox, capon, &c, or the way in which the gardener 
 nips off the flower-buds from his foliage plants. Taking a 
 further step, we recall the familiar and already repeated fact, 
 that favourable nutritive and other conditions enable the 
 aphides to continue parthenogenetic through the summer 
 months ; but both for the common plant-lice and for the vine- 
 insect phylloxera, it has been shown that a check to nutrition 
 causes the parthenogenesis to cease, and is associated with the 
 return of sexual reproduction. The above instances are ob- 
 viously not all upon the same plane. They illustrate however, 
 at different levels, the same great contrast. It is necessary, 
 however, to become more precise. 
 
 § 6. The Contrast between Growth and Reproduction in the 
 Individual. — (a) The Distribution of Organs. — The general 
 position of the flower at the end of the vegetative axis is so 
 obvious a fact that its import tends to be overlooked. The 
 end of the axis is furthest from the source of nutritive supply; 
 with exaggeration, we might call it the starvation-point. 
 There, with katabolic conditions tending relatively to pre- 
 dominate, the reproductive organs are situated. The flower 
 occupies a katabolic position, and is often the plant's dying 
 effort. 
 
 In the tiger-lily, growth at first tends to remain continuous. 
 and the base of the bulb bears simple vegetative buds 
 Further up, however, where nutrition reaches its maximum 
 the axils of the leaves contain buds, which are separable 
 though still asexual. Finally, further up still, where nutrition 
 is relatively less active and katabolism is maximised, the for- 
 mation of flowers indicates the appearance of sexual repro- 
 duction. 
 
 In many ferns, the contrast between the vegetative and re- 
 productive regions of the organism is as marked as in the 
 
240 
 
 THE EVOLUTION OF SEX. 
 
 flowering-plant. Thus the moonwort (Botrychitim) and the 
 adder's tongue (Ophioglossum) have their spore-bearing shoots 
 standing in conspicuous antithesis to the leafy portion, and a 
 similar contrast is well seen in the royal fern (Osmundd) and 
 some of its allies. 
 
 In animals, the contrast in position between reproductive 
 organs and the general body is never so marked. Yet the 
 
 The Moonwort Fern (Botrychium 
 liuiare), showing the con- 
 trasted frond (a), and fructi- 
 fication {l>).— After Sachs. 
 
 Diagram of the Tiger Lily, show- 
 ing bulbils (a) in lower axils, 
 and flower above. 
 
 generally posterior position of the organs, their frequent close 
 association with the excretory system, their occasional rupture 
 as external sacs, must not be lost sight of. 
 
 {b) The Contrast in the Individual Life. — Growth during 
 youth, sexual maturity at the limit of growth, the continued 
 alternation of vegetative and reproductive periods, are common- 
 
GROWTH AND REPRODUCTION. 24 T 
 
 places of observation which require no emphasis. If growth 
 and vegetative increase are the outcome of preponderant ana- 
 bolism, reproduction and sexuality as their antitheses must re- 
 present the katabolic reaction from these. But anabolism and 
 katabolism are the two sides of protoplasmic life; and the 
 major rhythms of the respective preponderance of these, give 
 the familiar antitheses we have been noting. These contrasts 
 of metabolism represent the swings of the organic see-saw; 
 the periodic contrasts correspond to alternate weightings or 
 lightenings of the two sides. Yet the contrast is less than it 
 seems. In previous chapters we have seen how growth, be- 
 coming overgrowth, turns into reproduction ; and how sexual 
 reproduction, dispensing with fertilisation, may degenerate till 
 we know it no longer from growth. Reproduction, moreover, 
 is as primitive as nutrition, for not only do hunger and love 
 become indistinguishable in that equal-sided conjugation which 
 has been curiously called " isophagy," but nutrition in turn is 
 nothing more than continual reproduction of the protoplasm. 
 Here, indeed, we have been anticipated by Hatschek, who 
 clearly states the more than verbal paradox, that all nutrition 
 is reproduction. 
 
 § 7. The Contrast between Asexual and Sexual Repro- 
 duction. — In plenty, the hydra buds ; in poverty, it reproduces 
 sexually. In the same way, the liverwort on the flower-pot 
 bears its pretty cryptogamic " flowers " when its exuberant 
 growth and budding have come to an end. On rich soil a 
 plant has luxuriant foliage ; but great abundance is the reverse 
 of conducive to the richest crop of flowers and fruit. Gruber, 
 Maupas, and others, have shown that abundant nutrition 
 favours the asexual multiplication, i.e., the division of in- 
 fusorians. In other words, the maximum size is rapidly 
 reached when food is abundant, but the conditions at the 
 limit of growth bring about reproduction. Preponderant ana- 
 bolism leads up to the possibility of multiplication, but we 
 need the onset of katabolism to bring about the reproductive 
 crisis. Gruber also notes, that in the very reverse of favour- 
 able conditions, rapid division with diminution of size and 
 resulting conjugation sets in; and Khawkine observes the 
 occurrence of division, both at an optimum and in famine. 
 In both cases a katabolic crisis is associated with reproduction, 
 though the crisis may be, and often is, preceded by an anabolic 
 preponderance. 
 
 16 
 
243 THE EVOLUTION OF SEX. 
 
 In regard to a common infusorian (Leucophrys patula), 
 Maupas observes that with abundant food the ordinary fission 
 continues, but with scanty nutrition a metamorphosis occurs, 
 followed by six successive divisions, which have for their end 
 conjugation. That is to say, we have positive proof that in 
 these lowest organisms, katabolic conditions determine the 
 beginning of sexual reproduction, a matter of no small import- 
 ance to the evolutionist. Generalising, M. Maupas concludes 
 that the reproductive power of ciliated infusorians depends, (i) 
 on the quality and quantity of the food; (2) on the tempera- 
 ture; (3) on the alimentary adaptation of the buccal organs. 
 He also demonstrates that with a vegetarian diet their rate of 
 asexual reproduction is much less, and the size smaller. Taking 
 these facts, along with his important demonstration that the 
 life of ciliated infusorians runs in cycles of asexual reproduc- 
 tion, necessarily interrupted (if the life of the species is to 
 continue) by conjugation or sexual reproduction, we again reach 
 the general conclusion that anabolic conditions favour asexual 
 reproduction, rather than sexual; and that while preponderant 
 anabolism is the necessary condition of the overgrowth which 
 makes the asexual reproduction possible, the onset of katabolic 
 preponderance is necessary to the act itself. 
 
 Semper quotes an interesting observation by Strethill Wright, 
 unfortunately somewhat vague, that certain polyps multiply 
 abundantly in the dark by buds, while in the light, and with 
 insufficient supplies of food, they bring forth sexual individuals 
 or medusae. More precise is the fact already cited fror.i 
 Zacharias, that the spontaneous asexual multiplication of 
 planarians went on apace when the food supply was copious 
 (anabolic condition), but if the amount of food was reduced 
 or altogether withdrawn (katabolic condition) the asexual re- 
 production completely ceased. Bergendal reports that in the 
 transverse division of another planarian worm (Bipalium), the 
 severed links were all sexually immature; and the results of 
 Rywosch demonstrate the same antithesis between the sexual 
 and the asexual process. 
 
 In the same way, sexual reproduction is contrasted with its 
 degenerate expression in parthenogenesis. The conditions of 
 the latter in aphides and phylloxera are demonstrably anabolic, 
 the normal sexual process recurs with the periodic return of hard 
 times, or in relatively katabolic conditions. In the lower crus- 
 taceans, a similar contrast of conditions has also been observed. 
 
GROWTH AND REPRODUCTION. 243 
 
 It is again, on the present view, readily intelligible why in 
 the exceptionally favourable anabolic environment of bacteria 
 and many parasitic fungi sexual reproduction should be absent. 
 Marshall Ward has pointed out that the more intimate the 
 degree of parasitism or saprophytism, the more degenerate the 
 sexual reproduction. The greater the anabolism, in other 
 words, the more growth and the less sexuality. That such 
 comparatively complex organisms can continue their asexual 
 reproduction, dispensing altogether with the acknowledged 
 stimulus of fertilisation, may probably be at least partially 
 explained on the assumption that the abundant waste products 
 of the host act as extrinsic stimuli. 
 
 On this view, moreover, alternation of generations loses 
 much of its uniqueness. The contrast between the vegetative 
 
 Pollen Grain : a., the two nuclei ; b, the general protoplasm ', 
 <r, the outer wall. — From Carnoy. 
 
 asexual hydroid or hydra-tuba, and the active sexual medusoid 
 or jelly-fish, is very marked. So too, on a higher plane, the 
 vegetative spore-producing fern-plant stands opposed to the less 
 nutritive sexual prothallus. The alternation is but a rhythm of 
 large amplitude between anabolic and katabolic preponderance. 
 
 What is so marked in the alternation is only a special- 
 isation of the reproductive or sexual parts of the organism as 
 against the growing or asexual ones, — a specialisation which 
 becomes exaggerated into separate existences, each dominated 
 by its own physiological bias. 
 
 In the fern or flowering plant the vegetative or asexual 
 existence has preponderated, and this is entirely consistent 
 with the characteristic passivity of plants. This is emphatically 
 
244 THE EVOLUTION OF SEX. 
 
 their line of development; but, be it observed, that though 
 in the flowering plants the nutritive generation has dwarfed, 
 and included the sexual, which seem indeed to be mere 
 organs, — the pollen-grain and embryo sac, — yet it is through 
 and for these that we have all the glory of the flower. In 
 animals, with their emphatically active line of development, 
 the reproductive generation is the higher; and in the higher 
 forms the separate asexual existence is wholly lost. 
 
 The experiments of Klebs may perhaps be regarded without 
 unfairness as marking the real beginning of a physiology of 
 reproduction in plants. For he has set himself to show how 
 definite environmental conditions of nutrition, temperature, &c, 
 are definitely associated with the occurrence of particular 
 modes of reproduction in Algse and Fungi. A Vauc/ieria-plant, 
 kept sterile for years, can be made sexual in a few days. A 
 form normally bisexual can be made unisexual. Asexual spore- 
 formation can be induced with certainty by one set of con- 
 ditions, e.g., in Hydrodiciyon, and the appearance of sexual 
 gametes by another. Only by definite experiments like these 
 can we pass from vague interpretations to a precise physiology. 
 
GROWTH AND REPRODUCTION. 245 
 
 SUMMARY. 
 
 1. Growth is characteristic of living organisms, though analogous pro- 
 cesses occur at the inorganic level. Hunger is an essential characteristic of 
 living matter. As certain as the fact of growth, is the definiteness of its 
 iimit alike for cell and for organism in the great majority of cases. 
 
 2. Spencer has analysed the limit of growth, in terms of the continual 
 tendency that increase of mass must have to outrun increase of surface. 
 
 3. Cell-division at the limit of growth, at the maximum or optimum of 
 size, restores the balance between mass and surface. The actual mechanics 
 of the process are at present beyond analysis. 
 
 4. Spencer's analysis may be restated in protoplasmic terms. Growth 
 expresses the preponderance of anabolism; increase of mass, with less 
 rapid increase of nutritive, respiratory, and excretory surface, involves a 
 relative predominance of katabolism. The limit of growth occurs when 
 katabolism has made up upon anabolism, and tends to outstrip it. What 
 is true of the unit, applies also to the entire multicellular organism. 
 
 5. Throughout organic life there is a contrast or rhythm between 
 growth and multiplication, between nutrition and reproduction, corre- 
 sponding to the fundamental organic see-saw between anabolism and 
 katabolism. 
 
 6. This contrast may be read in the distribution of organs, in the periods 
 of life, and in the different grades of reproduction. Yet nutrition and 
 reproduction are fundamentally nearly akin. 
 
 7. The contrasts between continuous growth and discontinuous multi- 
 plication, between asexual and sexual reproduction, between partheno- 
 genesis and sexuality, between alternating generations, are all different 
 expressions of the fundamental antithesis. 
 
 LITERATURE. 
 
 Delage, Y. — La Structure du Protoplasma, etc. Paris, 1895. 
 
 Klebs, G. — Ueber einige Probleme der Physiologie der Fortpflanzung. 
 
 Jena, 1895, 26 pp.; and Die Bedingungen der Fortpflanzung bei 
 
 einigen Algen und Pilzen. Jena, 1896 
 Spencer, Principles of Biology; and Haeckel, Generelle Morphologie. 
 
CHAPTER XVII. 
 
 Theory of Reproduction, 
 
 § i. The Essential Fact in Reproduction. — In the foregoing 
 chapters, the facts involved in the different forms of repro- 
 duction have been analysed apart, and separately discussed. 
 Male and female organisms have been interpreted as relatively 
 katabolic and anabolic; the origin of sex, in the individual 
 and in the race, has been traced back to the preponderance of 
 anabolic or katabolic conditions; the ultimate sex-elements 
 were seen to exhibit the same contrast in its most concentrated 
 expression ; fertilisation was regarded, not merely as a mingling 
 of hereditary characters, but as a katabolic stimulus to an 
 anabolic cell, and on the other side, of course, as an anabolic 
 renewal to a katabolic cell. Only by a separation of the 
 problem of " sexual reproduction" into its component problems 
 can clearness be reached. Sexual reproduction is like a 
 complex musical chord in the organic life, combining several 
 elements, all of which, however, admit of the same funda- 
 mental analysis. Two problems remain, — the psychical aspect 
 of the process ; and the import of that common feature of all 
 reproduction, the separation of part of the parent organism to 
 start a fresh life. The latter forms the subject of the present 
 chapter. 
 
 § 2. Argument from the Beginnings of Reproduction. — 
 Leconte and others have pointed out that reproduction really 
 begins with the almost mechanical breakage of a unit mass of 
 living matter,* which has grown too large for successful co-ordi- 
 nation. Reproduction, in fact, begins as rupture. Large 
 cells beginning to die, save their lives by sacrifice. Reproduc- 
 tion is literally a life-saving against the approach of death. 
 Whether it be almost random rupture or fragmentation, as 
 seen in some of the most primitive Protozoa, or the overflow 
 and separation of multiple buds as in Arcella, the organism, 
 which is becoming exhausted, saves itself, and multiplies in 
 reproducing. In some cases, reproduction is effected by 
 
THEORY Of REPRODUCTION. 24^ 
 
 Outflowing processes of the cell, which have gone a little too 
 far. Now, such primitive forms of multiplication, gradually 
 becoming more definite, express a relative predominance of 
 katabolism in the unit mass. Reproduction in its simplest 
 forms is associated with a katabolic crisis. 
 
 Nageli's hypothesis as to the origin of reproduction was (in brief 
 outline) as follows : — The original very simple organisms arose from a 
 natural synthesis of albuminoid molecules, a combination of these into 
 micellje, and a union of these into protoplasts. But the balance between 
 nutrition and expenditure was at first very insecure, and external changes 
 frequently tended to induce latent life, or death. But in the very article 
 of death came the beginning of a new life. Itself too large for coherence, 
 the protoplast broke into parts which lived on, or in other conditions it 
 died down in great part but lived on in a fragment. As differentiation 
 proceeded, the first mode of reproduction by rupture gave place to cell- 
 division, bursting into cells, discharge of special cells ; the second gave 
 place to various forms of free-cell formation, spore-building, gemmule 
 forming, &c. 
 
 § 3- Argument from Cell-Division. — Most unicellular 
 organisms reproduce by cell-division, and this is, of course, a 
 precedent of reproduction in multicellular organisms, whether 
 they multiply by asexual budding or by differentiated sex- 
 elements. But in the preceding chapter, following Spencer, 
 we have emphasised the connection between division and a 
 katabolic predominance within the cell. A constructive 
 period may precede, but a disruptive climax attends the 
 division. So far then as reproduction is either wholly included 
 in the process of cell-division, or has this as its necessary 
 precedent, it is associated with a katabolic crisis. 
 
 § 4. Argument from the Gradations between Asexual 
 Severance of Parts and the Liberation of Special Sex-Cells. — 
 Discussing asexual reproduction, we have noticed that some 
 worm-types break into two or more parts, which start new 
 individuals. That some nemerteans normally break up into 
 pieces, as they do in the feverish anxiety of capture, is most 
 probable; and this is certainly the case in certain annelids. 
 From a syllid, which sets free a sexual individual, the over- 
 growth of an asexual parent, to one which liberates a series of 
 joints, or even a single joint, bearing reproductive elements, is 
 but a slight step. From the last case, to the rupture which 
 liberates sex-elements, is again only a slight advance. A 
 similar series is well illustrated among the Hydromedusse. 
 The breakage or thinning away which sets a large portion free 
 
248 
 
 THE EVOLUTION OF SEX.. 
 
 is a katabolic process, in a sense a local death. The gentle- 
 ness of the gradient warrants us in concluding that the 
 liberation of sex-cells, in its earlier expressions at least, is 
 associated with a local or with a general katabolic crisis. 
 
 § 5. Argument from the Close Connection between Reproduc- 
 tion and Death. — Without going back to primitive disintegra- 
 
 Division of an Animal Cell, showing the nucleus (a) in process of forming 
 two daughter-nuclei, showing also the protoplasmic network (£). — 
 From Carnoy. 
 
 tions, or the asexual severance of more or less large portions, 
 we may point further to the close connection between repro- 
 duction and death, even when the former is accomplished by 
 specialised sex-cells. We shall presently discuss at greater 
 length this nemesis of reproduction, but it is important here 
 to emphasise that the organism not unfrequently dies in 
 continuing the life of the species. In some species of the 
 
THEORY OF REPRODUCTION. 249 
 
 primitive annelid Tolygordius, the mature females die in 
 liberating the ova. At a very different level, the gemmules of 
 the common fresh-water sponge are formed in the decay of 
 the asexual adult, while even the sexual summer forms, 
 especially the males, are peculiarly unstable and mortal. The 
 whole history of this form seems a continuous rhythm between 
 life and growth on the one hand, and death and reproduction 
 on the other. Or again, the flowering of phanerogams is often 
 at once the climax of the life and the glory of death. In his 
 ingenious essay on the origin of death, Goette has well shown 
 how closely and necessarily bound together are the two facts 
 of reproduction and death, which may be both described as 
 katabolic crises. 
 
 § 6.- Argument from Environmental Conditions which favour 
 Reproduction. — The rhythm between nutrition and reproduc- 
 tion, or between growth and multiplication, has been as it 
 were the refrain of the preceding pages. This " organic see- 
 saw " is determined by the very constitution of the organism ; 
 in other words, it expresses the fundamental characteristic of 
 living matter. It is an incomplete conception, however, unless 
 it be remembered that about this "organic see-saw" there 
 blows the wind of the environment, swaying it now to one 
 side, now to the other. It is important therefore to illustrate 
 how the play of external conditions accelerates or retards the 
 reproductive function. 
 
 The influence of heat upon the reproductive powers of 
 infusorians has been carefully investigated by Maupas. The 
 higher the temperature up to a certain limit, the faster do 
 these organisms reproduce. In favourable nutritive condi- 
 tions, Stylonichia pustulata divides once in twenty-four hours 
 at a temperature of 7° to 10° C, twice at 10° to 15°, thrice at 
 15° to 20°, four times at 20° to 24°, and five times at 24° 
 to 27° C. Illustrating the rapid rate of increase, Maupas 
 notes in the same paper, that at a temperature of 25° to 26° C, 
 a single Stylonichia would in four days have a progeny of a 
 million, in six days of a billion, in seven and a half days of a 
 hundred billions ! In six days the family would weigh one 
 kilogramme, and in seven and a half days one hundred 
 kilogrammes. 
 
 The action of heat may be twofold ; up to a' certain limit 
 it quickens development and the general life, favouring asexual 
 reproduction and parthenogenesis rather than the sexual pro- 
 
25° THE EVOLUTION OF SEX. 
 
 cess ; beyond that limit of comfortable warmth, so variable for 
 different animals, it may induce a feverish habit of body, and 
 hasten reproductive maturity and sexual reproduction. In 
 other words, heat may in some cases favour anabolism, in 
 others katabolism. It is intelligible enough to find increased 
 heat sometimes associated with increased asexual reproduction, 
 sometimes with accelerated sexuality. Instances of both may 
 be gathered from Semper's " Animal Life," the classical work 
 on the influence of the environment upon the organism. 
 
 Maupas supplies another vivid illustration of a yet more 
 important environmental influence, that of food. In another 
 ciliated infusorian (Leucophrys), so long as food is abundant, 
 fission obtains ; but when food grows scanty, there is a meta- 
 morphosis without encystation, followed by six successive 
 divisions. These are effected, however, "without vegetative 
 growth, and have for their final object not multiplication but 
 conjugation." In other words, abundant food is associated 
 with asexual reproduction; a check to the nutrition brings 
 about the sexual process. Maupas gives a vivid numerical 
 statement of the stimulus to reproduction by a sudden check 
 to the nutrition. Leucophrys at a temperature of 20 C, in 
 richly nutritive conditions, will give rise to sixteen thousand 
 three hundred and eighty-four individuals in three days ; but 
 if the food be then suppressed, this large number will in a few 
 hours be multiplied by sixty-four, resulting in a total of one 
 million forty-eight thousand five hundred and seventy-six 
 individuals ! 
 
 From cases already cited, which may be multiplied by con- 
 sulting Semper's "Animal Life," supplemented by a summary 
 of more recent researches by one of ourselves, the general con- 
 clusions may be drawn, — (a) That heat increases reproduction, 
 either directly or as the result of a preliminary acceleration of 
 growth ; (b) That increased food will, of course, favour growth, 
 but reproduction may follow all the more markedly as an 
 exaggerated nemesis; (<r) That checks to nutrition, especially 
 in the form of sudden scarcity, will favour sexual reproduction. 
 The clearest result of all is, that a sudden katabolic change 
 favours reproduction, especially in its sexual form. Anabolic 
 conditions favour reproduction indirectly; the reverse condi- 
 tions have a direct influence; in both cases, reproduction is 
 the expression of a katabolic crisis. 
 
 7. Conclusion. — Primitively, then, reproduction was a kata- 
 
THEORY OF REPRODUCTION. 2$ I 
 
 bolic rupture of a mass of protoplasm. This becomes more 
 definite in cell-division of various kinds, tending ever to occur 
 at the limit of growth when waste has made up on repair, or in 
 katabolic conditions due to the environment. In multicellular 
 animals, anabolic conditions favour overgrowth; a check to 
 this brings about discontinuous asexual reproduction. With 
 increasing differentiation, the asexual multiplication is replaced 
 by the liberation of special sex-cells, by which the life-saving 
 and life-continuing sacrifice is rendered less costly. Just as 
 asexual reproduction occurs at the limit of growth, so a check 
 to the asexual process is often seen to involve the appearance 
 of the sexual, which is thus still further associated with kata- 
 bolic preponderance. This is confirmed by the contrasts 
 observed in alternation of generations, where the two processes 
 in varying degrees of distinctness persist in the life-history of 
 the same organism. Corroboration is again afforded by the 
 association of sexual reproduction with sundry environmental 
 checks of a katabolic character. And thus the opposition 
 between nutrition and reproduction, which, after life and 
 death, is the most obvious antithesis in nature, admits of 
 being more precisely restated in the thesis, that as a continued 
 surplus of anabolism involves growth, so a relative preponder- 
 ance of katabolism necessitates reproduction. 
 
2$2 IHE EVOLUTION OF SEX. 
 
 SUMMARY. 
 
 1. The essential fact in reproduction is the separation of part of the 
 parent organism to start a fresh life. 
 
 2. Reproduction begins with rupture, — a katabolic crisis. 
 
 3. Cell-division, which sometimes sums up, and is always associated 
 with, the act of reproduction, occurs at a katabolic crisis. 
 
 4. The gradations between discontinuous asexual multiplication and 
 ordinary sexual reproduction, show a lessening of the sacrifice; but all 
 demand a disruption, or a katabolic preponderance. 
 
 5 From first to last reproduction is linked to death. 
 
 6. Environmental conditions of a katabolic character favour sexual 
 reproduction. 
 
 7. General conclusion, — a relative preponderance of katabolism is 
 associated with reproduction, though this may be prepared for by a 
 previous period of predominant anabolism. 
 
 LITERATURE. 
 
 Geddes, P. — Theory of Growth, Reproduction, Sex, and Heredity. Proc. 
 
 Roy. Soc. Edin., 1886. 
 IIaeckel. — Generelle Morphologie. 1866. 
 Nag eli, C. Von. — Mechanisch-physiologische Theorie der Abstammungs- 
 
 lehre. Mtinchen und Leipzig, 1884, pp. XI.-822. 
 Semper. — Animal Life. Int. Sci. Series. 1881. 
 Spencer. — Principles of Biology. 1866. 
 Thomson. — "Synthetic Summary of the Influence of the Environment 
 
 upon the Organism." Proc. Roy. Phys. Soc. Edin., 1887. 
 
CHAPTER XVIII. 
 
 Special Physiology of Sex and Reproduction. 
 
 It is no part of our purpose to discuss in detail the physiology 
 of sexual and reproductive functions. The fundamental physi- 
 ology of the essential functions has been the subject of preced- 
 ing chapters ; the details will be found in the standard works 
 on Physiology, Botany, and Zoology. For the sake of com- 
 pleteness, however, it is necessary to take a brief survey of 
 some of the most outstanding facts. 
 
 § i. JVeismann's Theory of " Continuity of the Germ- 
 Plasma." — Thanks, especially to Weismann, the view that 
 ordinary cells of the " body " become at a certain epoch 
 changed into special reproductive cells, may now be put aside 
 as exceedingly improbable. In a minority of cases, already 
 quoted, the reproductive cells, or the rudiments of sexual 
 organs, are demonstrably set apart at an early stage, before the 
 differentiation of the embryo has proceeded far. They thus 
 include some of the original capital of the fertilised parent 
 ovum intact, they continue the protoplasmic tradition un- 
 altered, and, when liberated in turn, they naturally enough 
 develop as the parent ovum did. Following out this important 
 fact, various naturalists have reached the conception of a con- 
 tinuous necklace-like chain of sex-cells from generation to 
 generation, — a continuous chain upon which the mortal indi- 
 vidual organisms arise and from which they drop away, like so 
 many separate and successive pendants. 
 
