CORNELL UNIVERSITY LIBRARY FROM 'the Estate of S^H.Gage DEPARTMENT of ZOOLOGY Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003079013 THE CONTEMPORARY SCIENCE SERIES. Edited by HAVELOCK ELLIS. THE EVOLUTION OF SEX. THE Evolution of Sex. BY Professor PATRICK GEDDES AND J. ARTHUR THOMSON. With 104 Illustrations. SCRIBNER & WELFORD, 743 & 745 BROADWAY, NEW YORK. '^^ . UK!|VU>.'.|1 Y ml 2, 6033 PREFACE. 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 eyolution. Hence this little book has the difficult task ) of inviting the criticism of the biological student, although i 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. VI 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 vety 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. t 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. t " Darwinism." Lond. 1889. PREFACE. VH 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 Hterature ; 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. ERRATUM. On page 146, instead of present title Qf figure, read :- ' The Process of Fertilisation. — After Boveri." CONTENTS. BOOK I.— MALE AND FEMALE. CHAPTER I. PAGE The Sexes and Sexual Selection 3-15 § I. Primary and secondary sexual characters. § 2. Illustrations from Darwin. § 3. Darwin's explanation by sexual selection. § 4. Criticisms of sexual selection — (fl.) Wallace. {//.) Brooks. {c.) St George Mivart. (rt".) Others. CHAPTER II. The Sexes, and Criticism of Sexual Selection 16-31 § I. Search for a broader basis. § 2. Differences in general habit, &c. Males active, females passive. § 3. Differences in size. Males smaller, females larger. Pigmies and exceptions. § 4. Secondary differences in colour, skin, &c. Males katabolic, females anabolic. § 5. Sexual selection : Its limits as an explanation. Postulate of extreme esthetic sensi- tiveness. Darwin and Wallace combined and supplemented. Sexual selection a minor accelerant, natural selection a retarding action, on constitutional differentiation. X CONTENTS. CHAPTER III. PAGE The Determination of Sex (Hypotheses and Obser- vations) 32-40 § I. The period at which sex is settled. Floss, Sutton, Laulanie, &c. § 2. Over five hundred theories suggested — Theological. Metaphysical. Statistical and hypothetical. Experimental. (Chap. IV.) § 3. Theory of male and female ova requires analysis. § 4. Theory of " polyspermy," or multiple fertilisation, dismissed. § 5. The theory of age of elements allowed. Thury, ■ Hensen, &c. § 6. Theory of parental age of secondary moment. Hofacker and Sadler. § 7. Theories of " comparative vigour," &c., require analysis. § 8. Theory of Starkweather, — many factors combined under " superiority." § 9. Darwin's position. § 10. Diising's synthetic treatment, and theory of self-regu- lation of numbers. § II. The sexes of twins. CHAPTER IV. The Determination of Sex (Constructive Treatment) 41-S4 § I. Nutrition as a factor determining sex. Favourable nutrition tends to females. (a. ) Yung's tadpoles. {/'. ) Cases of bees. (c.) Von Siebold's observations, (rf. ) Case of aphides. (e.) Caterpillars. (/.) Crustaceans. iff.) Mammals. Qi.) Human species. (i.) Plants. § 2. Temperature as a factor. Favourable conditions tend to females. § 3. Summary of factors : — {a. ) Nutrition, age, &c. , of parents affecting — (d.) Condition of sex celh, followed by — (c. ) Environment of embryo. § 4. General conclusion : — .Anabolic conditions favour pro- duction of females, katabolic conditions males. § 5. Hence corroboration of conclusion of Chap. II., that females were preponderatingly anabolic, males katabolic. § 6. Note on Weismann's theory of heredity. CONTENTS. BOOK II.— ANALYSIS OF SEX— ORGANS, TISSUES, CELLS. CHAPTER V. PAGE Sexual Organs AND Tissues S7-64 § I . Essential sexual organs of animals. § 2. Associated ducts. § 3. Origin of yolk-glands, &c. § 4. Organs auxiliary to impregnation. § S- Egg-laying organs. § 6. Brood-pouches. CHAPTER VI. Hermaphroditism 65-80 § I. Definition of hermaphroditism ; its varied forms. § 2. Embryonic hermaphroditism. Ploss, Laulanie, Sutton. § 3. Casual or abnormal hermaphroditism, from jelly-fish to mammal. § 4. Partial hermaphroditism, from butterflies to birds. § 5- Normal adult hermaphroditism, from sponges to toads. § 6. Degrees of normal hermaphroditism. § 7. Self-fertilisation and its preventives. § 8. Complemental males — cirripedes and Myzostomata. § 9. Conditions of hermaphroditism ; its association with passivity and parasitism. § 10. Origin of hermaphroditism ; the primitive condition ; persistence and reversion. CHAPTER Vn. The Sex-Elements (General and Historical) 81-96 § I. The ovum-theory. § 2. The history of embryology, "evolution" and " epigenesis. " Harvey's epigenesis and prevision of ovum- theory. Malpighi and early observers. Preformation school; "evolution" according to Haller, Bonnet, and Buffon ; ovists and ani- malculists. Wolff's demonstration of epigenesis. Wolff's successors. § 3. The cell-theory. § 4. The protoplasmic movement. § 5. Protozoa contrasted with Metazoa ; the making of the "body." § 6. General origin of the sex-cells in sponges. ,, ,, ,, ,, coelenterates. ,, ,, ,, ,, other Metazoa. § 7. Early separation of the sex-cells in a minority of cases. CONTENTS. PAGE §8. "Body" versus reproductive cells, and the continuity of the latter. Owen. Hzeckel. Rauber. Brooks. Jager. Galton. Nussbaum. § 9. Weismann's theory of the continuity of the germ-plasma. CHAPTER VIII. The Egg-Cell or Ovum 97-108 §1. Structure of ovum — Cell-substance and protoplasm. Nucleus and chromatin. §2. Growth of ovum — Transition from amoeboid to encysted phase. § 3. The yolk- Its threefold mode of origin. Its diifuse, polar, or central disposition. Resulting influence on segmentation. 4. Composite ova. § 5. Egg-envelopes — (ii.) From ovum itself. (b. ) From surrounding cells. (f.) From special glands. § 6. Birds' eggs — Concrete illustration of facts and problems. § 7. Chemistry of the ovum — Its capital of anastates. § 8. Maturation of ovum — Occurrence, formation, history of polar globules ; parthenogenetic ova. § 9. Theories of polar globules — 1. Minot, Balfour, Van Beneden, &c. 2. Biitschli, Hertwig, Boveri, &c. 3. Weismann. CHAPTER IX. The Male-Cell OR Sperm 109-116 § I. General contrast between sperm and ovum — An index to contrast between male and female. § 2. History of discovery — (a.) Hamm and Leuwenhoek. (h.) Animalculists. ' (c.) Classed as Entozoa or parasites. (d.) Kblliker's demonstration of cellular origin. CONTENTS. § 3. Structure of sperm — _ " Head," " tail," "middle portion," &c. § 4. Physiology of sperm — Locomotor energy and persistent vitality. §5. Origin of sperm — Theory of spermatogenesis. § 6. Further comparison of sperm and ovum — Processes comparable with formation of polar globules. § 7. Chemistry of the sperm. CHAPTER X. Theory of Sex : Its Nature and Origin - 117-134 §. I. Suggested theories of male and female — Rolph. Minot. Brooks. § 2. Nature of sex — seen in Sex-cells. The cell-cycle. Protoplasmic interpretation. § 3. Problem of origin of sex. § 4. Incipient sex among plants. § 5. Incipient sex among animals. 6. Corroborative illustrations. 7. General conclusions from foregoing chapters. BOOK III.— PROCESSES OF REPRODUCTION. CHAPTER XI. Sexual Reproduction 137-156 §1. Different modes of reproduction. § 2. Facts involved in sexual reproduction. § 3. Fertilisation in plants — From Sprengel to Strasburger. § 4. Fertilisation in higher animals — From Martin Barry and Biitschli, to Van Bene- den and Boveri. § 5. Fertilisation in Protozoa. § 6. Origin of fertilisation — {a.) Plasmodium. (*.) Multiple conjugation. (c.) Ordinary conjugation. ((/.) Union of incipiently dimorphic cells. (e. ) Fertilisation by differentiated sex-cells. § 7. Hybridisation in animals and plants. XIV CONTENTS. CHAPTER XII. PAGE Theory of Fertilisation 157-168 § I. Old theories — [a) Ovists, {d) animalculists, (c) the " aura sejuhialis. " § 2. Modern morphological theories — (a.) Nuclei all - important. Herlwig, Stras- burger, &c. (^.) Cell-substance also important. Nussbaum, Boveri, &c. § 3. Modern physiological theories — Sachs, De Bary, Marshall Ward, &c. Cienkowski and Rolph. Weismann's view. Critique and statement of present theory. § 4. Use of fertilisation to the species — (a.) Rejuvenescence — Van Beneden and Biitschli. Gallon and Hensen. Weismann's critique. {b.) The observations of Maupas. {c.) A source of variation. Brooks and Weismann. CHAPTER XIII. Degenerate Sexual Reproduction or Parthenogenesis 169-187 § I . History of discovery. § 2. Degrees of parthenogenesis — Artificial, pathological, occasional, partial, sea- sonal, total. § 3. Occurrence in animals — Rotifers, crustaceans, insects. § 4. Occurrence in plants — Phanerogams and fungi, § 5. The offspring of parthenogenesis. § 6. Effects on the species. § 7. Peculiarities of parthenogenetic ova — Weismann's discovery. § 8. Theory of parthenogenesis — Minot and Balfour. Rolph and Strasburger. Weismann. The present. § 9. Origin of parthenogenesis. §10. Case of bees. CHAPTER XIV. Asexual Reproduction 188-199 § I. Artificial division. § 2. Regeneration. § 3. Degrees of asexual reproduction. § 4. Asexual reproduction in plants and animals. CONTENTS. XV CHAPTER XV. PACE Alternation of Generations 200-215 §1. History of discovery. § 2. Rhythm between sexual and asexual reproduction. § 3. Alternation between sexual and degenerate sexual repro- duction. § 4. Combination of both these alternations. § 5. Alternation of juvenile parthenogenetic reproduction with the adult sexual process. § 6. Alternation of parthenogenesis and ordinary sexual reproduction. § 7. Alternation of different sexual generations. § 8. Occurrence of these alternations in animals. § 9. Occurrence of alternations in plants. § 10. The problem of heredity in alternating generations. § II. Hints as to the rationale of alternation. § 12. Origin of alternation of generations. BOOK IV.— THEORY OF REPRODUCTION. CHAPTER XVI. Growth and Reproduction 219-231 § I. Facts of growth. § 2. Spencer's analysis. § 3. Cell-division. § 4. Protoplasmic restatement. § 5. Antithesis between growth and reproduction. § 6. The contrast in the individual — (a. ) In distribution of organs. {i-l In the periods of life. § 7. The contrast between asexual and sexual reproduction. CHAPTER XVII. Theory of Reproduction — continued 232-238 § I. The, essential fact in reproduction. §2. The beginning of reproduction. § 3. Cell-division. § 4. Gradations from asexual severance to liberation of sex- cells. § 5. The close connection between reproduction and death. § 6. Reproduction as influenced by the environment. § 7. General conclusion. XVI CONTENTS. CHAPTER XVIII. PAGE Special Physiology of Sex and Reproduction 239-263 § I. The continuity of the germ-plasma. § 2. Sexual maturity. § 3. Menstruation. § 4. Sexual union. § 5. Parturition. § 6. Early nutriiion. § 7. Lactation. §• 8. Other secretions. § 9. Incubation. §10. Nemesis of reproduction. § 1 1. Love and death, or organic immortality. CHAPTER XIX. Psychological and Ethical Aspects 264-282 § I . Common ground between animals and men. § 2. The love of mates. § 3. Sexual attraction. § 4. Intellectual and emotional differences between the sexes. § 5. Love for offspring. § 6. Criminal habit of the cuckoo. § 7. Egoism and altruism. CHAPTER XX. Laws of Multiplication 283-299 § I. 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 300-315 § I. General history of evolution. § 2. The reproductive factor so far as hitherto recognised. § 3- Suggested lines of further construction. BOOK I. THE SEXES AND SEXUAL SELECTION. THE EVOLUTION OF SEX. CHAPTER I. The Sexes and Sexual Selection. THAT all higher animals are represented by distinct male 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, of the sexes often disappears; yet even naturalists have sometimes mistaken for different species, what were afterwards recognised to be but the male and female of a single form. § T.. 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 sexes, are included under the title oi primary sexual characters. Of less real importance, though often much more striking, are the numerous distinctions in size, colour, skin, skeleton, and Male and Female Bird of Paradise {Paradisea jninor). — From 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. S some cases at least to be truly part and parcel with the male or female constitution, they are only of secondary importance in the reproductive process. The beard of man and the mane of the lion, the antlers of stags and the tusks of elephants, the gorgeous plumage of the peacock or of the bird of paradise, are familiar examples of secondary sexual characters in males. Nor are the females lacking in special characteristics, which serve as indices of their true nature. Large size is one of the commonest of these ; while in some few cases the excellencies of colour, and other adornments, are possessed by the females rather 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, and, after a fresh survey, to the explanation by which we propose to supplement his theory. § 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, darker colours, 'and sometimes in the Winged Male and Wingless Female of a certain Moth {prgyiti antiqiia). — 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 THE EVOLUTION OF SEX. more brilliant than the females ; and many male beetles fight savagely for the possession of their mates. Passing to backboned animals, we find that among fishes the males are frequently distinguished by bright colours and ornamental appendages, as well as by structural adaptations for combat. Thus the "gemmeous dragonet " {Callionymus lyrd) is flushed with gorgeous colour, in great contrast to the " sordid " female, and is further adorned by a graceful elongation of the dorsal fin. In many cases, as in the sea-scorpion {Coitus scorpius), or in the stickleback {Gasterosteus), it is only at the 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 newts, 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 chamaeleons. 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 proud possessors before the eyes of their desired mates, with mingled emotions of eager love and pompous 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 differences between the magnificent male birds of paradise and their sober mates, between the peacock with his hundred THE SEXES AND SEXUAt SELECTION. 7 eyes and the plain peahen, between the musical powers of male and female songsters, are very familiar facts. Or again, the combs and "gills" of cocks, the "wattles" of turkey-cocks, the immense top-knot of the male umbrella-bird {Cephalopterus ornatus), the throat-pouch of the bustard,— illustrate another Male and Female Blackcocks. 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 sharply contrasted, and there are minor differences, in voice and the like, which cannot \)e ignored. Of weapons, the larger canine teeth of many male animals, such as boars ; the special tusks of, for instance, the elephant and narwhal ; the antlers of stags. The development of antlers in the successive years of a s'ag's life, or in the general history of stags. — From Cams Sterne. all but exclusively restricted to the combative sex ; the horns of antelopes, goats and sheep, oxen and the like,^which at least predominate in the males, — are well-known illustrations. The manes of male lions, bisons, and baboons ; the beards of 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 more 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. Danviris 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 doubt that female birds, by selecting during thousands of generations the most melodious or beautiful males, according . to their s tand ard of beauty, might produce a marked effect." ..." To sum up oil 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 lO 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." Had Darwin seen another inter- pretation of the facts, he would thus doubtless have given it frank recognition. § 4. Criticisms of Darwin's Explanation. — The above explanation may be summed up in a single "sentence, — a casual variation, advantageous to its possessor (usually a male) in courtship and reproduction, becomes established and perfected 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 supposed 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, may be grouped in four grades : — ( i ) Some, who allow great importance to both natural and sexual selection, are dissatisfied with the adequacy of Darwin's analysis, and seek some deeper basis for the variations so largely confined to the male sex. The position occupied by Brooks will be sketched below. (2) Others would explain the facts on the more general theory of natural selection, allowing comparatively little import to the alleged sexual selection exercised by the female. Wallace has on this basis criticised Darwin's theory. (3) Different from either of the above is the position occupied by St George Mivart, who attaches comparatively little importance to either natural or sexual selection. (4) We have to recognise contri- butions, such as those of Mantegazza, which suggest the organic or constitutional origin of the variations in question. It is this constructive rather than destructive line of criticism which we shall ourselves seek to develop. (a) Wallace's Objection. — It is more convenient to begin with Wallace's criticism, which precedes that of Brooks's in chrono- logical 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. According to Darwin, the gayness of male birds is due to selection on the part of the females ; according to Wallace, the soberness of female birds is THE SEXES AND SEXUAL SELECTION. II 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 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 sur- roundings ; while in those of birds where the nests are domed or covered, the plumage is gay in both sexes. At the same time, Wallace allows original importance to sexual selection on both sides in evolving bright colours and the like. We need not repeat Darwin's reply to Wallace's objections, as the reader will at once recognise considerable force in each position.* {b) 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. * Since the above was written, Mr Wallace's book on "Darwinism" has been published, in which the author proceeds yet 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 development," and such that it is "unnecessary to call to our aid so hypo- thetical a cause as the cumulative action of female preference." Or again, " if ornament is the natural product and direct outcome of superabundant h&fth and vigour, then no other mode of selection is needed to account fer the presence of such ornament." These conclusions are not only piiportant in relation to Darwin's theory, but obviously open up the pos- sibility of interpreting not only these as the "natural product and direct outcome of constitutional conditions " (see chap. xxi. ) but many other features 'also. This consideration, however, is fraught with serious consequences to Mr Wallace's main thesis. 12 THE EVOLUTION OF 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 males is due to their struggling with rivals, and to their selection by the females, but " I do not believe that this goes to the root of the matter." The study of domesticated pigeons, for instance, shows that "something within the animal determines 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 variations 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 tha 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- THE SEXES AND SEXUAL SELECTION. 1 3 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 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 finds an explanation of the greater diversity of the males in his theory 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 variations have by hypothesis most likelihood of being transmitted. Full consideration of this hypothesis would involve much discussion of the problems of inheritance, which will form the subject of a forthcoming volume ; but the general conclusion 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 of heredity. To refer preponderant male variability back to a power, ascribed to the male reproductive cells, of collecting and storing up assumed gemmules, is at best but a half-way analysis. Both the above critics are at one with Darwin on essential points. Though Wallace would explain by natural selection what Darwin explained by sexual selection, he does not deny the importance of the latter in many cases. Brooks, again, emphasises a deeper factor, without doubting the general truth of Darwin's account of the process. Different from both these positions is that {c) occupied by St George Mivart, who looks for some deeper reason than either Darwin or Wallace suggest. The entire theory of sexual selection appears to him an unverified hypothesis, only acquiring plausibility when sup- ported by quite a series of subsidiary suppositions. He submits 14 THE EVOLUTION OF SEX. 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 an internal force. The vague suggestions of Mantegazza and others are only of importance as indications of progress towards a- fundamental explanation. An obvious objection to the theory of sexual selection, that has been urged by many, 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 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. 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 hypothesis of sexual selection assumes the preservation and perfection of varia,tions, advantageous in courtship or in battles with rivals. 4. Wallace maintains that the females have been protectively retarded by natural selection ; Brooks, that the males predominate in power of transmitting variations, and are therefore more divergent; while Mivart demands a deeper analysis than is afforded by either sexual or natural selec- tion, — such a physiological rationale being 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. 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. MiVART (St George) — Lessons from Nature. London, 1876. Wallace (A. R.) — Contributions to the Theory of Natural Selection. London, 1 87 1. 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. To gain a firmer and broader foundation on which to base a theory of the differences between the sexes, it is ne- cessary to take another review of the facts of the case. Instead of considering the differences as they are expressed 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 completeness. ^ .^-y hy I ■"' . '^ Male and Female Coccus Insects, a, part of a cactus plant with the excrescences due to coccus insects ; i, male ; c, female. § 2. General Habit. — Let us begin with an extreme yet well-known case. The female cochineal insect, laden with reserve products in the form of the well-known pigment, 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 na^tural 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 femalesonly 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 (Strepsipterd) 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 pow:er 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 B 'i Female Ckondr acanthus^ a parasitic Crustacean, with pigmy male {a) attached just above the origin of the long egg- sacs (^) of the female. — From Claus. of the animal 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 diiference 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(Sarcij^pf^ia ^eKeirans); 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. (6) The same antithesis is seen, when we contrast, as we shall afterwards THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 19 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 20 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 antithesis 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. In other cases, though present, they entirely fail to accomplish their proper function of fertilisation, and, as parthenogenesis obtains, are not only minute, but useless. In a curious green marine Relative sizes of a male and feftiale Rotifer {Hyaatina sentd). — From Leunis. 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 viviparous coccus insect (Lecanium hesperidum), where the males are very degenerate, small, blind, and wingless. In spite of this condition, we should indeed think because of it, they are very male, for even the larvae, while still within the mother, havebeen shown to contain fully- developed spermatozoa. It would be unfair to argue from such an extreme case as that of Bonellia alone, but there is no doubt that up to the THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 21 level of amphibians at least the females are generally the larger. This then must be taken in connection with the conclusion of the previous paragraph. A sluggish conservative habit of body tends to an increase of size ; lavish expenditure of energy keeps down the accumulation of storage. Corroborative evidence will be afterwards forthcoming, as we contrast (a) the large and small spores which mark the beginnings of sex differences, or (6) the relatively large female cell or egg with the microscopic male cell or spermatozoon. Figure of the female Bonellia (from Atlas of Naples Aquarium), with its parasitic pigmy male enlarged. 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 and pregnancy. Further- more, we ,must recognise the strengthening influence of the 2 2 THE EVOLUTION OF SEX. combats between males, and the effect produced on the accumulative 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- 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. This path will appear less vague if reverted to after some of the succeeding chapters have been grasped. The point of view is simple enough. The agility of males is not . a special adaptation 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, are not, and cannot be (except teleologically), explained by sexual selection, but in origin and continued development 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.* 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 cliaracters 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 * That Mr Wallace has adopted the same explanation of the different sexual characters in his new book, has been already pointed out (see p. 11, note). THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 23 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 tend to remain undeveloped. Thus, as Darwin notes, stags never renew their 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 excep- tion 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 Sacculina 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 an approximation to the female form of appendage has been observed. Lastly, in aged females, which have ceased to be functional in reproduction, the minor peculiarities 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- 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, and others, our knowledge of the physiology of many of the pigments is still very scanty. Yet in many cases, alike among 24 THE EVOLUTION OF SEX. 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 commonly a natural expression of the male constitution. For the red pigment so abundant in the female cochineal insect, which appears to be of the nature of a reserve and not a waste product, and for similar occurrences, due exception must be made. 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 external objects, most conveniently upon its nest. Thus the curious weaving instinct does not demand or find rationale in the cumulative action of natural selection upon an inexplicable variation, and is traced back to a pathological and mechanical origin in the emphatic maleness of the organism. 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. THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 25 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 peri- odically 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." Male (c), Worker {b), and Queen (a) Ant. — From Chamber^ s Encyc, after Lubbock. 