 But in the majority of cases, such a conception, as Weis- 
 mann has justly insisted, gives a false simplicity to the facts. 
 A chain of insulated sex-cells, connecting the parental fer- 
 tilised ovum with the germ-cells which develop into offspring 
 is, so far as we yet know, only rarely demonstrable. In other 
 words, the rudiments of the reproductive organs often appear 
 at a relatively late stage in the development. Where do they 
 come from? Are somatic, or ordinary body-cells modified 
 
254 
 
 THE EVOLUTION OF SEX. 
 
 into reproductive elements? Weismann's answer is a decided 
 negative. Although no continuous chain of germ-like cells is 
 demonstrable, there is a strict continuity of germ-fllasm. To 
 quote Weismann's own words, "In each development a por- 
 tion of the specific germ-plasm which the parental ovum con- 
 tains, is not used up in the formation of the offspring, but is 
 reserved unchanged to form the germ-cells of the following 
 generation." In short, continuity is kept up by the plasm of 
 nuclei, rather than by a chain of cells. It will be observed, of 
 course, that while early insulation of definite germ-cells is a 
 
 The chromatin elements of the nuclei in coil (a), double star (b), and almost 
 divided stages (r). — After Pfitzner. 
 
 demonstrable fact, to be seen in a few cases, though perhaps 
 of wider occurrence than we know of, the continuity of germ- 
 plasm is strictly a hypothesis. 
 
 This being so, reproductive maturity may be defined as 
 the period when the reproductive cells (bearing the inherited 
 capital of germ-plasm) have established themselves to that 
 degree that they can start fresh organisms, and have multiplied 
 to an extent which in most cases makes their liberation a 
 physiological necessity. In the lower animals, the maturity of 
 the sexual functions is often as slightly marked as the liberation 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 255 
 
 of the elements is passive and random. In slightly differen- 
 tiated organisms, like sponges, there is little reason to suppose 
 that the distinction between cells preponderating in germ- 
 plasm and the ordinary cells of the body is much marked. 
 Nor in such cases is the anarchic opposition between body 
 and reproductive cells at all emphatic, especially as regards the 
 female cells. It is only as the differentiation increases, as the 
 contrast between body-cells and sex-cells becomes emphasised, 
 as the asexual mode of getting rid of surplus wanes, that the 
 typical liberation of sex-elements which marks sexual maturity 
 becomes a striking fact in the life. That the male-cells are 
 always more anarchic, usually mature before the female ele- 
 ments, and even in plants, and in such passive animals as a 
 sponge or a hydra, burst from the organism, while the female 
 cells remain in situ, is quite consonant with their relatively 
 katabolic character. 
 
 § 2. Sexual Maturation. — The maturation of the sexes not 
 only acquires increasing definiteness in the higher forms, but 
 becomes associated with various characteristic accompaniments. 
 The profound reaction of reproductive maturity upon the whole 
 system is best marked in birds and mammals, and perhaps 
 most of all in man. Thus in a young male bird, the circulation 
 in the testes is greatly increased, and these organs increase 
 greatly in size and weight, and commence to develop sperma- 
 tozoa. Meanwhile the "secondary sexual characters" of the 
 adult — gayer plumage possibly attractive to the female, or 
 weapons for contest with other males — appear, the voice and 
 note may alter, and a marked increase of strength and courage 
 may occur. Among mammals, the changes are of similar 
 order, the secondary sexual characters of course differing in 
 detail. The minor changes at puberty in man associated with 
 the commencement of spermatogenesis, are (besides the reflex 
 excitation of erection due to distension of the seminal vesicles, 
 and the more or less periodic expulsion of their contents during 
 sleep) the growth of hair on the pubic region and later on the 
 lower part of the face, and the rapid modification of the 
 laryngeal cartilages and the lengthening of the vocal chords, so 
 rendering the voice harsh and broken during the change, and 
 ultimately deepening it by about an octave. The marked 
 strengthening of bones and muscles, and the profound psychical 
 changes which accompany the whole series of processes, are 
 also familiar. 
 
256 THE EVOLUTION OF SEX. 
 
 Although we do not understand the facts, there is no doubt as to the 
 profound correlation which exists between the reproductive organs and the 
 body. This may be illustrated in various ways, e.g., by reference to the 
 effects of castration. On young cocks, according to Sellheim, this opera- 
 tion has very diverse results, sometimes decreasing the secondary sex- 
 characters, and sometimes, strange to say, increasing them. The capons 
 seldom crow, or do so abnormally; attempts at copulation are rare. But 
 one general result seems to be established, that the whole body is 
 affected: — the larynx is intermediate in size between that of cock and 
 hen; the syrinx is weakly developed; fat accumulates in the connective 
 tissue; the brain and heart are light in weight; even the skeleton shows 
 many abnormalities. ("Beitrage zur Geburtshilfe und Gynakologie," i., 
 1898, pp. 229-246.) The same investigator has also shown that in mam- 
 mals of both sexes the removal of the reproductive organs markedly pro- 
 longs the period of bone-development, and has thus an effect on the shape 
 of various parts. {Op. cit., ii., 1899, pp. 236-59.) 
 
 In regard to the correlation of antlers and reproductive organs ("Arch. 
 Entwickmech.," viii., 1899, pp. 382-447), A. Rorig has investigated five 
 points. His first question is : Does the absence of antlers or the development 
 of only one depend on an abnormality of the reproductive system ? He 
 answers that the condition may occur with both normal and abnormal 
 gonads. His second question is : Can the occasional development of 
 antlers in female Cervidaa be referred to the abnormal development of the 
 reproductive organs? He answers that a diseased state on one side may 
 be correlated with the development of one antler, on both sides with the 
 development of two antlers, and that one-sided disease has a correlation 
 operating transversely. If the ovaries are atrophied there are usually 
 antlers. Hermaphrodites seem always to have antlers, and these are the 
 more perfect the more the gonads incline towards maleness. Irritation of 
 the appropriate place may also evoke antlers in females. His third 
 question is: What effect has partial or total castration in the males? It is 
 answered that the effect varies according to the age of the animal and the 
 stage of antler development. In a young quite hornless male, castration 
 entirely inhibits the growth of antlers. Fourthly, Rorig points out that 
 atrophy of the testes is followed almost always by the formation of 
 " Periicken "-antlers, and injury to the testes by premature casting. 
 Fifthly, the excision of the antlers has no deleterious effect on the repro- 
 ductivity or health of the individual. It is obvious that this is a. very 
 important contribution to our knowledge of the correlation between gonads 
 and soma. 
 
 In higher vertebrates, the sexual maturity of the female is 
 marked by a cellular activity within the ovary, not less remark- 
 able than that in the testes. Associated therewith are minor 
 but often very important characteristics, such as the increased 
 mammary development in mammalia. In some of the lower 
 animals, such as certain marine annelids, the ova become so 
 numerous that their disruption or liberation is in great part a 
 mechanical necessity. The same might be said of fishes, 
 reptiles, and birds. At the same time the enlargement and 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 257 
 
 escape of the ova are doubtless expressions of a normal cellular 
 rhythm, of which hints are given in the frequent passage from 
 an amoeboid to an encysted phase, in the occasional relapse to 
 the former, and in the fatty degeneration or death of ova which 
 have not accomplished their destiny. 
 
 The primitive ova of vertebrates lie in clusters in the substance or 
 stroma of the organ, and are produced from the essential germinal 
 epithelium. Only a minority, however, grow into genuine ova; others, of 
 smaller size, form a nutritive sheath or follicle around them. In mammals, 
 each follicle forms a cavity containing a fluid. Into this the ovum, sur- 
 rounded by a mass of follicle cells, projects. When mature, the follicle with 
 its contained ovum has attained a superficial position. By the bursting of 
 the ripe follicle the ovum is expelled, and passes into the approximated and 
 ciliated upper end of the oviduct or Fallopian tube. The rupture of 
 blood-vessels in the substance of the ovary fills up the Graafian follicle with 
 blood. The white corpuscles form a framework resembling connective 
 tissue, in which the solids and corpuscles of the blood serum, with colour- 
 ing matter derived from the haemoglobin of the latter, are retained. The 
 whole constitutes the " corpus luteum," which, should pregnancy occur, 
 may persist and undergo further retrogressive changes, or otherwise 
 gradually disappear. 
 
 As to the direct causes of this process of ovulation there is some 
 difference of opinion. The congestion of the blood-vessels of the ovary, 
 its own internal turgidity, a slight contractility of its stroma, have been 
 regarded as determining factors. The process seems, however, rather to 
 depend upon the growth and turgescence of the individual follicle. The 
 question of the relation of ovulation to the process of copulation in the 
 higher animals has also been much discussed. Though we certainly know 
 that ovulation is of regular occurrence whether fecundation takes place or 
 not, it seems that in many cases copulation is speedily followed by the 
 liberation of an ovum ; nor is it difficult to see how the profound nervous 
 and circulatory excitement associated with the former process might 
 accelerate the bursting of a follicle. Leopold has conclusively shown, 
 however, that ovulation may also long precede impregnation, and Stratz 
 has shown, in the case of Tupaia. javanica, that after fertilisation of ova shed 
 into the oviduct, the follicles in the ovary undergo degeneration, and no 
 more mature eggs are formed till near the end of pregnancy (" Die Gesch- 
 lechtsreife Saiigethierstock. " Haag, 1898, pp. 67, 9 pis.). The experi- 
 ments of Heape("Proc. Roy. Soc," lxi., 1897, pp. 52-63) and others 
 show that artificial insemination may be effective in certain mammals, such as 
 mice and dogs ; on the other hand, Heape's observations on the rabbit point 
 to the conclusion that both copulation and the presence of spermatozoa in 
 fne uterus are necessary to induce ovulation in the virgin rabbit when she 
 is in "heat." 
 
 Since the oviduct, unlike its male counterpart, is not, in the vast 
 majority of vertebrates, continuous with its associated organ, it is often 
 difficult to see how the ova once liberated into the body-cavity find their 
 way safely into the small opening of the duct. The problem has been 
 worked out by Nussbaum in the frog, where the ova are moved by the 
 effective action of numerous muscles and by the ciliary activity of peritoneal 
 
 17 
 
258 THE EVOLUTION OF SEX. 
 
 epithelial cells which are disposed in tracts converging to the oviducal 
 aperture, so propelling the ova in the right direction. In reptiles, birds, 
 and mammals the open end of the oviduct is widened, fringed, and 
 ciliated, and lies close to or even touching the ovary; muscular fibres too 
 are present, and more or less active movements of this ciliated end over 
 the ovarian surface have been alleged to occur. The oviduct once reached, 
 the downward progress of the ovum is ensured by the cilia of the epithelial 
 lining, and probably also by peristaltic movements of its muscular coat. 
 
 There is no doubt that the advent of sexual maturity varies 
 with environmental conditions of climate, food, and the like. 
 Broadly speaking, sexuality becomes pronounced as growth 
 ceases. Especially in higher organisms, a distinction must 
 obviously be drawn between the period at which it is possible 
 for males and females to unite in fertile sexual union, and the 
 period at which such union will naturally occur or will result in 
 the fittest offspring. In the lower animals, where the individual 
 life is usually shorter, sexual maturity is more rapidly attained, 
 though we find cases such as that of the fluke (Polystomuni) so 
 commonly present in the bladder of the frog, where maturity of 
 the reproductive organs does not occur for several (three) years, 
 and maturity of growth for some years afterwards. In cestode 
 parasites, the bladder-worm stage remains indefinitely asexual 
 until in fact the stimulus of a new host admits of the develop- 
 ment of the sexual tapeworm. In plants, reproductive maturity 
 sets in at various ages; thus we have all gradations, at the 
 one extreme our characteristically short-lived but magnificent 
 annuals, then the biennials, and from these to a maturation at 
 still longer date, as in the well-known case of the American 
 aloe {Aloe ainericand), which even in Mexico takes from seven 
 to twelve years to reach the floral climax in which it expires, 
 and in our greenhouses as much as a generation or two, whence 
 its name of " century plant." 
 
 In contrast to such cases, precocious reproductive maturity 
 occasionally occurs. We have already referred to those 
 dipterous midges (Cecidomyia), in which the larvas for succes- 
 sive generations become reproductive, though only partheno- 
 genetically. Very striking too is the trematode worm Gyro- 
 dactylus, which recalls the mystical views of the preformation- 
 ists, in exhibiting three generations of embryos, one within the 
 other, while the oldest is yet unborn. The well-known axolotl 
 of Mexican lakes, though with its persistent gills in a sense 
 the larval form of Amblystoma, attains of course to sexual 
 maturity. A more marked precocity has been observed in the 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 2$q 
 
 Alpine salamander (Triton alpestris). In higher organisms, it 
 occasionally happens that long before growth has ceased or 
 adolescence been reached sexuality sets in, especially in the 
 male sex, but this is fortunately a comparatively rare patho- 
 logical occurrence. In one set of organisms precocious repro- 
 ductive maturity has been of paramount importance, viz., in 
 the flowering plants. Here the prothallus stage, as contrasted 
 with the vegetative, has been much reduced, and has remained 
 associated with or been absorbed by the asexual generation. 
 This is to be in part explained by the accelerated reproduction 
 of the prothallus, comparable to a similar process which has 
 reduced the separate medusoid sexual persons of a hydroid 
 colony to mere buds. 
 
 It would be interesting to consider in this connection the 
 profound changes of habit often associated with reproductive 
 maturity, but in most cases the facts are inadequately known. 
 It has been often said that it is a nisus generativus which 
 prompts the salmon to leave the sea where it feeds and to 
 pass up the rivers where it reproduces, but it must be noted 
 that salmon are seen ascending the rivers throughout the whole 
 year in all stages of reproductive development (D. Noel Paton, 
 Report on Life History of Salmon. Fishery Board for Scotland, 
 1898). It has been often said that it is a nisus generativus 
 which prompts the migratory birds to fly in spring from the 
 warmer areas where they winter to the colder areas where they 
 breed, and it is admitted that the adult males are the first to 
 leave, but the difficulty remains that a large proportion of the 
 migrants are in most cases immature, and do not breed in that 
 season. 
 
 § 3. Menstruation. — The process of menstruation (menses, catamenia), 
 although from the earliest times the subject of medical inquiry, is by no 
 means yet clearly understood. It occurs usually at intervals of a lunar 
 month in all women during their period of potential fertility (fecundity), and 
 is not confined to the human species, having been observed in a number of 
 mammals, e.g. , some monkeys (Macacus rhesus, Semnopithecusentellus), 
 the lemur Tarsius, Tufaia javanica, and the common shrew (in the last 
 case without proof of outBow of blood). See W. Heape, " Proc. Roy. 
 Soc, London," lx., pp. 202-205. . ., 
 
 Though thus clearly a normal physiological process, it yet evidently lies 
 on the borders of pathological change, as is evidenced not only by the pain 
 which so frequently accompanies it, and the local and constitutional disorders 
 which so frequently arise in this connection, but by the general systemic 
 disturbance and local histological changes of which the discharge is merely 
 the outward expression and result. In general terms, and apart from 
 
260 THE EVOLUTION OF SEX. 
 
 ovulation, menstruation may be described as a periodic discharge of blood, 
 glandular secretion, and cellular detritus from the lining of the uterus. 
 After from three to six days the blood ceases to appear, and the lost 
 epithelium is rapidly replaced, apparently by proliferation from the necks 
 of the glands. By the ninth or tenth day the mucous coat is fully healed, 
 and the beginnings of the next menstrual process recommence. 
 
 The age at which the process commences varies with race and climate, 
 with nutrition and growth, with habit of life (e.g., with difference between 
 town and country life), and with mental and moral characteristics. Of 
 these, however, climate seems most important; thus, while in Northern 
 Europe the age is reckoned at the beginning of the fifteenth year, in the 
 tropics it commences earlier, in the ninth or tenth year, according to some. 
 The cessation of menstruation usually takes place between the age of forty- 
 five and fifty, and, somewhat as the secondary characteristics of female 
 puberty coincide with its appearance, a less distinct reduction of these is 
 associated with its close; in many cases secondary resemblances to the 
 masculine type may supervene. 
 
 The old theories of menstruation were, that it served to rid the system 
 of impure blood, that it simply corresponded to the period of "heat" 
 observed in lower animals, or, later, that it was associated with ovulation, 
 — which indeed seems broadly to correspond with the end of the menstrual 
 period. In Tupaia, Stratz has shown that the eggs are mature at the 
 beginning of menstruation, and ready for fertilisation at its end. And 
 while it cannot be maintained that either " heat " or ovulation are neces- 
 sarily associated with menstruation in Homo, there can be little doubt of 
 the general physiological parallelism of all three processes. At present 
 there may be said to be two rival theories. According to the first of 
 these, the process is viewed as a kind of surgical " freshening " of the uterus 
 for the reception of the ovum, whereby the latter during the healing process 
 can be attached safely to the uterine wall. The other view is exactly the 
 reverse of this. Its upholders regard the growth of the mucous coat before 
 this commencement of the flow as a preparation for the reception of an 
 ovum if duly fertilised, and the menstrual process itself as the expression of 
 the failure of these preparations, — in short, as a consequence of the non- 
 occurrence of pregnancy. A decided majority of gynaecologists appear to 
 incline to the latter view. 
 
 The process may, however, be expressed in more general, and at the 
 same time more fundamental terms. If the female sex be indeed pre- 
 ponderatingly anabolic, we should expect this to show itself in distinctive 
 functions. Menstruation is one of these, and is interpretable as a means 
 of getting rid of the anabolic surplus, in absence of its consumption by the 
 development of offspring, — just as it is intelligible that the process should 
 stop after fertilisation, when replaced by the demands of the practically 
 parasitic fcetus. In the same way, the occurrence of lactation, after this 
 internal parasitism has been terminated by birth, is seen to be reasonable. 
 The young mammal is thus enabled to become what is practically a 
 temporary ecto-parasite upon the unfailing maternal anabolic surplus ; and 
 when lactation finally ceases, we have the return of menstruation, from 
 which the whole cycle may start anew. So in the widely different yet 
 deeply similar world of flowers, the distinctly anabolic overflow of nectar 
 ceases at fertilisation, and the surplus of continued preponderant- anabolism 
 is drafted into the growing seed or fruit. 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 261 
 
 § 4. Sexual Union. — In a previous chapter we have noted 
 the passive and random way in which the sex-elements of 
 many of the lower animals are liberated, and the chance 
 manner in which they are often brought together by water- 
 currents and the like, though this may not be quite so common 
 as our ignorance leads us to suppose, witness the alleged 
 occurrence of sexual union in Asterina and Antedon. Yet 
 more in plants is the liberation of male elements, and notably 
 that of pollen-grains, a passive dehiscence, and fertilisation a 
 matter of chance, only reduced by the prodigal wealth of 
 material. Secure as the methods of fertilisation of flowers 
 by the aid of insects often are, the margin of risk is wide ; and 
 this is yet more marked when the pollen is carried by the 
 wind. It is true that, both in plants and animals, there are 
 subtle attractions between the essential elements, but this is 
 only at a close range ; and the external union is in many 
 cases none the less random. 
 
 It is important to keep clearly in mind the different forms 
 of sexual relation which may occur. There may be mating 
 at random within the limits of the race ("pangamic" mating), 
 or there may be casual mating between members of different 
 races, or there may be jome form of selective or prefer- 
 ential mating within a race. Various forms of the latter 
 may be distinguished (following Pearson's classification and 
 terminology) — 
 
 {a) Autogamic, or self-fertilisation. 
 
 (p) Endogamic, within the family, brood, or clan. 
 
 (c) Homogamic or assortative, or the mating of like with 
 
 like, the two mates not being of the same breed, or 
 not necessarily so. 
 
 (d) Preferential or apolegamic — i.e., with sexual selection 
 
 in the narrower sense of Darwin. 
 
 (e) Heterogamic, or the mating of unlikes. (See Pearson's 
 
 "Grammar of Science," 2nd ed., pp. 423-424.) 
 As it is essential that there should be a timely encounter 
 of the ovum and spermatozoon, we naturally find a very varied 
 series of adaptations securing fecundation. And while the 
 physical adaptations are important, we have also to recognise 
 that the increasing differentiation of the sexes has in the 
 higher animals been enhanced by psychical as well as physical 
 attractions, thus more and more ensuring the continuance of 
 the species. 
 
262 THE EVOLUTION OF SEX. 
 
 A not unfrequent mode of fecundation is by means of spermatophores, 
 or packets of spermatozoa. These may be seen at times attached to the 
 earth-worm, or found within the leech and snail. Even in newts sperma- 
 tophores may be formed, and taken up as such by the females. 
 
 In the spider the spermatozoa are stored in a special receptacle on the 
 
 Male of Paper Nautilus (Argonauta), with its modified 
 arm. — From Leunis. 
 
 palp, and hence hastily transferred to the fierce female. In cuttlefishes 
 this mode of impregnation is yet more marked. One of the " arms " of 
 the male, much modified and laden with spermatophores, is thrust, or in 
 many cases bodily discharged into the branchial cavity of the female, where 
 it bursts. Such a discharged arm was, on first discovery, regarded as a. 
 
 Dijtlozoon paradoxum, a double organism 
 formed from the union of two distinct 
 hermaphrodite individual trematodes 
 (Diporpa) at an early stage in their life. 
 
 parasite, and hence received the name of Hectocotylus. A curious aberra- 
 tion from the ordinary relations is figured above, where two distinct 
 individuals of a species of fluke (Diphzoon) join in almost life-long union. 
 
 In many cases again, especially in bony fishes, there is a sexual 
 attraction between male and female, but without any copulation. The 
 female, accompanied by her mate, deposits ova, which he thereupon 
 
SPECIAL PHYSIOLOGY Of SfoC Atfi) REPRODUCTION. 263 
 
 fertilises with spermatozoa. A slightly more advanced stage is seen in the 
 frog. Fertilisation is still outside the body of the mother, but the male, 
 embracing the female, liberates spermatozoa upon the eggs, just as these 
 are laid. 
 
 In the majority of cases, however, special organs for emitting and for 
 receiving spermatozoa are developed, and copulation occurs. The male 
 organ is often an adaptation of some structure already existing, as in many 
 crustaceans, where modified appendages form external canals for the 
 seminal fluid. In skates and other gristly fishes, the remarkably complex 
 copulatory organs, the so-called "claspers," are in close connection with the 
 hind limb. The penis of higher vertebrates is virtually a new organ. The 
 copulation may be quite external, as in crayfishes, &c, where the male, 
 seizing the female, deposits spermatozoa upon the already laid eggs. 
 Oftener, however, it is internal, and the intromittent organ is inserted into 
 the genital aperture of the female. True copulation may occur without 
 the presence of special organs, — notably in the case of many birds, where 
 the cloaca of the male is apposed to that of the female. The spermatozoa, 
 forcibly expelled by the excited male organs, pass up the female ducts, 
 probably, in part, as the result of peristalsis, but chiefly at least by their 
 own locomotor energy, and one of them may eventually fertilise an ovum. 
 In addition to the intromittent organ, and the lower portion of the female 
 duct which receives it during copulation, there may be auxiliary structures, 
 such as true claspers for retaining hold of the females. The limy " cupid's 
 dart" or " spiculum amoris" of the snail, is usually interpreted as a 
 preliminary excitant. 
 
 Three further notes in regard to higher animals are requisite. (1.) 
 There is much reason to believe that the follicles tend to burst towards the 
 end of menstruation ; that this may be accelerated by copulation ; successful 
 fertilisation may occur at any period, but most frequently soon after 
 menstruation, and most rarely during the relatively infertile period most 
 distant from that process. (2.) After conception, when the fertilised egg 
 has begun to develop, the mouth of the uterus is closed by a secretion, 
 which prevents the entrance of other spermatozoa should further copulation 
 occur. (3.) The period of gestation — i.e., between the fertilisation of the 
 ovum and the extrusion of the fcetus, varies widely in mammals, from 
 about 18 days in opossum, or 30 in rabbit, to about 280 days in Homo or 
 600 in the elephant, being longer in the more highly evolved types. But 
 the length of the period should also be considered in relation to size, 
 being about 280 days in cow and 150 in sheep; in relation to number of 
 offspring, being about 350 in mare and 60 in dog ; and in relation to the 
 degree of maturity at birth, being 420 in giraffe and 40 in kangaroo. 
 
 § 5. Parturition. — In many cases — e.g., marine annelids, 
 mature ova burst, as we have already noted, from the mother 
 animal, who may thenceforth have nothing more to do with 
 them. Liberation of ova from the ovary and from the 
 organism may be almost coincident, as in most bony fishes. 
 In other cases, the ova are retained within the mother until 
 fertilised, but are expelled not long after, before development 
 has advanced to any marked degree. Such eggs are often 
 
264 THE EVOLUTION OF SEX. 
 
 furnished with the important capital of nutriment, so familiar 
 in the case of birds, and may be also surrounded by chitinous, 
 horny, membranous, or limy shells. All such forms of birth 
 are familiarly described as oviparous. 
 
 In numerous invertebrates, fishes, amphibians, and reptiles, 
 the ova develop within the mother, and the young are born 
 more or less actively alive. To such cases, where there is no 
 nutritive connection between parent and offspring, the term 
 ovo-viviparous used to be applied. They were contrasted with 
 oviparous birth, as in birds, on the one hand, and with the 
 viviparous birth of mammals, on the other. It is the well- 
 known characteristic of the latter that there is an intimate 
 nutritive connection between mother and offspring. The term 
 is of little use, however, for the cases to which it is applied 
 shade off towards the two other forms of birth. Thus among 
 gristly fishes (Afuslelus Imvis and Carcharias), in the curious 
 bony fish Anabkps, and in certain lizards {Trachydosaurus and 
 Cyclodus), a somewhat placenta-like function is discharged by 
 the yolk-sac -and the wall of the oviduct; while in fishes, 
 reptiles, &c, oviparous and ovo-viviparous birth may occur in 
 nearly related forms. The distinction involved in the term is 
 therefore abandoned, and it must also be recognised that the 
 difference between egg-laying and the production of young 
 actively alive is only one of degree. Even in mammals, which 
 are viviparous par excellence, the two lowest genera— the duck- 
 mole and the Echidna — are oviparous. The commotT grass- 
 snake, normally oviparous, has been induced, in artificial 
 conditions, to bring forth its young alive, and this is pjobably 
 true of other forms. The parthenogenetic generation? ti 
 aphides are usually viviparous, while the fertilised eggs are laid 
 as such. 
 
 Beard has maintained that there is a definite stage in 
 development when the embryo first begins to put on its specific 
 characters, and calls this the critical stage. He believes that 
 in the early days of mammalian evolution, before an allantoic 
 placenta had arisen, the birth period and the critical period 
 (on to the critical stage) coincided ; and this is still seen in 
 several marsupials. Then, too, the ovulation period must 
 have been almost equal to— really a little longer than — the 
 critical period, for a coming ovulation, with its reflex message 
 from ovary to uterus, was the direct cause of birth. In higher 
 mammals, the evolution of an allantoic placenta provided for 
 
SPECIAL PHYSIOLOGY OP SEX AND REPRODUCTION. 265 
 
 the nutrition of the foetus beyond the critical period, or, more 
 technically, beyond a single "critical unit." But the interesting 
 fact is that a correspondence between the length of gestation 
 and a certain number of critical and ovulation units (up to 
 eight) is still preserved. The critical period, multiples of this, 
 and the ovulation periods, must very frequently be times of 
 abortion in mammals ; and menstruation is comparable to 
 an abortion prior to a new ovulation. There are no doubt 
 difficulties connected with this theory, and Beard ingeniously 
 deals with some of these, but the idea is a luminous one that 
 the span of gestation, the ovulation period, and the critical unit 
 are all connected as expressions of the rhythm of reproduction 
 in mammals, — a rhythm which has its basis in the ovary. By 
 ovulation the rhythm is proclaimed throughout the reproduc- 
 tive life of the female ; in gestation the same rhythm is 
 maintained but in a modified fashion ; and as the span of 
 uterine life draws to a close, it again asserts itself, and induces 
 birth. " Thus harmony and law reign in the reproductive life 
 of Mammalia." 
 
 § 6. Early Nutrition. — The early nutrition of the embryo, 
 and even larva, is in most cases an absorption of the legacy of 
 yolk material, which is probably richest in the eggs of birds. 
 The tadpole of the frog grows and exerts itself for a short time 
 at the expense of its legacy; it then begins to feed for itself, but 
 it is interesting to notice that before metamorphosis is accom- 
 plished the growth of new structures appears to be provided 
 for by the nutritive absorption of the tail, the larva literally 
 living upon itself. The same is true in the elaborate meta- 
 morphosis of echinoderm larvae. In many cases, the cells of 
 the embryo, independently and actively, devour the yolk and 
 other available material, after the amoeboid fashion technically 
 known as intra-cellular. At the same time, osmotic currents 
 may more passively effect the like result. 
 
 In the buckie or whelk {Buccinum undatum) the eggs are 
 enclosed in capsules secreted from the sole-gland of the foot, 
 and the early nutrition is remarkable : a cannibalism occurs 
 among the crowd of embryos enclosed within each capsule. 
 The stronger and older devour the younger and weaker, — a 
 struggle for existence happily of exceptional precociousness. 
 
 The conception of a struggle for existence applies to the 
 earliest chapters of life. There is struggle among potential 
 ova, and struggle amid the crowd of spermatozoa; there is 
 
266 THE EVOLUTION OF SEX. 
 
 struggle between embryo and mother, and struggle between 
 adjacent embryos. Thus De Bruyne notes in regard to fresh- 
 water mussels, the struggle between the successful ova and 
 the adjacent cells, and the continuance of this between the 
 embryos and the maternal leucocytes, and even on to the time 
 when the larva? are temporarily parasitic in the skin of a fish. 
 (See "Arch. Biol.," xv., 1898, pp. 181-300, 5 plates.) 
 
 In the higher vertebrates (above amphibians), two foetal 
 membranes— amnion and allantois — are developed, in addition 
 to the yolk-sac which encloses the yolk. Of these the amnion 
 is mainly protective, and the allantois at first almost wholly 
 respiratory. But in birds (and to a slight extent in some 
 reptiles) the allantois begins to assume nutritive functions, 
 assisting in the absorption of the yolk. In placental mammals 
 this nutritive function becomes paramount, the allantois forming 
 the greater part of the embryonic side of the placenta. The 
 yolk-sac is here virtually yolk-less, but in some forms it serves 
 for a time to absorb nutriment as it did in birds, though from 
 a different source, — the maternal wall. In most cases, how- 
 ever, what was incipient on the part of the yolk-sac in the 
 exceptional elasmobranchs and lizards already mentioned, 
 becomes the emphatic function of the allantois, — namely, the 
 establishment of a vascular or nutritive connection with the 
 wall of the maternal uterus. By this means a very intimate 
 osmotic transfusion is effected. 
 
 § 7. Lactatio7i. — If menstruation be a means of getting rid 
 of anabolic surplus, in absence of the fcetal consumption, lacta- 
 tion is still more an anabolic overflow, adapted to, though not 
 of course originally caused by the offspring's demands. It is 
 at the same time evident enough, and easily verified by the 
 histologist, that in actual occurrence both processes are kata- 
 bolic, involving cellular disruption and death. That peculiar 
 liability of these uterine and mammary tissues to disease, 
 which furnishes the most tragic possibilities of the life of 
 woman, becomes thus less mysterious. We can understand 
 more readily the association of such diseases with much of 
 what we are pleased to generalise as civilisation, and view 
 more hopefully the possibilities of their enormous diminution 
 by the rational hygiene of civilisation properly so called. 
 
 The milk or mammary organs are modified skin-glands, 
 probably most nearly allied to the ordinary sebaceous type, 
 except in monotremes, where they seem to be allied rather to 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 267 
 
 the sweat glands. Every one knows that they are exclusively 
 characteristic of mammals, and are only normally functional in 
 the female sex. Rudimentary in the males, they may even 
 there produce milk ("witches' milk") at birth, puberty, and 
 under pathological conditions, while cases have been put on 
 record of males who have actually given suck. Merriam 
 (Hayden's U.S. Geol. Survey, VI., p. 666) gives a definite 
 account of male lactation in Lepus .bairdi. They vary greatly 
 in position and number, a large number being doubtless 
 the primitive condition. It has also been shown that the 
 nuclei of the gland cells undergo degeneration, disruption, and 
 expulsion, and they, in all likelihood, form the casein elements 
 of the nutritive fluid. Some of the abundant leucocytes or 
 white blood corpuscles also migrate through the epithelium, 
 but it is not proved that they actually share in forming the 
 milk, though they may bring fat globules into it. (See L. 
 Michaelis, "Arch. Mikr. Anat.," li., 1898, pp. 711-747, 2 pis.) 
 
 Before birth, the mammalian embryo has been nourished 
 through the placenta, by the transfusion already referred to. 
 The alimentary canal has obviously had no experience in 
 digestive function. Before it proceeds to digest the food of the 
 parents, it is put through a course of what Sollas neatly terms 
 "gastric education," by feeding upon the readily assimilated 
 mother's milk. 
 
 § 8. Other Secretions. — Every one has at least heard of 
 "pigeon's milk," and many are familiar with its administration 
 to the young birds. It is produced by both sexes, especially 
 just after the hatching of the young, and is the result of a 
 degeneration of the cells lining the crop. Some of the cells 
 break up, others are discharged bodily. The result forms a 
 milky emulsion-like fluid, which is regurgitated by the parents 
 into the mouth of the young bird. A similar substance is said 
 to occur in some parrots. 
 
 Of some interest also is the supra-salivation which occurs 
 at the breeding season in the swiftlets (Collocalia), which form 
 the edible birds' nests, the costly, though to us wofully insipid, 
 luxury of Chinese epicures. Certain salivary glands become 
 peculiarly active in these birds when breeding, and the secre- 
 tion, which, according to Green, consists chiefly of a substance 
 akin to mucin, is used to form the snow-white fibrous nest. 
 
 Take only one other instance of peculiar secretion, curiously 
 linked to the above by one of those profound physiological 
 
268 THE EVOLUTION OP SEX. 
 
 unities which show how superficial after all are the utmost 
 contrasts of organic form, — we refer to the viscid threads with 
 which the male stickleback weaves his nest. Mobius has 
 shown that the kidneys are greatly affected by the growth 
 of the testes ; that they produce, by a semi-pathological pro- 
 cess, special waste or katabolic elements, in the form of 
 mucous threads. The male gets rid of this uneasy encum- 
 brance (which has a somewhat parallel pathological equivalent 
 in higher animals), by rubbing itself against objects, and thus 
 almost mechanically has been evolved the familiar weaving of 
 the aquatic nest. 
 
 
 The Nest of the Stickleback (Gas(erosteus).~-¥iom Thomas Bolton. 
 
 § 9. Incubation. — The physiological sacrifice of the female 
 birds doesnot end with providing the large capital of nutritive 
 material with which the germ is endowed, but is continued in 
 all the patience of brooding. In passerine birds the male 
 relieves the female in her task of love, and in the ostrich and 
 Rhea he plays a prominent part. In the cuckoos and cow- 
 birds the parental care is shirked, and with varying degrees of 
 deliberateness the eggs are foisted into foster nests, and the 
 young thus put out to nurse. After the fatigue of reproduction 
 it is perhaps natural enough that the female should rest awhile 
 upon the eggs in the shelter of the nest, and since there is 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 
 
 269 
 
 observed to be an increased circulation in the skin of the 
 abdominal region at this time, it has been argued that the bird 
 merely sits to cool itself! Primitively, the incubatory instinct 
 may be traced back to the need for rest, as a reaction after the 
 fatigue of reproduction. 
 
 Here too one must include the retention of the young in 
 skin pouches, exhibited by the great majority of marsupial 
 mammals and by the Echidna. In the latter, the pouch is a 
 
 mtt^tt ®<&§fc* •or 
 
 •en c,r<Q:? « 
 
 The female Surinam Toad, with young ones on its back.— From Leunis. 
 
 simple and possibly periodic structure, arising from an insink- 
 ing of the skin in the mammary region of the abdomen. Here 
 the eggs are somehow or other stowed away and the young 
 developed. The milk glands simply open on the surface of 
 the depression. In most marsupials, the young, which are 
 born precociously after a very short uterine li r e, are sheltered 
 in similar, but more developed, pouches of the skin, within 
 which the teats open. 
 
!70 
 
 THE EVOLUTION OF SEX. 
 
 Ill oviparous reptiles, the eggs are usually left to hatch of 
 themselves, aided by the warmth of sun and soil. " The 
 female python disposes herself in coils round her eggs, and 
 incubates them for a prolonged period, during which the 
 temperature has been observed to rise as high as 96° F. within 
 the coils." 
 
 Some exceedingly curious parental adaptations occur among 
 amphibians, which seem to have made numerous experiments 
 in this direction. Thus in the Surinam toad (Pipa), the male 
 spreads the ova on the female's back, a sort of erysipelas sets 
 
 The female Kototi-ema. viarsupiatinii, — an amphibian, with 
 eggs in a dorsal sac, which is shown partly uncovered. 
 — From Carus Sterne, after Giinther. 
 
 iii, and each ovum becomes surrounded by a skin-cavity in 
 which the tadpole develops. After the process is over, the 
 skin of the back is renewed. In other cases this mode of 
 carrying the ova becomes somewhat more definite; thus in 
 Notodelphys and Nolotrema the eggs are stored in dorsal 
 pouches. Nor are the males without their share in the task 
 of parentage. In the obstetric frog (Alytes obstelricans), the 
 male helps to remove the eggs from the female, twists them in 
 strings round his hind legs, and buries himself in the water till 
 the tadpoles escape and relieve him of his burden. In Rhino- 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 27 1 
 
 derma darwinii, the croaking sacs, which were previously used 
 
 for amatory calling, become enlarged as cradles for the young. 
 
 In a Japanese tree-frog {RhacopJwrus schkgelii) the female 
 
 The Sea-horse {Hippocampus guttalatus).- 
 Atlas of the Naples Aquarium. 
 
 lays the eggs in a hole in the mud, and by curious kneading 
 and treading movements works the jelly which surrounds them 
 into a froth. The frothy envelope is like well-beaten white of 
 
 The female of the " Paper Nautilus" (Argonauta argo), with its 
 brood-ohamber. — After Leunis. 
 
 egg, and the outer surface dries into a crust. It protects the 
 eggs, and perhaps prevents overcrowding, but is of especial 
 service in facilitating the respiration of the eggs and embryos. 
 
2 72 THE EVOLUTION OF SEX. 
 
 (S. Ikeda, "Annot. Zool. Japon.," i., 1897, pp. 113-122, 2 
 figs.) 
 
 Among fishes, parental care is largely in abeyance, and 
 there are only slight hints of anything in the way of incuba- 
 tion. In a siluroid fish (Aspredo), the female deposits her ova 
 and lies upon them till they become attached to the spongy 
 skin of the belly, very much as happens in the dorsal attach- 
 ment of the Surinam toad. After hatching, the skin excres- 
 cence is smoothed away. In Soknostoma (allied to pipe-fish) 
 the ventral fins unite with the skin to form a pouch in which 
 the eggs are retained. In other cases, it is the male which 
 incubates or cares for the ova. Not a few form nests, as in the 
 stickleback, over which they keep a jealous guard. In some 
 species of Arius the eggs are carried about in the pharynx ; 
 while in the sea-horses a pouch is developed on the posterior 
 abdomen. 
 
 Among invertebrates, brood-chambers or cradles for the 
 young are not uncommon. The capsules of hydroids, the 
 tent of spines on a few sea-urchins, the depressions in the skin 
 in one or two sea-cucumbers, the modified tentacles of some 
 marine annelids, the dorsal shell-chamber in water-fleas, the 
 incurved abdomen of higher crustaceans, the gill-cavities of 
 bivalves, the beautiful brood-shell of the argonaut, illustrate a 
 habit even an outline of which is beyond our limits. 
 
 § 10. Nemesis of Reproduction. — We have already shown 
 how reproduction in its origin is linked to death. The primi- 
 tive ruptures by which the protozoon reduces encumbering 
 bulk, saves its own life, and multiplies its kind, are only a step 
 or two from more diffuse dissolution which is death. 
 
 The association of death and reproduction is indeed patent 
 enough, but the connection is in popular language usually 
 misstated. Organisms, one hears, have to die ; they must 
 therefore reproduce, else the species would come to an end. 
 But such emphasis on posterior utilities is almost always only 
 an afterthought of our invention. The statement must be 
 corrected by another; as Goette says, "it is not death that 
 makes reproduction necessary, but reproduction has death as 
 its inevitable consequence." This of course refers primarily to 
 the incipient forms of both these katabolic processes. 
 
 It is necessary to give a few illustrations. Goette refers to 
 Haeckel's Magosphczra, a protozoon which just as it had formed 
 for itself a multicellular body broke up into the component 
 
SPECIAL PHYSIOLOGY OF SEX AttD REPRODUCTION. 273 
 
 units. These lived on, and there was no corpse, but at the 
 same time the multicellular colony was no more. Again he 
 takes the case of the lowly and somewhat enigmatical ortho- 
 nectids, which Van Beneden has classed as Mesozoa, between 
 the single-celled and the stable many-celled animals. In some 
 of these the mature female forms numerous germ-cells, and 
 terminates her individual life by bursting. The germs are 
 
 
 
 ■■'•>. vv-;rs- V*% 'J%^rWr 
 
 A figure of cell division suggesting the internal disruptions and 
 rearrangements of the nucleus (a) and protoplasm. — From 
 Rauber. 
 
 liberated, the mother animal has been sacrificed in reproduc- 
 tion. "The death is an altogether inevitable consequence of 
 the reproduction." 
 
 Nor is this sacrifice confined to the incipient multicellular 
 organisms. Thus in some species of the annelid Polygordius, 
 the mature females break up and die in liberating their ova, 
 
 18 
 
274 
 
 THE EVOLUTION OF SEX. 
 
 In the " Heteronereis," or sexually modified form of Nereid 
 worms, the whole animal dies after the emission of the genital 
 products. This is approached, but suggestively avoided, in 
 some other Polychaet worms, e.g., the Syllidae and the Clito- 
 maslus. The whole organism is not sacrificed, but only a 
 modified portion of the body. This is probably also the case 
 with the famous Palolo-worm {Eunice viridis). Here in fact 
 we have one of the keynotes to reproductive differentiation, — 
 the sacrifice is lessened, and the fatality thus warded off. 
 
 Orthonectids, showing the rupture of the female in liberating 
 the germs. — From Goette, after Julin. 
 
 But again, we find in some threadworms or nematodes 
 (e.g., Ascaris dactyluris) that the young live at the expense of 
 the mother, until she is reduced to a mere husk. In fresh- 
 water Polyzoa, Kraepelin notes that the ciliated embryo leaves 
 the maternal body-cavity through a prolapsus uteri of the sacri- 
 ficed mother. In the precocious reproduction of some midge 
 larvae (Chironomus, &c), the production of young is fatal 
 through successive -generations. 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION, 275 
 
 Both Weismann and Goette, though with different interpre- 
 tations, note how many insects (locusts, butterflies, ephemerids, 
 &c.) die a few hours after the production of ova. The ex- 
 haustion is fatal, and the males are also involved. In fact, as 
 we should expect from the katabolic temperament, it is the 
 males which are especially liable to exhaustion. The males of 
 some spiders normally die after impregnating the female, a fact 
 perhaps helping to throw light upon the sacrifice of others to 
 their mates. The similarly tiny (ultra-katabolic) male rotifer — 
 an ideal but too unpractical lover, with not even an alimentary 
 canal — would seem usually to fail and expire prematurely, 
 leaving the female to undisturbed parthenogenesis. Every one 
 is familiar with the close association of love and death in the 
 common mayflies. Emergence into winged liberty, the love- 
 dance and the process of fertilisation, the deposition of eggs 
 and the death of both parents, are often the crowded events 
 of a few hours. In higher animals, the fatality of the repro- 
 ductive sacrifice has been greatly lessened, yet death may 
 tragically persist, even in human life, as the direct nemesis of 
 love. 
 
 The temporarily exhausting effect of even moderate sexual 
 indulgence is well known, as well as the increased liability to 
 all forms of disease while the individual energies are thus 
 lowered. 
 
 § it. Organic Immortality. — Comparatively little is yet 
 known about the length of life among lower animals, but there 
 is no reason to doubt that all multicellular organisms die. We 
 have just emphasised the view of Goette, and other naturalists, 
 that reproduction is the beginning of death ; which is not 
 inconsistent with the apparent paradox, that local death was 
 the beginning of reproduction. Allowing, then, that multi- 
 cellular organisms at any rate are mortal, and that the very 
 blossoming of the life in reproduction is fated with a prophecy 
 of death which is its own fulfilment, we have to face two 
 questions, — What of death in the Protozoa? and, In what 
 sense is there an immortality throughout the organic series ? 
 
 Often enough already, in the preceding pages, we have had 
 to reiterate the contrasts between the Protozoa and the higher 
 animals. These firstlings are single cells physiologically com- 
 plete in themselves, and have at least very great, if not un- 
 limited, powers of self-recuperation. They leave off where 
 higher animal life begins, that is to say, in a unicellular state. 
 
276 THE EVOLUTION OF SEX. 
 
 They do not form "bodies." Their reproduction, moreover, is 
 in the majority simple cell-division into two. If there be loss 
 of individuality, there is hardly loss of life. Death is not so 
 serious when there is nothing left to bury. Nor in most cases 
 can one half of the divided unit be the mother individual, and 
 the other the daughter, for the two appear indistinguishably 
 the same. Thus an idea, broached long ago by Ehrenberg, 
 has been revived and elaborated by several naturalists, and 
 especially by Weismann, that the Protozoa are virtually im- 
 mortal. 
 
 In Weismann's own words, "Natural death occurs only 
 among multicellular organisms, the single-celled forms escape 
 it. There is no end to their development which can be 
 likened to death, nor is the rise of new individuals associated 
 with the death of the old. In the division the two portions 
 are equal, neither is the older nor the younger. Thus there 
 arises an unending series of individuals, each as old as the 
 species itself, each with the power of living on indefinitely, 
 ever dividing but never dying." Ray Lankester puts the 
 matter tersely, "It results from the constitution of the proto- 
 zoon body as a single cell, and its method of multiplication by 
 fission, that death has no place as a natural recurrent pheno- 
 menon among these organisms." 
 
 Some limitations must be noticed, which make this idea of 
 pristine immortality yet more emphatic. . It is only asserted 
 that the Protozoa escape "natural death," a violent fate may 
 of course await them like any other organisms. They have no 
 charmed life, being as liable to be devoured as those of higher 
 degree. In relation to the environment, however, their sim- 
 plicity gives them a peculiar power of avoiding impending 
 destiny. The habit of forming protective cysts is very general, 
 and thus enwrapped they can, like the ova and a few of the 
 adults of some higher animals, endure even prolonged desic- 
 cation with successful patience, which is rewarded by a re- 
 juvenescence when the rain revisits the pools. But the 
 doctrine of the "immortality of the Protozoa" refers to a 
 defiance of natural, not violent, death. 
 
 The psychological objection that the original individuality 
 is extinguished when it divides into two, intrudes a con- 
 ception which is hardly applicable. The individualities are 
 doubled, nothing is really lost. Most seriously difficult are 
 those cases where the protozoon produces a series of buds, 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 277 
 
 spores, or division units, and leaves a residual core or unused 
 remnant behind to die. But in regard to the gregarines, for 
 instance, where such a remnant is often left, it has been fairly 
 answered that the residue is rather a kind of excretion than 
 the parent left to perish after its reproductive sacrifice. Weis- 
 mann is, however, willing to admit the possibility, that in the 
 suctorial Acinetse, and in the parasitic gregarines, which are 
 both somewhat removed from the normal protozoon type, 
 there may be cases of true mortality. 
 
 Another point in regard to which experts differ, is whether 
 the Protozoa are really quite self-recuperative. They suffer 
 injuries, they necessarily suffer waste, portions are used up 
 and ejected. The question then arises, Are those acquired 
 defects obliterated, or do they become intensified ? Is the 
 wasting only a local death, or is it the beginning of a true 
 senescence? This is a question which can only be answered 
 by observation ; a priori reasoning is here futile. The most 
 serious criticism of Weismann's view is due to Maupas. 
 Already we have noted his important result, that conjugation 
 is essential to the health of the species. Without this incipient 
 sexual reproduction, the individuals in the course of numerous 
 successive asexual generations grow old. The nucleus degen- 
 erates, the size diminishes, the entire energy wanes, the senility 
 ends in death. Maupas believes that all organisms are fated 
 to suffer decay and death, and protests strongly against Weis- 
 mann's theory that death began with the Metazoa. 
 
 It must be noted, however, that in natural conditions the 
 conjugation, prohibited in Maupas's experiments, occurs when 
 it is wanted, and the life flows on. Furthermore, in many 
 Protozoa conjugation has not been shown to occur. It seems 
 therefore more warrantable to insert Maupas's result as a saving 
 clause to Weismann's doctrine, than to regard it as contra- 
 dictory. The conclusion at present justifiable, is that Protozoa 
 not too highly differentiated, living in natural conditions 
 where conjugation is possible, have a freedom from natural 
 death. To this must then be added the demonstrated saving 
 clause, that in ciliated infusorians, conjugation, which here 
 means an exchange of nuclear elements, is the necessary con- 
 dition of eternal youth and immortality. 
 
 Accepting then, with an emphasised proviso, the general 
 conclusion that most, if not all, unicellular organisms enjoy 
 immortality, that in being without the bondage of a "body" 
 
278 THE EVOLUTION OF SEX, 
 
 they are necessarily freed from death, we pass to consider the 
 second question, What does the death of the higher and multi- 
 cellular organisms really involve? 
 
 If death do not naturally occur in the Protozoa, it is evident 
 that it cannot be an inherent characteristic of living matter. 
 Yet it is universal among the multicellular animals. Death, 
 we may thus say, is the price paid for a body, the penalty its 
 attainment and possession sooner or later involves. Now, by 
 a body is meant a complex colony of cells, in which there is 
 more or less division of labour, where the component units are 
 no longer, like the Protozoa, in possession of all their faculties, 
 but through division of labour have only restricted functions 
 and limited powers of self-recuperation. Like Maupas's isolated 
 family of infusorians, the cells of the body do not conjugate 
 with one another; and though they divide and re-divide for a 
 season, the life eventually runs itself out. 
 
 The relation between reproductive cells and the body. The horizontal chain of cells 
 represents a succession of the ova from which the "bodies" are produced. At 
 each generation, a spermatozoon fertilising the liberated ovum is also indicated. 
 
 A moment's consideration, however, will show that in most 
 cases the organism does not wholly die. Some of the cells 
 usually escape from the bondage of the body as reproductive 
 elements, — as, in fact, Protozoa once more. The majority of 
 these may indeed be lost; eggs which do not meet with male 
 elements perish, and the latter have even less power of inde- 
 pendent vitality. But when the ova are fertilised, and proceed 
 to develop into other individuals, it is plain that the parent 
 organisms have not wholly died, since two of their cells have 
 united to start afresh as new plants or animals. In other 
 words, what is new in the multicellular organism, namely, the 
 " body," does indeed die, but the reproductive elements, which 
 correspond to the Protozoa, live on. 
 
 This may be made more definite in the preceding diagram. 
 There it is seen that the organism starts like a protozoon, as a 
 single cell, or usually as a union of two cells in the fertilised 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 279 
 
 ovum. This divides, and its daughter-cells divide and re-divide. 
 They arrange themselves in layers, and are gradually mapped 
 out into the various tissues or organs. In division of labour, 
 they become restricted in their functions, and specialised in 
 their structure. They become differentiated as muscle-cells, 
 nerve-cells, gland-cells, and so on. The result is a more or 
 less complex " body," unstable in its equilibrium because of its 
 very complexity, composed moreover of competing cells far 
 removed from the protozoan all-roundness of function, limited 
 in their powers of recuperation, and emphatically liable to local 
 and periodic, or to general and final death. But the body is 
 not all. At an early stage in some cases, sooner or later 
 always, reproductive cells are set apart. These remain simple 
 and undifferentiated, preserving the structural and functional 
 traditions of the original germ-cell. These cells, and the results 
 of their division, are but little implicated in the differentiation 
 which makes the multicellular organism what it is; they remain 
 simple primitive cells like the Protozoa, and in a sense they 
 too share the protozoon immortality. The diagram shows how 
 one of these cells, separated from the parent organism (and 
 uniting in most cases with a germ-cell of different origin), 
 becomes the beginning of a new body, and, at the same time, 
 necessarily the origin of a new chain, or rather of a continued 
 chain of fresh reproductive cells. 
 