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 repects 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, in conclusion, as a literally illumined index of the contrasted physiology of the sexes. § 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 26 THE EVOLUTION OF SEX. 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 live at a loss, are more katabolic, — dis- ruptive changes tending to preponderate in the sum of changes in their living matter or protoplasm. The females, on the other hand, live at a profit, are 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 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 Mr W. E. Fothergill) : — -^ SUM OF FUNCTIONS. Nutrition. Reproduction. Anabolism. Katabolism. Female. Male. Here the sum-total of the functions are divided into nutritive and reproductive, the former into anabolic and THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 27 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, — the male reproduction is associated with preponderating katabolism, and the female with relative anabol- ism. In terms of this thesis, therefore, both primary and second- ary_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 com- bative males represent a role filled in the larger case by the foster- ing or eliminating action of the environment. As a special case of natural selection, Darwin's minor theory is open to the objection of being teleological, i.e., of accounting for structures in terms of a final advantage. Itis quite open to theological critic to urge, as a few have done, 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 fundamental physiological import, of the structures, must be explained 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, as is mentioned in the preceding historical chapter. One detailed objection which seems serious may also be urged. The evolution of coloured markings by selective pre- ference carries with it the- postulate of a certain level of aesthetic 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 undeveloped. It is true, doubtless, that * The reader whose physiological studies may not have been so recent as to familiarise him with that conception 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 Dr Michael Foster's article, " Physiology," in the Encyclopedia Britannica, or with Dr Burdon Sanderson's Presidential Address to Section D, British Association, 1889. The essential conception will, however, become clearer as we proceed (see pp. 89, 124). 28 THE EVOLUTION OF SEX. both insects and birds have so far and increasingly become educated in such sensitiveness; but when we consider the com- plexity of the markings 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 aesthetic development exhibited by no human being .without both special Eesthetic acuteness and special training. Moreover, the butter- fly, 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 aesthetic 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 unmistakable hints of real awakening of the aesthetic sense which are exhibited by the Australian bowerbird or by the common jackdaw in its fondness for bright objects, how very rude is this taste compared with the critical examination of infinitesimal variations of plumage on which Darwin relies. Is not, therefore, his essential supposition too glaringly anthropomorphic? 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 ^neas group of the genus Papilio, Darwin notes how there are frequent gradations in the amount of dif- ference 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 Eesthetic perception, this of an exquisitely subtle kind however, and without proportionate cerebral en- largement. 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 THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 29 or less cumbrous additional hypothesis of inheritance and so on to explain them, are intelligible enough if we regard them as-4^ illustrationsofincreasingkatabolism throughout a series of species. ] The third set may be supposed to be more male or 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 this particular case.* It is necessary once more to turn to the contrast between the positions of Darwin and Wallace. According to Darwin, sexual selection, for love's sake, lias accelerated the males into gay colouring ; according to Wallace, natural selection, for safety's sake, has retarded the females (birds or butterflies) and kept them inconspicuously plain. It is no longer difficult to establish a compromise. The true view seems to be, that both sexes have differentiated towards their respective goals, but the males faster, because so katabolic; the limits are constantly being fixed by natural selection in Wallace's cases, and as constantly ' increased by sexual selection in Darwin's. There is, in fact, no i reason why both should not be admitted as minor factors ; but T the greater part of the explanation is to be found in the view;,,' above stated, viz., in the physiological constitution of males and ^ females themselves. In short, the present position allows some truth in both these conclusions, but regards gay colouring as ,: the expression of the predominantly katabolic or male sex, and ^ quiet plainness as equally natural to the predominantly anabolic '- females. On this view, too, we are able to restate part of the position emphasised by Brooks. The greater variability of the males is indeed natural, if they be the more katabolic sex. In preponderant katabolism, the combinations and permutations of molecules which constitute variation, are necessarily more probable than in the quiescent, passive, or anabolic females. No special theory of heredity is required, — the males transmit the majority of variations, because they have most to transmit. At a later stage something more will be said of natural selection, and its limits as an explanation of facts. But it * For a discussion of the progressive development of colouring and markings, whether in butterflies or mammals, the reader may be referred O to the works of Professor Bimer, and especially to his forthcoming 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. 30 THE EVOLUtlON OF SEX. is here desirable to emphasise, that just as we adinit 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. It should also be noted, that both the retarding action of natural selection, and the accelerant action of sexual selection, become of increasing importance as we ascend the series. And thus, indeed, we are impelled towards a heresy which, as we shall see later, has bearings against the theory of natural selection, which overpass the limits of our present theme. Postscript.— Vli T. W. Kulton, Naturalist to the Scottish Fishery Board, has been good enough to furnish us with some of his results on the size and numerical proportions of male and female fishes. (l.) The females are usually considerably more numerous than the males, and never less numerous except in the angler and the cat-fish. The proportions of females to males among flat-fi.shes 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. The subject is being worked up by the above- named naturalist, and cannot fail to yield very valuable results. THE SEXES, AND CRITICISM 'OF SEXUAL SELECTION. 3 1 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 reproductive 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, are to be interpreted as expressions of the katabolic predominance in the con- stitution of males, as opposed to the anabolic preponderance of the females. 5. Sexual selection, as an explanation of secondary sexual characters, is limited, by being teleological rather than etiological, 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, con- stitutional or organismal origin in the katabolic or anabolic diathesis which preponderates in males and females respectively. LITERATURE. Brooks, Darwin, Mivart, Wallace. — As before. ElMER, G. H. T. — Die Enstehung der Arten .auf Grund von Vererben erworbener Eigenschaften, nach den Gesetzen organischen Wachsens. Jena, 1S88. Geddes, p. — Articles Reproduction, Sex, Variation and Selection. / Encycl. Brit. Also on the Theory of Growth, Reproduction, Sex, and Heredity. Proc. Roy. Soc. Edin. 1885-6. RoLPH, W. H. — Biologische Probleme. Leipzig, 1S84. Weismann, a. — Studies in the Theory of Descent (Meldola's Transla- tion). London, 1880-82. Wallace, A. R. — Darwinism. London, i88g. 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 Detei-mined. — 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 ; and when 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, so that it is quite possible for a germ-cell to have its future fate more than once changed. The constitution' of the mother, the nutrition of the ova, the constitution of the father, the state of thf; determination of sex. 33 the male element when 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 Laulani^ 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 (l) 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 einbryonic 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 concluded, 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 backboneless animals, that we can speak of prolonged neutrality of sex, or eniBFyonic hermaphro- ditism. § 2. Answers to the Question : What Determines Sex? — To the question what settles 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 so 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- 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. That the final physiological ex- planation is, and must be, in terms of protoplasmic metabolism, we must again, however, remind the reader (see p. 27, note). c 34 THE EVOLUTION OF SEX. § 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 that what is virtually decided at this early stage may be counteracted by later influences of an opposite character. 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 forthcoming. § 4. Numerous authors have attached great importance to the process of fertilisation as a determinant of the sex. One of the most crude positoins 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, Pfliiger, Hertwig, and others, that " polyspermy," or the entrance of more than one sperm, is extremely rare, is in fact generally impossible, and when it does in rare conditions occur, indicates a pathological condition of the egg-cell, and tends to produce abnormalities. Pfliiger 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 male 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 Diising (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 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." § 6. Age 0/ Parents.— Hoiacker (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 a fortiori if the male parent be the younger, female offspring appear in increasing majority. This conclusion, generally known as Hofaqker's and Sadler's law, has received both confirmation and perplexing contradiction. It has been confirmed by Gbhlert, Boulenger, Legoyt, and others, and by THE DETERMINATION OF SEX. 35 some breeders of stock and birds, but is denied by other practical authorities, and directly contradicted by the recent statistics of Stieda, from Alsace-Lorraine, and of Bemer, from Scandinavia. 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 ag:e. 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 90-6 107.5 Sadler 2,068 England 121.4 94.8 86.5 1 14. 7 Gohlert 4,584 108.2 93- S 82.6 105- 3 Legoyt 52,3" Paris 104,49 102.14 97- S 10Z.97 Boulenger 6,006 Calais 109.98 107.92 101.63 107.9 Noirot 4,000 Dijon 99-7 1 16.0 103-5 Breslau 8,084, Zurich 103.9 103. 1 117.6 186.6 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). 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 against 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 36 THE EVOLUTION OF SEX. Heyer : —In Leontarus domestica, according to Rumpf, the female plant may bear male blossoms before its proper female flowers. In Morus 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. 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 hypothesis 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 practicability of such inductions is, however, inevitable. § 8. Starkweather's Law 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 superiority THE DETERMINATION OF SEX. 37 of either sex. Few maintain that the sexes are essentially 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 pre- supposes a superiority and an entail in the male hne ; for Spencer, the development of woman is early arrested by pro- creative functions. In short, Darwin's man is as it were an evolved wotnan, 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 efificiency 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. Reacting from such speculations as to superiority of either sex, Starkweather firmly maintains 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 (j " 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. DarwirCs Position. — Neither in regard to the origin of 38 THE EVOLUTION OF SEX. 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 selfregula- tion of the sex-proportions, that reference to the latter is more profitable. § 10. Dusing on the Proportions of the Sexes, . and the Regulation of these. — In an important work, Diising has 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 conclusions; 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. THE DETERMINATION OF SEX. 39 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 compensa- tion, 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 seem to occur not uncommonly in the human species, and are either most markedly similar to one another or strongly dissimilar. The import of this is one of the minor problems of heredity, and cannot be here discussed, but we have to note the general fact, which holds without exception in the human species, that " true " twins are of the same sex. 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 (rt) the twins may be both female and then both normal, or (i) that the sexes may be different and normal, or (c) that bpth 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. It is now necessary; with Diising 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. 40 THE EVOLUTION OF SEX. 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 tvifo kinds of ova is still for the most part an assump- tion ; 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 Hansen 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 dismissed ; 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. 10. 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. Darwin, C. — The Descent of Man, Chap. VIII. London, 1871. The Variation of Animals and Plants under Domestication. Lond. DiJsiNG, C. — Die Regulierung des Geschlechtsverhaltnisses bei der Veirmehrung der Menschen, Thiere, und Pflanzen. Jena, 1884 ; or, Jen. Zeitsch. f. Naturw., XVII., 1883. Geddes, p. — As before. Hensen, V. — Physiologic der Zeugung. Hermann's Handbuch der Physiologic, Bd. VI., pp. 304, with references to Ploss, Schultze, &c. Leipzig, 1 88 1. His, W. — Theorien der geschlechtlichen Zeugung. Arch. f. Anthropologic. Bde. IV.-VI. Hofacker. — Ueber die Eigenschaften, welche sich bei Menschen und Thieren auf die Nachkommen vererben. Tiibingen, 1828. LAULAN16, F. — Comptes Rendus, CI., pp. 593-5. 1885. RoLPH, W. H. — As before. Roth, E. — Die Thatsachen der Vererbung (historical). Berlin, 1885. Pfluger, E. — Ueber die das Geschlecht bestimmenden Ursachen und die Geschlechts-verhaltnisse der Frosche. Arch. f. d. ges. Physiol. XXIX. Sadler. — The Law of Population. London, 1830. SCHLECHTER. — Ueber die Ursachen welche das Geschlecht bestimmen. Rev. f. Tierheilkunde. Wien, 1884. Biol. Centralblt., IV., pp. 627-9. Starkweather. — The Law of Sex. London, 1883. Stieda. — Das Sexual Verhaltniss bei Geborenen. Strasburg, 1875. Sutton, J. B. — General Pathology. London, 1886. Thury. — Ueber das Gesetz der Erzeugung der Geschlechter. Leipzig, 1863. Wappceus. — Allgemeine Bevolkerungs-Statistik. Leipzig, 1861. 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 I'ensemble constant de res alternatives de la nutrition ; c'est la nutrition consideree dans sa rdalite, em- brassde d'un coup d'oeil k 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 most 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, ip) distinct females, and (ir) 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 57 in the hundred. 42 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 especially nutritious flesh of frogs was supplied, the per- centage rose from 56 to 92. That is to say, in the last case the result of high feeding 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. (1^) 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 the 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 Commiin Hive-Bee. will turn out the possible mother of a new generation, or remain at the lower level of a 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. 43 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 13-55 20.39 4-06 55-9' It. 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 larvEe 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 44 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 Siebold's 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 vahiable 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 larvje reared from fertilised ova. The results of a series of observations may be condensed in a table : — END OF Larval period ( Pupation). Percentage of Females, No. of Females. No. of Males. 15th June ]t ;: :: :: August End of August September 14 269 340 500 100 19 66 579 136 66 Z15 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 (AssimilalionsleisUmg), 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 (Stoffwechsel) and the nutrition, the greater THE DETERMINATION OF SEX. 45 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 Hxperiments. Duration of Embryonic and Larval State. Sex. II 12 13 15 16 17 21 days 11 :; 17 » 17 „ 18 ,, =4 ,. All Males. All Males. 493 Males. 2 Females. 26s „ 2 „ 374 „ 8 „ 168 „ I ,, z ,, 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 appear to introduce females. Two Forms of a Common Plant-Louse or Aphis. — This figure equally well illustrates 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 Aphides. — 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 46 THE EVOLUTION OF SEX. somewhat in the various species, but the general facts are re- cognised 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 parthenogenetic 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-introduce males and sexual reproduction. (,' 48 THE EVOLUTION OF SEX. go to show, for some cases, that abundant moisture and nourish- ment 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. 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 or female organs. The botanical evidence, though by no means very strong, certainly corroborates the general result that good nourishment produces a preponderance of females. The contrast of the sexes in our common diaecious plants is here very instructive. Taking for instance the dog-mercury {Menurialis perennis) of any shady dell, or the day lychnis (Z. diurna), so often hardly less abundant on its sunnier slopes, experiments are still certainly wanting with regard to given plants, as to what cir- Male and Female Flowers of Pink Campion {Lychnis diurna). cumstances 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 conditions, whether of the embryo or of the seedling, are sufi&- cient 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 THE DETERMINATION OF SEX. 49 caution in such matters. A water-melon was grown in a heated glass-house, where the temperature sometimes rose on warm days to iio° 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 ])receding 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. In the human species, Diising 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, must of course be noted ; nor must it be forgotten that temperature may have its influence indirectly through the nutritive functions. § 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, with increasing penetration of analysis, may be more and more resolved into plus or minus nutrition, operating upon parent, sex elements, embryo, and in some cases larvs. {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. (b) As to the reproductive elements, a highly nourished D 5° THE EVOLUTION OF SEX. 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- 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 temperature, deficient light, moisture, and the like, are obviously such as would tend to induce a preponderance of waste over repair, — a katabolic 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 moisture, favour constructive processes, i.e., make for |n . anabolic habit, and these conditions result in the production 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 determination of sex, influences inducing katabolism tend to result in production of males, as those favouring anabolism similarly increase the probability of females. § 5. This is not all, however; the above conclusion is indeed 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 induction, that the males were forms of smaller size, more active habit, higher THE DETERMINATION OF SEX. 5 1 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 conception, 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 kata- bolism 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 pre- dominant- katabolism, and the female of equally emphatic 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 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-plasma, 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 emphasized 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. Thus an increase of calcareous matter in an animal might well be wholly of constitutional origin ; but a change to a new diet, or to a new medium, might be followed by modifications arising, in one sense, from without. But all such functional and environ- mental variations are, according td Weismann, restricted to the individual organism ; they are not transmissible. 52 THE EVOLUTION OF SEX. And why not ? This denial of the inheritance of dints from without, and of acquired habits other than constitutional, can be no mere optimism on Weismann's part. It is, he maintains, 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 changes pro- duced as above explained reacting from the "body" on the reproductive cells. 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 or muscle can, as such, be transmitted in inheritance. In short, the reproductive protoplasm must be in a sense insulated, and leads a charmed life away from external disturbance. This view, supported as it is by many authorites, is obviously of the utmost importance, both for the general theory of evolu- tion, and for such practical problems as those of the pathologist and the teacher. Its full consideration is here impossible, involving matter enough for a special treatise on heredity. The difficulty of any yea or nay lies in the relative scarcity of experimental data, in the divergence of opinion as to the pathological evidence, and very largely in the difficulty of applying our logical or anatomical distinctions to the intricate facts of nature. Thus the distinction between "acquired," and germinal or constitutional, is easily made on paper, but is difficult in actual practice ; nor is the line between a variation of the reproductive cells, along with the body, and one produced by the body, readily drawn in concrete cases. One criticism is suggested by the present chapter. The assumed insulation or separateness of the reproductive elements from the general life of the body, how far is this real ? In view of the genuine unity of the organism, a charmed life of one of the systems seems to some a "veritable physiological miracle;" and therefore we point to such a case as Yung's tadpoles, where an outside 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. THE DETERMINATION OF SEX. 53 SUMMARY. 1. Nutrition is one of the most important factors in determining sex. In illustration, note (a) the experiments of Yung, which raised the percentage of females from 56 to 92 by good feeding; (*) 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 ; [, Frog or other Amphibian. I. Fertilised ovum ; 2. Segmented ovum, a ball of cells, morula, or blastosphere ; 3. The same, after further division or in section ; 4. 'J'he 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. More 86 THE EVOLUTION OF SEX. than that, he followed the disposition of these primitive elements to the upbuilding of some of the important organs. He undoubtedly reached too far in his emphasis on the entire simplicity 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 organisation was gradually acquired by an observable process of development. (if) Wolff's Successors. — The important conclusion reached by Wolff remained for about sixty years without effect. In i8i 7, Christian Pander took up embryological research exactly where Wolff had left it, and worked out the history of the chick in more exact detail. In 1824, Prdvost 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 Graafs 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 Prdvost 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. Protoplasmic 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. 87 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 ^^ /_ — — _\ B ^ Figl. + Ground Plan of Protoplasmic Changes. this level, in fact where biology 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- 88 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, disruptic 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. The upper figure (a) 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 (see pp. 122-4). Protospongia, a colonial infufiorian, showing the difference between outer and inner cells. — From Saville Kent. § 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 start at THE ULTIMATE SEX^ELEMENTS. 89 the level of the simplest animals or Protozoa, which (with the exception of very loose colonies) remain always unicellular. The simplest organisms leave off where the higher plants and animals begin, z.«., as unit masses of living matter. I'hey corre- spond, in fact, to the reproductive cells of higher animals, and may be called, according to their predominant character, protova and protosperras. A fertilised ovum, as we have seen, pro- ceeds 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 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- Ophrydium, a colonial infusorian.— From Saville Kent. histories, they express phases in the same cell cycle which recurs in higher forms iri 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 and amceboid stage in the life-history of many forms, is a forecast of the contrast between a ciliated cell and a white blood corpuscle, between a mobile spermatozoon and a young ovum. That is to say, a predominance of the same protoplasmic processes is the common explanation of such similarities of form (see p. 121). (d) It is among the Protozoa that we must presently look, if we hope to understand the origin and import either of "male and female," or of fertilisation (see pp. 119, 128). (c) Among the loose colonies which some Protozoa form. 90 THE EVOLUTION OF SEX. and which bridge the gulf between the unicellular animals and the Metazoa, there is seen the beginning not only of the formation of a "body," but also the setting apart of special reproductive cells (see figs, on pp. 88, 89). On this point more emphasis must be laid. The ordinary Protozoon is a single cell, and forms no body. It divides indeed, and multiplies accordingly, but the products - of division go asunder, whereas in the segmentation of the ovum they remain connected. In most Protozoa, there is continual self-recuperation ; 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 justi- fiable to speak with Weismann and others of the "immortality of the Protozoa." In a certain sense too, as we shall see, it is justifiable 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. 