 " The body or soma" Weismann says, " thus appears to a 
 certain extent as a subsidiary appendage of the true bearers of 
 the life, — the reproductive cells." Ray Lankester has again 
 well expressed this: — " Among the multicellular animals, certain 
 cells are separated from the rest of the constituent units of the 
 body, as egg-cells and sperm-cells; these conjugate and continue 
 to live, whilst the remaining cells, the mere carriers as it were 
 of the immortal reproductive cells, die and disintegrate. The 
 bodies of the higher animals which die, may from this point of 
 view be regarded as something temporary and non-essential, 
 destined merely to carry for a time, to nurse, and to nourish 
 the more important and deathless fission-products of the 
 unicellular egg." 
 
 In most cases, as Weismann insists, it is more correct to 
 speak of " the continuity of the germinal protoplasm " than of 
 the continuity of the germ-cells; but, with this proviso, the 
 diagram expresses a fact most important in understanding 
 reproduction and heredity, that the chain of life is in a real 
 
280 THE EVOLUTION OF SEX. 
 
 sense continuous, and that the " bodies " which die are 
 deciduous growths, which arise round about the real links. 
 The bodies are but the torches which burn out, while the living 
 flame has passed throughout the organic series unextinguished. 
 The bodies are the leaves which fall in dying from the con- 
 tinuously growing branch. Thus although death take inexor- 
 able grasp of the individual, the continuance of the life is still 
 in a deep sense unaffected; the reproductive elements have 
 already claimed their protozoan immortality, are already 
 recreating a new body; so in the simplest physical, as in the 
 highest psychic life, we may say that love is stronger than 
 death. 
 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 28 1 
 
 SUMMARY. 
 
 I. According to Weismann's theory of the continuity of the germ-plasm, 
 a portion of the specific hereditary substance which the fertilised ovum 
 contains is not used up in the development of the offspring's body, but is 
 reserved unchanged to form the germ-cells of the following generation. 
 
 2. Sexual maturity generally occurs towards the limit of growth, is marked 
 by liberation of reproductive elements and by secondary characteristics, in 
 part due to the reaction of the .reproductive function on the general system. 
 Precocious maturity may be due to constitutional or environmental con- 
 ditions, and has been of much importance in the evolution of flowering 
 plants. 
 
 3. Menstruation may be interpreted as a means of getting rid of the 
 anabolic surplus of the female in absence of its fcetal consumption. 
 
 4. Sexual union, at first very passive and random, becomes active and 
 definite with the gradual evolution of sex and secondary sexual organs. 
 
 5. Birth is at first accomplished by rupture, but becomes a definite 
 process usually effected through special ducts. Oviparous and viviparous 
 birth only differ in degree. 
 
 6. Early nutrition is usually an absorption of the yolk, but in mammals 
 is accomplished by osmotic transfusion from the blood of the mother to that 
 of the foetus. 
 
 7. Lactation may be interpreted as an anabolic overflow. 
 
 8. Besides milk, there are other secretions associated with the nutrition 
 and sheltering of the young. Pigeon's milk, edible birds' nests, and the 
 mucous threads of sticklebacks, are illustrations. 
 
 9. Incubation, reaching * climax in birds, is paralleled in many other 
 classes. 
 
 10. Reproduction and death both represent katabolic crises. Primitively, 
 they are nearly akin. Reproduction may ward off death from the Proto- 
 zoon, but in the simplest Metazoa it helped to cause it. 
 
 II. The Protozoa come nearer immortality than other organisms. The 
 fact of germinal continuity involves an organic immortality. 
 
 LITERATURE. 
 
 For the special physiology of sex and reproduction, consult standard text- 
 books, such as those of Foster, Landois and Stirling, and especially Hensen's 
 work already often cited. - 
 
 On the continuity of the germ-plasma, consult recent translation of 
 Weismann's papers — " Heredity," Oxford, 1889; while a full bibliography 
 will be found in " History and Theory of Heredity," by J. A. Thomson, 
 Proc. Roy. Soc. Edin., 1888; and, since 1886, in the Zoological Record. 
 
 On the nemesis of reproduction, and on organic immortality, see A. 
 Goette, " Ueber den Ursprung des Todes" Hamburg and Leipzig, 1883; 
 and A. Weismann, " Ueber die Dauerdes Lebens," Jena, 1882; " Ueber 
 Leben und Tod," Jena, 1S84; E. Maupas, "Archives de Zoologie 
 expe'rimentale," 1888. 
 
252 THE EVOLUTION OF SEX. 
 
 Beard, J. — The Span of Gestation and the Cause of Birth : a Study of the 
 Critical Period and its Effects in Mammalia. Jena, 1897, ix. and 
 
 132 PP- 
 The Rhythm of Reproduction in Mammals. Anat. Anzeiger, XIV. , 
 
 1897. PP- 97-102- 
 Beeton, Alice, and Pearson, K. — Inheritance of Longevity. Proc. 
 
 Roy. Soc. London, 1899; Nature, LX., 1899, pp. 356-357. 
 Ellis, Havelock. — Psychology of Sex, Vol. II., 1899. (In great part 
 
 devoted to a discussion of pathological sexual conditions.) Man and 
 
 Woman. London, 1894. 
 FerIS, Ch. — Les Perversions Sexuelles chez les Animaux. Revue Philo- 
 
 sophique, XXII., 1897, pp. 494-503. 
 IIeape, W. — The Artificial Insemination of Mammals. Proc. Roy. Soc. 
 
 London, LXL, 1897, pp. 52-63. 
 Kohlwey, H. — Arten- und Rassenbildung. Eine Einfiihrung in das 
 
 Gebiet der Tierzucht. Leipzig, 1897, 72 pp., 5 figs. 
 Lee, F. S. — Physiology of Reproduction in W. H. Howell's Text-book of 
 
 Physiology. Philadelphia, 1896. 
 Mackenzie, J. N. — Physiological and Pathological Relations between the 
 
 Nose and the Sexual Apparatus of Man. Bulletin Johns Hopkins 
 
 Hospital, IX., 1S98, pp. 10-17. 
 Morgan, C Lloyd. — Habit and Instinct. London, 1896, pp. 351. 
 
 (See chapter on habits of birds, etc., in mating.) 
 Pearson, Karl. — Law of Ancestral Heredity. Proc. Roy. Soc. London, 
 
 LXII. , 1898, pp. 386-412. The Chances of Death, and other Studies 
 
 in Evolution, 2 vols. London, 1897. 
 Webster, J. C. — The Biological Basis of Menstruation. Montreal Med. 
 
 Journal, April 1897, 19 pp. 
 
CHAPTER XIX. 
 
 Psychological and Ethical Aspects. 
 
 § i. Common Ground between Animals and Men. — Hitherto 
 we have been justifying the orthodoxy of an anatomical train- 
 ing, by almost wholly ignoring the fact that animals have a 
 psychic life, or only mentioning the mere neural aspect of 
 functions. Only in discussing sexual selection, and the general 
 facts of sexual union and of parentage, have we intruded words 
 like "care," "sacrifice," and "love." A purely physiological 
 treatment of sex and reproduction is, however, obviously 
 incomplete. It would be rejected with scorn in reference to 
 human life; it must be equally rejected in regard to the 
 higher animals, which, taken together, exhibit ihe analogues 
 of almost every human emotion, and of all our less recondite 
 intellectual processes. It is with emotions that we have here 
 most to do ; and without raising the difficult question whether 
 animals exhibit any emotions exactly analogous to those which 
 in man are associated with the " moral sense," "religion," and 
 "the sublime," we accept the conclusion of Darwin, followed 
 by Romanes and others, that all other emotions which we 
 ourselves experience, are likewise recognisable in analogous 
 expression in the higher animals. Those which are associated 
 with sex and reproduction are indeed among the most patent; 
 love of mates, love of offspring, lust, jealousy, family affection, 
 social sympathies, are undeniable. 
 
 § 2. The Love of Mates. — In the lowest animals, where 
 two exhausted cells flow together in incipient sexual union, 
 there is apparently only one component of that most complex 
 musical chord in life which we call "love." There is physical 
 attraction, and the whole process is very much a satisfaction of 
 protoplasmic hunger. 
 
 In multicellular animals, the liberation of sex-elements is 
 at first very passive. It concerns the individual alone. Fertili- 
 sation is a random matter; and though sex exists, sexual 
 attraction does not, 
 
284 THE EVOLUTION OF SEX. 
 
 A grade higher, true sexual union begins to appear. But 
 at first this simply occurs between any male and any available 
 female. The psychological factor is still but feebly expressecTf 
 there is no genuine pairing, and it would be folly to use the 
 word love in such cases. 
 
 Gradually, however, for instance among insects, the sexes 
 associate in pairs. There is some psychic sexual attraction, 
 often accompanied with no little courtship, but much more 
 important is the occasional maintenance of the association for 
 a lengthened period. There may even be co-operation in 
 work, as in dung-rolling beetles such as Ateuchus, where the 
 two sexes pursue their somewhat disinterested labours together. 
 The male and female of another lamellicorn beetle {Lethrus 
 cephalotes) inhabit the same cavity, and the virtuous matron 
 is said greatly to resent the intrusion of another male. As 
 degenerate offshoots from the path of psychic progress, or as 
 illustrations of the predominance of merely physical attraction, 
 one must regard such prolonged associations of the two sexes 
 as are seen in the formidable parasitic worm Bilharzia, where 
 the male carries the female about, or in some parasitic crusta- 
 ceans where the positions are reversed. 
 
 Among the cold-blooded fishes, the battles of the stickle- 
 back with his rivals, his captivating manoeuvres to lead the 
 female to the nest which he has built, his mad dance of 
 passion around her, and his subsequent jealous guarding of 
 the nest, have often been observed and admired. In one of 
 the sunfishes the male and female alternate in guarding the 
 ova. The monogamous habits of the salmon, and the 
 frequently fatal contests between rival males are well known. 
 Carbonnier has beautifully described the elaborateness of 
 sexual display and the ardency of passion in the male butterfly- 
 fish, and also in the rainbow-fish of the Ganges. 
 
 The amatory croaking of frogs, the love-gambols of some 
 newts, the curious parental care of some male amphibians 
 mentioned in the preceding chapter, and the like, illustrate 
 the continuance of more than crude physical attraction 
 between the sexes. Of many amphibians it may be said that 
 it is only in their sexual and reproductive relations that they 
 seem to wake up out of their constitutional sluggishness. 
 
 In regard to reptiles, little is known beyond the exhibition 
 of sexual passion and the jealous combats of rival males. Yet 
 Romanes refers to the interesting fact that when a cobra is 
 
PSYCHOLOGICAL AND ETHICAL ASPECTS. 285 
 
 killed, its mate is often found on the same spot a day or two 
 afterwards. 
 
 Among birds and mammals, the greater differentiation of 
 the nervous system and the higher pitch of the whole life is 
 associated with the development of what pedantry alone can 
 refuse to call love. There is often partnership, co-operation, 
 and evident affection beyond the limits of the breeding periods, 
 there are abundant illustrations of regard for conventions, 
 there are close analogues of human flirtation, courtship, 
 jealousy, and even crime, though, so far as we understand the 
 matter, there is no convincing evidence of what may be called 
 a distinctively moral judgment. There is no doubt that in 
 the two highest classes of animals at least, the physical 
 sympathies of sexuality have been enhanced by the emotional, 
 if not also intellectual, sympathies of love. Those sceptical 
 on this point should consult such a work as Biichner's " Liebe 
 und Liebesleben in der Thierwelt," or Sutherland's "Origin 
 and Growth of the Moral Instinct" (1898), which contain an 
 overflowing wealth of instances. 
 
 § 3. Sexual Attraction. — Mantegazza has written a work 
 entitled " The Physiology of Love," in which he expounds 
 the optimistic doctrine that love is the universal dynamic ; 
 and from this Biichner quotes the sentence, that " the whole 
 of nature is one hymn of love." If the last word be used very 
 widely, this often-repeated utterance has more than poetic 
 significance. But even in the most literal, sense there is much 
 truth in it, since so many animals are at one in the common 
 habit of serenading their mates. The chirping of insects, the 
 croaking of frogs, the calls of mammals, the song of birds, 
 illustrate both the bathts and glory of the love-chorus. The 
 works of Darwin and others have made us familiar with the 
 numerous ways, both gentle and violent, in which mammals 
 woo one another. The display of decorations in which many 
 male birds indulge, the amatory dances of others, the love- 
 lights of glow-insects, the joyous tournaments or furious duels 
 of rival suitors, the choice which not a few females seem to 
 exhibit, and the like, show how a process, at first crude 
 enough, becomes enhanced by appeals to more than merely 
 sexual appetite. But it is hardly necessary now to argue 
 seriously in support of the thesis that love — in the sense of 
 sexual sympathy, psychical as well as physical — exists among 
 animals in many degrees of evolution. Our comparative 
 
286 THE EVOLUTION OP SEX. 
 
 psychology has been too much influenced by our intellectual 
 superiority ; but while this, no doubt, has its correspondingly 
 increased possibilities of emotional range, it does not neces- 
 sarily imply a corresponding emotional intensity ; and we 
 have no means of measuring, much less limiting, that glow of 
 organic emotion which so manifestly flushes the organism with 
 colour and floods the world with song. Who knows whether 
 the song-bird be not besici^^g-Jiairwhat thTcBiiafefiigsician 
 is to the ordinary dulness of our daily toil and thought ? :KJie_ 
 fact to be insisted upon is this, that the vague sexual attraction 
 of the lowest organisms has been evolved into a definite repro- 
 ductive impulse, into a desire often predominating over even 
 that of self-preservation; that this again, enhanced by more 
 and more subtle psychical additions, passes by a gentle 
 gradient into the love of the highest animals, and of the 
 average human individual. 
 
 But the possibilities of evolution are not ended, and 
 though some may shrink from that comparison of human 
 love with its analogues in the organic series, the theory of 
 evolution offers the precise compensation such natures require. 
 Without recognising the possibilities of individual and of racial 
 evolution, we are shut up to the conventional view that the 
 poet and his heroine alike are exceptional creations, hopelessly 
 beyond the everyday average of the race. Whereas, admitting 
 the idea of evolution, we are not only entitled to the hope, but 
 logically compelled to the assurance, that these rare fruits of 
 an apparently more than earthly paradise of love, which only 
 the forerunners of the race have been privileged to gather, or 
 it may be to see from distant heights, are yet the realities of a 
 daily life towards which we and ours may journey. 
 
 § 4. Intellectual and Emotional Differences between the 
 Sexes. — We have seen that a deep difference in constitution 
 expresses itself in the distinctions between male and female, 
 whether these be physical or mental. The differences may be 
 exaggerated or lessened, but to obliterate them it would be 
 necessary to have all the evolution over again on a new basis. 
 What was decided among the prehistoric Protozoa cannot be 
 annulled by Act of Parliament. In this mere outline we 
 cannot of course do more than indicate the relation of the 
 biological differences between the sexes to the resulting 
 psychological and social differentiations ; for more than this 
 neither space nor powers suffice. We must insist upon the 
 
PSYCHOLOGICAL AND ETHICAL ASPECTS. 287 
 
 biological considerations underlying the relation of the sexes, 
 which have been too much discussed by contemporary writers 
 of all schools as if the known facts of sex did not exist at all, 
 or almost if these were a mere matter of muscular strength or 
 weight of brain. 
 
 The reader need not be reminded of the oldest and most 
 traditional views of the subjection of women inherited from the 
 ancient European order; still less perhaps of the attitude of 
 the ordinary politician, who supposes that the matter is one 
 essentially to be settled by the giving or withholding of the 
 franchise. The exclusively political view of the problem has 
 in turn been to a large extent subordinated to that of economic 
 laissez-faire, from which of course it consistently appeared that 
 all things would be settled as soon as women were sufficiently 
 plunged into the competitive industrial struggle for their own 
 daily bread. While, as the complexly ruinous results of this 
 inter-sexual competition for subsistence upon both sexes and 
 upon family life have begun to become manifest, the more 
 recent economic panacea of redistribution of wealth has 
 naturally been invoked, and we have merely somehow to raise 
 women's wages. 
 
 All disputants have tolerably agreed in neglecting the 
 historic, and still more the biological factors ; while, so far as 
 the past evolution of the present state of things is taken into 
 account at all, the position of women is regarded as having 
 simply been that in which the stronger muscle and brain of 
 man was able to place her. The past of the race is thus 
 depicted in the most sinister colours, and the whole view is 
 supposed to be confirmed by appeal to the practice of the 
 most degenerate races, and this again as described with the 
 scanty sympathy or impartiality of the average white traveller, 
 missionary, or settler. 
 
 As we have already said, we cannot attempt a full discus- 
 sion of the question, but our book would be left without 
 point, and its essential thesis useless, if we did not, in con- 
 clusion, seek to call attention to the fundamental facts of 
 organic difference, say rather divergent lines of differentiation, 
 underlying the whole problem of the sexes. We shall only 
 suggest, as the best argument for the adoption of our stand- 
 point, the way in which it becomes possible relatively to 
 harmonise the very diverse outlooks. We shall not so readily 
 abuse the poor savage, who lies idle in the sun for days after 
 
2SS THE EVOLUTION OF SEX. 
 
 his return from the hunting, while his heavy-laden wife toils 
 and moils without complaint or cease ; but bearing in view the 
 extreme bursts of exertion which such a life of incessant 
 struggle with nature and his fellows for food and for life in- 
 volves upon him, and the consequent necessity of correspond- 
 ingly utilising every opportunity of repose to recruit and eke 
 out the short and precarious life so indispensable to wife and 
 weans, we shall see that this crude domestic economy is the 
 best, the most moral, and the most kindly attainable under the 
 circumstances. Again, the traveller from town, who thinks 
 the agricultural labourer a greedy brute for eating the morsel 
 of bacon and leaving his wife and children only the bread, 
 does not see that by acting otherwise the total ration would 
 soon be still further lowered, by diminished earnings, loss of 
 employment, or loss of health. 
 
 The actual relations of fisherman and fishwife, of the 
 smallest farmer and his wife, seem to us to give a truer as well 
 as a healthier picture of antique industrial society, than those 
 we find in current literature ; and if we admit that such life 
 is deficient in refinement (although, on all deeper grounds, 
 from religion to ballad poetry, we might even largely dispute 
 this), it has still much to teach in respect of simplicity and 
 health. 
 
 The old view of the subjection of women was not, in fact, 
 so much of tyranny as it seemed, but roughly tended to 
 express the average division of labour; of course hardships 
 were frequent, but these have been exaggerated. The abso- 
 lute ratification of this by law and religion was merely of a 
 piece with the whole order of belief and practice, in which 
 men crushed themselves still more than their mates. Being 
 absolute, however, such theories had to be overthrown, and 
 the application of the idea of equality, which had done such 
 good service in demolishing the established castes, was a natural 
 and serviceable one. We have above traced the development 
 of this, however, and it is now full time to re-emphasise, this 
 time of course with all scientific relativity instead of a dogmatic 
 authority, the biological factors of the case, and to suggest 
 their possible service in destroying the economic fallacies at 
 present so prevalent, and still more towards reconstituting that 
 complex and sympathetic co-operation between the differen- 
 tiated sexes in and around which all progress past or future 
 must depend. Instead of men and women merely labouring 
 
PSYCHOLOGICAL AND ETHICAL ASPECTS. 289 
 
 to produce things as the past economic theories insisted, or 
 competing over the distribution of them, as we at present 
 think so important, a further swing of economic theory will 
 lead us round upon a higher spiral to the direct organic facts. 
 So it is not for the sake of production or distribution, of self- 
 interest or mechanism, or any other idol of the economists, 
 that the male organism organises the climax of his life's 
 struggle and labour, but for his mate ; as she, and then he, 
 also for their little ones. Production is for consumption ; the 
 species is its own highest, its sole essential product. The 
 social order will clear itself, as it comes more in touch with 
 biology. 
 
 It is equally certain that the two sexes are complementary 
 and mutually dependent. Virtually asexual organisms, like 
 Bacteria, occupy no high place in Nature's roll of honour ; 
 virtually unisexual organisms, like many rotifers, are great 
 rarities. Parthenogenesis may be an organic ideal, but it is 
 one which has been rarely realised. Males and females, like 
 the sex-elements, are mutually dependent, and that not merely 
 because they are males and females, but also in functions not 
 directly associated with those of sex. To dispute whether 
 males or females are the higher, is like disputing the relative 
 superiority of animals and plants. Each is higher in its own 
 way, and the two are complementary. 
 
 While there* are broad general distinctions between the in- 
 tellectual, and especially the emotional, characteristics of males 
 and females among the higher animals, these not unfrequently 
 tend to become mingled. There is, however, no evidence that 
 they might be gradually obliterated. The males of the sea- 
 horse, the obstetric frog, and many birds discharge maternal 
 functions, and there are females who fight for the males, and 
 are stronger, or more passionate than their mates. But these 
 are rarities. It is generally true that the males are more active, 
 energetic, eager, passionate, and variable ; the females more 
 passive, conservative, sluggish, and stable. The males, or, to 
 return to the terms of our thesis, the more katabolic organisms, 
 often seem more variable, and therefore, as Brooks has empha- 
 sised, may have frequently been the leaders in evolutionary 
 progress, while the more anabolic females tend rather to pre- 
 serve the constancy and integrity of the species. 
 
 There are some cases, as illustrated notably by the contrast 
 between ruffs and reeves, where the greater variability of the 
 
 l 9 
 
290 THE EVOLUTION OF SEX. 
 
 males along certain lines seems obvious, but, according to Karl 
 Pearson, the doctrine that man is more variable than woman 
 is a pseudo-scientific superstition, based on inadequate or in- 
 admissible data. The examination of seventeen groups of 
 measurements of different parts of the body shows that in 
 eleven groups the female is more variable than the male, and 
 in six the male more than the female. The differences of 
 variability, however, are slight, less than those between 
 members of the same race living in different conditions; 
 they are perhaps due to differences in the severity of the 
 struggle for existence. (See "The Chances of Death, and 
 other Studies in Evolution," 2 vols., London, 1897, pp. 388 
 and 460.) 
 
 Along paths where the reproductive sacrifice was one of 
 the determinants of progress, the females must have the credit 
 of leading the way. The more active males, with a conse- 
 quently wider range of experience, may have bigger brains and 
 more intelligence; but the females, especially as mothers, have 
 indubitably a larger and more habitual share of the altruistic 
 emotions. The males being usually stronger, have greater 
 independence and courage ; the females excel in constancy of 
 affection and in sympathy. The spasmodic bursts of activity 
 characteristic of males contrast with the continuous patience 
 of the females, which we take to be an expression of con- 
 stitutional contrast, and by no means, as some would have us 
 believe, a mere product of masculine bullying. The stronger 
 lust and passion of males is likewise the obverse of pre- 
 dominant katabolism. 
 
 That men should have greater cerebral variability and 
 therefore more originality, while women have greater stability 
 and therefore more "common sense," are facts both con- 
 sistent with the general theory of sex and verifiable in common 
 experience. The woman, conserving the effects of past varia- 
 tions, ha? what may be called the greater integrating intel- 
 ligence ; the man, introducing new variations, is stronger in 
 differentiation. The feminine passivity is expressed in greater 
 patience, more open-mindedness, greater appreciation of subtle 
 details, and consequently what we call more rapid intuition. 
 The masculine activity lends a greater power of maximum 
 effort, of scientific insight, or cerebral experiment with im- 
 pressions, and is associated with an unobservant or impatient 
 disregard of minute details, but with a stronger grasp of 
 
PSYCHOLOGICAL AND ETHICAL ASPECTS. 291 
 
 generalities. Man thinks more, women feels more. He dis- 
 covers more, but remembers less ; she is more receptive, and 
 less forgetful. 
 
 § 5. The Love for Offspring. — Just as it is impossible to 
 point to the stage where psychical sympathies enhance the re- 
 productive impulse into the love of mates, so we cannot tell 
 where parental care becomes disinterested enough to warrant 
 our calling it love of offspring. For, as no one can be foolish 
 enough deliberately to ignore the sexual or physical basis of 
 " love " in the higher and highest organisms, so it must be 
 allowed that even maternal care has its selfish side. To take 
 only one example, that of lactation. The unrelieved pressure 
 in the mammary glands of a mother animal robbed of her 
 young is no doubt largely concerned in prompting her to 
 adopt young ones not her own, yet we soon see these estab- 
 lished in her affections. So in normal cases, there naturally 
 remains an alloy which prevents us from regarding even ma- 
 ternal care as altogether disinterested. In all such cases, our 
 interpretations risk an undue materialism on the one hand, 
 and an undue transcendentalism on the other ; and while our 
 modern temper may habitually incline us to the former, we 
 must not be too fond of taking for granted that all the 
 common-sense is on that side, for we must remember that the 
 course of evolution not only has been, but must be, towards 
 the other. 
 
 Among animals low down in the organic series there is 
 often a close association between mother and offspring. Even 
 in some ccelenterates and worms the offspring cling about the 
 mother animals, and may be protected in various kinds of 
 brood-chambers. The little freshwater leech, Clef sine, carries 
 its young about with it, fixed to its ventral surface. A marine 
 leech, known as the skate-sucker (Pontobdella muricatd), 
 mounts guard for weeks over the eggs which are laid in a 
 bivalve shell or the like. It is probable that this habit has 
 protective value, but whether from active enemies or from 
 accumulations of sand and mud is uncertain. In an aquarium 
 one of these leeches continued to incubate for one hundred 
 and twenty-three days. 
 
 In some sea-urchins and starfishes there are simple forms 
 of brood-care, but the case of Holothurians is perhaps more 
 interesting. Prolonged attachment between the young ones 
 and the mother is known in at least nine species, of which five 
 
292 
 
 THE EVOLUTION OF SEX. 
 
 are antarctic and one arctic. The mode of attachment differs 
 markedly in different forms, thus each of the five antarctic 
 
 A Sea-cucumber, or Holothurian (Citcnmaria crocca), with numerous young attached 
 to the skin. — From Cams Sterne, after " Challenger" Narrative. 
 
PSYCHOLOGICAL AND ETHICAL ASPECTS. 293 
 
 species has its young attached in a different way. In Fsolus 
 ephippifer the young develop among the dorsal plates; in Psolus 
 antarcticus, on the ventral surface; in Cucumaria crocea, on the 
 modified dorsal ambulacra; in Cucumaria Itzvigata, in ventral 
 pouches; and in Chirodota contorta, in the genital tubes. (See 
 H. Ludwig, "Zool. Anzeiger," xx., 1897, pp. 217-219.) The 
 
 A Male " Sea-spider," or Pycnogonid. carrying the ova. — 
 After Carus Sterne. 
 
 interpretation here evidently does not lie with the morpho- 
 logist; is he not compelled to speculate on the beginnings of 
 psychic life in the strangely rudimental nervous system of 
 these forms from which even definite ganglia seem absent? 
 Yet even here we have motherhood protecting offspring, 
 perhaps all the more because of the prevailing cold; perhaps 
 
>94 
 
 THE EVOLUTION OK SEX. 
 
 of course also as a protection from premature burial in soft 
 muddy bottoms. 
 
 In some lowly crustaceans, the young may return to the 
 shell-cavity of the mother after hatching, and even after they 
 have undergone a moult. The young crayfish are said to return 
 to the maternal shelter after they have been set adrift. The 
 care of the nurse-bees for their charge, though not exactly 
 maternal, deserves to be recalled; and the way in which ants 
 save the cocoons when danger threatens is well known. De 
 
 [-Clusters ofn 
 
 cies of Cuttlefish.— Fi 
 
 i Von Hayek. 
 
 Geer describes how one of the insects infesting plants behaves 
 to her young brood exactly like a hen with her chickens: and 
 Bonnet vividly describes a case where a mother spider, at the 
 mercy of an ant-lion, fought for her eggs at the sacrifice of her 
 own life. Some spiders, too, carry their young; and some 
 crustaceans swim along with their young ones. Some cuttle- 
 fishes are careful in keeping their egg clusters clean and safe; 
 while even the headless fresh-water mussel retains her young, 
 
Psychological and ethical aspects. 
 
 295 
 
 when there is no fish present to which they may attach them- 
 selves. In fishes, it must be allowed that the care, if at all 
 evident, is usually paternal; in amphibians, it is rare; in 
 
 Ideal unity! 
 
 society. 
 
 of family. 
 
 Jj offspring. 
 
 mates. 
 
 N V R 
 
 Protoplasmic identity. 
 Diagrammatic Representation of the Relations between Nutritive, 
 Self-Maintaining, or Egoistic, and Reproductive, Species- 
 Regarding, or Altruistic Activities. 
 
 reptiles, somewhat more marked. In birds and mammals, 
 however, parental care is general, and unquestionably grows 
 into love for offspring. 
 
 § 6. Egoism and Altruism. — The optimism which finds in 
 
296 'HIE EVOLUTION OF SEJi. 
 
 animal life only " one hymn of love " is inaccurate, like the 
 pessimism which sees throughout nothing but selfishness. 
 Littre, Leconte, and some others less definitely, have more 
 reasonably recognised the co-existence of twin streams of 
 egoism and altruism, which often merge for a space without 
 losing their distinctness, and are traceable to a common origin 
 in the simplest forms of life. In the hunger and reproductive 
 attractions of the lowest organisms, the self-regarding and 
 other-regarding activities of the higher find their starting-point. 
 Though some vague consciousness is perhaps co-existent with 
 life itself, we can only speak with confidence of psychical 
 egoism and altruism after a central nervous system has been 
 definitely established. At the same time, the activities of even 
 the lowest organisms are often distinctly referable to either 
 category. 
 
 A simple organism, which merely feeds and grows, and 
 liberates superfluous portions of its substance to start new exist- 
 ences, is plainly living an egoistic and individualistic life. But 
 whenever we find the occurrence of close association with 
 another form, we find the first rude hints of love. It may still 
 be almost wholly an organic hunger which prompts the union, 
 but it is the beginning of life not wholly individualistic. Hardly 
 distinguishable at the outset, the primitive hunger and love 
 become the starting-points of divergent lines of egoistic and 
 altruistic emotion and activity. 
 
 The differentiation of separate sexes; the production of 
 offspring which remain associated with the parents; the 
 occurrence of genuine pairing beyond the limits of the sexual 
 period; the establishment of distinct families, with unmistak- 
 able affection between parents, offspring, and relatives; and 
 lastly, the occurrence of animal societies wider than the 
 family, — mark important steps in the evolution of both egoism 
 and altruism. 
 
 The diagram sums up the important facts. There are two 
 divergent lines of emotional and practical activity, — hunger, 
 self-regarding, egoism, on the one hand ; love, other-regarding, 
 altruism, on the other. These find a basal unity in the primi- 
 tively close association between hunger and love, between 
 nutritive and reproductive needs. Each plane of ascent marks 
 a widening and ennobling of the activities; but each has its 
 corresponding bathos, when either side unduly preponderates 
 over the other. The actual path of progress is represented by 
 
psychological and ethical aspects. 297 
 
 action and reaction between the two complementary functions, 
 the mingling becoming more and more intricate. Sexual 
 attraction ceases to be wholly selfish; hunger may be over- 
 come by love; love of mates is enhanced by love for offspring; 
 love for offspring broadens out into love of kindred. Finally, 
 the ideal before us is a more harmonious blending of the two 
 streams. 
 
298 THE EVOLUTION OF SEX. 
 
 SUMMARY. 
 
 1. In most of the emotions, and in the simpler intellectual processes, 
 there is common ground between animals and men. This is especially true 
 of the emotions associated with sex and reproduction. 
 
 2. The love of mates has its roots in physical sexual attraction, but has 
 been gradually enhanced by psychical sympathies. 
 
 3. The modes of sexual attraction rise from the crude and physical to 
 the subtle and psychical. 
 
 4. The intellectual and emotional differences between the sexes are 
 correlated with the deep-seated constitutional differences. Males and 
 females are complementary, each higher in its own way. 
 
 5. The love for offspring has grown as gradually as the love for mates. 
 Even lactation and maternal care may be in part egoistic. Apart from 
 exceptional cases, genuine love for offspring is only emphatic in birds and 
 mammals, where the reproductive sacrifice of the mother has also been 
 increased. 
 
 6. Egoism and altruism have their roots in the primary hunger and 
 love, or nutritive and reproductive activities. The divergent streams of 
 emotion and activity have a common origin, subtly mingle at various 
 turning-points, and ought to blend more and more in one. 
 
 LITERATURE. 
 
 See works on Sexual Selection cited at Chap. I. 
 
 See also Carus Sterne's most admirable of general natural history 
 
 books — Werden und Vergehen. Third edition. Berlin, 1886. 
 BiicHNER, L. — Liebe und Liebesleben in der Thierwelt. Berlin, 1879. 
 Eimer, G. H. T. — Die Entstehung dcr Arten auf Grund von Vererben 
 
 Erworbener Eigenschaften nach den Gesetzen Organischen Wachsens 
 
 Jena, 1888. 
 Ellis, H. — Man and Woman. Contemporary Science Series. 
 Groos, K.— The Play of Animals. (Translation), 1898. 
 Ma\iegazz;\, P. — Die Physiologie der Liebe; Die Hygiene der Liebe ; 
 
 and Anthropologisch-Kulturhistorische Studien ilber die Geschlechts- 
 
 verhaltnisse des Menschen. Jena. 
 Ploss. — Das Weib in der Natur und Volkerkunde. Second edition. 
 
 Leipzig, 1887. 
 Rolph, W. H.— Op. at. 
 Romanes, G. J. — Animal Intelligence. Internat. Sci. Series. Fourth 
 
 edition, 1886 ; and Mental Evolution in Animals, by the same. 
 Sutherland, A.— The Origin and Growth of the Moral Instinct -> vols 
 
 189S. 
 
CHAPTER XX. 
 
 Laws of Multiplication. 
 
 § i. Rate of Reproduction and Rate of Increase. — We know- 
 much more about the rate at which organisms reproduce, than 
 about the rate at which the number of adults in reality increases 
 or decreases. The one fact may be ascertained by observation ; 
 the other involves comparative statistics, which are difficult 
 enough to obtain, even for the human species. The rate of 
 reproduction depends upon the constitution of the individual 
 and its immediate environment, including, above all, its nutri- 
 tion. The rate of increase or decrease depends upon the wide 
 and complex conditions of the entire animate and inanimate 
 environment, or upon the degree of success in the struggle for 
 existence. 
 
 That there are enormous differences in the rates of repro- 
 duction is very evident. Maupas tells us how a single infu- 
 sorian becomes in a week the ancestor of a progeny only 
 computable in millions,— of numbers which the progeny of a 
 pair of elephants, supposing they all lived their natural term of 
 years, would not attain to in five centuries. Again, Huxley 
 calculates that the progeny of a single parthenogenetic plant- 
 louse — supposed again to live a charmed life — would in a few 
 months literally outweigh the population of China. The geo- 
 metrical ratio of reproduction, so often emphasised, would 
 indeed have startling results if it involved real, and not merely 
 potential, increase. 
 
 That it does sometimes realise itself for short periods or 
 special areas of favourable conditions is well known ; for in- 
 stance, in the periodic plagues of insects, or in the still unmas- 
 tered rabbit pest of Australia. But in the established fauna 
 and flora of a country, without intruded importations or marked 
 climatic changes, the rise and fall of population is seldom 
 emphatic. The rate of reproduction is only one factor in the 
 
3°0 THE EVOLUTION OP SEX. 
 
 numerical strength of the species or in its increase. The 
 common tapeworm produces myriads of embryos, but these 
 have only one chance in eighty-five millions (it is said) of 
 succeeding. Many common and numerous animals repro- 
 duce very slowly. That some species are on the increase, e.g., 
 bacteria, under the unprecedentedly favourable conditions which 
 our recent " industrial progress " affords, while other species 
 are on the decrease, e.g., many birds, is certain; but the rate 
 of reproduction is not a direct condition in either case. 
 
 § 2. History of Discussion on Rate of Reproduction. — In this, 
 as in not a few other cases, the biologist is profoundly indebted 
 to the student of social questions, for no adequate attention was 
 paid to the laws of multiplication before the appearance of the 
 epoch-making "theory of population" of Malthus, nor is it yet 
 possible or profitable to isolate the human question from the 
 general one. Malthus's fundamental proposition is indeed 
 usually softened from its earliest form — that population tends 
 to increase in geometrical, subsistence only in arithmetical 
 ratio — into the simple statement that population tends to out- 
 run subsistence, but has none the less served as a base of 
 weighty deductions for both the naturalist and the economist. 
 From Darwin's standpoint, the " positive checks " to population 
 (disease, starvation, war, infanticide), and the "prudential" 
 (moral or birth-restricting) checks, come to be viewed as special 
 forms of natural or artificial selection, while the fundamental 
 induction has been extended throughout nature as the essential 
 condition of the struggle for existence. After long dispute, the 
 induction of Malthus gained acceptance, followed by wide 
 deductive use and abuse, among economists. Yet, fundament- 
 ally important as the subject thus is to naturalist and economist 
 alike, the former has not as yet effected any thorough investi- 
 gation of the conditions of multiplication, or even usually 
 incorporated the keen analysis which we owe to Spencer, while 
 the economic theorist or disputant frequently still employs the 
 doctrine even in its pre-Danvinian form. It is thus doubly 
 needful to summarise, as briefly as may be, Spencer's elaborate 
 statement of the laws of multiplication. 
 
 § 3. Summary of Spencer's Analysis. — Different species exhibit different 
 degrees of fertility, which have become established in process of evolution 
 like the organisms themselves. To understand this particular adaptation 
 of function to conditions of existence, of organism to environment, we may 
 analyse these into their respective factors. It is evident that in the environ- 
 ment of any species there are many conditions viith which its individuals 
 
LAWS OF MULTIPLICATION. 30T 
 
 establish a moving equilibrium, sooner or later overthrown in death. To 
 prevent extinction, the organism meets these environing actions in two 
 distinct ways, — (1) by individual adaptations, active thrusts or passive 
 parries ; (2) by the production of new individuals to replace those over- 
 thrown, — in other words, by genesis. The latter may occur, as we have 
 seen, in varied forms, sexual or asexual, and at various rates, which depend 
 upon age, frequency, fertility, and duration of reproduction, together with 
 amount and nature of parental aid. These actions and reactions of environ- 
 ment and organism admit of another grouping in more familiar terms, into 
 two conflicting sets, — (a) the forces destructive of race ; (b) the forces pre- 
 servative of race. 
 
 Leaving aside cases in which permanent predominance of destructive 
 forces causes extinction, and also, as infinitely improbable, cases of perfectly 
 stationary numbers, the inquiry is : — In races that continue to exist, what 
 laws of numerical variation result from these variable conflicting forces 
 that are respectively destructive or preservative of race ? How is the 
 alternate excess of one or other rectified? A self-sustaining balance must 
 exist ; the alternate predominance of each force must initiate a compensa- 
 tory excess of the other ; how is this to be explained ? 
 
 When favourable circumstances cause any species to become unusually 
 numerous, an immediate increase of destructive influences, passive as well 
 as active, takes place ; competition becomes keener and enemies more 
 abundant, and conversely. Yet this is not the sole, much less the perma- 
 nent, means of establishing a balance ; nor does it explain either the 
 differences in the rate of fertility and mortality, or the adaptation of one to 
 the other. This minor adjustment in fact implies a major one. 
 
 The forces preservative of race were seen above to be two, — power to 
 maintain individual life, and power to generate the species. Now, in a 
 species which survives, given the forces destructive of race as a constant 
 quantity, those preservative of race must be a constant quantity also ; and, 
 since the latter are two, the individual plus the reproductive, these must 
 vary inversely, one must decrease as the other increases. To this law 
 every species must conform, or cease to exist. Let us restate this at greater 
 length. A species in which self-preservative life is low, and in which the 
 individuals are accordingly rapidly overthrown in the struggle with the 
 destructive forces, must become extinct, unless the other race-preservative 
 factor be proportionally strengthened, — unless, that is to say, its reproductive 
 power become proportionally great. On the other hand, if both preserva- 
 tive factors be increased, if a species of high self-preservative power were 
 also endowed with powers of multiplication beyond what is needful, such 
 success of fertility, if extreme, would cause sudden extinction of the species 
 by starvation, and if less extreme, and so effecting a permanent increase of 
 the numbers of the species, would next bring about such intenser competi- 
 tion, such increased dangers to individual life, that the great self-preserva- 
 tive power would not be more than sufficient to cope with them. 
 
 In short, then, we have reached the a priori principle, that in races 
 which continuously survive, in which the destructive forces are balanced 
 by the preservative ones, there must be an inverse proportion between the 
 power to sustain individual life and the power to produce new individuals. 
 But what is the physiological explanation of this adjustment, and how has 
 it arisen in process of evolution ? Spencer has elsewhere enlarged upon the 
 proposition, which we have already illustrated, that genesis in all its forms 
 
302 THE EVOLUTION OF SEX. 
 
 is a process of disintegration, and is thus essentially opposed to that process 
 of integration which is one element of individual evolution. The matter 
 and energy supplied for the young organism represent so much loss for 
 the parent ; while, conversely, the larger the amount of matter and energy 
 consumed by the functional actions of the parent, the less must be the 
 amount remaining for those of the offspring. The disintegration which 
 constitutes genesis may be complete or partial, and in the latter case the 
 parent, having reached considerable bulk and complexity before reproduc- 
 tion sets in, may survive the process. In the same way, individual evolution 
 may be expressed in bulk, in structure, in amount or variety of action, or in 
 combinations of these ; yet, in any case, this progress of each individuality 
 must correspondingly retard the establishment of the new ones. 
 
 While in the first portion of the argument, then, it was shown that a 
 species cannot be maintained unless self-preservative and reproductive 
 power vary inversely, it is now evident that, irrespective of an end to be 
 subserved, these powers cannot do other than vary inversely, and the one 
 a priori principle is thus seen to be the obverse of the other. And if we 
 group under the term individuation all those race-preservative processes by 
 which individual life is completed and maintained, and extend the term 
 genesis to include all those processes aiding the formation and perfecting of 
 new individuals, the result of the whole argument may be tersely expressed 
 in the formula, — Individuation and Genesis vary inversely. And from this 
 conception important corollaries open ; thus, other things equal, advancing 
 evolution must be accompanied by declining fertility ; again, if the diffi- 
 culties of self-preservation permanently diminish, there will be a permanent 
 increase in the rate of multiplication, and conversely. 
 
 In attempting the inductive verification of these a priori inferences, 
 practical difficulties arise, owing to the high complexity of each of our two 
 sets of factors and the independent variability of their details, and thus the 
 total cost of individuation and of genesis alike is hard of estimation and 
 comparison. For this purpose, however, there are successively to be in- 
 vestigated, — (i) the antagonism between growth and genesis, sexual and 
 asexual ; (2) that between development and genesis ; (3) that between ex- 
 penditure and genesis ; and (4) the coincidence between high nutrition and 
 genesis. It is impossible to summarise the wealth of evidence drawn from 
 a wide survey of the animal and vegetable world contained in the chapters 
 devoted to those various heads, but attention may be called to the last and 
 most obscure of these. It is indeed evident a priori that, if the cost of 
 individuation be once provided for, a higher nutrition will render possible 
 a greater propagation, sexual or asexual, and this may be abundantly veri- 
 fied by observation and experiment. Witness the case of aphides, in which 
 the rate of parthenogenetic reproduction is found to be directly proportional 
 to temperature and food-supply ; or, again, that of domestic animals, such as 
 the sheep, whose fertility is in direct relation to richness of pasture and 
 warmth of climate ; or, finally, and most obviously of all, that of field or 
 fruit crops, upon which the influence of increased liberality of manuring 
 will not be disputed. Yet it is sometimes maintained, for both plants and 
 animals, that overfeeding checks increase, while limited nutriment stimu- 
 lates it ; and to support this view there are cited such cases as that of the 
 barrenness of a very luxuriant plant, and the fruitfulness which appears on 
 its depletion. But if this objection really held, manuring would in all cases 
 be inexpedient, instead of only in plants where the growth of sexless axes 
 
LAWS OF MULTIPLICATION. 
 
 3°3 
 
 V 
 
 is still too luxuriant ; and a tree which has borne a heavy crop should, by 
 this depletion, bear again yet more heavily, instead of being more or less 
 barren next year unless manured. Or the difficulty may also be met by 
 interpreting such vegetative luxuriance, not as a case of 
 higher individuation at all, but simply as a case of asexual 
 multiplication of secondary axes ; or again, and perhaps 
 most simply, by regarding the appearance of sexual re- 
 production on depletion simply as a case of the previously 
 demonstrated antagonism between genesis and growth. 
 
 But again, since fatness is associated with sterility, it 
 is often argued that high feeding is unfavourable to gene- 
 sis. Obesity, however, is now known to be associated 
 with imperfect assimilation, with physiological impoverish- 
 ment or degeneration, — by no means with that constitu- 
 tional wealth which is favourable to fertility. If, in short, 
 we bear in mind that truly high nutrition means only due 
 abundance of, and due proportion among, all the sub- 
 stances which the organism requires, and that their per- 
 fect assimilation by the organism is also needful, such 
 objections to the generalisation not only disappear, but 
 such a phenomenon as the coincidence of returning fer- 
 tility with disappearing obesity affords a confirmatory 
 argument. 
 
 Organisms having aberrant modes of life are next ap- 
 pealed to for crucial evidence bearing on these general 
 doctrines. Thus, turning to vegetable and animal para- 
 sites, which combine superabundant nutrition with greatly 
 diminished expenditure, the enormous fertility exhibited 
 by all such forms is seen to be the necessary correlative 
 of such a state of nutrition and expenditure, and not 
 merely an acquired adaptation to their peculiar difficulties 
 of survival. The reversion exhibited by so many species 
 (especially among the higher arthropods, e.g., Aphis, 
 Cecidomyia) from sexual reproduction to primitive forms 
 of genesis, is explained by pointing out that such species 
 are peculiarly situated in obtaining abundant food with 
 little exertion. Among bees, ants, and termites alike, 
 the enormous fertility of the inactive and highly nourished- 
 queen-mother are obviously also cases in point. 
 
 The inverse variation of genesis with individuation has 
 now been demonstrated inductively as well as deductively, 
 and that for each element of the latter (growth, develop- 
 ment, or activity). Yet before discussing its application 
 to the problems of the multiplication of the human species, 
 two points remain, — a question has to be answered, and 
 a qualification made. The question, only partially 
 answered in course of the preceding argument, is, How is 
 the ratio between individuation and genesis established in 
 each special case ? and the answer is, By natural selec- 
 tion. This may determine, whether the quantity of 
 matter spared from individuation for genesis be divided into many small ova 
 or a few larger ones ; whether there shall be small broods at short intervals, 
 
 A speciesof Onion 
 with asexual vege- 
 tative bulbils (/>) 
 among the flowers 
 
304 THE EVOLUTION OF SEX. 
 
 or larger broods at longer intervals ; or whether there shall be many unpro- 
 tected offspring, or a few carefully protected by the parent. Again, survival 
 of the fittest has a share in determining the proportion of matter subtracted 
 from individuation for genesis. Yet this operation of natural selection goes 
 on strictly under the limits of the antagonism above traced. 
 
 The needed qualification arises on introducing the conception of evolu- 
 tionary change. If time be left out of account as hitherto, — or, what is the 
 same thing, if all the species be viewed as permanent, — the inverse ratio 
 between individuation and genesis holds absolutely. But each advance in 
 individual evolution (it matters not whether in bulk, in structure, or in 
 activities) implies an economy ; the advantage must exceed the cost, else it 
 would not be perpetuated. The animal thus becomes physiologically 
 richer ; it has an augmentation of total wealth to share between its in- 
 dividuation and its genesis. And thus, though the increment of individua- 
 tion tends to produce a corresponding decrement of genesis, this latter will 
 be somewhat less than accurately proportionate. The product of the two 
 factors is greater than before ; the forces preservative of race become 
 greater than the forces destructive of race, and the species spreads. In 
 short, genesis decreases as individuation increases, yet not quite so fast. 
 
 Hence every type that is best adapted to its conditions — every higher 
 type — has a rate of multiplication that ensures a tendency to predominate. 
 For though the more evolved organism is the less fertile absolutely, it is 
 the more fertile relatively. 
 
 The whole generalisation admits of the simplest graphic 
 illustration. For if the line AB represents the aggregate 
 
 C 
 
 A ! — , B 
 
 matter or energies, the structures or the functions, of the 
 organism, of which AC denotes the amount devoted to in- 
 dividuation and CB to reproduction, the inverse variation of 
 AC to CB is obvious, as also if AC and CB represent the 
 psychological obverse of these two classes of function. Nor 
 does an increase in total energy modify this, as when the 
 stronger members of a species frequently also exhibit greater 
 .reproductive power ; for if in one case AB = 20, of which 
 CB=4,'and in another AB = 25, CB may become 5 without 
 any rise of reproductive ratio, since -±1 = 4s- But if the species 
 be evolving, the advance in individuation implies a certain 
 economy, of which a share may go to diminish the decrement 
 to genesis, as above explained. 
 
 § 4. Spencer's Application of his Results to Ma?i. — In ex- 
 tending this hard-won generalisation to the case of man, the 
 concomitance of all but highest total individuation with all but 
 lowest rate of multiplication (the enormous bulk of the elephant 
 involving a yet greater deduction from genesis) is at once 
 apparent, Comparing different races or nations, or even 
 
LAWS OF MULTIPLICATION. 305 
 
 different social castes or occupations, the same holds good; 
 while the prevalence of high multiplication in races of which 
 the nutrition is in obvious excess over the expenditure is also 
 evident, witness the Boers or French Canadians. Such an 
 apparent difficulty as that of the Irish, in whom rapid multipli- 
 cation occurs despite poor food, is accounted for by the re- 
 latively low expenditure in obtaining it (since the " law of 
 diminishing return " implies its converse for diminishing labour), 
 though, no doubt, also in part by the habit of early marriage, if 
 not by some measure of lowered individuation as well. The 
 main position being established, Spencer proceeds to discuss 
 the question of human population in the future, and insists 
 strongly on the importance of pressure of population, which he 
 regards as the main incentive to progress alike in past, present, 
 and future. Reviewing the possibilities of progress in bulk, 
 complexity of structure, multiplication and variation of func- 
 tion, he concludes that the more complete moving equilibrium, 
 and more perfect correspondence between organism and 
 environment, which such evolution involves, must take place 
 mainly in the direction of psychical development. Yet this 
 development, while stimulated by pressure of population, con- 
 stantly tends to diminish the rate of fertility ; in other words, 
 this cause of progress tends- to disappear as it achieves its full 
 effect. The acute pressure of population, with its attendant 
 evils, thus tends to cease as a more and more highly individu- 
 ated race busies itself with its increasingly complex yet normal 
 and pleasurable activities, its rate of reproduction meanwhile 
 descending towards that minimum required to make good its 
 inevitable losses. 
 
 § 5. Summary of the Population Question. — The general 
 question, so far as yet developed, may now be conveniently 
 summarised in the accompanying tabular form. Here the 
 stage of knowledge reached by each author, together with 
 any practical applications therefrom deduced, may be read 
 horizontally, while the historic development of each separate 
 line of conceptions may be traced vertically. 
 
 From such a summary, brief as it is, the main steps in the 
 development of our knowledge are clear enough, but a deeper 
 analysis is required before final exposition or complete appli- 
 cation is possible. Nor, when we note how vast the progress 
 of science through the advance in precision and extension 
 
 20 
 
306 
 
 THE EVOLUTION OF SEX. 
 
 effected upon the conception of Malthus* by Darwin, will the 
 utility of such increasing elaboration be disputed. Thus the 
 full inductive verification of Spencer's law involves a detailed 
 
 Author. 
 
 Development of Theory of Population, 
 
 Practical 
 
 Action 
 Deduced. 
 
 I. 
 
 Non- bio- 
 logical 
 writers 
 (prede- 
 cessors 
 and op- 
 ponents 
 of Mal- 
 thus). 
 
 Increase of population 
 
 does not tend to out- 
 run subsistence. 
 
 
 
 
 II. 
 Malthus. 
 
 1798. 
 
 Increase of population 
 tends to outrun that 
 of subsistence. 
 
 But meets checks : 
 
 A. Positive. 
 
 B. Preventive. 
 
 
 To avoid A, 
 adopt B. 
 
 III. 
 Darwin. 
 
 1S59. 
 
 Do. 
 
 Hence struggle 
 for existence ; 
 
 A. Natural 
 selection. 
 
 B. Artificial 
 selection. 
 
 Leading to 
 evolution. 
 
 Laissez -/aire, 
 i.e., on ac- 
 count of ad- 
 vantage to 
 species from 
 A, avoid B. 
 
 IV. 
 Spencer. 
 
 1852-66. 
 
 Do. 
 Rate of multiplication 
 investigated for dif- 
 ferent species, and 
 shown to vary inverse- 
 ly as individuation. 
 