130). 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 con- stant in higher animals. The only marked differences 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. § 6. General Origin of /he Sex-Cells. — Except in the lowest invertebrates, the sponges and coelenterates, the reproductive elements almost always arise in connection with the middle layer (mesoderm or mesoblast) of the body. Neither in sponges nor in coelenterates 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 mesoglseal cells ; the piimitive male cells, which divide into numerous minute sperma- tozoa, are the reverse. THE ULTIMATE SEX-ELEMENTS. 9 1 In coelenterates 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 Beneclen 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 Tuhilaria mesemhryanthemum ; while in the Eudendrium ramosum the ova appeared to arise from the ectoderm, and the male elements from the endoderm, 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 medusoids (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, lessening the danger of its extinction ; and this has doubtless to be con- sidered, though it can hardly be regarded as a 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 coelenterates, 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 how widely early separation occurs, nor how far the fact is of much sig- nificance. The question will have to be discussed in the volume treating of heredity ; only a brief reference is here possible. 92 THE EVOLUTION OF SEX. In the case of a well-known fly, Chironomus, Prof. Balbiani, 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 pre- sumed to have retained intact the characters which they received when first divided off from the ovum. At a certain stage, however, the insulated 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 therefore 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 be; it divides, and all it* cells must at first more or less share these characteristics ; the body-cells lose them, the insulated reproductive cells must retain them. The ovum of the next generation has thus also the characteristics a b c, and must therefore produce an organism essentially like the parent. An early isolation of the reproductive cells, though never so striking as in Chironomus, has been observed in many cases, — e.g., in other insects, in the aberrant worm-type Sagitta, in leeches, in thread-worms or nematodes, in some Polyzoa, in some small crustaceans known as Cladoeera, in the water-flea Moina, and in some spiders {Phalafigidm),^r\& probably in other cases. As the series is ascended, the reproductive THE ULTIMATE SEX-ELEMENTS. 93 organs are later in making their appearance, or at least they are •only detected at a later stage ; and it must also be pointed out that, in cases of alternation of generations, an entire asexual generation, or more than one, may intervene between one ovum and another. . § 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 apari as reproductive organs. (fl) 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. (b) In 1866, Hseckel 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. () 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 successive genera- tions ; Ovists zi«'«« Animalculists. (/as)/ia (a specific nuclear matter), as opposed to continuity by a chain of undifferentiated cells, 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, Geddes, p. — Encyclopcedia Britannica articles already referred to; also Morphology, ibid. Hensen, V. — Op. cit. M'Kenurick, J. G. — Text-book of Physiology. Lond., 1888. Q Thomson, J. A. — Arts. Cell and Embryology, new Edition of Chambers's Encyclopasdia. History and Theory of Heredity. Proc. Roy. Soc. Edin., 1888. Waldeyer, W. — Die Karyokinese, &c. Arch. Mikr. Anat., 1888. Weismann.— Q*/. cit. 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. It is now necessary to attend more carefully to the essential characters and history of this " primordium commune," this common starting-point of life, leaving the details. Animal Cell, showing the chromatin elements of nucleus la) in a long coil, and the protoplasmic network (i) round about. — From Carnoy, along with the other problems of development, to a special volume devoted to Embryology. 8 I Structure of the Ovum.—i:h.Q ovum presents all the essential features of any other animal cell. There is the G 98 THE EVOLUTION OF SEX. t cell-substance, consisting in part of genuine living matter or protoplasm ; and there is the nucleus, or " germinal vesicle," which plays such an important part in the ripening, fertilising, and subsequent division of the cell. The cell-substance exhibits, when highly magnified, a homo- geneous matrix, traversed by a delicate network, with minute yolk-balis, pigment, and other granules strewn about the meshes. So much of it is genuine protoplasm, of course, but then there are also substances in process of ascent and even descent from the climax of living matter, and there is in more or less abund- Ovum of a Threadworm (Ascaris), showing (a) the chromatin elements of the nucleus, and the appear- ance of the surrouiKling yolk. — From Carnoy. ance a reserve capital of yolk nutriment for the future embryo. Delicate observations, by the modern masters of microscopic technique, have detected ' many marvels in the egg-cell, into which which we cannot at present enter. Thus, within the last year, Boveri has drawn attention to a special element in the pro- toplasm, which he calls archoplasm, a substance which, as its name suggests, seems to have an altogether marvellous architec- tural function in relation to the changes of the nucleus in segmentation. THE EGG-CELL OR OVUM. 99 When Purkinje, in 1825, discovered the nucleus of the fowl's egg, he could have little idea that the little "vesicle" to which he directed the attention of investigators was in reality an intricate microcosm. Little more than ten years elapsed, before R. Wagner began to complicate matters by the discovery of the nucleolus or germinal "spot" within the "vesicle." We new know that the nucleus has not only a very complex structure, but in a sense a curious internal life all 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 disappears as the period of activity begins. Inside this membrane, 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 essential constituent of the nucleus, a system of strands, coils, or loops, which stain deeply with various dyes, and are there- fore known as the chromatin elements. In contrast thereto, the less stainable and less 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 disordered ball of twine, while in other cases they appear rather as a living network. One thing about them seems very certain, and that is that they are in no disorder, but really preserve a very thorough definite- ness. 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 sub- stance too that the germ-plasma, on which Weismann and others have so much insisted, has its seat. § 2. Growth of the Ovum. — When the ovum is very young, it very generally 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 amoeba. Even in the simplest animals, however, the amoeboid phase lOO THE EVOLUTION OF SEX. constantly shows a tendency to pass into greater quiescence, to become in fact more or less encysted. So is it with ova, which though at first often resembling various forms of amoeboid 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 pas- sivity, to one in which passivity undoubtedly preponderates, is associated with an increase of nutriment and reserve-products. Th^ ovum feeds, becomes heavy with stored capital, becomes less active, and more encysted 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 (^pyornis) 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 ovU, 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 ways : — (a.) Very generally it feeds on the nutritive elements in the general lymph or vascular fluid of body, (b.) At the same time, or in another case, it avails itself of the debris of surround- ing cells. In many instances, e.g., in the minute ovary of hydra, or in the ovarian tubes of insects, the ovum is but the THE EGG-CELL OR OVUM. lOI surviving competitor among a crowd of surrounding cells, which to start with were all potential ova. (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." But we have already pointed out that this yolk- gland is usually interpreted as a degenerate portion of the essential organ. The relation between the disposition of the yolk and the mode of segmentation ;— A, diffuse yolk, e.^., sponge; B, polar, «.^.,frog; C, central yolk, « ^f. , crayfish ; 1>, predomin- ant, «.£-., bird :— A', Total and equal segmentation ;^B', total and unequal; C, peripheral ; D', partial segmentation. The yolk, gained in the above ways, 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 ro/e. We cannot, of course, enter here into the difficult embryological I02 THE EVOLUTION OF SEX. question as to the extent in which the yolk ever shares in directly contributing to embryonic structures. The possibility of distinguishing between formative protoplasm and the nutritive material, 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, (d.) 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 pro- toplasm. 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, (f.) A dis- tinct mode of yolk arrangement occurs in arthropods (crusta- ceans, insects, &c.), where the centre, not a pole, of the ovum is occupied by the nutritive material. In this case the forma- tive protoplasm divides round about the nutrient core, (d.) In the majority of fishes, 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 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. § 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 histoiy this becomes associated with several nutrient cells derived from the yolk-gland. These sur- round 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 neighbours it may happen to absorb. § S- ^,?S' 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 THE EGG-CELL OR OVUM. I03 general facts can here be stated. The envelopes may be derived {a) from the ovum itself, (/') 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 chemic-al peculiarities, so it is with the ovum. What are called yolk or viteUine membranes are generally pro- duced by the ovum itself. Furthermore, the outer protoplasm often forms a distinct firm zone, known as the Zona pellucida. This may lie 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. (i.) 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. According to some investigators {e.g., Will), the follicular cells sometimes arise from within the ovum, as the result of an early activity in the nucleus. This view, however, cannot be said to be confirmed. (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 liiny egg-shells of birds, so often stained with pigments, afford good illustrations of these secondary investments. Quite distinct are cocoons, such as those of earthworm and leech, which surround several eggs, and are produced from the skin of the animal. § 6. Birds' Eggs. — 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 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 ro"uM~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 ; and finally, the hard but porous limy shell. There arises the difficult problem of the origin of the shell, in regard to which it is to be noted that I04 THE EVOLUTION OF SEX. Mr Irvine, of Granton, has recently shown that fowls kept with access to no carbonate, 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 investi- gator'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 protective; and whether Lucas is right in supposing, that the colour of the surroundings can actually influence the deposition of pig- ment, by acting on the nervous system of the mother bird. 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 interpreted 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 hatch- ing. 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. 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, nuclein, lecithin, and, in association with the latter, neuriii. As we cannot 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 Otntm. — 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 EGG-CELL OR OVUM. 105 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, which the majority of investigators regard as one of normal cell-division or cell-budding, is known as the extrusion of the polar globules. Of general, and probably of universal occurrence, it has been but rarely observed in fishes and amphibians, and not as yet demonstrated in reptiles or birds. It was for long thought to be absent in arthropods, but the researches of Weismann, Blochmann, and others, have shown that this is not the case. An interesting peculiarity, which we shall afterwards notice, has been demonstrated by Weismann in regard to parthenogenetic ova. There is con- siderable 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 ^ny history, though they occasionally linger for a considerable time on the outskirts of the ovum. As an exception, 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. By the twofold division just described it has been considerably reduced in size, though not a whit in complexity, or in the number of its chromatin elements. 4- At this point, awaiting the essential moment of fertilisation, we ) shall for the present leave it. Within the last two years, 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 ostracodes) and rotifers, — and is believed by this eminent authority to be a general fact. Blochmann, who has been successful 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 was formed, while the eggs, which only developed after fertilisation, two occurred as usual. To these facts we must afterwards recur in connection with parthenogenesis. Io6 THE EVOLUTION OF SEX. § 9. Theories of the Polar Globules. — The polar globules appear to have been first observed in 1848 by Fr. Muller and Loven, but it is only within recent years that much has been made of them. Thanks to the masterly researches of Biitschli 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. The latest results of Boveri, Zacharias, and others, however, confirm the older view, that the process is essentially one of normal cell-division. But while this structural fact may be regarded as certain, there is no unanimity as to what the process means. The chief opinions on this subject, only a mere outline 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 ex- pressed 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 parthenogenesis." To this it must now be pointed out, that so far as one polar globule is con- cerned, 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 parthenogenetic ova, he supposes that enough male element is retained, since only 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. Sabatier 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. (b) A very different view — morphological rather than jDhysiological — has been maintained by Biitschli, Whitman, and others. The formation of polar globules is an atavistic reminiscence of the 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. Yet parthenogenetic ova, so fkr as polar-globules are concerned, show this least, nor can we well conceive an atavism so universally present without some important physio- logical necessity directly'behind it. To Biitschli's view, however, such an authority as Hertwig inclines, and Boveri likewise interprets the polar globules as "abortive ova." THE EGG-CELL OR OVUM. 1 07 {() Weismann's view is different from either of the above, though nearer the first. He distinguishes in the nucleus of the ovum two kinds of plasma, — (i) the ovogenetic or histogenetic substance, which enables the ovum to accumulate yolk, secrete membranes, and the like ; and (2) the germ-plasma, which enables the ovum to develop into an embryo. When the ovum is mature, the ovogenetic substance has served its turn ; it is henceforth only an encumbrance ; it is extruded as the first polar globule. This is all that is extruded in parthenogenetic ova. The second extrusion is a reduction of the germ-plasma itself by half, and the same must occur in the male germ cell too. What is lost in the second polar globule is supplied by the fertilising sperm. The beginning of development depends upon the presence of a definite quantity of germ-plasma. This the normal egg attains by first losing half and then regaining it, while the partheno- genetic egg attains the same resiilt by never losing any at all. In this too there is much hypothesis. The two kinds of nuclear plasma, the difference between the two polar globules, the necessity for a definite quantity before development begin, are all assumptions. Nor is it at all evident how the advantage of fertilisation (as a source of progressive change and so on) could operate, so as to induce the ovum to go through the circuitous process of losing half its " germ-plasma," and then gaining it again. ((/) It appears simpler to us to suppose that the ovum, like any other cell, tends to divide or bud at the limit of growth, a view in no way inconsistent with regarding the process as an extrusion of male elements. The precise homologies of the process will be clearer on reference to the diagram at page 114. t Io8 THE EVOLUTION OF SEX. SUMMARY. 1. The ovum presents all the essential features of a cell ; its suhstance and nucleus described. The chromatin-elements of the latter are the essential parts. 2. The ovum usually grows 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 neighbours. This hardly affects its unicellular character. S- 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 or budding, known as the extrusion of polar globules. In parthenogenetic ova only one seems to occur. 9. This polar globule formation has been interpreted variously : — (a) As an extrusion of male elements (Minot, Balfour, Van Beneden); {/>) as an atavistic occurrence of cell-division (Biitschli, Whitman, Hertwig, &c.); {c) by Weismann's more complex hypothesis. It seems to be a case of cell- - division at the limit of growth. LITERATURE. Balfour, F. M. — Op. cit. Van Beneden, E. — Recherches sur la Fecondation. Arch, de Biologie, IV., 1883. Carnoy.— La Cellule II., 1886, &c. Geddes, p. — Of. cit. Haddon, a. C. — Op. cit. 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. Thomson, T. A. —Recent Researches on Oogenesis. Quarterly Journ. Micr. Sci., XXVI., 1886. ■ Art. Embryology, Chambers's Encyclopaedia. Weismann, a. — Die Continuitat des Keimplasmas. Jena, 1885. Die Bedeutung der sexuellen Fortpflanzung. Jena, 1886. And other papers recently translated, " Heredity." Oxford, 1889. 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 a cellular expression of female characteristics, so the smaller size, less nutritive habit, and predominant activities of the male are summed up in the sperm. As the ovum is usually one of the largest, the sperm is one of the smallest of cells. The yolk or food-capital, and encysting membranes, which are often so pro- minent in the former, are as conspicuously absent in the latter. The contrast, though less accented, is still quite discernible in plants. In fact, the two kinds of cells are just as widely op- posed in their general features, as they are fundamentally com- plementary in their history. Before this opposition and comple- mentariness can be fully understood, however, we must briefly sum up the characters and history of the male eleriients. § 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, depreciat- ing 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 after- wards 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 Cyclopadia of Anatomy and Physiology, of about the same date, he will find tha:t the veteran Owen includes the spermatozoa under that strange 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 gibsence in infertile male THE EVOLUTION OF SEX. hybrids ; Von Siebold demonstrated their presence in many of the lower animals ; and lastly, in 1841, KoUiker 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 Sperm. — The sperm, then, is a cell. Though some, such as KoUiker, have inclined to regard it rather as a nucleus, its truly cellular character may be regarded as proven beyond dispute. We have, as in the ovum, to deal with cell-substance and nucleus, with this marked difference, that the cell- substance is generally reduced to a minimum. " Spermatic Animalculi " of the Kabbit and the Dog. — From BufFon, after Leeuwenhoek. The sperm is almost always, moreover, a cell of a very definite type or phase. It is like one of the highly motile 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. Oc- casionally, as the diagram shows, there is a departure from the predominant phase of cell-life. Thus in the threadworm Ascaris, the sperm has a blunt pear-shaped form, and exhibits slight amoeboid 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 amoeboid movements. Zacharias has made some interesting experiments, showing the modifiability of sperms under reagents ; thus, in a little crustacean (Poly- phetnus 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 con- gruent with other experiments and observations on the passage of cells from one phase of the cell-cycle to another. 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 MALE-CELL OR SPERMATOZOON. the more important of these, the reader is referred to the Encyclopedia Britannica, article Reproduclion. 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 undulating or vibratile band. Complexities such as axial filaments, stria- tions, and the like abound. In a few cases, as in the threadworm, the sperm Spermatozoa of crayfish (a), lobster (5), crab (c), ascarid (rf), water-flea— moina (g), man (y^, ray (^), rat (A), guinea-pig (/), a beetle — immature stage (/O, sponge {I), 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. Important perhaps is the observation, mainly due to Flemming, that the head of the sperm not only arises from the nucleus of the mother-cell, but almost wholly consists of the chromatin-elements of the same. § 4. Physiology of the Spermatozoon. — A few facts in regard to the physiology of the sperm demand notice, (a) It is speciahsed 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, and its resemblance to a flagellate monac^has already been noted, {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 fertilising power after remain- ing 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 THE EVOLUTION OF SEX. special reservoirs or spermathecge. 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 fertilising 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 ap- parently 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, {c) Remarkable too, and again like monads, is the power the sperms have of suc- cessfully resisting great deviations from the normal temperature. The presence of acids has usually a paralysing influence, but alkaline 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). Li, an amreboid sex-cell ; A, ovum, with germinal vesicle, n ; B, ovum extruding first polar hody, ;>' and leaving nucleus reduced by half; C, extrusion of second polar body, ^2^ nucleus n^, now reduced to one-fourth of original, i, a mother -sperm-cell, dividing (2, 3) into immature and mature spermatozoa (j/.). D, the entrance of a spermatozoon ; E, the male and female nuclei sp. n and n^ approach one another. §5. Origin of the Sperms. — A primitive female cell in the ovary grows in bulk and nutriment, and remains intact, but a primitive male cell in the testis undergoes repeated division into secondary cells, which either themselves, or by further division, form the 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 THE MALE-CELL OR SPERMATOZOON, 113 of which the subject is now emerging. In a general way, the process is simply that of the varied segmentation of a mcther-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, 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, who has worked per- sistently at the subject for over twenty years. 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. C" Comparison of Spermatogenesis and Ovum Segmentation. Explanation. — The first line, A-E, exhibits types of ovum segmentation ; — A, regular morula ; B, unequal segmentation, e.g;.^ in some Molluscs ; C, centrolecichal or peripheral type, e,g., in a shrimp Peneus ; D, partial 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, 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 nutritive blastbphore, after Gilson, &c. ; E' and E", the fame, with the sperm cells less definitely separated off, after Von Ebner and his followers. H 114 THE EVOLUTION OF SEX. 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 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 Herrmann 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 formation 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 as obvious to us as to any ; but here, as elsewhere, the morphologist's comparisons are strictly inde- pendent of the approval of the physiologist. § 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 ele- _ ^ ments are indeed, in a general way, of Diagrammatic comparison-I. female B equal rank, and are obviously comple- and male a> cell formed from the mentary. But even in this respect, the division of a single cell in the de- two elements, which unite in equal pro- ^rXTve "orSTn'-oTl^'worm PO^ions in the essential act of fertilis- Sagitta; II. ovum V and polar ation, are not exactly sperm and Ovum, hody a^; III. stump of mother- but (fl) the head or nucleus of the sperm sperm-cellos and the spermatozoon a', ^nd (/;) the female nucleus doubly re- duced by the extrusion of two polar globules. The accurate structural resem- blance or homology is not between ovum and sperm, but between ovum and mother-sperm-cell.* This fact, pointed out by Reichert in 1847, corrobor- ated by Von la Valette St George, Nussbaum, and others, is fundamental to a clear comparison of the history of ovum and sperm, and is postulated as an accepted fact in the rationale of spermatogenesis suggested in this chapter. * Since the above was written, Platner has in a remarkable manner demonstrated the unity between the division of the ovum in extruding , polar globules and the division of the spermatocytes. In both cases occurs the unique phenomenon of a' second nuclear division following on the heels of the first without the intervention of the usual resting phase. THE MALE-CELL OR SPERMATOZOON. I I 5 It is possible to follow out the homology into even further detail ; thus the antithesis seen in polar-globule formation may be fairly collated with similar separations occurring in spermatogenesis. Van Beneden and Julin, in their researches in oogenesis and spermatogenesis in Asmris, have noted the morphological correspondence of the polar globules, as we may call Iheni, of both ovum and sperm. Again we have a recent micro-chemical demonstration of the similar staining reactions of polar globules in ova, and the correspond- ihg remnant of the parent cell in spermatogenesis. In the differentiation of the reproductive cells in plants, both higher and lower, similar extrusions are to be observed. Of this Strasburger has given numerous illustrations, crowned by his own demonstration, that the nucleus of the pollen grain, in its germination upon the stigma, separates into a vegetative, relatively unimportant, and a generative or essential nucleus. Even in Protozoa, Blochmann and others have found analogues. A process so general is capable of a unified explanation, more specific than that of simply referring the matter to the mysterious necessities of cellular physiology. Just as in the development of the " worm " Sagitta a single cell divides into two, which become the starting-points of male and female organs respectively, so the cell divisions above alluded to express antitheses between more katabolic and more anabolic protoplasmic constituents. § 7. Chemistiy of the Sperm. — Comparatively little has been done in 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, fal, 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 protamin, which occurs in association with the nuclein already noted as present in the yolk. Albuminoid material, and products of decomposition, such as sarkin and guanin, were demon- strated, according to Hensen, by Picard. Miescher. emphasised the interesting fact, that while the sperm is being formed in the Rhine salmon, the animal is fasting. As no food whatever is taken, and as the muscularity of the fish is well known to decrease greatly, Miescher directly connected the degeneration of the lateral muscles with the development of the spermatozoa. Zacharias has more recently made a micro-chemical comparisoft of the male and female elements in Characea;, 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, ii larger nuclear mass than the female elements. It is interesting to notice that two investigators have recently pointed out, that an analysis of two different kinds of pollen showed a great analogy of composition between these male reproductive cells and those of the salmon and ox. Il6 THE EVOLUTION OF SEX. SUMMARY. 