 Do. 
 
 Do. 
 Also lead- 
 ing to 
 e v 1 u- 
 tion of 
 species. 
 
 Do. 
 \Individuate. ] 
 
 comparison of the rates of reproduction of each group of 
 organic species, with their observed degree of individuation 
 (first in each of its factors, and finally in their sum), devia- 
 tions from the inverted symmetry of the theoretic curves 
 (see fig. opposite) having to be separately discussed. Natural 
 selection also requires a yet deeper analysis ; the limits and 
 possibilities of artificial selection are but little known, while 
 
 * It is also interesting to compare Malthus's view of population, tend- 
 ing to increase in geometrical proportion and substance only in arithmetical - 
 with Spencer's demonstration of the limit of growth already summarised 
 (see p. 220), the more so when we bear in mind that reproduction is dis- 
 continuous growth. The precise statement of Malthus becomes confirmed 
 as regards the cell, if not the cell aggregate. 
 
LAWS OF MULTIPLICATION. 
 
 307 
 
 a theory of variation is still far from agreed upon. If how- 
 ever we bear in mind that the amount of evolution in given 
 time is but small our knowledge seems not insufficient for the 
 practical deductions which are so pressingly demanded; yet it 
 is here that the most serious disagreement has prevailed. 
 Thus the Malthusian position is obviously inadequate, in not 
 allowing for the Darwinian one ; yet the converse also is 
 undeniable, for the position of laissez-fab-e, upon which Darwin 
 and Spencer alike take their stand, not 
 only almost ignores the wellbeing of the 
 individual in considering the advance- 
 ment of the species, but is even then 
 too optimistic, since it not only fails 
 to accelerate the progressive evolution 
 which is alone considered, but also fails 
 to provide against the equal possibility 
 of degenerative change. Are we then 
 simply to return to the somewhat crude 
 proposals and excessive hopes for the 
 increase of individual wellbeing due to 
 Malthus or his followers, based too as 
 these have been on imperfect pre- 
 Spencerian knowledge ? 
 
 The answer is not far to seek, — it 
 lies in the generalisation above estab- 
 lished ; yet it is remarkable that Mr 
 Spencer, after not only establishing the 
 inverse variation of individuation and 
 genesis among species in general, but 
 even showing for the human species in 
 particular that it is essentially upon 
 increase of the psychical activities that 
 the increased individuation and dimin- 
 ished genesis of the future must depend, 
 should not have proceeded to a fuller application. For unless 
 the main generalisation be abandoned, it is obvious that the 
 progress of the species and of the individual alike is secured and 
 accelerated whenever action is transferred from the negative 
 side of merely seeking directly to repress genesis, to the 
 positive yet indirect side of proportionally increasing individua- 
 tion. This holds true of all species, yet most fully of man, 
 since that modification of psychical activities in which his 
 
 Let the perpendiculars above the 
 line A B denote the increasing 
 degree of total individuation 
 of a series of forms 1, 2, 3, 4, 
 5,_ 6 (say Worm, Fish, Frog, 
 Bird, Man, Elephant), and 
 similarly let the perpendicu- 
 lars to C D represent the rate 
 of multiplication of the same 
 forms ; the curves joining 
 these two series of points 
 respectivelyillustrate by their 
 inverted symmetry the inverse 
 ratio _ of individuation and 
 genesis. 
 
308 the evolution of sex. 
 
 evolution essentially lies, is far excellence and increasingly the 
 respect in which artificial comes in to replace natural selection. 
 Without therefore ignoring the latter, or hoping ever wholly 
 to escape from the iron grasp of nature, we yet have within our 
 power more and more to mitigate the pressure of population, 
 and that without any sacrifice of progress, but actually by 
 hastening it. Since then the remedy of pressure and the hope 
 of progress alike lie in advancing individuation, the course for 
 practical action is clear, — it is in the organisation of these 
 alternate reactions between bettered environment (material, 
 mental, social, moral) and better organism in which the whole 
 evolution of life is defined, in the conscious and rational 
 adjustment of the struggle into the culture of existence. 
 
 The practical corollaries of the Malthusian view are celibacy, 
 late marriage, and moral control ; the objections are vice, in- 
 creased mortality in childbirth, and the present low evolution 
 of our moral nature. The practical corollary of the Darwinian 
 doctrine is virtually nil ; the objection, that the survival of 
 what we consider the best types is doubtful, and that the 
 survival of the fit is apt to be cruel. The practical corollaries of 
 the Spencerian principle, although Mr Spencer can hardly be 
 said to have insisted upon these, are individuate and educate. 
 The objection is, that the pressure of population is already felt, 
 and that individuation is a matter of centuries. Furthermore, 
 the effect of education, for instance in reducing sexuality, will 
 tell most where it is least wanted, viz., among the best types. 
 
 We are therefore bound to include, as a continuation of the 
 above table, the amendment of some of the most thoughtful ex- 
 ponents of what is generally called neo-Malthusian doctrine. 
 This advocates the use of artificial preventive checks to fer- 
 tilisation. Discussion of this proposal is at present difficult, 
 because of the comparative absence of distinctly expressed 
 opinion on the part of medical experts, and because of strong 
 superficial prejudices, not only against the scheme, but against 
 its discussion. These prejudices are, however, dying out, and 
 that is well, for they do nothing but obscure appreciation alike 
 of the merits and demerits of the doctrine. An increasing 
 realisation of the plain facts of reproduction and population 
 must rapidly exterminate the persistently theological absurdities 
 which people utter, if they do not believe on the subject. The 
 vague feeling that control of fertilisation is " interfering with 
 nature," in some utterly unwarrantable fashion, cannot be 
 
LAWS OF MULTIPLICATION. 
 
 3°9 
 
 consistently stated by those who live in the midst of our highly 
 artificial civilisation. The strongest prejudice seems to be 
 based in a moral cowardice, which gauges a scheme by its 
 " respectability," while even more culpable is that consciously 
 or unconsciously derived from the profitableness to the 
 capitalist classes of unlimited competition of cheap unskilled 
 labour. For never did the proletariat more literally deserve its 
 name than since the advent of the factory period, their rapid 
 and degenerative increase, indeed, primarily representing " the 
 progress of investments." 
 
 The general attitude of the modern Malthusian may first of 
 all be roughly indicated by quoting the mottoes which head 
 the organ of their league. " To a rational being, the prudential 
 check to population ought to be considered as equally natural 
 with the check from poverty and premature mortality" (Malthus, 
 1806). " Little improvement can be expected in morality until 
 the production of large families is regarded in the same light as 
 drunkenness, or any other physical excess " (John Stuart Mill, 
 1872). "Surely it is better to have thirty-five millions of 
 human beings leading useful and intelligent lives, rather than 
 forty millions struggling painfully for a bare subsistence " 
 (Lord Derby, 1879). Starting from the familiar induction 
 that " population has a constant tendency to outrun the 
 means of subsistence," they recognise in this over-population 
 " the most fruitful source of pauperism, ignorance, crime, 
 and disease." To counteract this there are checks, posi- 
 tive or life-destroying on the one hand, prudential or birth- 
 preventing on the other. " The positive or life-destroying 
 checks comprehend the premature death of children and adull£ 
 by disease, starvation, war, and infanticide." As these positive 
 checks are happily reduced with the progress of society, 
 attention must be concentrated on the other side. " This 
 consists in the limitation of offspring by abstention from 
 marriage, or by prudence after marriage." But as to the first, 
 prolonged abstention from marriage, as advocated by Malthus, 
 this is " productive of many diseases, and of much sexual vice," 
 while " early marriage, on the contrary, tends to secure sexual 
 purity, domestic comfort, social happiness, and individual 
 health." The check that remains to be advocated is thus 
 " prudence after marriage," and by this the neo-Malthusians 
 most distinctly mean attention to methods which will secure 
 that sexual intercourse be not followed by fertilisation. For 
 
310 THE EVOLUTION OF SEX. 
 
 the details of the various methods, we must refer to the 
 Malthusian literature ; but a brief outline is imperative, even 
 for an approximate understanding of the problem. 
 
 (a.) Thus we have the suggestion that intercourse should be 
 limited to the relatively infertile period most remote from 
 menstruation, when conception may indeed occur, but with 
 less probability than at other periods. Although gynaecologists 
 are disagreed as to the degree of this probability, there can be 
 little doubt that such limitation would have a useful influence, 
 although in itself confessedly incomplete. The so-called 
 artificiality of control is here reduced to a minimum, and the 
 suggestion is obviously in harmony with that increased 
 temperance which all must allow to be desirable. 
 
 (b.) In the second place, there are methods employed by 
 the males, such as that of withdrawal before the emission 
 of the seminal fluid, a habit common enough both in savage 
 and civilised communities. Fertilisation is in this way ab- 
 solutely prevented, but apart from a more general objection to 
 be afterwards emphasised, such a practice is maintained by 
 some to be injurious to the male, and yet more to the female. 
 Moreover, although the risks of over-population and female 
 exhaustion by child-bearing are here minimised, there is still 
 risk of male exhaustion. 
 
 (c.) Thirdly, although again under the severe criticism of 
 some of the medical experts, there are means employed by the 
 females, for securing by means of pessaries that the spermatozoa 
 do not come into contact with the ovum, or by means of washes 
 that the male elements are rendered ineffectual. In reply to 
 fche medical objections to both these methods of artificial check, 
 it is answered ((7) that it may in many cases be necessary to 
 choose between two evils, of which the risk involved in the 
 artificial check may be much less than that involved in con- 
 tinued child-bearing ; (b) that it is hardly a fair argument as 
 yet to urge that the proposed checks of neo-Malthusianism 
 are fraught with danger. As to the popularly supposed pre- 
 ventive check of prolonged nursing one baby in the hope of 
 thereby preventing a new conception, it is necessary to em- 
 phasise that nursing does not effect this, and that the prolonga- 
 tion of the lacteal function and diet beyond their natural limits 
 is seriously injurious alike to mother and offspring. 
 
 Even recognising some of these objections, the neo-Malthu- 
 sians urge the number of distinct advantages, — the reduction 
 
LAWS OF MULTIPLICATION. 31 1 
 
 of the present rapid rate of increase ; the possibility of earlier 
 marriages, and a probable diminution of vice ; an increase in 
 the fitness of the race by lessening the propagation of unfit 
 types and the exhaustion of the mothers by too frequent child- 
 bearing. Supposing, again, the general adoption of the pro- 
 posal, the neo-Malthusians insist upon the possibility of a 
 heightened standard of comfort among the poorer members 
 of the community, and the removal of obstacles to marriage 
 which stand in the way of those who ought to marry but ought 
 not to be parents. 
 
 Without urging medical objections above referred to, — 
 for in regard to the discussion of these, professional experts 
 must bear the responsibility, — we must emphasise several 
 counter-arguments. Thus it has been maintained, though with 
 no great degree of certitude, that a proposal involving some 
 deliberate and controlled action would tend to be adopted 
 most where least wanted, viz., among the more individuated 
 types, whose numbers would in consequence be proportionately 
 reduced. The diminished rate of increase, which is the most 
 obvious social result of the extensive adoption of neo- 
 Malthusian practices, has long been known to the student of 
 population ; and in some countries, particularly France, — 
 although here, no doubt, to some extent the result Of peculiarly 
 high individuation, — is a recognised national danger, especially 
 since the diminished population, in being largely freed from the 
 normal acuteness of the struggle for existence, loses many of 
 the advantages of this as well. 
 
 The statistician will doubtless long continue his fashion of 
 confidently estimating the importance and predicting the sur- 
 vival of populations from their quantity and rate of reproduction 
 alone ; but at all this, as naturalists we can only scoff. Even 
 the most conventional exponent of the struggle for existence 
 among us knows, with the barbarian conquerors of old, that "the 
 thicker the grass, the easier it is mown ; " that " the wolf cares 
 not how many the sheep may be." It is the most individuated 
 type that prevails in spite, nay, in another sense, positively 
 because of its slower increase ; in a word, the survival of a 
 species or family depends not primarily upon quantity, but 
 upon quality. The future is not to the most numerous popu- 
 lations, but to the most individuated. And as we increas- 
 ingly see that natural history must be treated primarily from 
 the standpoint of the species-regarding sacrifice rather than 
 
312 THE EVOLUTION OF SEX. 
 
 from that of the individual struggle, we see the importance of 
 the general neo-Malthusian position, despite the risks which the 
 particular modes of its practice may involve. 
 
 Apart from the pressure of population, it is time to be learn- 
 ing (i) that the annual childbearing still so common, is cruelly 
 exhaustive to the maternal life, and this often in actual duration 
 as well as quality ; (2) that it is similarly injurious to the 
 standard of offspring ; and hence (3) that an interval of two 
 clear years between births (some gynecologists even go as far 
 as three) is due alike to mother and offspring. It is time there- 
 fore, as we heard a brave parson tell his flock lately,. " to have 
 done with that blasphemous whining which constantly tries to 
 look at a motherless " (ay, or sometimes even fatherless) " crowd 
 of puny infants as a dispensation of mysterious providence." 
 Let us frankly face the biological facts, and admit that such 
 cases usually illustrate only the extreme organic nemesis of 
 intemperance and improvidence, and these of a kind far more 
 reprehensible than those actions to which common custom 
 applies the names, since they are species-regarding vices, and 
 not merely self-regarding ones, as the others at least primarily 
 are. To realise the social consequences of sexual intemperance 
 is enough to obviate any hasty criticism of neo-Malthusianism, 
 whatever conclusion may be arrived at as to its sufficiency. 
 
 It is time, however, to point out the chief weakness in neo- 
 Malthusian proposals, which are at one in allowing the gratifica- 
 tion of sexual appetites to continue, aiming only at the preven- 
 tion of the naturally ensuing parentage. To many doubtless 
 the adoption of a method which admits of the egoistic sexual 
 pleasures, without the responsibilities of childbirth, would mul- 
 tiply temptations. Sexuality would tend to increase if its respon- 
 sibilities were annulled ; the proportion of unchastity before 
 marriage, in both sexes, could hardly but be augmented ; 
 while married life would be in exaggerated danger of sinking 
 into " monogamic prostitution." On the other hand, it seems 
 probable that the very transition from unconscious animalism 
 to deliberate prevention of fertilisation, would tend in some to 
 decrease rather than increase sexual appetite. 
 
 It seems to us, however, essential to recognise that the ideal 
 to be sought after is not merely a controlled rate of increase, 
 but regulated married lives. Neo-Malthusianism might secure 
 the former by its more or less mechanical methods, and there 
 is no doubt that a limitation of the family would often increase 
 
LAWS OF MULTIPLICATION. 313 
 
 the happiness of the home; but there is danger lest, in re- 
 moving its result, sexual intemperance become increasingly 
 organic. We would urge, in fact, the necessity of an ethical 
 rather than of a mechanical " prudence after marriage," of a 
 temperance recognised to be as binding on husband and wife 
 as chastity on the unmarried. When we consider the inevit- 
 able consequences of intemperance, even if the dangers of too 
 large families be avoided, and the possibility of exaggerated 
 sexuality becoming cumulative by inheritance, we cannot help 
 recognising that the intemperate pair are falling towards the 
 ethical level of the harlots and profligates of our streets. 
 
 Just as we would protest against the dictum of false physi- 
 cians who preach indulgence rather than restraint, so we muut 
 protest against regarding artificial means of preventing fertilisa- 
 tion as adequate solutions of sexual responsibility. After all, the 
 solution is primarily one of temperance. It is no new nor 
 unattainable ideal to retain, throughout married life, a large 
 measure of that self-control which must always form the organic 
 basis of the enthusiasm and idealism of lovers. But as old 
 attempts at the regulation of sexual life have constantly fallen 
 from a glowing idealism into pallor or morbidness, it need 
 hardly be said that the same fate will ever more or less 
 befall the endeavour after temperance, so long as that lacks 
 the collaboration of other necessary reforms. We need a 
 new ethic of the sexes ; and this not merely, or even mainly, 
 as an intellectual construction, but as a discipline of life ; 
 and we heed more. We need an increasing education and 
 civism of women, — in fact, an economic of the sexes very 
 different from that nowadays so common, which, while attack- 
 ing the old co-operation of men and women because of its 
 manifest imperfections, only offers us an unlimited and far 
 more mutually destructive industrial competition between them 
 instead. The practical problems of reproduction become in 
 fact, to a large extent, those of improved function and evolved 
 environment; and limitation of population, as we are begin- 
 ning to see in regard to the more individual forms of intem- 
 perance, is primarily to be reached, not solely by individual 
 restraint, but by a not merely isolated and individual, but aggre- 
 gate and social, reorganisation of life, work, and surroundings. 
 And while our biological studies of course for the most part 
 only point the way towards deeper social ones, they afford also 
 one luminous principle towards their prosecution, — that thorough 
 
314 THE EVOLUTION OF SEX. 
 
 parallelism and coincidence of psychical and material considera- 
 tions, upon which moralist and economist have been too much 
 wont respectively to specialise. 
 
 § 6. Rate of Reproduction "Nil" — Sterility. — When we view 
 reproduction in terms of discontinuous growth, — that is, as a 
 phenomenon of disintegration, — it is obvious that complete 
 integration of the matter acquired by the organism into its own 
 bulk, and for its own development, precludes reproduction, — 
 that is, involves sterility, — and similarly as regards the energies 
 of the organism. This is only a re-statement of Spencer's 
 generalisation above discussed ; for it is evident that, if genesis 
 vary inversely as individuation, it must be suppressed altogether 
 if individuation becomes complete. The actual phenomena, 
 however, by no means usually admit of explanation as such 
 realisations of the ideal of evolution, and hence the cause and 
 treatment of sterility mainly pass into the provinces of the 
 experimental naturalist and the physiological physician. From 
 the earliest times, indeed, physician and naturalist, priest and 
 legislator, alike devoted attention to the subject; and it was 
 probably in this way, as a recent monographer remarks, that 
 research became directed to the larger problem of repro- 
 duction in general. The general biological questions — 
 e.g., the relations between sterility within the limits of 
 a species to changes in the environment, or that of sterility 
 among hybrids — are extensively discussed in the copious 
 literature which centres around Darwin's "Variation of Animals 
 and Plants under Domestication " ; while with regard to the 
 human species, an extensive medical literature of course exists, 
 to which any encyclopaedia of medicine, or conveniently the 
 recent careful monograph of P. Miiller (" Die Unfruchtbarkeit 
 der Ehe," Stuttgart, 1885), will furnish bibliographical details. 
 
LAWS OF MULTIPLICATION. 3 15 
 
 SUMMARY. 
 
 J. The rate of reproduction is chiefly determined by the constitution of 
 the organism ; the rate of increase, by its relations to the animate and 
 inanimate environment. 
 
 2. The naturalist has to thank the sociologist for directing emphatic 
 attention to the laws of multiplication. 
 
 3. Summary of Spencer's analysis. Individuation and genesis vary 
 inversely. 
 
 4. In regard to man, Spencer urges the importance of pressure of popu- 
 lation as an incentive to progress, and concludes that man's future evolution 
 must continue mainly in the direction of psychical development, and pre- 
 dicts with the increase of individuation a diminution of fertility. 
 
 5. Predecessors and opponents of Malthus denied that increase of 
 population tended to outrun subsistence ; Malthus successfully demon- 
 strated his thesis, and noted the checks which curbed the increase ; Darwin 
 emphasised the advantage of the pressure and checks ; Spencer shows the 
 inverse ratio of degree of development and rate of reproduction ; neo- 
 Malthusians advocate the use of artificial preventive checks to fertilisation. 
 Discussion of these various generalisations and proposals. 
 
 6. Completed individuation, were that possible, would be theoretically 
 associated with sterility. 
 
 LITERATURE. 
 
 Malthus. — Theory of Population. 1806. 
 
 Spencer. — Principles of Biology. Lond. 1866. 
 
 Geddes. — "Reproduction," Ency. Brit.; and Lecture on Claims of 
 
 Labour. Edin. 1886. 
 Drysdale. — The Population Question. Lond. 1878. 
 Besant. — The Law of Population. Lond. n.d. 
 Clapperton. — Scientific Meliorism. Lond. 1885. 
 
CHAPTER XXI. 
 
 The Reproductive Factor in Evolution. 
 
 § i. General History of Evolution. — The history of the doctrine 
 of evolution is essentially modern ; for though the idea glim- 
 mered before the minds of many ancient philosophers from 
 Empedocles to Lucretius, it was not till the eighteenth century 
 that naturalists began seriously to apply the conception to the 
 problem of the origin of our fauna and flora. In thinking of 
 the history, it is necessary to distinguish, on the one hand, the 
 gradual growth of the conviction that the theory of evolution 
 is a satisfactory modal interpretation of the origin of animate 
 nature as we know it, and, on the other, the inquiry into the 
 real mechanism of the process. The value of the evolution 
 doctrine as a thought-economising formula was made quite 
 clear by the labours of Spencer, Darwin, Wallace, Haeckel, 
 and others; the real aetiology of organisms, the "how" of the 
 evolution process — is still the subject of searching inquiry and 
 keen debate. 
 
 The idea of evolution, for so many centuries a latent germ, 
 first took definite shape, so far as biology is concerned, in the 
 mind of Buffon (1749), who not only urged the general con- 
 ception with diplomatic skill and powerful irony, but sought to 
 elucidate the working out of the process. He illustrated the 
 influence of new conditions in evoking new functions; showed 
 how these in turn reacted upon the structure of the organism; 
 and how, most directly of all, changes of climate, food, and 
 other elements of the environment, were external factors 
 evoking internal change, whether for progress or for degene- 
 ration. 
 
 Contrasted with Buffon in many ways, both in his mode of 
 treatment and in his view of the factors, was Erasmus Darwin 
 (1794), the grandfather of the author of the " Origin of Species." 
 In rhyme and reason, with all the humour and common-sense 
 of a true Englishman, and with a vivid appreciation of life as 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 3 I 7 
 
 more than mechanism, he stated the general conception of 
 evolution, and emphasised the organism's inherent power of 
 self-improvement, the moulding influence of new needs, 
 desires, and exertions, and the indirect action of the environ- 
 ment in evoking these. 
 
 To Treviranus (writing in 1802-31) — a biologist too much 
 neglected both in his lifetime and since — organisms appeared 
 almost indefinitely plastic, especially however under the direct 
 influence of external forces. His keen analysis of possible 
 factors did not fail to recognise— what Brooks, Galton, Weis 
 mann, and others have since elaborated — that the union of 
 diverse sexual elements in fertilisation was in itself a fountain 
 of change. " Every form of life," he says, " may have been 
 produced by physical forces in either of two ways, either from 
 formless matter, or by the continuous modification of form. 
 In the latter case, the cause of change may be either in the 
 influence of the heterogeneous male reproductive matter on the 
 female germ, or in the influence of other potencies after 
 generation." 
 
 His contemporary Lamarck (writing in 1801-9) — of greater 
 posthumous fame — fought in poverty like a hero for the evolu- 
 tionary conceptions of his later years. He is well known to 
 .have emphasised the importance of changed conditions in 
 evoking new needs, desires, and activities, urging at the same 
 time the perfection wrought upon organs by increased practice, 
 and conversely the degeneration which follows as the nemesis 
 of disuse. ^Asregards the evolution of plants, he laid the main 
 emphasis on the modifications brought about by the environ- 
 ment. Evolution seemed to him to be due to the interaction 
 of two fates, — an internal progressive power of life ; and the 
 external force of circumstances, encountered in the twofold 
 struggle with the ihanimate environment and with living 
 competitors. The keynote of his system was that adaptive 
 modifications in the bodies of organisms are brought about by 
 changes in function or in environment, or in both, and that 
 these modifications are in some degree at least inheritable. 
 
 Among the philosophers too, and especially in the minds 
 of those who had been disciplined in physical or historical 
 investigations, the speculations of the ancients were ever taking 
 fresh form, gaining moreover in concreteness. Thus Kant 
 viewed the evolution of species mainly in terms of the 
 mechanical laws of the organism itself, but allowed also for 
 
318 THE EVOLUTION OF SEX. 
 
 the influence of environment, noted the importance of selection 
 in artificial breeding, and, like such ancients as Empedocles 
 and Aristotle, had glimpses of the notion of the struggle for 
 existence. The same idea is more distinct in Herder's 
 " Philosophy of History," where, probably under Goethe's 
 influence, he speaks of the '' struggle, each one for itself, as if 
 it were the only one," of the limits of space, and of the gain to 
 the whole from the competition of individuals. Oken (1809) 
 saw the light of the evolution idea dancing like a will-o'-the- 
 wisp in the mist of his "Urschleim" speculations, and seemed 
 chiefly to interpret the organic progress in terms of action and 
 reaction between the organism and its surroundings ; while in 
 the noble epic of evolution which we owe to his contemporary 
 Goethe, the adaptive influence of the environment and the 
 inherent growth-tendencies of the organism are especially 
 emphasised. 
 
 Wells in 1813, and Patrick Matthew in 1831, forestalled 
 Darwin in suggesting the importance of natural selection ; but 
 their virtually buried doctrines, however interesting historically, 
 were of less practical importance than those of Robert 
 Chambers, the long unknown author of the " Vestiges of 
 Creation " (1844-53). His hypothesis of evolution emphasised 
 the growing or evolving powers of the organisms themselves, 
 which developed in rhythmic impulses through ascending 
 grades of organisation, modified at the same time by external 
 circumstances, which acted with most effect on the generative 
 system. It is difficult indeed to refrain from amusement or 
 irritation at the naive simplicity with which he evolves a 
 mammal from a bird, by the short and easy method of pro- 
 longing the period of uterine life in favourable nutritive condi- 
 tions ; but though a goose could not so simply give rise to a 
 rat, the emphasis laid on the influence of prolonged gestation 
 is full of suggestion. Apart from his common-sense view of 
 evolution as a process of continued growing, Chambers 
 deserves to be remembered as one of the first to appreciate 
 "the force of certain external conditions operating upon the 
 parturient system." 
 
 In France, Etienne and Isidore Geoffroy St Hilaire — father 
 and son — denied indefinite variations, regarded function as of 
 secondary importance, and laid special stress upon the direct 
 influence of the environment. To them it seemed not so 
 much the effort to fly, as the (supposed) diminished proportion 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 319 
 
 of carbonic acid in the atmosphere, which had determined the 
 evolution of birds from ancient reptiles. A complete history 
 of evolution theories, up to the publication of the " Origin of 
 Species" (1859), would have to take account further of the 
 opinions of the geographer Von Buch and the embryologist 
 Von Baer, of Schleiden and Naudin, Owen and Carus, and 
 many others ; but no such survey is here our purpose. 
 