1. The contrast between the elements is that between the sexes. The large, passive, highly-nourished anabolic ovum ; the small, active, katabolic sperm. 2. Hamm's discovery, 1677 > Leeuwenhoek's interpretation ; the school of animalculists ; KoUiker'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 in reality comparable to a monad or flagellate infusorian, only with less cell-substance. Its occasional degra- dation 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 homologous with the ovum. The different modes of " spermatogenesis " may be collated with the diiferent 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. p~, Geddes, p., and Thomson, J. A. — History and Theory of Spermato- genesis. Proc. Roy. Soc. Edin., 1886, pp. 803-823, I pi. See also Zoological Record from 1886. 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 DreUncourt'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." I 1 8 THE EVOLUTION OF SEX. 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 becom.e 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 emphasis on the greater variability of males is of much importance. These three positions must be taken as representative ; others, which appeal to superiorities, polarities, and like mys- teries, can hardly claim scientific standing, and have been already sufficiently referred to at p. 33. To those which in- terpret the sexes in terms of the advantages of sexual repro- duction, and to those which deal almost exclusively with the problem of fertilisation, we shall afterwards return. The truth in fact is, that it is difficult to find any answer at once serious ■and direct to the question of the fundamental difference between male and female. §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 interpretation of sex should start at this level. Here, assuredly, the difference between male and female has its fundamental and most con- THEORY OF SEX — ITS NATURE AND ORIGIN. 119 centrated expression. For the bodies, after all, as Weismann has so clearly emphasised, are but appendages to this immortal chain of sex-cells. We have already pointed out that the sex- cells are more or less on a level with the Protozoa. If we only knew, they pro- bably 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. Even a student, shown an extended series of unicellular forms, amoebae, foraminifers, The divergence of male and female cells from primitive amosboid indifference. sun -animalcules, infusorians, gregarines, and some of the simplest algs as well, might gradually begin to group these in his mind under three divisions. First there are highly active cells, — infusorians of all sorts ; at the opposite extreme there are quiescent forms, in which the life seems to sleep, and loco- motion is almost absent, — the gregarines, and some unicellular algse ; and between these there are forms which in a via media have effected a sort of comprornise between activity and pas- sivity, which are without the cilia of the one or the self-contained stagnancy of the other, but possess outflowings of their living substance, — the familiar amoeboid processes. He would thus reach, almost by inspection, a rough and ready classification of the Protozoa, into active, passive, and amoeboid cells, — a f THE EVOLUTION OF SEX. classification however which, under varying titles, is more or less distinctly recognised by all the authorities on the subject. But if he went further than casual inspection, and studied - the life-history of some of the very simplest forms, such as some of the primitive moulds or Myxomycetes, and followed Haeckel's The encysted Proto7nyxa^ and its division into numerous individuals within the cyst. • — From Heeckel. account of the life-cycle in Frotomyxa, he would gain new light on his classification. For in these life-histories he would find the cells now encysted, now active lashed spores, and again sinking down into the compromise of equilibrium effected by The cyst of Frotofftyxa bursting, the flagellate young stages becoming at once amoeboid, eventually to unitein a composite amoeboid mass, oi" " Plasmodium.' — After Hscckel. THEORY OF SEX — ITS NATURE AND ORIGIN. amoebae. He is now in a position to recognise that the chapters in the Hfe-history of the simplest forms are, as it were, prophecies of each of his three groups. Before final differentiation has taken place, the organisms pass through a cycle of phases, one of which is accented by each of the different groups of the Protozoa. Thus an infusorian 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. ''^i^U Diagram of the Cell-cycle,— of encysted, ciliated, and amoeboid phases. I., 11., HI., in Protozoa ; IV., ovum and sperm of fern prothallus ; . V., encysted, ciliated, and anraboid animal cells; VI., ciliated animal cell pathologically becoming amoeboid; VII., sperm and amoeboid sperm; VIII., ama'boid and encysted ovum.— From Geddes. A conviction that the triple division really meant much would grow in our student's mind, if he passed from the Protozoa to the cells which compose higher animals. There he would find active ciliated cells in most of the classes, from the cihated chambers which lash the water into a sponge, to the cells lining the air passages in man; passive encysted cells would be illustrated in some forms of connective, fatty, and 122 THE EVOLUTION OF SEX. skeletal tissue ; while the white blood corpuscles would be at once recognised as amoebse. Extended observation here also would show him the cells passing from one phase to another. His rough classification of the Protozoa would be verified in the histology of higher animals, and would reappear in the study of their diseases. He would be thus at length in a position to say, that however these three phases were brought about, the forms characteristic of them were of such wide occurrence through nature as to justify his 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 unit mass of living protoplasm, amoeboid, encysted, or ciliated, as the case might be, he would 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. All this time, however, our student has remained a mor- phologist, his use of terms, like active and passive, simply expressing change of place. Not on this path of structural observation alone is it possible to understand what the forms and phases of cells really mean. A final corroboration of the cell-cycle, and at the same time a rationale of it, is obtainable only on physiological lines, when we begin to inquire into the protoplasmic processes which lie behind any change in the form and habit of a cell. 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, it is being continually reconstructed by an income of nutritive material, which, at first more or less simple, is worked up by a series of chemical changes till it reaches the climax of complexity and instability. These upbuilding, constructive, synthetic processes are summed up in the phrase anabolism. But, on the other hand, the proto- plasm is continually, as it "lives," breaking down into more and more stable compounds, and finally into waste products. There is a disruptive, descending series of chemical changes known as katabolism. Both constructive and disruptive changes occur in manifold series. The same summit (see p. 89) 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 THEORY OF SEX — ITS NATURE AND ORIGIN. I 23 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 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 more katabolic than the other, or vice versa on the opposite supposition. 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 cell of the body. Income too may continuously preponderate, and we increase in wealth, or similarly, in weight, or in anabolism. Conversely, expenditure may predominate, but business may be prosecuted at a loss ; and similarly, we may live on for a while with loss of weight, or in katabolism. This losing game of life is what we call a katabolic habit, tend- ency, or diathesis; the converse gaining one being, of course, the anabolic habit, temperament, tendency, or diathesis. The words anabolic and katabolic are, of course, new, unfamiliar, and un- deniably ugly. Habit and temperament have very vague associa- tions, 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 is now-a-days quite scientific and definite in speaking of gouty or neurotic diathesis, of bilious habit, strumous tendency, or the like. The metaphysical vagueness is no longer chargeable to him ; still less, we trust, to us. We are now in a position profitably 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, may we express the plain and indubitable facts of anabolism and katabolism within the living matter. The student may think of the processes, with some 124 THE EVOLUTION OF SEX. degree of accuracy, under the metaphor of an eddy in a stream, or of a ceaseless fountain, which, while remaining approximately constant, is the expression of continual ascent and descent of drops. The protoplasm 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 temporary phases, there is predominant katabolism, — predominant when compared with the life expen- diture of a passive, quiescent, enclosed, or encysted cell. In amoeboid organisms these extremes are avoided ; there is cer- tainly 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 katabolisin, 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 predominant, the increasing libera- tion 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 adapta- tion 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 katabolism respec- tively. Here again we reach the same formula as before ; or, more cumbrously in words — the functions are either self-main- taining or species maintaining, individual or reproductive ; the former are divided into anabolic and katabolic, the latter into male and female. But the second set of products and processes, THEORY OF SEX — ITS NATURE AND ORIGIN. 125 SO far from being unrelated to the other as is commonly supposed, are in complete parallelism. Femaleness is anabolic preponderance in reproduction, hence the ovum has necessarily the general character which this " diathesis " produces in non- reproductive cells; and, similarly, katabolic preponderance stamps its character of active energy upon spermatozoon as naturally as upon the ciliated cell or the monad. Diagram showing the divergence of ovum and spermatozoon from a undifferentiated amoeboid type of cell. Rolph's characterisation of the male cells as hungry and starving (katabolic), has been experimentally confirmed by their powerful attrac- tion to highly nutritive fluids, and is every day illustrated in their persistent attraction to the ova. Plainer has suggested, in the intimately hermaphrodite 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-sperm-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.ff. , when the sperms lie closely packed in the special storing reservoirs ; {/>) 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 126 THE EVOLUTION OF SEX. protoplasmic explosives, which may remain long inert, but on the presence of the required stimulus are able to start again into extraordinary activity. § 3. The Problem of the Origin 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. Some people, like children, wish everything at once. Yet it must be admitted that there has been a lack of any sure and certain voice on this question. Apart from the simple fact that evolutionist biology is still young, there are three reasons for the comparative silence in regard to the origin of sex. (i.) The first of these is the still curiously prevalent opinion, that when you have explained the utility or advantage of a fact, you have accounted for the fact, — an opinion which the theory of natural selection has done more to foster than to rebuff. Darwin was, indeed, himself characteristically silent in regard to the origin of sex, as well as of many other "big lifts" in the. organic series. Many, however, have from time to time pointed out that the existence of male and female was a good thing. Thus Weismann finds in sexual reproduction the chief, if not the sole source of progressive change. Be that as it may at present, it is evident that a certain pre-occupation with the ulterior benefits of the existence of male and female, 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 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, those questions must be kept distinct, though in the final synthesis they are all answerable in a sentence. THEORY OF SEX — ITS NATURE AND ORIGIN. I 27 (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 varia- tion 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 Prof. Vines in his article Reproduction — Vegetable, in the Encyclopedia Britannica, at each stage, however, endeavouring to interpret the facts, physiologically, in the light of protoplasmic processes. (l.) The simple alga, Protococcus — which, in the widest sense of that term, every one knows in some form or other, 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 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, Ulothrix — 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 case's at least, is a weakly plant. They have what Prof. 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. 128 THE EVOLUTION OF SEX. 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, Edocarpus, 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 Icatabolic 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 development. (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 most clearly our general conclusion that preponderant katabolism or anabolism are the ruling characteristics 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 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. THEORY OF SEX — ITS NATURE AND ORIGIN. 1 29 In the bell-animalcule, which grows so commonly 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, Engel- mann has described how an individual divides first of all into two cells. One of these remains as such (like an ovum), the Vorticella, the Bell-animalcule, — a, the normal individual ; 5, its division into two ; c, the division accomplished ; rfj the further division of one of the halves into eight small (male) units ; e, a minute individual uniting with one of normal size. 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 (1?.^., CoUozouni), dimorphic spores — large and small — have been described, although their history has not yet been fully traced. Even in Foraminifera, as Schlumberger, De la Harpe, and H. B. Brady have shown, a marked dimorphism may occur ; and here again the distinction seems to lie between preponderant anabolism and katabolism. As another illustration, it will be instructive to select the case of volvox. In this colonial organism, which is best re- garded as a multicellular protist, the component cells are at first all alike. They are united TjjTprotoplasmic bridges, and simply form a vegetative colony. In favourable environmental con- ditions 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 130 THE EVOLUTION OF SEX. from their neighbours ; and if their area of exploitation be sufficiently large, emphatically anabolic cells or ova result; Volvox glohator, a colonial Alga or Infusorian, showing the ordinary cells (c) that make up the colony (or body), and the special reproductive cells (a, d), both male and female. — After Cohn. 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- tive greenness and becoming yellow. In some species, distinct colonies may, in the same way, become predominantly anabolic or katabolic, and be distinguishable 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. Corroborative Illustrations. — If the anabolic and kata- bolic contrast, so plainly seen in the sex-elements, be the funda- mental one, we must expect to find it saturating through the entire organism. We have already drawn attention to the occurrence of yolk glands in association with ovaries. Or again, in the cells of a developing anther an enormous number THEORY OF SFX — ITS NATURE AND ORIGIN. 131 of crystals may be often observed to occur. Crystals are, how- ever, usually regarded as accumulations of waste products, and these anther crystals are, in fact, comparable to urinary deposits. Such accumulations do not, however, occur, at least to any similar extent, in the embryo-sac or in the female organs, in spite of the homology in male and female development. They occur as results of katabolism, where we would naturally expect them — in the tissue of male organs. \ Stonewort (C^arayVfl^/ j) show g n two stnges adult and embryonic, the female organ (^), and the male organ {a). — From Sachs, after Pringsheim. In the stoneworts Chara or Nitella there is, as is well known, an alternation between nodal and internodal cells. The internodal cells are actively vegetative, and go on increasing in size; they do not divide, and may be justly regarded as emphatically anabolic. The nodal cells, on the other hand, are much smaller, and do divide. That is to say they are relatively more katabolic. A crucial test of the present theory thus suggests itself. 132 THE EVOLUTION OF SEX. Since the reproductive organs are simply, as every morphologist knows, shortened branch-structures, we should predict that the cell from the segmentation of which the antheridium is derived must correspond in position to a nodal and katabolic cell {i.e., be based upon an internode), while the corresponding essentially female cell or ovum must be internodal or apical in origin {i.e., based upon a node, and this relatively more anabolic). It is therefore not a little noteworthy that an examination, alike of classical figures and fresh specimens, will show that this imper- fect homology, but perfect physiological correspondence, is invariably the fact (see figure). § 7. Conclusion. — In conclusion, in defiance of Dr Minot's recent dictum, that "such speculation passes far beyond the present possibilities of science," we believe that the consideration {a) 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 general con- clusion, that the female is the outcome and expression of pre- ponderant anabolism, and in contrast the male of predominant katabolism. Further corroborations will gradually appear in the succeeding sections, as we discuss fertilisation, partheno- genesis, or special facts like menstruation and lactation. The whole thesis may be once more summed up diagraramatically. SUM OF FUNCTIONS. Reproduction. Anabolisni. Katabolism. Female. Male. In this way we see, with reference to the three speculations ~ THEORY OF SEX — ITS NATURE AND ORIGIN. 1 33 outlined at the beginning of the chapter, — (i.) that the penetrat- ing insight of Rolph, of females as the more, and males as the less nutritive, is fully justified ; (2.) that the view of Minot of the differentiation of both sex-cells from a primitive hermaphroditism becomes similarly developed, and acquires greater definiteness ; while (3.) the view of Brooks, which ascribes variability primarily to the males, at least acquires considerable support from the inter- pretation of the males as preponderatingly katabolic. For it is rather in connection with the destructive changes of protoplasm than with the constructive, that variations might be expected to arise. 134 THE EVOLUTION OF SEX. 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 ; (/') Minot's theory of the differentiation of both kinds of sex-cells from a primitive her- maphroditism ; [c) the conclusion of Brooks, that the males are more vari- able, and 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 physiolo- gical 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 blinding influence of teleological or utilitarian inquiries, {&) the number of separate problems involved, {c) 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 preponderant katabolism and anabolism, 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. Corroborative illustrations, — anther cells and Chara. 7. General conclusion, — (a) from the sex-cells, [6) from incipient sex, (t) from organs and tissues, {d) from the determination of sex, (e) from the characters of the sexes, — that male and female are the results and expressions of predominant katabolism and anabolism respectively, with consequent confirmation of the speculations of Rolph and Minot, and in some measure also of that of Brooks. LITERATURE. Brooks, W. K.— The Law of Heredity. Baltimore, 18S3. Geddes, p. — 0/>/>. cit., especially " Theory of Growth, Reproduction, Sex, and Pleredity," Proc. Roy. Soc. Edin., 1886; and Article "Sex," Encyc. Brit., also "Restatement of Cell Theory," Proc. Roy. Soc. Edin., 1883-84. Minot, C. S.— Theorie der Genoblasten. Biolog. Centrallilatt, II., P- 365- Rolph, W. H.— Biologische Probleme. Leipzig, 1884. 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. Weismann, a. — 0pp. cit. 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 a leg or a tail can be made good. In a great variety of cases, a kind of physiological forgiveness is shown in the repara- tion of even' serious injuries. Now this " regeneration," as it is called, is in a certain degree a process of reproduction. 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 the division is artificial, the result shows how very far from unique is the process which we usually speak of as reproduction. In fact, as Spencer and Hasckel said long ago, 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 deliberate and orderly division, both multiple and binary : while finally, in colonial forms, the liberation of special repro- ductive units may be observed. We shall afterwards have to discuss the relations of these and other processes ; but just as we began the study of sex with the familiar contrast of male and female, so we shall begin our investigation of the reproductive processes with the most obtrusive mode, known as sexual reproduction. 138 THE EVOLUTION OF SEX. § 2. Fads Involved in Sexual Reproduction. — It is necessary, at the outset, to be quite clear as to the concurrence of several distinct facts in any ordinary case of sexual reproduction among many-celled organisms, (i.) 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 produce 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, atten- tion 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 gene- rations. 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- SEXUAL REPRODUCTION. I 39 fold devices for ensuring that the unconscious insects carry the fertilising pollen from one flower to another, but has also emphasised the beneficence of cross-fertilisation for the health 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 Professor D'Arcy Thompson's valuable edition of Miiller's " Fertilisation of Flowers," Sir John Lubbock's " Flowers in Relation to Insects," and the classic works of Darwin. Reference must, however, also be made to Meehan's protest (see pp. 75, 76), that self-fertiUsation is neither so rare nor so "abhorrent" as is now generally believed. Bees visiting White Deadnettle and Broom. In a great number of cases, cross-fertilisation by means of insects does occur ; in many it must occur. In another by no 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 cer- tainly take place ; in some this is necessarily so. Interesting in this connection is the indubitable self-fertilisation which 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 I40 THE EVOLUTION OF SEX. X along capillary spaces between the leaves, to the passive female cells. In some cases there is a curvature of the male organ towards an adjacent female organ, apparently in obedience 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. As 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 0), liberating pollen (a), B, Diagrammatic section of a Flower, showing female parts (c), — receiving stigma, conduct- ing style, ovary with seed (H) ; the male parts, stamens iji) witll pollen. C, The Pollen-tube (a) growing down to the ovule i^ and female cell {e). The pollen grain is here represented as distinctly two-celled, cf. pp. 142 and 229. 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 mini- SEXUAL REPRODUCTION. 141 ature embryo. The view of 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 feniale 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 OTrv Illustrating the-contrast between male and female'flowers in the pink campion {Lychnis diuma). 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 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 142 THE EVOLUTION OF SEX. 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 surrounds the nucleus has no direct influence in the essential act. Fer- tilisation 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 fertilisation, are two very important facts, showing the unity of the process not only in higher and lower plants but in all organisms. § 4. Fertilisation in Animals. — That the sperms were essen- tial 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 Abb6 Spallanzani 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 Provost 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 confirmed about ten years afterwards by Bischoff and Meissner; in the frog ovum by Newport (1854) ; and in successive years it was gradu- ally recognised in a great variety of animals. The external devices which secure that the sperms shall reach the ova are very varied. Sometimes it seems more a matter of chance than of device, 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 SEXUAL REPRODUCTION. 143 already laid eggs of her neighbours. Meanwhile she is attended 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, surrounded 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.f;., 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. Different Forms of Conjugation in Plants, a, zoospores ; It, mould ; c, d, conjugate algae ; e, /, desmid. 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 (fig. c, d) 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 144 THE EVOLUTION OF SEX. 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 observation, that round the egg-shells of 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. 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, patho- logical development is at least often the result. In the lamprey's egg quite a number of sperms find their way into a watch-glass- shaped space at the upper pole of the ovum, but only one gets further, the rest remain imprisoned without further history of any importance. 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 vary with each species ; what takes place in the act of fertilisa- tion, however, is always essentially the same. The head of the sperma- tozoon becomes the male nucleus (or pro-nucleus) of the fertilised ovum, entering into close association with the female nucleus. The latter, as we have already noted, has had its own history ; it is no longer the original germinal 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 Btill exhibited by the male. Whit- SEXUAL REPRODUCTION. 