 For it must be already evident from the above brief sketch 
 of representative opinions, that successive naturalists have 
 emphasised now one factor and now another in the evolu- 
 tionary process. To one it seemed as if the organism had a 
 motor power of development — often a metaphysical one, it 
 must be allowed — within itself, and that evolution was to be 
 explained, in Topsian fashion, " according to the laws of 
 organic growth;" to another, function appeared all-important, 
 perfecting organs on the one hand, allowing them to wane in 
 disuse on the other; to a third, organisms were seen under 
 the hammers of external forces and circumstances, being con- 
 tinuously welded into more and more perfectly adapted forms. 
 The intrinsic character of the organism, its function, and its 
 environment, on each of the three factors emphasis was in turn 
 laid. 
 
 At this juncture Darwin elaborated his theory of " The 
 Origin of Species by means of Natural Selection and the 
 Preservation of Favoured Races in the Struggle for Life," and 
 was independently and simultaneously corroborated by Alfred 
 Russel Wallace. They did not indeed deny a spontaneous 
 power of change in the organism itself, nor the influence of 
 function and environment; but, without definitely discussing 
 the origin of variations, sought to show how the destructive or 
 eliminating, and the conservative or selecting agency of the 
 animate and inanimate environment, were the directive factors 
 in evolution. Given a sufficient crop of indefinite variations, 
 — unanalysed or unanalysable as to their origin, — the struggle 
 for existence separated the minority of wheat ears from the 
 majority of tares, and secured a finer and finer harvest. 
 
 So much had Darwin in his magistral labours to do with 
 making the general conception of evolution current coin, that 
 we can readily understand how not only the educated laity, 
 but the majority of professed naturalists, identified their 
 adherence to the general doctrine with a subscription to the 
 specific principle of natural selection, and in becoming evolu- 
 
320 THE EVOLUTION OF SEX. 
 
 tionists became at the same time Darwinians, that is to say, 
 natural selectionists. Of late years, however, as conflict has 
 passed from the outworks to the very citadel of evolution, — has 
 come, that is to say, to centre round the problem of the origin 
 of variations,- — history has repeated itself. Naturalists such as 
 Nageli, Mivart, and Eimer have championed the cause of 
 internal organismal variations, of evolution in terms of the con- 
 stitution of the organism, of progress according to the definite 
 laws of organic growth. An active school of neo-Lamarckians, 
 such as Cope and Packard, has arisen in America; while 
 Spencer has re-emphasised the importance both of function and 
 of environment as factors in organic evolution, supported more- 
 over in this position by the experimental work of Semper and 
 others. The last published essays of Spencer may be referred 
 to in illustration of the unended state of the controversy, but 
 at the same time, of the growing tendency to limit the importance 
 of natural selection, and as a good instance of successful 
 endeavour to recognise the measure of truth in the different 
 theories. Wallace remains staunchest among the upholders of 
 the theory of natural selection, for his share in which he seems 
 ever to refuse to take to himself sufficient credit; but it is 
 interesting to notice, that in his " Darwinism" (1889), in re-in- 
 forcing his old objections against the importance which Darwin 
 attached to sexual selection, he has made admissions welcome 
 to those of us who believe that the shoulders of natural selection 
 have also been overburdened. As we have already noticed, 
 the phenomena of male ornament are discussed and summed 
 up as being "due to the general laws of growth and develop- 
 ment," so that it is " unnecessary to call to our aid so 
 hypothetical a cause as the cumulative action of female pre- 
 ference." Again, " if ornament is the natural product and direct 
 outcome of superabundant health and vigour," — a view to which 
 the reader of the preceding pages can be no stranger, — -"then 
 no other mode of selection is needed to account for the presence 
 of such ornament." But if the origin of characters so im- 
 portant as those often possessed by males is to be ascribed 
 to internal constitution rather than to the external selection of 
 indefinite variations, the suggestion seems obvious that the 
 origin of this, that, and the other set of characters may also be 
 explained in the same way. A vivid historical account of the 
 evolution of evolution-theory will be found in Osborn's " From 
 the Greeks to Darwin," but a much larger work will be neces- 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 
 
 sary if justice is to he done to many who have contributed to 
 working out the most characteristic idea of the nineteenth 
 century. Thus Stuart-Glennie's insight in seeking to bring 
 the laws of inorganic processes into line with the processes 
 of organic evolution has never received due recognition. 
 
 Before we conclude this historical sketch, we must however 
 refer to the subject of debate re-opened by Weismann, to 
 whom, as one of the foremost of European naturalists, the 
 reader's attention has already been so frequently directed. To 
 a very large extent at least, we and our fathers have believed 
 that characters acquired by the individual organism from 
 
 Two adjacent animal cells, showing communications through 
 adjacent intercellular substance; also the protoplasmic 
 network, and the nucleus. — After Pfitzner. 
 
 functional or environmental conditions might be transmitted as 
 a legacy to the offspring. According to Weismann, and not a 
 few others independent of and dependent on him, this has 
 been a delusion. Not only is positive proof of such transmis- 
 sion of acquired characters, i.e., other than those of constitu- 
 tional, congenital, or germinal origin, so scanty and unsatisfactory 
 that His has not hesitated to call the catalogue of cases a mere 
 "handful of anecdotes," but the connection between the body- 
 cells and the sex-elements seems to Weismann and his school 
 so far from close or dependent, that there is a great probability 
 
 21 
 
32 2 THE EVOLUTION OF SEX. 
 
 against any " somatic " modification specifically and represen- 
 tatively affecting the reproductive elements, — or, what comes to 
 the same thing, the offspring. If the reproductive elements, in 
 spite of the close connection between all parts of the body, 
 or even between cell and cell (see above fig.), are unaffected 
 directly and specifically by changes in the other parts of the 
 body, then the functional and environmental " modifications " 
 of the body, however important to the individual, are only of 
 indirect importance in the evolution of the race. It has been 
 suggested by Baldwin, Osborn, and Lloyd Morgan that advan- 
 tageous modifications may serve in the struggle for existence as 
 an individual shield until such time as congenital and therefore 
 transmissible variations in the same direction may be estab- 
 lished; but even if this be demonstrated, it remains true that 
 the evolutionary importance of modifications is indirect. If 
 individually acquired characters or " modifications " are of 
 importance only to the individual body, they are obviously of 
 no direct moment in the evolution of the species, — above the 
 level of the Protozoa at least; and, as Weismann himself says, 
 the ground is thus taken from under the feet of Buffonians, 
 Lamarckians, neo-Lamarckians, etc. The ground is left clear for 
 natural selectionists, and the struggle for existence acting on 
 variations remains as the chief factor in the mechanism of 
 evolution. Though we cannot demonstrate that a logical possi- 
 bility has been the vera causa of past evolution, much has been 
 done to establish the probability. But much still requires to 
 be done by actual observation in the present to show that dis- 
 criminate elimination is of frequent occurrence, for it is on the 
 assumption of discriminate, as opposed to indiscriminate, elimi- 
 nation that the case for natural selection rests. To the ques- 
 tion, What starts these variations which natural selection 
 eliminates or fosters? Weismann has suggested various 
 answers: — (a) That the action of the environment on the bodi- 
 less Protists established a multitude of differences of which all 
 subsequent variations in the multicellular organisms are simply 
 permutations and combinations; (b) that a prolific source of 
 variation is to be found in the intermingling or amphimixis of 
 the sex-cells in fertilisation, and in the reduction-processes 
 associated with the maturation of these sex-cells; and (c) that 
 the germ-plasm is provoked to vary by nutritive and other 
 stimuli acting on it from without, or from the enclosing body 
 of the parent-organism. There are many, however, who would 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 323 
 
 still say, that even if none but constitutional or germinal varia- 
 tions are transmissible, we are not shut up to the exclusive 
 adoption of the natural selectionist position. It is still open 
 to the naturalist to demonstrate that many adaptations at least 
 are not explicable as the result of a long process of fostering 
 and eliminating selection among a host of sporadic indefinite 
 variations, but are rather the direct and necessary results of 
 "laws of growth," of "constitutional tendencies," or of the 
 precise chemical nature of the protoplasmic metabolism in the 
 organisms in question. If constitutional variations occur along 
 a few definite lines, as Eimer, Geddes, and others have main- 
 tained in certain cases, then we can understand the origin, 
 though not perhaps the distribution, of species apart from any 
 long process of selection, for which indeed, if variations be 
 strictly definite, the material must be vastly reduced. In 
 other words, we can think of the organism not merely under 
 the moulding influence of its functions, nor solely as the pro- 
 duct of environmental hammering, least of all as the survivor 
 from a crowd of unsuccessful competitors, but as the expression 
 of an internal fate, no longer mystical, but expressible in terms 
 of the dominant chemical constitution. 
 
 § 2. The Reproductive Factor. — Without further discussion 
 of the still open controversy as to the various factors of evolu- 
 tion, which would not be relevant to such a work as this, we 
 must summarily collate the more prominent opinions as to the 
 share reproduction has in the process. To most of these we 
 have already alluded in the body of the book. 
 
 (a.) First of all, as to the origin of variations, we find that 
 what Treviranus recognised in the first years of this century — 
 viz., the influence of fertilisation in evoking change — has been 
 emphasised by several, such as Brooks and Galton, and has 
 been especially elaborated by Weismann. As we have already 
 noted, Weismann has suggested that the intermingling of two 
 "germ-plasmas," which is at least part of the essence of 
 fertilisation, may be an important fountain of congenital 
 variations. In apparent contrast is the view advocated by 
 Hatschek, who sees in the intermingling essential to fertilisa- 
 tion a counteractive of idiosyncrasies, a means of controlling 
 and checking disadvantageous individual peculiarities. The 
 two positions are not antagonistic, but rather complementary. 
 
 (p.) No impartial student of Darwinism can fail to admit, 
 that in the "struggle for existence" stress is laid upon the 
 
324 THE EVOLUTION OF SEX. 
 
 nutritive and self-maintaining functions and strivings, yet we 
 must remember Darwin's own words: — "I should premise 
 that I use this term [struggle for existence] in a large and 
 metaphorical sense, including dependence of one being on 
 another, and including (which is more important) not only the 
 life of the individual, but success in leaving progeny" ("Origin 
 of Species," p. 50). Similarly, Herbert Spencer says : "If we 
 define altruism as being all action which, in the normal course 
 of things, benefits others instead of benefiting self, then from 
 the dawn of life altruism has been no less essential than 
 egoism. Though primarily it is dependent on egoism, yet 
 secondarily egoism is dependent on it." " Self-sacrifice is no 
 less primordial than self-preservation" ("Principles of Ethics" 
 and " Principles of Psychology"). 
 
 (c.) Darwin also insisted upon the role of "sexual selection," 
 which implies a recognition of the reproductive factor. We 
 have seen, however, that sexual selection is only a special case of 
 natural selection ; that it seeks to explain the elaboration, not 
 the origin of sexual peculiarities ; and lastly, that Darwin's 
 arguments in favour of the mechanism which he emphasised, 
 have been seriously impugned by Wallace, in an attack which 
 reacts strongly upon the critic's own position. It must not be 
 overlooked, however, that the existence of any form of selective, 
 as opposed to indiscriminate mating, will, if natural selection 
 be at work, tend to accelerate the process of differentiation. 
 (See Pearson's "Grammar of Science," 2nd edition, 1900, 
 
 PP- 423-4370 
 
 (d.) Romanes has recently elaborated, what others seem 
 also to have suggested, the importance of mutual sterility in 
 splitting up one species into several. "Whenever any varia- 
 tion in the highly variable reproductive system occurs, tending 
 to sterility with the parent form without impairing fertility with ' 
 the varietal form, a physiological barrier must interpose, 
 dividing the species into two parts, free to develop distinct 
 histories, without mutual intercrossing, or by independent 
 variation." The reproductive system is very apt to vary, — 
 why, he does not say ; the consequence might readily be, 
 that among the progeny of a parent stock some were fertile 
 inter se, but infertile with the consistent members of the 
 parent stock; these will be isolated by a physiological barrier, 
 just as they might be insulated by a geographical one, and 
 left free to develop along divergent paths of their own. Here 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 325 
 
 again there is recognition of the reproductive factor in evolu- 
 tion ; but how far, and in what cases species have so originated, 
 is obviously a question which would involve discussion of each 
 individual instance. 
 
 (<?.) When pairing is restricted in its range to more or less 
 closely related forms — i.e., when inbreeding is practised — it 
 seems to have up to a given limit the effect of fixing characters 
 and developing prepotency. By prepotency is meant a relative 
 strength in the power of transmitting character, or, in other 
 words, the strong persisting power which certain characters 
 have over others in inheritance. Thus if a certain character 
 of a sire is invariably transmitted to the offspring, irrespective 
 of the character of the dam, we say that the male is prepotent 
 as regards that character. But if inbreeding occurs in nature 
 — e.g., as the result of some form of isolation, as it does in 
 domestication ; and if it has as one of its results the develop- 
 ment of prepotency, we have here an important factor in 
 evolution. For the prepotency will account for the persistence 
 of variations in their early stages. (See "The Penycuik 
 Experiments," by Professor J. Cossar Ewart, London, 1899, 
 xciii. and 177 pp., 46 figs.) 
 
 (f.) There is among organisms, and even in allied species, 
 an enormous variety in the actual fertility and in the potential 
 fertility (fecundity) ; even among members of the same species 
 there are great differences. The importance of this is increased 
 when we note that Professor Karl Pearson, with the assistance 
 of Miss Alice Lee and Mr. Leslie Bramley-Moore, have shown 
 that fertility is inherited in man and fecundity in the horse — a 
 conclusion which probably admits of great extension. The 
 fact gives basis to Pearson's theory of " reproductive or genetic 
 selection " — a phrase which he uses to describe the selection 
 "of predominant types owing to the different degrees of repro- 
 ductivity being inherited, and without the influence of a 
 differential death-rate If fertility is inherited, and if fertility 
 is correlated with other characters, there will be a continual 
 tendency to progress in a definite direction, unless there be 
 some counteracting factor which takes the form of a differential 
 death-rate more intense for the offspring of the more fertile. 
 (" Proc. Roy. Soc. London," lxiv., 1899, pp. 163-167.) 
 
 (g.) Worthy of consideration is the suggestion of Robert 
 Chambers, crudely illustrated as ' it may have been, that 
 environmental influences acted with special power upon the 
 
326 THE EVOLUTION OF SEX. 
 
 generative system, and that the prolongation of gestation was a 
 maternal sacrifice which brought its own reward in the higher 
 evolution of the offspring. With this Beard's interesting essay 
 on "The Span of Gestation" may be profitably compared. 
 Miss Buckley has well pointed out how the increase of parental 
 care was a factor in, as well as a result of, the general ascent; 
 how the success of birds and mammals especially must in part 
 be interpreted in reference to the noteworthy deepening of 
 parental affection, and strengthening of the organic and 
 emotional links between mother and offspring. In emphasis- 
 ing the progressive value of prolonged infancy, especially in 
 the evolution of the emotions, Fiske has also recognised the 
 importance of the reproductive factor. The same idea is 
 prominent in Drummond's "Ascent of Man." 
 
 § 3. Farther Construction. — The general tendency of all 
 theories of evolution has been to start with the individual 
 organism as the unit, and to consider the self-maintaining and 
 nutritive activities as primary, the reproductive and species- 
 regarding as only secondary. But along many lines of 
 research, such as those indicated in the preceding paragraphs, 
 the importance of the reproductive factor has been recognised, 
 and the centre of gravity of the evolutionary inquiry has already 
 been to some degree shifted. Recent investigations on heredity, 
 for instance, forbid that attention should any longer be con- 
 centrated on the individual type, or reproduction regarded as a 
 mere repetition process ; the living continuity of the species 
 is seen to be of more importance than the individualities of the 
 separate links. Physiologists and evolutionists are coming to 
 see the most complex individual lives, in Foster's phrase, as 
 "but the bye-p!ay of ovum-bearing organisms." The species 
 is a continuous undying chain of unicellular reproductive 
 units, which indeed build out of and around themselves 
 transient multicellular bodies, but the processes of nutritive 
 differentiation, and other individual developments, are second- 
 ary, not primary. 
 
 Thus it is the central generalisation of botany that, despite 
 the individual differentiation of fern, selaginella, cycad, conifer, 
 and flower, these turn out, on deepest analysis, to be but the 
 surviving phases of a continuous and definite increase in the 
 subordination of the sexual parents to their asexual offspring. 
 
 Or if we take in particular the origin of the flower, which 
 many botanists agree in regarding as a shortened branch, the 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 327 
 
 natural selectionist explanation would seem to be, that the 
 flower had arisen by selection from the two other alternatives 
 of lengthened and unshortened axes. But this point of view 
 must be corrected in relation to the physiological theory that 
 shortening of the axis was inevitable, since the expense of the 
 reproductive functions necessarily checks the vegetative ones. 
 It is evident that we cannot speak of selection where the 
 imaginable alternatives are physically impossible. So too the 
 shortening of the inflorescence from raceme to spike or flower- 
 head, or still further into the hollowed form of a fig, with the 
 corresponding reduction in the size of the flowers, is again the 
 result of the check imposed by reproduction on the growth of 
 axis and appendages. 
 
 The same simple conception of a continuous checking of 
 vegetation by reproduction unlocks innumerable problems of 
 floral structure, large and small alike, from the inevitable 
 development of gymnosperm into angiosperm by the con- 
 tinuous subordination of the reproductive carpellary leaf, to 
 the variations of cabbages as seen in the transitions between 
 leafy kale and cauliflower. Or again, the origin of floral 
 colour, as primarily an inevitable consequence of the same 
 principle of vegetative subordination through reproductive 
 sacrifice, was long ago pointed out by Spencer, and admits of 
 detailed elaboration without attaching more than secondary 
 importance to selection by insects. 
 
 In another way, the antithesis between reproduction and 
 nutrition may be illustrated among the existing orders and 
 species of flowering plants. Just as the lilies, for instance, 
 range on the one side towards the characteristically vegetative 
 grass, or on the other towards the reproductive orchid, so it is 
 with the main variations of every natural alliance. Thus, the 
 Ranunculacese have their grassy and their orchid-like types in 
 meadow-rue and larkspur respectively, while the species of 
 these very genera show, within narrower limits, similar swings 
 of variation. What we call higher or lower species are thus the 
 leaders or the laggards along one or other of these two lines of 
 variation. 
 
 Among animals, the importance of the reproductive factor 
 may be illustrated in the most diverse series. Thus the 
 greatest step in organic nature, that between the single-celled 
 and many-celled animals, bridged as it is by loose colonies, 
 some of which are at a very low morphological level, is not 
 
3 2§ 
 
 THE EVOLUTION OF SEX. 
 
 due to the selection of the more individuated and highly 
 adapted forms, but to the union of relatively unindividuated 
 cells into an aggregate, in which each becomes dimnishingly 
 competitive and increasingly subordinated to the social whole. 
 The colonial or multicellular forms, originating pathologically 
 in all probability, may of course have rapidly justified their 
 existence in the struggle for existence, just as unions of many 
 kinds do in human society, but the Protozoa cannot be accused 
 
 Formation of the Gastrula. — From Haeckel. 
 
 of any prevision of future advantage in remaining clubbed 
 together in co-operation, nor indeed credited with much 
 primitive altruism in so doing. 
 
 No structure is more emphatically nutritive in its adult 
 result than the gut-cavity of the embryonic gastrula. It is 
 worth inquiring whether this important step in differentiation 
 was attained in history in response to nutritive needs. The 
 usual supposition is certainly that the gastrula cavity, by what- 
 ever peculiarities of growth it may have arisen, justified itself 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 329 
 
 from the first in an additional nutritive advantage. But 
 Salensky, in his studies on the primitive form of the Metazoa, 
 has given strong arguments in favour of the theory that the 
 primitive cavity, arising in a volvox-like form, was originally a 
 brood-cavity or "genitoccel," and that it only secondarily acquired 
 nutritive significance. It would be indeed striking if this 
 important morphological step in the establishment of the 
 nutritive system was reached along the road of reproductive 
 modification; for if this most fundamental of nutritive and 
 self-maintaining advantages, the belly itself, be but a secondary 
 resultant of an originally reproductive and species-regarding 
 progress, the lower Utilitarianism, which has so long been 
 arguing from economics to biology and back again, is evidently 
 a step nearer exposure. 
 
 Or again, that increase of reproductive sacrifice, which at 
 once makes the mammal and marks its essential stages of 
 further progress through oviparous monotreme, prematurely- 
 bearing marsupial, and various grades of placental; that in- 
 crease of parental care; that frequent appearance of sociality 
 or co-operation, which even in its rudest forms so surely 
 secures the success of the species attaining it, be it mammal or 
 bird, insect or even worm, — all these phenomena of survival of 
 the truly -fittest, through love, sacrifice, and co-operation, need 
 far other prominence than they could possibly receive on the 
 hypothesis of the essential progress of the species through 
 internecine struggle of its individuals at the margin of subsist- 
 ence Each of the greater steps of progress is in fact associated 
 with an increased measure of subordination of individual com- 
 petition to reproductive or social ends, and of interspecific 
 competition to co-operative association. 
 
 The corresponding progress in the historic and individual 
 world, from sex and family up to tribe or city, nation and race, 
 and ultimately to the conception of humanity itself, also 
 becomes increasingly apparent. Competition and survival of 
 the fittest are never wholly eliminated, but reappear on each 
 new plane to work out the predominance of the higher, i.e., 
 more integrated and associated type, the phalanx being 
 victorious till in turn it meets the legion. But this service no 
 longer compels us to regard these agencies as the essential 
 mechanism of progress, to the practical exclusion of the asso- 
 ciative factor upon which the victory depends, as economist 
 and biologist have too long misled each other into doing. For 
 
330 THE EVOLUTION OF SEX. 
 
 we see that it is possible to interpret the ideals of ethical pro- 
 gress, through love and sociality, co-operation and sacrifice, 
 not as mere Utopias contradicted by experience, but as the 
 highest expressions of the central evolutionary process of the 
 natural world. The ideal of evolution is indeed an Eden; 
 and although competition can never be wholly eliminated, 
 and progress must thus approach without ever completely 
 reaching its ideal, it is much for our pure natural history to 
 recognise that " creation's final law " is not struggle but love. 
 The fuller working out of this thesis, however, would lead us 
 far beyond our present limits, towards a restatement of the 
 
 An Opossum (Didclphys doTsigcra) carrying its young on its back. — 
 From Cams Sterne. 
 
 entire theory of organic evolution. Suffice it here, in con- 
 clusion, to indicate an important change in the general point of 
 view. The older biologists have been primarily anatomists, 
 analysing and comparing the form of the organism, separate 
 and dead; however incompletely, we have sought rather to be 
 physiologists, studying and interpreting the highest and intensest 
 activity of things living. From the study of individual structure 
 they were wont to pass, indeed, to that of reproductive 
 structures, and thence even functions; hence, too, the pair 
 and the totality of the species did at length come successively 
 into view, but this with the individualistic theory of natural 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 33 1 
 
 selection bulking as practically all-important in the foreground. 
 For us, however, this perspective has become entirely reversed. 
 The individual is a mere link in the species, and its repro- 
 ductive processes are thus of fundamental importance to the 
 interpretation even of its self-maintaining ones. Hence we no 
 longer regard, with Darwin and the majority of our brother 
 naturalists, the operation of natural selection upon individual 
 characters as the simplest of problems, looking for residual 
 explanation to sexual selection, and only in extreme difficulty 
 invoking the aid of " principles of correlation," " laws of 
 growth," and the like, viewed as almost inscrutably mysterious. 
 On the contrary, it is the continual correlation yet antithesis — 
 the action and reaction — of vegetative and reproductive pro- 
 cesses in alternate preponderance, which seems to us of 
 fundamental importance, since to this the general rhythm of 
 individual and racial life runs fully parallel. Hence it is that 
 we have the primeval lily developing on the one hand the 
 ideally vegetative grass, yet also the supremely specialised re- 
 productive orchid; and that we can trace (as we hold) the same 
 swing of divergent evolution, of definite variation, in every 
 natural order, nay, in every genus, often even in the very 
 varieties of a species. Hence, too, it is that the rhythm of 
 hydroid and medusoid in the individual life of the typical 
 forms becomes fixed in coral or ctenophore as a racial tempera- 
 ment. This preponderance of passivity or activity (which we 
 can read throughout, in barnacle and insect, as well as in tortoise 
 and swallow) once set up, goes on accumulating till it meets 
 reversal through environment or other causes, and limitation 
 or extinction through the agency of natural selection, which, 
 however, is more frequently a retarding force than an accel- 
 erant of evolution. The problem of organic progress is thus 
 to be interpreted not merely as on conventional lines, by help of 
 an analogy derived from an age of mechanical progress which 
 gives us the watch, or sewing-machine, or tricycle, — by the 
 cumulative patenting, as it were, of useful improvements in 
 detail. The essential problem is not one of mechanism but 
 of character, to which incident is accessory but not fundamen- 
 tal, — not of details put together, but of aggregate organic life 
 or temperament. The life of the individual or the species is 
 essentially a unity, of which the specific characters are but 
 the symptoms, be their subsequent measure of importance 
 and utility in adaptation, their modification by environment, 
 
332 EVOLUTION OF SEX. 
 
 their enhancement or diminution by natural selection, what 
 they may. Our special study of the reproductive process has 
 thus fairly brought us to the threshold of a larger inquiry, the 
 primary one of the organic sciences, that of the factors of 
 organic evolution. For it is in nature, as Schiller saw long ago 
 in the human life, which this foreshadows: "While philo- 
 sophers are disputing about the government of the world, 
 Hunger and Love are performing the task." 
 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 333 
 
 SUMMARY. 
 
 1. A brief review of the history of evolution theories, and of the present 
 state of the question. 
 
 2. A reproductive factor in evolution has been hinted at by a few 
 naturalists. 
 
 3. Further indications of the importance in evolution of reproductive 
 and species-regarding, as opposed to the nutritive and self-maintaining 
 activities, 
 
 LITERATURE. 
 