1 45 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). This result, however, appears to have been overlooked, till the same fact was reobserved in threadworm ova by Biitschli in 1874. Since that date the f^ct has been continuously studied. Some observers still doubt whether what can be accurately called fusion of nuclei ever occurs ; and if fusion means inextricable 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 component 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 megalocephala) which infests the horse. Since 1883 about a dozen important memoirs have dealt with this subject, and with the same material. The results of competent observers have varied enormously in detail, but on the essential points there has been (with some few exceptions) an increasing congruence of opinion. The most important work on the subject has been that of Prof, van Beneden, whom most of those who have followed him unite in regarding as a master. The discrepancies and .contradictions have been accompanied at times by not a little warmth of asseveration, but with ever-increasing perfection of method many of these are disappearing. To one alone shall we here allude. According to Van Beneden, the normal ovum of this 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 investi- gators had unravelled the structure and behaviour of the nuclei, the dis- crepancy seemed serious enough. Now, however, 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 recent 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 by three-fourths, but the number of nuclear elements remains the same. Only one sperm pene- trates 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. 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 146 THE EVOLUTION OF SEX. 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 anastomo.sing processes, after the fashion of little rhizopods ; gradually the rods thus resolve themselves into a network, in the meshes of which minute " nucleoli " are also demonstrable. I f4^ _./ *- * 7 \ Diagram of the Process of Fertilisation, following Boveri's figures.^a, female pro- nucleus ; b^ polar bodies ; c, male nucleus ; rf, sperm-cap ; oc, chromatin elements of uniting united female and male nuclei {a and c) ; £■, protoplasmic centres ;yj archoplasmic threads. The \yNO 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. VI.-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. Both Van Beneden and Boveri have recently agreed on the existence of two "central corpuscles" (centrosomata) in the protoplasm, which serve as " points of insertion " for protoplasmic threads, which exert a "muscular action" upon the nuclear elements in the forthcoming division. Boveri SEXUAL REPRODUCTION. 147 vt -e 1. T/m 1 V \ 148 THE EVOLUTION OF SEX. 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 contractile fibrils (/), which moor themselves to the nuclear elements. 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, the thought of which makes one dumb. In the spindle the nuclear elements, still distinguishable in their orderly behaviour as male and female, eventually form what is known as the "equa- torial plate" (VI. ), lying across the centre of the spindle. This is awell-marked stage, and one characterised by apparent equilibrium. "It is the resting- stage/a;' excellence in' the life of the cell. Movement is at an end, a state of stability has set in, and this would continue ad injinihim, did not-a factor, which hitherto has played no part, assert itself and bring about fresh move- ment. This new movement is the longitudinal division of the chromatin elements, an independent expression of life — indeed, a reproductive act — on the part of the nuclear elements." The above short sketch will show how intricate, and yet at the saine 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 has shown, for plants. One marvellous fact, showing the closeness of union in fer- tilisation, 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 purely male, the other purely female, and this is true not only for Ascaris (by Van Beneden) and other thread-worms (by Carnoy), but for representatives of other worm-types, ccelenterates, echinoderms, molluscs, andtuni- cates." In division to form daughter-cells (IX., X.), half of each set of constituents goes to either cell, and the dualism is kept up. Furthermore, though hardly yet quite certain, it is most probable, that " of the four chromatin loops observed in the division figure of a daughter-cell, two are derived from the male parent, and two from the female." The importance of this fact, in relation to the influence of both parents upon the offspring, is very obvious. § 5. Fertilisation in Protozoa. — In the nascent sexual union observed in many Protozoa, — not, however, as yet in foraminifers or radiolarians, — con- siderable 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 Vorticella. 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. In both cases the nuclear elements play an important part, disrupting and reconstructing SEXUAL REPRODUCTION. 1 49 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 " paranucleus." 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 paranucleus divides in a regular way, and with the results there is interchange between the two indiviclu.ils. According to Maupas, who has investigated the subject in most detail, 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 : — (i.) The para-nucleus increases in size. (2, 3.) It then divides twice, and eliminates certain corpuscles. (4.) This effected, it 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 j aijd 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 ani- malcule {Parainacium), and in species of Stylonichia, Leucophrys, Euplotes, Onychodromus, Spirostotnum, &c. , the facts are as above stated . It is of interest to cite the facts in regard to the common bell-animalcule ( J^; //«//«), because here the conjugating individuals are likeovum 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 more male than the other. In the nearly allied Carchesiitm polypinuni, 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 nor- mal size, first to the stalk, and then to the body (fig. p. 129). The accessory 150 THE EVOLUTION OF SEX. nuclear bodies divide as usual ; the large individual ceases to feed, and hermetically 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 para-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, {/>) 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 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 Protomyxa (see fig. at p. 120), the units which burst forth from the cyst sink down into tiny amoebae, and unite together in numbers to form a composite spreading mass of protoplasm, technically known as a plasmodiuvi. 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 Protomyxa^ 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 micro- scope, and the formation of a plasmodium is seen. The dying cells fuse into composite masses, just like the units di Protomyxa ; 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 artiado 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 plasmodiuin 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 patho- logical processes. Now it is from this primitive union of cells, as illustrated in the lowest organisms, that we start in explain- SEXUAL REPRODUCTION. 151 ing the origin of fertilisation. Just as the very beginning of. reproduction may be detected in the almost mechanical break- age of a form like Schizogenes, so the very beginning of fertili- j sation is found in the almost mechanical flowing together of 1 exhausted cells. 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.) fertilis- ation of ovum by spermatozoon. {b) Between this and the process usually described as con- jugation, there are some interesting hnks. Sometimes as many as three or four spores of lowly Algae club together, as if to gather sufl5cient momentum to make a combined start in life. The young forms of the sun-animalcule {Adinosphmriuni) usually unite in twos, but Gabriel has observed in some cases a multiple union. 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 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 «>;?//«?• unicellular organisms occurs, IS2 THE EVOLUTION OF SEX. as we have seen, very generally in the Protozoa, and is also a common fact in the life-history of simple Algae. It is open to every one possessed of a microscope to observe what conjuga- tion means in such a common fresh-water alga as Spirogyi-a. Opposite cells of adjacent filaments are attracted to one another by what a recent observer calls a " purely physical process," and the contents of the one cell pass bodily over into the other. 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 (see fig. p. 143). '^k Diagrammatic representation of the contrast between conjugation (horizontal line) and fertilisation (vertical line). (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, "especi- ally 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 dimorphism has been already noted in discussing the origin of sex, and need not be re-emphasised. ((?) 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 pro- tist colonies like Volvox or AmpuUina, which suggest the bridge SEXUAL REPRODUCTION. 153 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. (&) Multiple conjugation. (c) Conjugation of two similar cells. (d) Union of incipiently dimorphic cells. («) 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 has since withdrawn this view mainly on the /c 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 ; (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. A reinvestigation of the whole question of plasmodium formation, from this point of view, is however very desirable, especially since the recent progress of microscopic technique has rendered the study of the nucleus in the lowest forms much more practi- cable than it was a few years ago. § 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. Only to a very limited extent is such a notion justified. Every one is aware that by direct human control animals lijce 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 very little of the occurrence of any such hybridisation. It seems to occur in some fishes ; different species of toad are often seen in sexual union, but the result is unknown ; in higher animals it seems confined to varieties of a species. The demarcation of a species is the vague line which marks the 154 THE EVOLUTION OF SEX. physiological range of natural and successful intercrossing. Domesti- cated breeds are usually fertilisable mutually, and their progeny is fertile ; we regard them as mere varieties. Nearly related species, however, only rarely admit of being crossed, even when under man's control ; and species hybrids tend to be themselves sterile. In structural characters two varieties of dog may seem more widely separate than two nearly allied species, yet the varieties of dog may be intercrossed, while this very rarely occurs with the two species. The difference in the reproductive elements must often be greater than the structural _ differences of the adults would suggest. Hertwig has experimented of late with the artificial hybridisation of echinoderms, and Born with that of amphibians. Both emphasise the difficulties of the process, and the varying degrees of success that may result. In three cases Born brought about reciprocal hybridisa- tion, but this is by no means always the case. Sometimes real fertilisation took place, but nothing followed ; in other cases the ova segmented ; in a few the larval stages were reached ; and in two cases metamorphosis was survived. The hybridisation is the more readily effected the nearer the elements are to perfect maturity. Sometimes the success seemed greatly to depend on the concentration of the sperm fluid, — the more dilute this was, the fewer sperms were there to overcome the difficulties of effecting entrance to the ovum. 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 the females may be successfully impregnated by horse or ass. In many cases the 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 con- stitution is more or less 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. 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 therefore 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 the pure parent to throw off gemmules. The fact that variation is due to the male influence, and that the action upon the male parent of unnatural or changed conditions results in the variability of the child, 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. " When we regard the male as katabolic, his variability becomes intel- ligible; while in hybridisation, which means the sexual union of organisms SEXUAL REPRODUCTION. 155 with a more than usually divergent life-experience, the reproductive elements which are intermingled have probably a corresponding divergence in chemical constitution. The early researches of Kcilreuter (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 hybridisation in - animals. IS6 THE EVOLUTION OF SEX. SUMMARY. 1. Reproduction is but more or less discontinuous growth. 2. Sexual reproduction normally implies (a) special reproductive cells, distinct from the body ; (/>) the dimorphism of these cells ; {c) their physiological dependence, — the ovum being unproductive without the sper- matozoon, and 7J7i:e 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, [h] multiple conjugation, [c) ordinary con- jugation, [d] union of dimorphic cells, («) fertilisation of ovum by sperma- tozoon. 7. Both in plants and animals hybridisation is often successful, but the offspring frequently tend to be sterile. This, however, must not be exaggerated. LITERATURE. See the already noted works of Balfour, Van Beneden, Carnoy, Gediles, Haddon, Hensen, Hertwig, M'Kendrick, Sachs, and Vines. For recent papers see Boveri, Th., Zellen Studien ; Jenaische Zeitschrift fiir Naturwissenschaften, 1887-88 ; Zoological Record, from 1886 ; and Journal of Royal Microscopical Society. CHAPTER XII. Theory of Fertilisation. In his 49th Exercitation on the " efificient cause of the chicken," Harvey thus quaintly e.xpresses what has always been, and still is, a baffling problem : — " Although it be a known thing sub- scribed by all, that the foetus' 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 dis- cerning brain have disclosed the manner how the cock and its seed doth mint and coine the chicken out of the egge." The old theories on the subject are more curious than profitable, a fact not to be wondered at since it is really only within the last fifty years that the fundamental fact of the union of the sex-cells has been observed. § I. Old Theories of Fertilisation. — {a) P'rom Pythagoras and Arisfotle on to the " Ovists," of whom we have already spoken (p. 84 ), 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. This will be less insisted on, however, when it is recognised that in reality the ovum is not so fairly comparable with the spermatozoon as with the mother- sperm-cell. It must be allowed, too, that there is much to warrant us in thinking of the sperm as an element which stimulates the ovum to division ; yet this will be recognised as only approximate language, when the facts of the intimate nuclear union are fully appreciated. (^) In contrast to the above opinion, we find ingenious thinkers, so widely separate in time as Democritus and Paracelsus, regarding the male fluid as very important, forestalling Buffon and Darwin in fact in considering it in a sense an extract or 158 THE EVOLUTION OF SEX. 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 "animalcuhsts." 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 supplied more than modern magnifiers to those observers who detected in the spermatozoon the members and lineaments of the future organism. After this the discovery that the sperm supplies half the nucleus of the fertilised ovum, and half the nuclei of the two first daughter-cells, seems almost a little thing. The polemic of modern science has this advantage at least, that when two competent authorities on the same subject assert the same thing, we may generally believe them. (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 within the last fifty years, an old conception of the male in- fluence lingered persistently. I'his 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 contagium 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 obser- vations, 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 THEORY OF FERTILISATION. 1 59 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 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) Hertwig's Vieiv. — Professor O. Hertwig, who was one of the first carefully to follow out the details of fertilisation in animals, thus sums up his " Theorie der Befruchttuig" : — "In fertilisation, distinctly demonstrable y morphological processes occur. Of these the important and essential 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 fer- tilising 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) Sirasburger' s View. — VSTiat Hertwig maintains for animals, Strasburger does for plants. "The process of fertilisation depends upon the union of the sperm nucleus with the nucleus of the egg-cell ; the cell- substance (cytoplasm) does not share in the process." "The cell-sub- stance 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 differentiated 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 l6o THE EVOLUTION OF SEX. 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. (/') There are a iew 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 important 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, (r/) The researches of Boveri show, that though the union of nuclei is so essentia], the protoplasmic activity and share in the process are also considerable. It appears to us a fact well worthy of con- sideration, that according to this authority the sperm brings with it into the ovum a protoplasmic centre^a " centrosoma" — 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 archoplasma) 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 'equa- torial plate ' is the result of the action of the archoplasmic sphere exerted through the fibrils." Now this specially active protoplasm, which the skilful observer seems to have succeeded in fixing, has its centre. There are two central corpuscles, each " ruling a sphere of archoplasma." Where then do these centres come from ? "It is probable," Boveri says, " that the spermatozoon brings a centrosoma into the ovum, and that this by division forms two centres. Since these two corpuscles condition the division, the dependence of this upon the presence of the spermatozoon is for the Ascaris ovum explained." We have given these details, technical as they are, because they seem to us to show clearly that it is 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 mor- phological 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 difiScult ; only a few incidental results are as yet available ; the suggestions thrown out by various naturalists must therefore 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 Rolph, who regarded the process THEORY OF FERTILISATION. l6l 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 mimimal 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 quiescent organism, the female. Therefore too is it, that the small starv- ing male seeUs out the large well-nourished female for purposes of conjuga- tion, to which the latter, the larger and better nourished it is, is on its own motive less inclined." Cienkpwski has also inclined to a similar view, regarding conjugation as equivalent to rapid assimilation. 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 constructed 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 properties 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 the sudden doubling of the mass 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 con- jugation of similar nuclei. " The germ-plasma in the male and female reproductive cells is identical." Previous to the essential moment of fertilisation, half of the germ-plasma is given off from the germinal vesicle of the ovum in forming the second polar body. Development will not take place unless the loss be made good, and the original mass restored. This is what the sperm does in fertilisation. In short, to Weismann the process is quantitative rather than qualitative. l62 THE EVOLUTION OF SEX. This supposition appears to us to be open to criticism. (l.) 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 Strasburger 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 difificult to under- stand the phenomena of conjugation, whether permanent or transitory, from which we believe fertilisation to have originated. (4. ) That the normal ovum should lose half its quantity of germ-plasma, only to regain a similar quantity in fertilisation, certainly appears a curiously circuitous process. (5.) The occasional possibility of inducing division by replacing the sperms with other stimuli, seems to point to a dynamical or chemical action, which Weismann denies. We are bound, of course, to admit the importance of the established facts of nuclear union, and agree with Boveri, that the complexity of the morphological facts shows the present impossibility of supposing that they can be fully expressed in chemical terms. But a due impression of the marvellous "individuality" of the nuclear elements may be combined with a general physiological interpretation of the entire process. 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 is comparable to mutual digestion, and, though bound up with reproduction, has 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 waste products or katastates, 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. Rolph's suggestion is thus included and defined. THEORY OF FERTILISATION. 1 63 § 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 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 advantageousness. The two naturalists who have recently reached the most valuable results in regard to the uses 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 constancy of the species. Several naturalists of the highest reputation 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 supply 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 rejuvenesced 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 g^ndrateur ^chappe k k mort." Hensen, in his 1 64 THE EVOLUTION OF SEX. admirable "Physiology of Reproduction," expresses the same when he says : — " By normal fertilisation, death is warded off (ferngehalten) from the germ and its 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." (Some may be obliged to plead guilty to a similar impression in regard to Weismann's Keimplasma.) " 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 one assumes that in each animal there persists only half the repro- ductive 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 (Vererbungstendenzen)." (Does Pro- fessor Weismann not feel that there is something " indefinite and misty" even about this?) He sarcastically compares the two exhausted individuals to two exhausted rockets, which are supposed' to rejuvenesce in mutually affording the constituents of nitroglycerine. More forcibly he urges the difficulty sug- gested by continued parthenogenesis, — a difficulty which we shall afterwards have to discuss. " To the conception of rejuvenes- cence," 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 Uttle as the reverse.'' But Weismann 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. Such cases have, happily, come to hand, as we shall now see. We have already referred to Maupas's proof of true sexual union in ciliated infusorians. By an elaborate process of nuclear division, disruption, elimination, interchange, union. THEORY OF FERTILISATION. 1 65 and reconstruction, two "slipper animalcules" 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 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 {Onychodromus 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 {Stylontchta pustulata), 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 himself They were not old exactly, but they 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 richness 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 conjugates did not even recover from the effects of their forlorn hqpe. 1 66 THE EVOLUTION OF SEX. Without the normal sexual union, then, the family becomes senile. Powers of nutrition, division, and conjugation with unrelated forms, come to a standstill. This senile degeneration is very interesting. The first symptom is decrease in size, which may go on till the individuals only measure a quarter of their normal proportions. Various internal structures then follow suit, "until at last we get formless abortions, incapable of living and reproducing themselves." 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." Physiolo- gically 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 in- evitably ends in death. The general result is evident. Sexual union in those infus- orians, 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 phoenix of the species renew its youth. 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 element is much the more important in this connection. Similarly, though on somewhat different lines, Weismann finds in the mingling of male and female Keimplasmas the source of those variations on which natural selection operates. Rejecting as he does the alleged inheritance of acquired characters, he finds' the fountain of change in sexual repro- duction. " Sexual reproduction is well known to consist in the fusion of two contrasted reproductive cells, or perhaps even in the THEORY OF FERTILISATION. 1C7 fusion of their nuclei alone. These reproductive cells contain the germinal material or Keimplasma, and this again, in its specific molecular structure, is the bearer of the hereditary tendencies of the organisms from which the reproductive cells originate. Thus in sexual reproduction, two hereditary tend- encies are in a sense intermingled. In this mingling, I see the cause of the hereditary individual characteristics ; 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." But this very reasonable contention hardly appears to consist with Weismann's quantitative interpretation of the process of fertilisation. Nor is it evident how the diversities of the male and female plasmas became such as their results indicate them to be, if Weismann be correct in maintaining that no modifications of the body influence the reproductive elements. Brooks and Weismann have at any rate maintained a thesis which few will be inclined to oppose, that sexual union is productive of variation. To discuss the relations of this view to other theories of variation is not here relevant, nor can we do more than mention the reasonable sugges- tion of Hatschek, that sexual reproduction is a remedy against the operation of injurious variations. For we can readily imagine, that the excess of some particular line of anabolic or katabolic differentiation may be neutralised through fertilisation. In this way one is led to speculate, whether the constant pairing of diseased individuals may not sometimes be more mercifully condoned by nature than we have been accustomed to think. 1 68 THE EVOLUTION OF SEX. SUMMARY. 1. 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 Strasburger are strongly in favour of this view. The claims of the cell-substance and general proto- plasm must not, however, be overlooked. Many facts, such as those demon- stiated by Boveri, show that the protoplasm is also important. 3. Modern physiological theories of fertilisation are necessarily .very tentative. Sachs compares it to fermentation ; Rolph to mutual digestion. To Weismann, the process appears quantitative rather than qualitative, so far as the subsequent division is concerned. Half of the Keimplasma which the ovum has surrendered with the second polar globule is restored by the sperm-nucleus. The two nuclei are alike, and thus there is virtually no sex. Protest against this. The male cell brings to the ovum a supply of char- acteristic katastates. 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, fertilisation is an important source of variation ; according to Weismann, it is really the sole source. LITERATURE. Works already cited. Hertwig, O. — Das Problem der Befruchtung, &c. ; Jenaische Zeitschrift fiir Naturwissenschaften, XVIII., 1885. Maupas, E.— Comptes Rendus, 1886, 1887 ; and Archives de Zoologie exp^rimentale, 1888. Strasburgee, E. — Neue Untersuchungen iiber den Befruchtungsvorgang . bei den Phanerogamen, als Grundlage fiir eine Theorie der Zeugung. Jena, 1884. Weismann, A. — Of p. cit. , especially Die Eedeutung der sexuellen Fortpflan- zung fiir die Selektions-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 sine concubifu, 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 difificulty, 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 70 THE EVOLUTION OF SEX. Meanwhile Schaffer had observed the occurrence of partheno- genesis in minute aquatic crustaceans, the study of which has since shed some vivid hght 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. Degrees 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 development without fertilisation may occur. (a) Artificial Farthenogenesis. — There are a few curious observations which go to show that in exceptional circumstances 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 Ranafusca and R. esculenta, and of the tree-frog (Hyla arbored). But it must be noted that Leuckart long ago noted the occurrence of spontaneous division in frog ova. Similarly, Tichomiroff, experimenting with the un- fertilised ova of the silkmoth, which are occasionally partheno- genetic, was surprised to observe that ova, which would not of themselves develop parthenogenetically, might be induced to do so by certain stimuli. These consisted 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 de- veloped. It must be remembered that occasional partheno- genesis occurs in this insect, and all that Tichomiroff did was to incite this. There is no doubt that reagents may con- DEGENERATE SEXUAL REPRODUCTION. 171 siderably 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 ap- parently replaced by the stimulus afforded from the waste pro- ducts 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. " {d) Fathological 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. Such cases must be regarded as rare abnormalities, comparable perhaps to patho- logical formations which not unfrequently take place in the ovary, and it is hardly necessary to say that in no case did the development proceed far. Balfour has also cited a remarkable observation of Greeff, who saw unfertilised ova of the common starfish developing in ordinary sea water, in a perfectly normal fashion, only more slowly than usual. (f) Occasional Parthenogenesis. — In some of the lower animals, which are not themselves normally parthenogenetic, but have relatives so addicted, 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. " A whole series in 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 perish- ing. Examples of successful occasional parthenogenesis (to the extent at least of producing males) are furnished by those worker bees, wasps, and ants which exceptionally become fertile." (d) Partial Parthenogenesis. — The queen-bee, as has been already mentioned, is impregnated by a drone in her nuptial h 172 ■ THE EVOLUTION OF SEX. flight. The sperms thus received are stored up, and used to fertiHse 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 similiar, are un- fertilised, and these, as Dzierzon first clearly showed, develop solely into drones. We cannot, however, say that the absence or presence of fertilisation is the sole difference, though if fertilis- aflon 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 sometimes happens that what are called " fertile workers " crop up, which in con- sequence of some accident or misdirected intention in the nutrition, become less abortive than the host of semi-females which make up the body of workers. They are 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 bees, is also true of some wasps and ants. («) Seasonal Parthenogenesis. — In some of the minute aquatic crustaceans (Cladocerd), 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 succes- sion 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, Rdaumur and Kyber succeeded in rearing as many as 'fifty continuous parthenogenetic generations. In the gall-wasps {Cynipida) there is usually only one parthenogenetic generation between the DEGENERATE SEXUAL REPRODUCTION. 1 73 normal sexual reproductions, but in many insects besides Aphides there are several. It ought to be noted, that the parth- enogenetic 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 larvae. 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 repro- ductive rudiment develop into larvffi within the mother-larva's body. The mother falls 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, only how- ever to fall themselves 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 larvse become too constitu- tionally poor to be precociously parthenogenetic, and develop into adult midges — male and female, the latter producing how- ever 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 V6n Siebold to occur among the Strepsiptera, little insects which infest bees. (g) Total Parthenogenesis. — Lastly, in some of the minute aquatic crustaceans and in many rotifers no males have ever been found. There is every probability that the parthenogenesis is thus total ; and as the numbers are abundant, it has apparentiy been established without detriment to, at least, the continuance of the species. 174 THE EVOLUTION OF SEX. § 3. Occurrence of Parthenogenesis. — In these 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 (Pkilodinadce) 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. (/;) 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 Cypris 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 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 Apis, 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 successfully 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 relations are very varied. Thus in Cypris ovum and Notodromus jiionachus 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. ptibera, and the males are rare, appearing usually in spring. (f) 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 butterflies (Psyche helix and Solenobia, DEGENERATE SEXUAL REPRODUCTION. I7S 2 sp.) and a beetle {Gastrophysa) ; some coccus-insects and Aphides; certain saw-flies {Tenthredinida) and gall-wasps (Cynipidce), are normally patthenogenetic. In the butterflies just noticed, the males seem to dis- appear 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 SoUnobia trinquetrella, it is interesting to notice that they may predominate in numbers over the ■females. A whole brood may be male ; they are brought back with a rush. About a score of moths, including the silkmoth (Bomhyx mori) and death's-head {Sphinx atropos) have been known to exhibit casual partlienogenesis ; 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 regarded as quite distinct, and had received different generic titles, have been shown in about a score of cases to be merely the parthenogeneticand 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 pas- sive bias is so strong in plants, that it is easy to understand the rarity of parthenogenesis. The egg-cell which develops of itself must re- tain 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 par- thenogenesis has repeatedly been described, especially in regard to a native of New Hol- land, known as Ccekbogyne. When cultivated in Europe, the male flowers degenerate, and according to Braun and Hanstein disappear. Yet fertile seeds are produced. 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 partheno- genesis in a Menisperm found by him in Caracas, and named Disciphania Ernstii. "Female plants, which bore no male flowers, and which were grown perfectly isolated where there was no pos- sibility of the access of pollen from another plant, produced in three succes- sive years an increasing number of fertile fruits. " In the lower plants, however, there is no doubt on the subject. Owen's figure of the Genera- tions 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. 176 THE EVOLUTION OF SEX. Parthenogenesis frequently occurs 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 (Saprolegnice and Peronosporea). 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 sort of protective sheaths. His series may be briefly summed up. (i.) In Pythium, the male organ discharges most of its protoplasm into the female, — the usual story. (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. (3.) In Peronospora, 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 Saprolegnice, there are indeed the usual antheridia or male organs, which are directed towards the female organs, but do not 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 degener- ate the sexual reproduction, and all trace of it is often lost. The Fungus fertilises itself from its host. In the Fungus on the coffee plant, for example, the stimulus of fertilisation is replaced as it were by an "essence of coffee." Male parthenogenesis, paradoxical as it sounds, is really exhibited among lowly alg«. 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 (Magosp/ieera) which Hteckel saw, which did its best to get beyond the Protozoa, but failed as soon as it had succeeded. A DEGENERATE SEXUAL REPRODUCTION. 177 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 Parthetwgenesis. — The fate of parthenogenetic ova is very diverse. They may all perish, or all succeed ; they may turn out wholly males or wholly females, flensen notes the following suggestive series, with decreasing reproductive, as opposed to constitutional, energy at each level : — (l.) 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 («.^., Silkmoth). (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 excep.tional or periodic males [e.g., Apus, Artemia). (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 : — Result. Example. Nil Partial and pathological development Great mortality in a mixed brood. . iJ 's alone ...... (J 's mostly, a few 9 's . i 's and ? 's (one generation) i 's, and more than a few 9 's 9 9 9 (a succession), then a predomin- ance of (J 's . . . ; . . 9 9 9, then equal numbers of i 's and 9 's 9 9 9, then a minority of i 's among 9 's 9 9 9 9, very rare (5 's 9 9 9 9, non-functional <5 's among 9 's , 9 9 9 9 , ad infinitum, no iJ 's § 6. Effects of Parthenogenesis. — Since dominant in rotifers, and well established and plant-lice, it is very plain that whatever anything but prejudicial to numbers. An M Most organisms. Rarities mentioned. Many insects. Hive-bee and some other forms. Nematus (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, parthenogenesis is among water-fleas else it affects, it is aphis will continue 178 THE EVOLUTION OF SEX. 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 ancester 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 parthenogenesis favours the general life and progress of the species? It will be at once recognised that rotifers, brine-shrimps, water-fleas, aphides, coccus-insects, and so on, are relatively low forms. Only two or three butterflies and one beetle are parthenogenetic. Higher up in the scale virgin birth never occurs except in a very partial and pathological degree. But we can go further. 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 the intermingling of the male and female " germ-plasmas " in fertilisation is really the sole source of variation. That it is a source, all will admit. If it be removed therefore, as in rotifers, the species will be so much the less likely to progress. Weis- mann holds that it will not progress at all ; and though we should not go quite so far, we are bound to allow that the establishment of parthenogenesis is a handicapping of evolution. We cannot, however, follow Weismann in his next step. If all change springs from the sexual intermingling, the rotifer species cannot change at all. They cannot go forwards, nor yet backwards. Having attained to a physiological state when males became superfluous, they remain in statu quo. So he emphasises that superfluous organs, such as the sperm-receptacle, do not become rudimentary in parthenogenetic species, — " rudi- mentary organs can only occur in species with sexual reproduc- tion." This is a corollary of Weismann's contention that no individually acquired characters, either plus or minus, can be transmitted, and that the sexual intermingling is the sole source of change affecting the species. Were the main propositions proven, the corollary would follow, but there are still many dissentient voices. Without going into the general question at present, let us take the corollary by itself, (i.) Cases where males are quite unknown are comparatively few; in most cases they reappear at intervals. It is not possible, therefore, as Weismann will allow, to be certain that the sperm-receptacle DEGENERATE SEXUAL REPRODUCTION. 1 79 becomes superfluous to the species. (2.) He also allows that it does degenerate in the summer aphides, where the periodic disappearance of males is well known. (3.) In spite of the absence or else futility of impregnation in rotifers, we find the males obviously in process of degeneration. In conclusion, we believe with Weismann and others, that the absence of fertilisation is a minus in evolution, but see no warrant for supposing that it absolutely precludes either pro- gress or the reverse. The power of parthenogenetic birth has two different results. (i.) The female cell has a certain maleness about it ; it retains the stimulus which the male ele- ment usually affords ; the species will therefore be frequently of active male-like habit, e.g., rotifers and water-fleas. (2.) On the other hand, the long continued production of females means an anabolic preponderance, a weighting of the species ; and this is seen in the sluggish plant-lice, coccus insects, and the like. § 7. Peculiarity 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 that 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 earUness in the embryonic separation of the germ-cells with a most precocious reproductioii. 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. This, unfortunately, is not known to be true of some of the most signal cases of parthenogenesis {e.g., rotifers) ; but 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 l8o THE EVOLUTION OF SEX. is unknown (e.g., leeches and Sagitta), 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. This discovery is due to Weis- mann, who, with the assistance of Herr Ischikawa, has verified it in about a dozen species, Leptodora hyalina, Sida crystallina, Cypris reptans, and other water-fleas. Blochmann has also corroborated Weismann's discovery, in his observations on aphides. What theoretical importance Weismann attaches to the fact will be immediately noticed.* § 8. Theory of Parthenogenesis. — 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. It is only necessary to change cells into cell to make it reasonable to-day. One must not forget, however, that in higher animals, where parthenogenesis is unknown, polar cells have not been found often as yet, nor ever seen in birds and reptiles. And one would fain get further back still, and know why only one polar globule is formed in parthenogenetic ova. " 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 re- tained." 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 intra-cellular hermaphroditism. Nor can it yet be said that there are always two polar globules in ova * Blochmann, however, claims" to have demonstrated the formation of two polar bodies in those unfertilised eggs which are to give birth to drones. DEGENERATE SEXUAL REPRODUCTION. l8l which are impregnated. The discovery referred to is histori- cally Weismann's, while a corroboration is due to Blochmann. It is more important, however, to notice how Minot cleverly adapts himself, and rightly too, to increased knowledge of the facts. The parthenogenetic ovum only retains one polar globule, — one male element is enough; two would be "polyspermy," which is abhorred. 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 minimum, 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 have 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 algo 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 dif- ferent 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. («) Weismann has a peculiar right to be heard on the nature of partheno- genesis. For not only has he been for many years an investigator of the tiny daphnids or water-fleas, but he has recently 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 an absolute fact, but the probabilities are strong that it is. Before stating his theory, it is necessary to remember that the "germ-plasma" ofWeismann is a specific and essen- tial portion of the nucleus of ovum or sperm, part of which keeps up the 1 82 THE EVOLUTION OF SEX. continuity of heredity, by passing intact into the reproductive cells of the next generation. Besides this all-important " germ-plasma," the nucleus of the ovum contains, according to Weismann, an " ovogerietic nuclear plasma," which is of no direct importance in development, but is useful to the ovum simply as an ovum. It is the substance 'which is supposed to have to do with the general upbuilding of the egg-cell, with the accumu- lation of yolk, secreting of membranes, and the like. " The first polar body implies the removal of the ovogenetic nuclear plasma, which has become superfluous when the egg has attained maturity. The second polar body, on the other hand, implies the removal of a portion of the germ-plasma itself. This is so effected that. the number of ancestral elements (A/inen-idioplasmen) which compose it is reduced to a half. A similar reduction must also take place in the number of the male germ- elements. ' ' 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 fef tilisation, this is afforded by the importation of the sperm-nucleus, and development follows on the heels of fertilisation. The parthenogenetic 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 ovo- genetic nuclear-plasma." Now if it be true that a constant difference between an egg which can develop of itself and one that cannot, is that the former extrudes one tiny cell, and the latter, so far as yet observed, two, Weismann must be right in emphasising that part at least of the secret of parthenogenesis lies here. Partly hidden still, however, if one dare ask what there is about the par- thenogenetic ovum which limits its primitive budding to once instead of twice. Not altogether so subversive of Minot's theory either, as Weismann would niake out. Minot, as we saw, accepts the facts, but ingeniously supposes that the polar element retained in parthenogenetic ova is a male element. It is necessary, however, to examine Weismann's theory more closely, not only in its direct relation to the problem of parthenogenesis, hut because of its postulates, which run so directly counter to our reading of the phenomena of sex. (I.) Weismann's theory obviously differs very emphatically from those previously suggested. The first polar body is no skimming of antagonistic male material ; the very reverse, it is an extrusion of ovogenetic nuclear material which had to do with the upbuilding of the ovum, an emphatically female function. Nor is the second polar extrusion in any way an expulsion of male elements ; it is a giving away of some of the precious germ-plasma, the bearer of hereditary characteristics. Furthermore, even the sperm nucleus is in no peculiar sense male material ; it might as well be another ovum-nucleus. It has only a quantitative value, to restore to the nucleus of the ovum an amount of germ-plasma equivalent to that which has been so recklessly squandered. (2.) But Weismann's theory, based on the observation of facts, is in itself full of hypotheses. This distinction between ovogenetic and germ- DEGENERATE SEXUAL REPRODUCTION. 183 plasma within the germinal vesicle is an unveriliable myth. That the first polar body is an extrusion of one kind of nuclear substance, and the second something quite different, is another unproved hypothesis. Were the extru- sions markedly different, one might believe it, but they are the same. When a large cell divides very unequally, as in polar body formation, there is some vi'arrant for supposing that the little bud is different from the large cell ; but that two successive divisions, entirely similar in character, are conspicuously different, requires faith. It is allowed by all that each polar division lessens the mass (not the number) of the chromatin elements in the nucleus by a half, but so far as nucleus is concerned there is nothing whatever to show that the first division is qualitatively different from the second. The first may have more cell-substance extruded along with it, and the second may be rather a nuclear than a cell-division, but as regards "plasma" the two are, so far as the facts go, absolutely alike. The second division also follows on the heels of the first without the inter- vention of the usual resting stage. Nor of course is there any proof that a partheiiogenetic ovum does not part with half its "germ-plasma" in the first division. The distinctidn between the two kinds of nuclear plasma is, in plain words, a myth. (3.) 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 has this and keeps it. The ordinary ovum has it too, but extrudes it, to get it back again from another source. If this is all the sperm does, one cannot help wondering that such a circuitous pro- cess could ever arise. The entrance of the sperm must be looked at in two ways, — (a) It bears with it certain hereditary characteristics, doubtless in the nucleus for the most part ; (i) 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. Yet in spite of this virtual denial of sex,' — i.e., of any deep difference between male and female whether elements or organisms, — he does admit a qualitative action after all, for it is out of the mingling of the male and female germ- plasma that all variations arise. (4. ) Bbveri makes an interesting note in regard to Weismann's discovery and theory. There is a tendency, illustrated in ascarids, for the second polar division to limit itself to the chromatin elements, to be a nuclear division rather than a genuine cell-budding. Such a second division may possibly occur in the parthenogenetic ova, while there may be in reality one extrusion. A second nucleus may be formed, and retained, and act the part of a spermatozoon, very much as Minot's theory supposes. {g) Our theory of parthenogenesis is not so subtle as Weismann's nor so simple as Minot's. Just as the spores which illustrate the beginnings of sex may sometimes dispense with conjugation and germinate independently, so may ova develop parthenogenetically. These are to be regarded as incompletely differentiated female cells, which retain a measure of katabolic (relatively male) products, and thus do not need fertilisation. Such a successful balance between anabolism and 1 84 THE EVOLUTION OF SEX. katabolism is indeed the ideal of all organic life. That the extrusion of one polar globule still occurs, only shows that some katabplic products are still expelled. In parasitic fungi, sexual reproduction disappears, and surrounding waste products presumably help the purpose otherwise effected by sexual organs, so peculiarities in the conditions 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 pre- ponderance of reproductive over vegetative constitution (Hen- sen), their liberation before the anabolic bias has carried them too far, are among these favouring conditions. The incipient {disease (d) . sex (s). parthenogenesis (p). Male ( parthenogenesis (p). ■I sex (s). ( disease (d). Diagram illustrating the theory of parthenogenesis. 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 dormant. 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 im- perfectly 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 originated as a degeneration from the ordinary sexual process. It is no direct persistence of a primitive ideal state, though to some degree a recapitulation of it. One hypothetical mode of origin, which may well apply to the rotifers, is easily sketched. DEGENERATE SEXUAL REPRODUCTION. 1 85 In conditions favouring katabolism the males wore themselves out, the females became katabolic enough to do without them. We find the males, where they persist, much smaller than the female rotifers, often extremely degenerate, in one section wholly unknown. Again, from the fact that the interruption of a parthenogenetic series of females by the appearance of ma.les . usually occurs in hard times, we may infer that prosperous vital conditions induced parthenogenesis. Why then are not internal parasites parthenogenetic? They are very generally herma- phrodites, and have moreover gone beyond parthenogenesis to prolific asexual multiplication. ^ It is ■ niileaffihg to interpret the occurrence of partheno- genesis as due to "motives" and "important advantages." 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 partheno- genesis a " device." Such casual words are of little account; but 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 (Zweck- massigkeitsgrunden)," is to say the least misleading. A species of crustacean is being decimated by enemies, increased multi- plication would lessen the danger of extinction, parthenogenesis is establised, and for every one before producing eggs there are now two— w?7a tout. Against this short and easy methodwith nature we emphatically protest, and maintain that the origin of parthenogenesis was not for any subsequent advantage, but purely from necessary internal conditions. § 10 The Case of Bees.— ^e have already spoken of the "voluntary parthenogenesis " of bees. All the eggs are supposed to have the power of parthenogenesis, but all are not allowed so to develop. The fertilised eggs develop into queens and workers, the unfertilised give rise to drones. Weismann emphasises the fact that the ova are all alike. ' There is no difference between those which are, and are not to be fertilised. ine difference first appears after the maturation of the egg, and the removal ot the ovogenetic plasma." The state of the polar bodies is not known, so the question need not be complicated by suppositions about them. Writing before his discovery in regard to parthenogenesis, he says the sine qua non of development is that the nucleus acquire a certain quantity of germ-plasma ; the fertilised egg gets its quantum in the usual way by aid of the sperm, the unfertilised gets it by simple growth ; the difference of sex in the result need not be further taken into account. Again we remark, that this matter of a quantum of "germ-plasma," and the two ways of getting it, is a pure * See, however, p. 180, nole. 1 86 THE EVOLUTION OF SEX. supposition, both in general and in this particular case. Again we must note, that if parthenogenesis be decided on utilitarian principles, and if the difference of sex need not be taken into account, and if the eggs are all the same to start with, we see some difficulty in understanding the persistence of drones and sexual reproduction at all. It is a laborious and expensive way of attaining no obvious gain. But we should, indeed, like to be- sure that the ova are all the same to start with. Von Siebold said that the queen was moved by the sight of the different size of the cells to fertilise or refrain from fertilising. This may be so. Impressive as a queen's cell is, the difference between a worker's and drone's is much less striking. We suspect the impulse lies somewhere else. But barring this, the eggs laid first, when the queen is at its prime, develop into females ; the eggs which give rise to drones come later, when the mother is more exhausted. They have had less chance of differentiation — they are parthenogenetic ova. So with old queens, when the stock of sperms is also of course exhausted. Weismann quotes the experiment which Bessels made, after Dzierzon. The nuptial flight was prevented, and ova which, in the course of nature, would have been fertilised and given rise to queens and workers, were of course unfertilised, and developed parthenogenetically into males. This proves, he says, that the ova are all the same to start with. But one would like to know whether the prevention of the nuptial flight had not also its effect upon the ova, and whether the parthenogenetic ova are not always less differentiated. DEGENERATE SEXUAL REPRODUCTION. 