 See articles by the writers in "Chambers's Encyclopaedia," especially 
 Biology, Botany, Environment, Evolution, and minor articles, 
 such as Ccelenterates, Flower, Fruit, &c. ; also " Encyclopedia 
 Britannica," Sex, and Variation and Selection; also Geddes, 
 A Restatement of the Theory of Organic Evolution (Summary in 
 Proc. Roy. Soc. Edin., 1888-89, still unpublished). 
 
 Spencer, Mivart, Eimer, Wallace, Weismann, &c. — Of. cit. 
 
 Thomson, J. A. — The Endeavour after Well-being. Natural Science, 
 VIII. (1896), pp. 21-26. 
 
INDEX. 
 
 Age of parents, influence of, on sex, 
 
 37 
 Albrecht, parthenogenesis of silk- 
 moth, 183 
 Aldrovandus, 89 
 Algse, reproduction in, 135, 136 
 Allantois, 266 
 
 Alternation of generations, 213, 225 
 Alternation between sexual and 
 
 degenerate sexual reproduction, 
 
 216 
 Altruism, egoism and, 295, 296 
 Amici on pollen-grain, 148 
 Amnion, 266 
 Amphibians, parental care in, 270, 
 
 271 
 Anabolism, see Protoplasm 
 Angiostomum, 77 
 Animal cell, division of, 248 
 Animalculists, 89, 169 
 Anther, section of, 148 
 Antlers, 26 ; and reproductive 
 
 organs, 256 
 Aphides, 49 ; parthenogenesis in, 
 
 187 ; generations of, 189 
 Apospory, 219 
 
 Aquapendente, Fabricius ab, 89 
 Argonauta, 262 
 Artemia salina, 50 
 Artificial division, 200 
 Artificial parthenogenesis, 184 
 Arthropods, hermaphroditism in, 
 
 77 
 
 Ascaris, sperms of, 118; segrega- 
 tion of germ-cells in, 99 ; Prof. 
 Wilson's account of, 123 
 
 Asexual reproduction, 200-211; in 
 plants, 203 
 
 Asexual propagation of a grass, 20 
 " Aura seminalis," 170 
 Aurelia, alternation of generations 
 in, 215 
 
 Baer, Von, discovery of mammalian 
 
 ovum, 92 
 Balbiani on Chironomus, 98 ; conju- 
 gation in infusorians, 158 
 Balfour on polar bodies, 113; 
 
 theory of parthenogenesis, 194 
 Barfurth, parthenogenesis in hens' 
 
 eggs, 185 
 Barry, Martin, 150 
 Bary, De, 172 ; parthenogenesis 
 
 in fungi, 190 
 Beard, hermaphroditism in Myzos- 
 
 toma, 83 ; rhythm in vertebrates, 
 
 223, 264, 265 
 Bees, sex in, 46 ; origin of sex-cells, 
 
 97 
 
 Beneden, Van, polar globules, 113; 
 ovum of threadworm, 153, 155 
 
 Berner's statistics, 38 
 
 Berthold, 137 
 
 Bichat's analysis, 92 
 
 Bidder's organ, 74 
 
 Bilharzia, 77 
 
 Bird of Paradise, 4 
 
 Birds, eggs of, 1 10, III; courtship 
 of, 6 
 
 Blackcock, 7 
 
 Blochmann, polar bodies, 112; 
 parthenogenetic ova, 194 
 
 Body and reproductive cells-, rela- 
 tion between, 101, 278 
 
 Boerhaave, 89 
 
 Bonellia, 20, 21 
 
336 
 
 INDEX. 
 
 Bonnet, parthenogenesis in aphides, 
 
 183 
 Bordage, regeneration, 202 
 Born, hybridisation in amphibians, 
 
 163 
 Botrychium, 240 
 Boveri, researches, 153, 154 
 Brauer, parthenogenetic ova, 194 
 Brodie, Dr T. G., on spermatozoa, 
 
 124 
 Brooding and young-feeding organs, 
 
 69 
 Brooks, W. K. , sexual selection, 
 
 11-13 ; variability of males, 127 ; 
 
 fertilisation, 178 ; alternation of 
 
 generations, 222 
 Bryophyllum calycinum with adven- 
 titious buds, 211 
 Biichner on love among animals, 
 
 2S5 
 Buckley, Miss, on parental care, 
 
 326 
 Buffon, 316 
 
 Biitschli on polar globules, 114 
 Butterflies and moths, sex in, 50 
 
 Callionymus lyra, 6 
 
 Camerarius, 148 
 
 Canestrini, 36 
 
 Carnoy on fertilisation, 153 
 
 Castration, 23, 24, 256 
 
 Casual hermaphroditism, 72 
 
 Cecidomyia, 258 
 
 Cell, animal, 104 
 
 Cell-cycle, 127, 130 
 
 Cell-division, 235, 273 
 
 Cell-theory, 92 
 
 Cells, body and reproductive, 99 
 
 Cephalopterus ornatus, 7 
 
 Chambers, Robert, on prolonged 
 gestation, 318 
 
 Chamisso, alternation of genera- 
 tions, 212, 213 
 
 Chemistry of the egg, 1 1 1 
 
 Chironomus, 98 ; parthenogenesis 
 in, 188 
 
 Chondracanthus, 17 
 
 Chromatin elements, 254 
 
 Cladocera, seasonal parthenogenesis 
 in, 187 
 
 Claspers, 263 
 
 Coccus insects, 16 
 
 Ccelebogyne, 190 
 
 Ccelenterates, reproductive organs 
 in, 64, 65 ; hermaphroditism in, 
 76 ; asexual reproduction in, 205- 
 207 ; • alternation of generations 
 in, 222 
 
 Coiter, Volcher, 89 
 
 Colour, animal, 25 
 
 Colour, floral, 327 
 
 Comet form of starfish, 210 
 
 Comparative vigour, theory of, 38 
 
 Complemental males, 82 
 
 Composite ova, no 
 
 Conjugation in Protozoa, 157-160; 
 multiple, 157 
 
 Continuity of germ-plasm, 100-102, 
 252 
 
 Copulation, 263 
 
 Corals, budding of, 208 
 
 Crustaceans, sex in, 50; partheno- 
 genesis in, 50 
 
 Cucumaria crocea, 292 
 
 Cupid's dart, 263 
 
 Cutleria, reproduction in, 137 
 
 Cuttlefishes, reproductive organs of, 
 68 
 
 Cynipidre, parthenogenesis in, 187 
 
 Cyprids, reproduction in, 189 
 
 Daphnids, 189 
 
 Darwin, 5; sexual selection, 8-14; 
 and Wallace, 30 ; determination 
 of sex, 40; fertilisation of flowers, 
 146, 147 ; the population ques- 
 tion, 300, 306; natural selection, 
 319. 320 
 Darwin, Erasmus, 316, 317 
 Death, origin of, 248, 276, 278 
 Degenerate sexual reproduction, 
 
 183-198 
 Degrees of asexual reproduction, 203 
 Delage's experiments on fertilisa- 
 tion, 172 
 " Descent of Man,'' 5 
 Determination of sex, 34 
 Development, stages of, 91 
 Dewitz, 151 
 Dichogamy, 78 
 Didelphys dorsigera, 330 
 Diplozoon paradoxum, 262 
 Divergence of sex-cells, 128, 134 
 Division of labour, 206 
 
INDEX. 
 
 337 
 
 Doliolum, alternation of generations 
 
 in, 216 
 Drelincourt, 130 
 Drones, 19 
 
 Ducts of reproductive organs, 66, 67 
 Dufour, parthenogenesis in aphides, 
 
 183 
 Diising, 36 ; on proportions of 
 
 sexes, 40, 41 
 Dzierzon, parthenogenesis in bees, 
 
 184 
 
 Echinoderma, hermaphroditism 
 among, 77 
 
 Ectocarpus, reproduction in, 137 
 
 Edible birds' nests, 267 
 
 Egg, chemistry of, 1 1 1 
 
 Egg-cell, 104- 1 14 
 
 Egg envelopes, 1 10 
 
 Egg-laying organs, 63 
 
 Egoism, 295; 296 
 
 Eimer on sex in bees, 47 
 
 Embryology, history of, 88 
 
 Embryonic hermaphroditism, 71 
 
 Encystation, 206, 276 
 
 Engelmann on dimorphism in Pro- 
 tozoa, 138 
 
 Environment, influence of, 249, 250 
 
 Epigamy, 209 
 
 Epigenesis, 88; Wolff's reassertion 
 of, 91 
 
 Epistylis, 138 
 
 Ernst, Dr A., parthenogenesis in 
 plants, 190 
 
 Ethical aspects of reproduction, 283- 
 
 , 2 97 . . . 
 
 Eunice viridis, reproduction in, 210 
 Eupagurus bernhardus, 21 
 " Evolution" in old sense, 90 
 Evolution, reproductive factor in, 
 
 316-332 
 Ewart, Prof. Cossar, experiments in 
 
 hybridisation, 164, 165 
 
 Factors in determination of sex, 54 
 
 Fallopian tube, 257 
 
 Females, 1-52 passim ; size, 19; 
 passivity, 18; preponderant ana- 
 bolism of, 28 ; psychical char- 
 acters of, 286-291 
 
 Ferns and mosses, alternation of 
 generation in, 217-219 
 
 Fertilisation, time of, 36; in plants, 
 146; in animals, 150-157; pro- 
 cess of, 154, 155; origin of, 158- 
 162; contrasted with conjugation, 
 161; theory of, 1 69- 1 80; a source 
 of variation, 317 
 
 Fishes, sex differences in, 25 ; 
 parental care in, 272-295 
 
 Flower, 239 
 
 Fluke, alternation of generations in, 
 216, 217 
 
 Follicular cells, 1 10 
 
 Free-martin, 41 
 
 Fulton, Dr W. T., on male and 
 female fishes, 22 
 
 Fungi, parthenogenesis in, 191 
 
 Gall-wasps, parthenogenesis in, 190 
 Galton on body and sex-cells, 100; 
 
 on use of fertilisation, 170 
 Gastroblasta raffaelii, 206 
 Gastrula, 91, 328 
 
 Gegenbaur on origin of hermaphro- 
 ditism, 84 
 Gemmules of fresh-water sponge, 
 
 222 
 Genesis, 285 
 Genetic selection, 325 
 Genoblasls, theory of, 127 
 Gentry on sex in moths, 50 
 Geoffroy, E. F., 148 
 Germiparity, 71 
 Gestation, period of, 263; influence 
 
 of, prolonged, 318 
 Giard on polar bodies 114 
 Girou on comparative vigour, 3?; 
 
 on sex in sheep, 51 
 Glyciphagus cursor, 206 
 Goette on nemesis of reproduction, 
 
 272. 275 
 Gonads, or essential reproductive 
 
 organs, 63-67 
 Graaf, De, 89, 170 
 Growth, facts of, 233; Spencer's 
 
 theory of, 234. 
 Gruber on reproduction in Protozoa, 
 
 2 4 T 
 Guaila, hybridisation in mice, 164 
 
 Gyrodactylus, 220 
 
 Haeckel on contrast between body 
 and reproductive cells, 90; on 
 22 
 
338 
 
 INDEX. 
 
 Protomyxa, 129; on alternation 
 of generations, 226 ; on mago- 
 sphaera, 272 
 
 Haller, 89, 90 
 
 Hamm, discovery of spermatozoon, 
 117 
 
 Hartog, nuclear reduction, 196 
 
 Harvey, 88, 171 
 
 Hatchett Jackson on union in in- 
 fusorians, 138 
 
 Hatschek on fertilisation, 179; on 
 nutrition and reproduction, 241 
 
 Heat, influence of, on reproduction, 
 249 
 
 Hectocotylus, 68, 162 
 
 Helix, reproductive system of, 68 ; 
 hermaphroditism in, 79 
 
 Hensen, theory of sex, 36, 38 ; 
 length of life of sperms, 120; use 
 of fertilisation, 175 ; partheno- 
 genesis in bees, 186 
 
 Heredity, see Weismann, in alter- 
 nating generations, 224, 225 ; 
 acquired characters, 321, 322 
 
 Hermaphroditism, 7 1 -84 ; conditions 
 of, 83; origin of, 84 
 
 Heme, J. van, 89 
 
 Hertwig on hermaphroditism, 74; 
 theory of fertilisation, 171 
 
 Heterodera schachtii, 17 
 
 Heterogamy, 219 
 
 Heterophagy, 172 
 
 Heyer on sex in plants, 52 
 
 Hofacker and Sadler's law, 38 
 
 Hoffman, 38 
 
 Human species, influence of nutri- 
 tion on sex of, 5 1 
 
 Hunger and love, 296, 297 
 
 Hybrids, variability of, 164 
 
 Hybridisation, 162-166 
 
 Hydatina, 20; sex in, 51 
 
 Hydractinia, division of labour in, 
 206 
 
 Hydroids, alternation of generations 
 in, 218, 219 
 
 Immortality, organic, 275 
 Inbreeding, 179, 180 
 Incubation, 268 
 Individuation, 305, 307 
 Infusorians, 158; reproductive 
 power of, 249 
 
 Insects, sex characters of, 5, 11, 16- 
 19; determination of sex in, 46- 
 50; hermaphroditism in, 75 ; par- 
 thenogenesis in, 189 
 
 Isophagy, 172 
 
 Iwanzoff on fertilisation in Echino- 
 derms, 173 
 
 Jager, continuity of germ proto- 
 plasm, 100 
 
 Janosik, parthenogenesis in mam- 
 mals, 185 
 
 Jaworowski, parthenogenesis in 
 Chironomus, 188 
 
 Jhering, von, 223 
 
 Joseph on fertilisation, 171 
 
 Joukowsky, conjugation in in- 
 fusorians, 178 
 
 Juvenile parthenogenesis, 187 
 
 Karyogamy, 162 
 
 Kennel, Professor von, on antlers, 
 26 
 
 Kerner von Marilaun, partheno- 
 genesis in Antennaria alpina, 
 190 
 
 Kerville on hybridisation, 165 
 
 Kirby and Spence, parthenogenesis 
 in aphides, 183 
 
 Klebs, reproduction in Algse, 162, 
 244 
 
 Kolliker on origin of sperms, n8 
 
 Kblreuter, 146 ; hybridisation in 
 plants, 166 
 
 Kossel on Protamin, 124 
 
 Lactation, 266, 267 
 
 Lamarck, 317 
 
 Laulanie on embryonic organs, 35 ; 
 
 hermaphroditism, II, 22 
 Lecanium hesperidum, 20 
 Leeuwenhoek, 89 ; on spermatozoa, 
 
 "7 
 
 Lessona on regeneration, 145 
 
 Leuckart, frog ova, 184, 185; alter- 
 nation of generations, 214 
 
 Limax, reproductive system of, 78 
 
 Limit of growth, 234 
 
 Liver-fluke, life-history of, 217 
 
 Liverwort, 241 
 
 Loeb on sea-urchins, 174, 185 
 
 Love, 284-294 
 
INDEX. 
 
 339 
 
 Love of offspring, 291 
 Lubbock on queen ants, 120 
 Luciola, 27 
 
 Magosphsra, 272 
 
 Males, 1-52 passim; variability, 
 12, 13; activity, 18; size, 19; 
 preponderant katabolism, 28 ; 
 complementai males, 82 ; psychi- 
 cal characters of, 2S6-291 
 
 Male-cell or spermatozoon, 1 17-128 
 
 Male parthenogenesis in Algae, 1 9 1 
 
 Malpighi, 89 
 
 Malthus, theory of population, 300- 
 306 
 
 Mammals, weapons of, 7 ; sex in, 
 51 ; love among, 285 
 
 Mammalian ovum, 87 
 
 Mammary glands, 266, 267 
 
 Mantegazza, 13, 285 
 
 Marchal, Paul, 223 
 
 Mark on polar bodies, 1 1 5 
 
 Marsupials, 269 
 
 Maternal sacrifice, 274-275 
 
 Mates, love of, 283-285 
 
 Maturation of ovum, 112 
 
 Maupas on conjugation and multi 
 plication in infusorians, 157, 158, 
 177, 242, 249, 250; use of ferti- 
 lisation, 175 
 
 Mayflies, 275 
 
 Mechanism of cell-division, 236 
 
 Meehan on sex in plants, 52 ; on 
 fertilisation, So, 147 
 
 Menstruation, 259 
 
 Mesozoa, liberation of germs in,' 
 
 273 
 
 Metazoa and Protozoa, 95 
 
 Metabolism, see Protoplasm 
 
 Micropyle, no 
 
 Microstomum, 208 
 
 Miescher on milt of salmon, T?4 
 
 Migration of sex-cells, 66 
 
 Milk, 267 
 
 Millington, 149 
 
 Minot on polar bodies, 113 ; theory 
 of genoblasts, 127-139 ; partheno- 
 genesis, 194 
 
 Mivart on sexual selection, 13 
 
 Mobius, 25 ; effects of vegetative 
 multiplication, 179 ; stickleback, 
 268 
 
 Molliard, influence of temperature, 
 
 54 
 Molluscs, hermaphroditism among, 
 
 78 
 
 Moonwort, 240 
 
 Morgan, T. H , hybridisation in 
 echinoderms, 164 ; regeneration, 
 202 
 
 Morphological theories of fertilisa- 
 tion, 170 
 
 Mosses, alternation of generations 
 in, 217-219 
 
 Miiller, Johannes, 92 
 
 Multiplication, laws of, 299-314 
 
 Multiple conjugation, 160 
 
 Myrianida, 209 
 
 Myzostomata, 81 
 
 Nageli, origin of reproduction, 247 
 Natural selection, a check on diver- 
 gence of sexes, 31 
 Needham, 89 ; scope of, 319, 320, 
 
 331 
 
 Nematodes, alternation of genera- 
 tions in, 222 
 
 Nemesis of reproduction, 272 
 
 Neo-Malthusianism, 309-314 
 
 Nereis, reproduction in, 209 
 
 Normal adult hermaphroditism, 75 
 
 Notodelphys, 270 
 
 Nototrema, 270 
 
 Nucleus, behaviour of, in fertilisa- 
 tion, 154, 155; essential elements 
 of, 254 
 
 Nussbaum, continuity of germ- 
 plasm, 100 
 
 Nutrition, influence of, on sex, 45 
 
 Nutrition of embryo, 265 
 
 Nutrition and reproduction, 238, 
 295 
 
 Obstetric frog, 270 
 
 Occasional parthenogenesis, 186 
 
 Oellacher, parthenogenesis in hens' 
 
 eggs. 185 
 Offspring, love of, 291 
 Offspring of parthenogenesis, 191- 
 
 192 
 Oken, 318 
 
 Onion with bulbils, 303 
 Ophrydium, 95 
 
34° 
 
 itabEx. 
 
 Organs auxiliary to impregnation, 
 67, 68 
 
 Organic immortality, 275 
 
 Orgyia, 5 
 
 Orthonectids, 274 
 
 Oudemans, J. Th., on castration, 24 
 
 Oviparous animals, 264 
 
 Oviparous mammals, 264 
 
 Ovipositors, 68 
 
 Ovists, 89, 169 
 
 Ovo-viviparous animals, 264 
 
 Ovulation, 257 
 
 Ovum or egg-cell, 104-114; matura- 
 tion of, 112; liberation of, 257 
 
 Ovum theory, 88 
 
 Owen, alternation of generations, 
 213; on residual spermatic force, 
 224, 225 
 
 Pedogenesis or juvenile partheno- 
 genesis, 187 
 
 Pairing, early forms of, 2S4 
 
 Paper nautilus, brood shell of fe- 
 male, 271 
 
 Paramecium, conjugation of, 158 
 
 Parental care, 291-295 
 
 Parthenogenesis, 183 194 
 
 Parthenogenetic ova, peculiarity of, 
 
 193 
 
 Partial hermaphroditism, 72 
 
 Parturition, 263 
 
 Pearson, Karl, forms of sexual re- 
 lation, 261 ; variability, 290; 
 genetic selection, 325 
 
 Peckham on spiders, 1 4 
 
 Penis, 263 
 
 Penycuik experiments, 165 
 
 Perez, parthenogenesis in silk- 
 moths, 185 
 
 Pfluger on polyspermy, 36, 45 
 
 Physiological theories of fertilisa- 
 tion, 172 
 
 Pieri, 174 
 
 " Pigeon's milk,'' 267 
 
 Pigments as waste products, 25 
 
 Placenta, 264 
 
 Planarians, 238 
 
 Planta, A. von, diet of bees, 47 
 
 Plants, determination of sex in, 52 ; 
 conjugation in, 160, 161 ; fertili- 
 sation, 146-150; parthenogenesis 
 in, 190; alternation of genera- 
 
 tions in, 223 ; asexual reproduc- 
 tion of, 203 ; origin of sex among, 
 
 136 
 
 Plasmodium, 159 
 
 Plastogamy, 160, 162 
 
 Platner on parthenogenetic ova, 194 
 
 Ploss on embryonic hermaphrodi- 
 tism, 35 
 
 Polar globules, 112, 113 
 
 Pollen grain, 243 
 
 Polyspermy, 195 
 
 Polyzoa, building in, 208 
 
 Population question, summary of, 
 305 ; development of theory of, 
 306 
 
 Precocious reproduction, 258, 259 
 
 Preformation theory, 89 
 
 Prepotency, 325 
 
 Prevost and Dumas, 92 
 
 Primary sexual characters, 3 
 
 Protamine, 124 
 
 Proterospongia, 94 
 
 Protococcus, reproduction in, 136 
 
 Protomyxa, 129 
 
 Protoplasm, metabolism of, 92-94, 
 131, 132, 237, 238; infertilisation, 
 171, 172; in cell-division, 235, 
 236 
 
 Protoplasmic movement, the, 93 
 
 Protozoa contrasted with metazoa, 
 95, 96 ; illustrating cell-phases, 
 : 3 2 > J 33 5 fertilisation in, 157, 
 158; conjugation in, 162; im- 
 mortality of, 275-280 ; alternation 
 of generations in, 221 
 
 Psychidse, sex differences in, 27 
 
 Puberty, 255 
 
 Purkinje, 92 
 
 Pycnogonid, male carrying the 
 young, 293 
 
 Rate of increase, 299 
 
 Rate of reproduction, 299, 300 
 
 Rath, Dr O. vom, on telegony, 
 
 166; experiments in breeding, 
 
 180 
 Rauber on body and reproductive 
 
 cells, 100 
 Reaumur on aphides, 183 
 Regeneration, 201, 202 
 Reibmayr, inbreeding, 180 
 
INDEX. 
 
 341 
 
 Reproduction, different modes of, 
 145; sexual, 145-166; growth 
 and, 233, 244; theory of, 245- 
 251 ; in relation to environment, 
 249 251 ; nemesis of, 272-275 
 
 Reproductive cells, 87-124 passim; 
 v. somatic cells, 99, 100, 278- 
 280 
 
 Reptiles, amatory emotions of, 284, 
 285 
 
 Rhumbler, evolution of fertilisation 
 processes, 162 
 
 Rhythms of life, 238, 239 
 
 Richarz, 39 
 
 Rolph on antlers, 26 ; sex in wasps, 
 48 ; theory of sex, 1 30 ; theory of 
 fertilisation, 172, 173 ; partheno- 
 genetic ova, 192, 195 
 
 Romanes, G. J., 140; on physio- 
 logical selection, 324 
 
 Rorig on castration, 24 : antlers and 
 reproductive organs, 256 
 
 Rotifers, parthenogenesis in, 1 87 ; 
 male, 275 
 
 Sabatier, theory of polarities, 1 14 
 
 Sachs, origin of fertilisation, 161, 
 162, 172 
 
 Sadler on determination of sex, 38 
 
 Salensky on primitive metazoa, 329 
 
 Salmon, 259 
 
 Salpa, alternation of generations in, 
 2t6 
 
 Schaffer, parthenogenesis in crusta- 
 ceans, 185 
 
 Schaudinn on reduction division, 
 
 "3 
 
 Schenk's theory, 52 
 
 Schizogamy, 209 
 
 Schlechter on sex in horses, 54 
 
 Schleiden, 92, 14S 
 
 Schultze, two kinds of ova, 36 
 
 Schwann, 92 
 
 Sea-anemones, asexual multiplica- 
 tion in, 207 
 
 Sea-horse, parental care in male, 
 271, 272 
 
 Seasonal parthenogenesis, 187 
 
 Secondary sexual characters, 3-8, 
 r6-27 
 
 Self-fertilisation, 79 
 
 Sellheim on castration, 24, 256 
 
 Seminal vesicles, 255 
 
 Sex, determination of, 34-57 ! 
 theory of, 130-140; origin of, 
 135; special physiology of, 253- 
 282 
 
 Sexes, differences between the, in 
 general habit, 16-19; in size, 19; 
 in other characters, 21 ; intel- 
 lectual and emotional differences, 
 286 
 
 Sex-elements, the ultimate, 87-102, 
 general origin of, 97 ; early separa- 
 tion of, 98 
 
 Sexual attraction, 285 
 
 Sexual diathesis, 23 
 
 Sexual maturation, 255 
 
 Sexual organs and tissues, 63-69 
 
 Sexual reproduction, 145-158 
 
 Sexual selection, 8-14 ; its limits as 
 an explanation, 27 
 
 Sexual union, 245 247 
 
 Shufeldt on sex in birds, 36 
 
 Siebold, Von, experiments on 
 wasps, 48; on sperms, 118; 
 parthenogenesis, 184, 186, 188 
 
 Silkmoth, parthenogenesis in, 185 
 
 Silvestri on sperms, 1 19 
 
 Simon, origin of sex, 173 ; par- 
 thenogenetic ova, 193 
 
 Siphonophore colony, 207 
 
 Skin-outgrowths, 26 
 
 Snail, reproductive specialisation 
 in, 68 
 
 Spallanzani on fertilisation, .170 
 
 Spencer, theory of growth, 234 ; 
 laws of multiplication, 30J ; 
 population question, 299 ; factors 
 in evolution, 320 
 
 Spermatogenesis, 122 
 
 Spermatic animalcule, 118 
 
 Spermatozoon, 117-128; discovery 
 of, 117; structure of, 118-120; 
 physiology, 120 ; contrasted with 
 ovum, 123; influence of, 317 
 
 Spermatozoa of crayfish, 119 
 
 Spiculum amoris, 263 
 
 Spiegelberg, 41 
 
 Spirogyra, conjugation in, 160 
 
 Sponge, hermaphroditism in, 76 ; 
 colony of, 201 ; asexual multipli- 
 cation in, 205 ; alternation of 
 generations in Spongilla, 222 
 
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