187 * SUMMARY. 1 . _ 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 for clearness 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 of the lower forms. 5. The offspring of parthenogenetic ova is very diverse. 6. The effects of parthenogenesis on the species deserve consideration, especially by those who find in sexual intermingling the sole fountain of specific variation. 7. Parthenogenetic ova, so far as observed, form only one polar body. 8. Parthenogenetic ova are here regarded as imperfectly differentiated [j female cells, retaining certain male or katabolic characteristics. ' 9. In origin parthenogenesis is regarded as a degeneration from the ordinary sexual process. 10. The voluntary parthenogenesis of bees is taken as a concrete illustration. LITERATURE. See especially the already cited works of Balfour, Brooks, Hensen, Minot, Rolph, Sachs, Weismann ; also— Owen. — Parthenogenesis; or, The Successive Production of Procreating Individuals from a Single Ovum. London, 1849. Von SiEBOLD. — Beitrage zur Parthenogenesis. Leipzig, 1871. Leuckart. — Art "Zeugung" in Wagner's Handwbrterbuch d. Physiol., Bd. IV., 1853. Gerst^cker. — Bronn's Klassen und Ordnungen des Thierreich, Vol. V., Arthropoda. Brooks, W. K. — Law of Heredity. Baltimore, 1883. Simon, F. — Die Sexualitat, &c., Inaug. Dissertation. Breslau, 1883. Blochmann — Ueber die Richtungskorper bei Insekteneiern, Biolog. Cen- tralblatt, VII., and Morpholog. Jahrbuch, XII. Weismann, A. — Beitr. zur Naturgeschichte der Daphnoiden. Leipzig, 1876-79. Ueber die Zahl der Richtungskorper und iiber ihre Be- deutung fiir die Vererbung. Jena, 1887. Weismann, A., and Ischikawa, C. — Berichten der naturforsch. Gesell- schft., Freiburg, III., 1887. Hudson and Gosse. — ^The Rotifera. London, 1886. Plate. — Beitrage zur Naturgeschichte der Rotatorien, Jenaische Zeitschft. f. Naturwiss, XIX., 1886. Karsten, H. — Parthenogenesis und Generations-Wechsel im Thier und Pflanzenreiche. Berlin, 1 888. 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, or in other words artificial asexual multiplica-' tion. So too, only more naturally, the Canadian pond-weed has spread prodigiously in our lochs, canals, and rivers, never A group of Sea-Anemones. — From Andres. flowering, but owing its increase wholly to the asexual process. Every one knows how the gardener increases his stock by slips and cuttings, thus taking advantage of the power a part has to reproduce the whole. Quite in the same way, cultivators of bath sponges bed out little fragments to keep up a convenient ASEXUAL REPRODUCTION. 189 supply. In the last century, the Abbe Trembley delighted himself and others by the often repeated observation, that to get many hydra polypes out of one, the simplest and quickest way was to cut it in pieces. Though the fragment be very small, it will reproduce the whole, provided always that it have to start with fair samples of the different kinds of cells in the body. The same may be done any day with the much larger sea-anemones. So the earthworm, curtailed by the spade, does not necessarily suffer loss, though it suffer pain. The head por- tion grows a new tail, and even a decapitated portion may reproduce a head and brain, not that this is saying much for these. § 2. Regeneration. — Spades and knives are not exactly instruments of natiire, but they have their counterparts. Fight- ing with a rival a crab may lose its claw, or the same may The Formation of a Sponge Colony {Olynthus) by budding. — After Hseckel. happen in the frequently fatal moulting, which seems almost like a mistake in nature. Slowly, however, forgiving nature makes good the loss ; 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 patiently to regenerate an amputated eye-bearing horn twenty times running. Sometimes one is tempted to think that the animals almost understand that it is better for one member to perish than for the whole life to be lost, so readily does a starfish surrender an arm, or a lizard its tail. Yet it must be recognised that animals, like men, are often wiser than they wot of. In the panic of capture, strong con- vulsions may occur, which surprise and perhaps shock the 190 THE EVOLUTION OF SEX. molester of a sea-cucumber by the ejection of its viscera ; or a tetanic contraction 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 regrowth of part of a lizard's leg is the chef-d' ceuvre in this line. Beyond that, regeneration is restricted to little things. We constantly regenerate the skin of our lips, but we cannot naturally replace an amputated limb. It is more marvellous that we cannot, than that the lizard can. That the cells of an irritated stump should divide and multiply, and that the result should be the same as it was at the first, is really no marvel, or rather as much as, but no more than the original development. The dividing cells of the growing stump are simply repeating their original development. § 3. Degrees of Asexual Reproduction. — The keynote of the subject was truly struck by Spencer and Haeckel, when they defined asexual reproduction as discontinuous growth. All growth is a reproduction of the protoplasm and its nuclear elements, or in short of the cells ; 'all reproduJki^n(excluding the important fact of fertilisation)! is growth. The ovum, asexually produced from the parent 5vum or its lineal de- scendant cellSj 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 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. To these we must add 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 continuous 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 ev^tually 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- ASEXUAL REPRODUCTION. 191 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 typical vegetative constitution, the asexual process is common, particu- larly among the lower forms. The most familiar of all cases is afforded by the common liverworts {Marchantia and Lunularia), Asexual Propagation of Grass — (a) the bulbils rooting on the ground ; {b) their appear- ance in the inflorescence ; (c) a small portion enlarged. — From nature. which through the formation of asexual buds or gemmse in the cups so familiar upon their thallus, are enabled to overrun our flower-pots, and so rapidly 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. 226 and 287). The alliums, and some of our common grasses also, furnish us with examples of the replacement of flowers by separable buds. 192 THE EVOLUTION OF SEX. Asexual reproduction or multiplication by more or less dis- continuous growth, without the differentiation of special and mutually dependent sex-cells, occurs frorn 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 a primitive form like Schizogenes, but it also occurs in a few of the relatively high infusorians. That the breakage sometimes means dissolution is certain ; nor is reproduc- tion 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 budj are constricted off all round. But the process is oftener concentrated in one extrusion or overflow. In budding, the separated daughter-cell is in varying degree smaller than the parent, and the process resembles an over- flow. 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 processes. The nucleus almost always shares in them in an orderly and deliberative fashion. There are variations in its behaviour as in higher animals, but there is no doubt that cell-division, with a gradient of progress 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 grow and repair their losses for a little. 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 siniple extension of the parent mass, and the budding off of new individuals, no sure distinction can in many cases be made out. Sponges do not divide, though they may be cut up, yet they give off discontinuous buds. An out- grown 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 lifeof theotherwise'dying sponge. They are complex enough, with sheaths and spicules, and some- times 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 ASEXUAL REPRODUCTION, 1 93 respect is the freshwater sponges (Spongilla) ; they have also been described Y in other common sponges, e.g., in Clione, the borer in oyster shells. Calenteraies. — In such names as zoophytes, sea-firs, sea-roses, there is a prevision of the undoubtedly plant-like character of many of the coelenterates. 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 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 nutritive . One of the acarids or lice {plyci^hagits cursor) forming a life-saving cyst, while the individual itself dies. 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- awimming embryo becomes exhausted, settles down, and exhibits pre- dominant vegetativeness with postponed sexuality. In many cases, however, 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- medusce) 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 lately described a remarkable compound medusoid (Gastroblasta raffaelii), which has sometimes as many as nine stomachs, and may be assumed to be highly nutritive. The remarkable point, however, is that the compound adult is the result not only of continued budding, but of a process of rectangular incomplete division. Along with some others it leads on towards the Portuguese man-of-war, or N 194 THE EVOLUTION OF SEX. siphonophore series. Here the larva develops at first into a simple medusa-like individual, but this buds off a manifold series of " persons," which, by dislocation or even migration, become arranged in allthe 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 " persons "—which Hseckel calls " Medusomes"— and mere organs like protective bracts, which are also budded off. I U ( < Siphonophore Colony, showing the float (a), the swimming-bells (6), and the nutritive, repro- ductive, and other " persons " beneath. — From Lang, after HEeckel. 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 {Lticemarida) may be described as sedentary ; between these we find the common aurelia, which settles down in its youth, and gives rise by division to what afterwards become the large sexual jelly-fishes (see fig. p. 202). ^ There remains two classes of coelenterates, — the Ctenophora, like Beroe, which represent a climax of activity, and never divide ; and the Actinozoa ASEXUAL REPRODUCTION. 195 (sea-anemones and corals), which lead to a passive terminus again, and exhibit profuse asexual multiplication. A few sea-anemones divide normally, 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 overflow buds of sponges. The division may be either longitudinal or cross\yise_in sea-anemones, and 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 multipliaticon of a young jelly-fish. In another of the corals {Antipatharia) Brook has recently observed how a nutritive "person" may by constriction form ii. re- productive individual on either side. Worms. — The lower worm-types are roughly dis- tinguishable 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 in- dividual 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 exceeds this form 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 nutri- tion was checked the vegetative increase ceased, and sexual reproduction set in. Not quite parallel with the above, but quite asexual, is the prolific multiplication characteristic of the flukes and tapeworms. The common liver-fluke has often several asexual generations before it finds its final host in the sheep, and is surpassed in this respect by some of its relatives. The bladder-worm, in passive ease, with a plethora of nutrition, may form asexually many "heads," each of 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 to boot. 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 Diagrammatic repre- sentation of the for- mation of a chain of individualsin the Turbellarian worm Microstojnmn lin- eare. — From I.eu- nls. 196 THE EVOLUTION OF SEX. statoblasts may further resemble sponge-gemmules in elaborateness of external equipment, a common characteristic of passive resting structures. A Sea-worm {Myrjanida) which has budded off a chain of individuals. — After M line- Ed wards. In the higher bristle-footed worm-types (C/^^/o/oa'a:), asexual multiplica- tion occurs in great variety of expression. Some, when alarmed, break up Syllis ramosa^ a ringed marine worm, in which asexual multiplication has pro- duceda branched appearance. — From M'Intosh, " Challenger " Rep. on Annelida. ASEXUAL REPRODUCTION. ,197 in a panic, but a few are also known to do this in apparently normal life. Each part — there may be more than two^reproduces the whole. Thus, at a comparatively high level among animals, reproduction 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 (Syllis 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 n series Comet form of a Starfish, showing how one arm ' ' regenerates " or reproduces other four. — From Carus Sterne, after HEeckel. 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 free. In many of these 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 individuals. ^ a=> ^^-t-.>s ', ^>' \ 'IX- \ f Adventitious buds forming at the sides of a leaf of Bryophyllum calycinum. — From nature. Starfishes and the like surrender- their " arms " so readily, that some have supposed that they might, in this way, normally multiply. A voluntary surrender of parts as a mode of multiplication is, however, in this case difficult to prove. So while crustaceans, insects, spiders, and molluscs may lose and regrow certain parts, no asexual multiplication occurs. 198 THE EVOLUTION OF SEX. In the tunicates the asexual process has again full play. It is not confined 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. ASEXUAL REPRODUCTION. I 99 SUMMARY. 1. Artificial division may be 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, leading on to the sexual process. 4. It occurs throughout the series from Protozoa to Tunicata. LITERATURE. General Works cited ; the ordinary Zoological and Botanical Text-books ; and, Lang, A. — Der Einfluss des Festsitzen auf denThieren, and der Ursprung der ungeschlechtlichen Fortpflanzung. Jena, 1886. Spencer. — Principles of Biology. London, 1866. H/«;CKEI,. — Generelle Morphologie. Berlin, 1866. Fredericq. — La Lutte pour I'Existence chez les Animaux Marins. Paris, 1889. (For " Regeneration of Parts," &c.) 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, Lovdn, 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 ALTERNATION OF GENERATIONS. 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 suborclinated 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. § 2. The Rhythm between Sexual and Asexual Reproduction. — The 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 202 THE EVOLUTION OF SEX. ova an embryo develops, which eventually settles down 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, — ^ - A -^ - A -^ ^— i A, bisexual 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 Aurelia ; i, the free-swimming embryo, or planula ; 2, the embryo settled down ; 3, 4, 5, 6, the developing asexual stage, or liydra- 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 Hasckel. ALTERNATION OF GENERATIONS. 203 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 multiplication is only a phase preceding the acquisition of sexuality. The above cases are again expressible in the simplest formula. {b) Now take a more complex case, from among the tunicates, the highest point at which the genuine alternation can be said to occur. From a fertilised ovum in Salpa, a nurse or asexual individual develops. This has a root-like process or stolon, on which buds are formed. These are set free together, and forrh a chain of sexual salps. The chain finally breaks up. Thefertihsed ova of the sexual salps grow up into nurses again. Now the only emphatic complication here is the liberation of a chain of individuals at once ; otherwise the formula holds perfectly good. In the allied Doliolum, however, the case is different. From a fertilised ovum a nurse, or asexual individual, develops as before. This produces a number of primitive buds, which cluster about the nurse. Many of them form nutritive in- dividuals, and these we may leave alone. But others become "foster-mothers," and go free, carrying with them a few of the primitive buds, — as it were their younger sisters. The foster- mother remains asexual, is a bearer merely, and need not further complicate the series. But the primitive buds which have been carried away give rise asexually to secondary buds ; these become sexual, and their fertilised ova give rise to the original ".nurse " forms. There are therefore several asexual generations between the sexual, and our formula must run, — 204 THE EVOLUTION OF SEX. § 3. Alternation between Sexual and Degenerate 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 ; these must be distinguished, vague as the boundary must be, from alternation between the ordinary sexual process and a degenerate form of the same. The adventurous history of some of the flukes ( Trematodci) may be taken as a first illustration. The common liver-fluke (Distomum or Fasciola hepaticd) which causes the disastrous " rot" in sheep has a life of vicis- situdes. The fertilised ovum gives rise to an embryo, which passes from the sheep which its sexual parent infested to the water by the field side. There it leads for a while an active life, knocking against many things, but finally attaching itself to a minute water-snail. Into this it bores, losing its covering of active cilia with change of habit, and becoming 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 form germs, and these serve in the place of genuine ova. They produce within the body of the sporocyst another brood of what are called Redice. There may be several generations of them, and the final result is a brood of minute tailed organisms \Cercaria:'), 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. The somewhat complex story may be written in lines : — The fertilised ovum gives rise to an aquatic embryo (l). This enters a water-snail, and becomes a '^sporocyst." (The sporocyst may divide. ) Within the sporocyst, cells develop into '' Redia"\2'). There may be several generations of redise (3, 4). The last generation ifiercaria) may become adult sexual liver-flukes (5). This cannot be accurately ranked as parallel to what occurs among the above-mentioned tunicates, for the redias arise from precocious repro- ductive 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 occa- sional division of the "sporocyst." A, asexual larvae ; S, sexual fluke ; the upper circles represent the special germ cells ; fertilised ova at the base. The germ-cells, which behave like ova, and yet do not rise to that level, appear sometimes in a central mass within the asexual individual, some- ALTERNATION OF GENERATIONS. 205 times simply in the epithelium lining the body walls. There may be a long series of generations producing and produced in this way, and these are often unlike one another. Flulce, embryo, sporocyst, redia, and cer- caria, are all markedly different in structure, though embryo changes into sporocyst, and cercaria into fluke. This alternation between sexual reproduction with the usual fertilisation, and reproduction by means of special cells which yet require no fertilisa- tion, prevails in many plants, e.g., ferns and mosses. From a fertilised egg-cell the ordinary fern-plant, with which everyone 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 inconspicuous 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 : — 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 con- spicuous " moss-plant" is sexual. It bears male and female organs, and an egg-cell is fertilised by a male element. The fertilised egg-cell, how- ever, does not lose its hold of the mother plant, but grows like an encum- bering 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 (see also fig. p. 201) : — Where A= inconspicuous sexless parasitic generation upon the " moss-plant." sp. = the special parthenogenetic reproductive cell or spore produced by A. S=the conspicuous sexual "moss-plant," budded from the threads developed from the spore. 2o6 THE EVOLUTION OF SEX. 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 the 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 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 be 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 redias (as it sometimes does), and these to continue the species without ever becoming really sexual, solely by means of the special cells above described. § 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. 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. ALTERNATION OF GENERATIONS. 207 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. 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, call them what you will — 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 Parthenogenetic 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 larvse, which prey upon the mother larva, eventually kill her and leave her, only themselves to become in turn similar victims of precocity. This may continue for a series of generations, with continuous decrease in the size of the reproduc- tive cells, till finally true sexuality and adult life is attained. The repro- ductive 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, we 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 organisms which produce them are adults, not juveniles. The formulae will differ 2o8 THE EVOLUTION OF SEX. mainly in the number of generations through which the parthenogenesis 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 5 = 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 (Leptodera appendiculata) found in the snail gives rise, by the or- diaary 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 (Angiostonium 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 {lihabdonema 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 %lso 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- tains that all the colonial radiolarians, known as Sphserozoa, form on the one hand isospores, which are all equal and apparently parthenogenetic, and on the other hand aHisospores, 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 sphserozoa is further complicated (a) by division of the colonies, (b) by division of the individuals of young vegetative colonies, and (c) by the fomiation of special " extra-capsular " reproductive bodies in young colonies. The history of the common fresh-water sponge (Spongilla), as told by ALTERNATION OF GENERATIONS. 209 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 coelenterates. 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 recently described by W. K. Brooks in a remarkable medusa {Epenthesls macradyi). On the reproduc- tive organs of this swim-bell there grow, like parasites, what are exactly comparable to the reproductive buds (blastostyles) of a hydroid, and these form medusoids 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 chsetopods (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 case is well stated by Hatchett Jackson in his monumental edition of RoUeston's "Forms of Animal Life," and we accept his verdict that there is really one individual throughout, except when asexual multiplication of O 2IO THE EVOLUTION OF SEX. 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 some tunicates, 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 hyasna-like carnivore Praopus, 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. § 9. Occurrence of Alternations in Plants. — In the lower plants, algae 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 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. § 10. Heredity hi 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, either in itself or through its nucleus, 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 difficillty, by supposing that what really keeps up the protoplasrnic tradition or con- tinuity between the parental ovum and the next generation, is a specific and stable portion of the nucleus, — the " germ-plasma." When a medusoid goes off from a hydroid, it carries with it a legacy of this germ-plasma, continuous with that which gave rise to the hydroid. This legacy forms the reproductive elements of the medusoid, which in turn give rise to hydroids. ALTERNATION OF GENERATIONS. The nfedusoid itself is a modified asexual growth, into which some of the germ-plasma of the hydroid has migrated ; it is literally only the bearer of the hydroid germ-plasma. Weis- mann's classic researches on hydroids have shown that the I. The hermaphrodite fern prothallus contrasted with (2 a) the male and (2 i) the female tliallus of liverwort, and (3 a and i) male and female prothallus of horsetail. Above are the correspondmg reductions of the sexual prothallia in (4) Salvinia, (5) Isoetes, (6) Cycad and Conifer, and (7) Phanerogam. reproductive cells, which by hypothesis bear the germ-plasma, 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 2 12 THE EVOLUTION OF SEX. process and multiplication by special parthenogenetic cells, as is the case in many flukes, we are in the same way bound to suppose that the cells within a sporocyst which give rise to redife are, like ova, charged with this reproductive germ-plasma. 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 reproductive-cells, of which so much is now made, but of the essential idea of the " germ- plasma." Speaking of the recurrence of a parental form_ after numerous interpolated generations, he says, " the essential con- dition 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 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 somewhat over-weighted sentence, if we read " germ-plasma " instead of " spermatic force," we have a close approximation to the modern conception of Weismann. So again, he 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." § II. Hints as to the Rationale of Alternation. — We shall have to take a fresh view of alternation of generations after the general theory of growth and reproduction has been discussed ; meanwhile, however, the physiological aspect of the facts may be simply indicated. A fixed hydroid contrasted with a swimming-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 preponderatingly 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 ALTERNATION OF GENERATIONS. 213 spore-bearing fern-plant and inconspicuous sexual prothallus is again fundamentally 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. Anabolism. Katabolism. Female. Male. Although it has just been shown that the process of alter- ', nation demands a much more thorough analysis and discrim- ination 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 214 THE EVOLUTION OF SEX. processes, and an organism in an environment where katabolism is, at an early stage, likely to gain the ascendant. 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 ex- pression of predominant anabolism, and the sexual as equally emphatically katabolic. Alternation of generations is, in fine, a rhythm between |a relatively anabolic and katabolic prepon- derance. § 12. 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, the sexual reproductive dominates. But the origin in each particular case is involved in the pedigree of the organism. Thus Hseckel 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 organisms are returning or degenerating from dominant sexuality to an asexual pro- cess. It is safe to say that the latter is more frequently the right inter- pretation 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. Where we have alternating series of similar^ forms with different degrees of sexuality, e.g., the rhythm between parthenogenesis and true sexual reproduction in aphides, Weismann once interpreted the facts as associated with the periodic action of external influ- ences (" 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" CeciJomyia ; {b) the case of the Hydromedusse, where sexuality is postponed in early life, and asexual reproduction dominates ; and (c) an origin from division of labour within a colony. Without entering upon a discussion 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 con- tinuous alternation between growth and multiplication, nutrition and repro- duction, asexuality and sexuality, anabolism and katabolism, may express itself in the life-history of the organism. Postscript. — From Mr R. J. Harvey Gibson's valuable paper on " The Terminology of the Reproductive Organs of Plants " (Proc. Liverpool Biol. Soc, Vols. III. and IV.), we take the following scheme : — A. Asexual stage or sporophyte, produces spores in sporangia (pvospor- angia and spermosporangia in higher Cryptogams and Phanerogams). B. Sexual stage ox gamophyie {oophyte and spermophyle where the thallus is unisexual), produces ova and sperms in ovaries and spermaries ; the pro- duct of union of ovum and sperm being an oosperm. ALTERNATION OF GENERATIONS. 215 SUMMARY. 1. The fact that successive generations may be marlcedly 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 swimming-bells, 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 special cells like parthenogenetic egg-cells, 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. In animals these alternations occur from sponges up to tunicates. 9. In plants they occur in algse and fungi, are almost constant in ferns and mosses, but are inconspicuous in higher plants. 10. The problem of heredity is somewhat complicated by such alter- nations. 11. Alternation of generations is but a rhythm between a relatively anabolic and katabolic preponderance. 12. The origin has varied considerably in different cases. LITERATURE. See the general works already cited ; also, Steenstrup " On the Alternation of Generations," transl. RaySoc, 1845 ; Owen's "Parthenogenesis," &c., 1849; Hzeckel's " Generelle Morphologic," 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.; and the ordinary Text-books of Zoology and Botany. BOOK IV. THEORY OF REPRODUCTION. CHAPTER XVI. Growth and Reproduction. § I. Facts of Growth. — In a well-known aphorism Linnaeus noted that living organisms .were 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 it can hardly be said that an inorganic body has any control over or credit in its growth, nor does the latter follow as the almost necessary consequence of previous waste or liberation of energy. It is one of the oldest generalisations, 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 sur- face of already established material, as is the case with a crystal, but are intercalated in the interstices of previous particles. It is of course unnecessary to 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 charac- teristically 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 nutrition. Some have even maintained that a simple organism may be physically attracted to, as well as psychically by, its food. The supply which the lifelong hunger of the protoplasm demands, is frequently afforded in greater abundance than the THE EVOLUTION OF SEX. actual necessities require. There is a surplus for further up- building after mere reparation has been made. This surplus is the condition of growth. Popularly, but yet accurately, it may be said that 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. We may fairly call giants a few of the Protozoa, 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 accummulation of yolk. Yet the unit masses generally remain very small. They have their maximum size, ooO Cell-division at the limit of growth. approximately constant for each species. Up to this point they grow, but no further. The same, as every one knows, is true of 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 GROWTH AND REPRODUCTION. 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 tEeform of the unit may be altered, and surface gained by flattening out, or very frequently by 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. Here, in fact, the famous law of Malthus holds good. § 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 ; but in the vast majority of cases it is an orderly and definite VI ' ■Diagram of the changes in the nucleus during cell-division :— coil stage (a), the formation of a double star {b, ) directly as the surplus of nutri- tion 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 persistence of matter and conservation of energy. Some apparent exceptions are readily explained. Thus, many plants seem to grow in- definitely, 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 diminishing rate, but it again expends relatively little energy in proportion to its high nutrition. Birds which expend most energy, have their size most sharply defined. § 5. T/ie 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 perennial 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. In some cases, such as salmon and frog, periods of active and preponderant nutrition are followed by times of fasting, at the end of which reproduction occurs. Foliage and fruiting, periods GROWTH AND REPRODUCTION. 225 of nutrition and crises of reproduction, 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, produces numerous buds, and even these sometimes begin themselves 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 de- velopment of the sexual organs and the occurrence of sexual reproduction. In planarian worms, the asexual multiplication of which we have already noted, Zacharias observed that favourable nutritive conditions were associated with the forma- tion of asexual chains, while a check to the nutrition brought about both the separation and the sexual maturity of the links. Rywosch corroborates this, noting in Microsfomum 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 repro- ductive 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 partheno- genetic 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 obviously not all upon the same plane. They illustrate however, at different levels, the same great con- trast. 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 p 226 THE EVOLUTION OF SEX. 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 predominate, the reproductive organs are situated. The flower occupies a kata- bolic 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 The Moonwort Fern (^Botrychitcm lunare)^ showing the con- trasted frond (a), and fructi- fication (i^). — After Sachs. Diagram of the Tiger Lily, show- ing hulhils (a) in lower axils, and flower ahove. 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 formation of flowers indicates the appearance of sexual reproduction. In many ferns, the contrast between the vegetative and re- GROWTH AND REPRODUCTION. 227 productive regions of the organism is as marked as in the flower- ing plant. Thus the moonwort {Botrychiuni) and the adder's tongue {pphioglossuni) have their spore-bearing shoots standing in conspicuous antithesis to the leafy portion, and a similar contrast is well seen in the royal fern {Osmunda) and some of its allies. In animals, the contrast in position between .reproductive organs and the general body is never so marked. Yet the"! 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. (/5) 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- 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 their 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 light- enings of the two sides. Yet the contrast is less than it seems. In previous chapters we have seen how growth, becoming over- growth, 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 indis- / tinguishable 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 repro- duction. § 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 the plant has luxuriant foliage ; but great abundance is the reverse of conducive to the richest crop of flowers and fruit. Gruber, 2 28 THE EVOLUTION OF SEX. Maupas, and others, have shown that abundant nutrition favours the asexual multiplication, i.e., the division of infusorians. 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 anabolism 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 favourable 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. 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 importance 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 temperature ; (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 reproduction, necessarily interrupted (if the life of the species is to continue) by conjugation or sexual repro- duction, 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 medusEe. More precise is the fact already cited from Zacharias, that the spontaneous asexual multiplication of GROWTH AND REPRODUCTION. 229 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 (Bipalmni), 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 tiijnes, or in relatively katabolic conditions. In the lower crus- taceans, a similar contrast of conditions has also been observed. Pollen Grain ; «, the two nuclei ; h, the general protoplasm c, the outer wall. — From Carnoy. 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 ackiaowledged 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 230 THE EVOLUTION OF SEX. 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 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 (see fig. p. 211). In animals, with their emphatically active line of de- velopment, the reproductive generation is the higher ; and in the higher forms the separate asexual existence is wholly lost. GROWTH AND REPRODUCTION, 23 1 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 limit alike for cell and for organism. 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 optinmm 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-| spending 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. Spencer, Principles of Biology ; and Meckel, Generelle Morphologic. 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 as a katabolic stimulus to an anabolic cell, and on the other side, of course, as an anabolic renewal to a katabolic cell, as well as the union of opposed hereditary characteristics. Only by a separation of the problem of "sexual reproduction" into its component problems can the solution 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 fundamental 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 Reprodiution. — Leconte and others have pointed out that reproduction really begins with thte almost mechanical breakage of a unit mass of living mg,tter, 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. Reproduction is literally a life-saving against the approach of death. AVhether it be the almost random rupture of one of the more primitive forms such as Schizogenes, or the overflow and separation of multiple buds as in Arcella, or the dissolution of a few of the infusorians, an organism, which is becoming exhausted, saves itself, and multiplies in reproducing. In some cases, reproduc- THEORY OF REPRODUCTION. 233 tion is effected by outflowing processes of the cell, which have gone a little too far. Now, such primitive forms of multiplica- tion, gradually becoming more definite, express a predominant katabolism in the unit mass. Reproduction in its simplest forms is associated with a katabolic crisis. § 3- Argument from Cell- Division. — Most unicellular organisms reproduce by cell-division, and this is, of course, a Division of an Animal Cell, showing the nucleus (a) in process of forming two daughter-nuclei, showing also the protoplasmic network {b). — From Carnoy. 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 2 34 THE EVOLUTION OF SEX. cell-division, or has this as its necessary precedent, it is associated with a katabolic crisis. § 4. Argument from the Gradations between Asexual Sever- ance of Parts and the Liberation of Special Sex- Cells. — Discuss- ing 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 overgrowth 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 HydromeduscR. The breakage or thinning away which sets a large portion free is a katabolic process, in a sense a local death. The gentleness 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. Argume7tt from the Close Connection between Reproduc- . tion and Death. — Without going back to primitive disintegra- tions, or the asexual severance of more or less large portions, we may point further to the close connection between reproduc- tion 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 continu- ing the life of the species. In some species of the primitive annelid Polygordius, 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 r\ 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- THEORY OF REPRODUCTION. 235 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, Jiowever, 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 conditions, Stylo- nichia pustulata divides once in twenty-four hours at a tem- perature of 7° to ro° 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 Siylonichia 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- 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 metamor- phosis without encystation, followed by six successive divisions. These are effected, however, "without vegetative growth, and have for their final object not multiplication but conjugation." 236 THE EVOLUTION OF SEX. In Other words, abundant food is associated with asexual repro- duction ; 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 Samper's "Animal Life," supplemented by a summary of more recent researches by one of ourselves, the general con- clusions may be drawn, — («) That heat increases reproduction, either directly or as the result of a preliminary acceleration of growth ; (/;) That increased food will, of course, favour growth, but reproduction may follow all the more markedly as an exaggerated nemesis ; {c) 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 condf- tions have a dir^t influence ; in both cases, reproduction is the expression of a latabolic crisis. 7. Conclusion. — Primitively, then, reproduction was a kata- 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 in- creasing 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 katabolic pre- ponderance. 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 THEORY OF REPRODUCTION. 237 katabolic character. And thus the opposition between nutri- tion, which, after hfe and death, is the most obvious antithesis in nature, admits of being more precisely restated in the thesis, that as a continued surplus of anaboUsra involves growth, so a relative preponderance of katabolism necessitates reproduction. Or this may be summed up once more in our fundamental diagrams : — SUM OF FUNCTIONS. Anabolism. Katabolism, Female. Male. 238 THE 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 de- mand 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 re- production. 7. General conclusion, — a relative preponderance of katabolism neces- sitates reproduction. LITERATURE. Geddes, p. — Theory of Growth, Reproduction, Sex, and Heredity. Proc. Roy. Soc. Edin. 1886. H^CKEL. — Gerierelle Morphologic. 1866. Spencer. — Principles of Biology. 1866. Semper. — Animal Life. Int. Sci. Series. 1881. 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 facts, which are in themselves of supreme importance, and which further elucidate the general biology of the subject. § Weismann'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 im- probable. In a minority of cases, already quoted, the repro- ductive 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 con- tinue the protoplasmic tradition unaltered, 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 continuous necklace-like chain of sex-cells from generation to generation, — a continuous chain upon which the mortal individual organisms arise and drop away, like so many separate and successive pendents. But in the majority of cases, such a conception, as Weismann has justly insisted, gives a false siitiplicity to the facts. A chain of insulated sex-cells, connecting the parental fertilised 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 into reproductive 240 THE EVOLUTION OF SEX. elements? Weismann's answer is a decided negative. Although no continuous chain of germ-like cells is demonstrable, there is a strict continuity of gerva-plasma. Part of the double nucleus of the fertilised ovum keeps its characteristics unaltered, in spite of manifold divisions persists intact, and is finally established in the rudiment of reproductive organs. Or in other words, those cells in which the original germ-plasma most predominates become the reproductive cells. To quote Weismann's own words, " In each development a portion of the specific germ- plasma which the parental ovum contains, is not used up in the formation of the offspring, but is reserved unchanged to The chromatin elements of the nuclei in coil (a), double star (5), and almost divided stages (c), — After Pfitzner. form the germ-cells of the following generation." In short, con- tinuity is kept up by the jjlasma of nuclei, rather than by a chain of cells. It will be observed, of course, that while early insul- ation of definite germ-cells is a demonstrable fact, to be seen in a few cases, though perhaps of wider occurrence than we know of, the continuity of germ-plasma is strictly an hypothesis. This being so, reproductive maturity may be defined as the period when the reproductive cells (bearing the inherited capital of germ-plasma) have established themselves to that degree SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 241 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 of the elements is passive and random. In slightly differentiated organisms, like sponges, there is little reason to suppose that the distinction between cells preponderating in germ-plasma 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 elements, 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 con- sonant with their predominantly 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 for alluring the female, or weapons for contest with other males — make their appearance, the voice and note may alter, and a marked increase of strength and courage may appear. 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 Q 242 THE EVOLUTION OF SEX. Strengthening of bones and muscles, and the profound psychical chaiiges which accompany the whole series of processes, are also familiar. 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 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. Since the oviduct, unlike its male counterpart, is not, in the vast majority of vertebrates, continuous with its associated organ, it is often SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 243 difficult to see how the ova once liberated into the body-cavity find their way safely into the small opening of the duct. In the frog, however, tracts of the peritoneal epithelium become ciliated, 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 cilated 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, sexuahty 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 {Polystomum) 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 americand), 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 {Cecidomyice), in which the larvae for succes- sive generations become reproductive, though only partheno- genetically. Very striking too is the trematode worm Gyrodadylus, which recalls the mystical views of the prefor- mationists, 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 2 44 THE EVOLUTION OF SEX. to sexual maturity. A more marked precocity has been ob- served in the 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 pathological occurrence. In one set of organisms precocious reproductive maturity has been of paramount importance, viz., in the flowering plants. Here the prothallium stage, as con- trasted 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. § 2. 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 females during their period of potential fertility (fecundity), and so far from being confined to the human species, has been observed at the period of " heat " in a large number of mammals. 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 fre- quently 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 ovulation, menstruation may be described as a periodic discharge of blood, glandular secretion, and cellular detritus from the lining of the uteius. 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 begin- nings 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. And while it cannot be maintained that either "heat " or ovula- SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 245 tion are necessarily 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 gynsecologists 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 continual preponderant anabolism is drafted into the growing seed or fruit. § 3. 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 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 recent observation of the sexual intertwining of Asterina and of 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 must be allowed that the primary importance of the timely encounter of the ovum and spermatozoon has per- 246 THE EVOLUTION OF SEX. petuated in the various groups a varied series of adaptations securing fecundation. At the same time, 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. Male of Paper Nautilus (Argonauta), with its modified arm, — From Leunis. 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 are formed, which are taken up by the females. In the spider the spermatozoa are stored in a special receptacle on the palp, and hence hastily transferred to the fierce female. In cuttlefishes Diplozoon fiaradoxujn^ a double organism formed from the union of two distinct hermaphrodite individual trematodes {^Diporpa)a\. an early stage in their life. 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 parasite, and hence received the name of Hectocotylus. A curious aberra- SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 247 tion from the ordinary relations is figured above, where two distinct animals (Diplozoon) join in almost life-long union. In many cases again, especially in bony fishes, there is a sexual attrac- tion between male and female, but without any copulation. The female, accompanied by fher mate, deposits ova, which he thereupon 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, which are at the same time 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 copula- tory organs, 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 crustaceans, where the male seizing the female deposits spermatozoa upon the already laid eggs. Oftener, how- ever, 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. ( I . ) There is much reason to believe that the follicles tend to burst towards the end of menstruation ; that this may be accelerated by copulation ; success- ful 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 copula- tion 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 it also depends on size, being about 280 days in cow and 150 in sheep ; on number of offspring, being about 350 in mare and 60 in dog ; and on the degree of maturity at birth, being 420 in giraffe and 40 in kangaroo. § 4. 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 248 THE EVOLUTION OF SEX. cases, the ova are retained within the mother until fertiHsed, but are expelled not long after, before development has advanced to any marked degree. Such eggs are often 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 nutri- tive 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 (Mustelus Icevis and Carcharias), in the curious bony fish Anableps, 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 terra 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 common grass- snake, normally oviparous, has been induced, in artificial condi- tions, to bring forth its young alive, and this is probably true of other forms. The parthenogenetic generations of aphides are usually viviparous, while the fertilised eggs are laid as such. § 5. 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 some time before it begins to feed at the expense of this inheritance of yolk. Later on, in the frog division of amphibians, 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 metamorphosis of echinoderm SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 249 larvae. In many cases, the cells of the embryo, independently and actively, devour the yolk and other available material, doing so after the amoeboid fashion technically known as intra-cellular. At the same time, osmotic currents may more passively effect the like result. In the whelk and related forms, a curious cannibalism is well known to occur among the crowd of embryos enclosed within a common capsule. The stronger and older devour the younger and weaker,— a struggle for existence happily of exceptional precociousness. In ' the higher vertebrates (above amphibians), 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 probably to a slight extent in reptiles) the allantois begins to assume nutritive functions, assisting in the absorption of the yolk. In placental mammals, however, a 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 lower orders may absorb nutriment as it did in birds, though from a different source, — the maternal wall. In most cases, however, 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, though no drop of blood ever passes from mother to offspring, a very intimate osmotic transfusion is effected. § 6. Lactation. — If menstruation be a means of getting rid of anabolic surplus, in absence of the foetal 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 ho]je- fuUy the possibilities of their enormous diminution by the rational hygiene of civilisation properly so-called. The milk or mammary organs are modified skin-glands, ^ 250 THE EVOLUTION OF SEX. probably most nearly allied to the ordinary sebaceous type, except in monotremes which appear to be divergent. Every one knows that they are exclusive characteristics of mammals, and are only normally functional in the female sex. Rudi- mentary 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 men who have actually given suck.* They vary greatly in position and number, a large number being doubtless the primitive condi- tion. In function, after the birth of offspring, the surrounding tissue is specially rich in white blood-corpuscles, which probably form some of the structural elements of the milk. It has also been shown that the nuclei of the gland cells undergo degene- ration, disruption, and expulsion, and that they in all likelihood form the casein elements of the nutritive fluid. 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. § 7. Other Secretions. — Every one has heard at least of " pigeon's milk," and many are familiar with its administration to the young birds. This is produced by both sexes for a week or so 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 * Merriam (Hayden's U.S. Geol. Survey, VI., p. 666) gives a definite account of male lactation in Lepus bairdi. SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 25 1 linked to the above by one of those profound physiological 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 mature testes ; that they produce, by a now normal pathological pro- cess, special waste or katabolic elements, in the form of mucous threads. The male gets rid of this uneasy encumbrance (which has a somewhat parallel pathological equivalent in higher ani- mals), by rubbing itself against objects, and thus almost mechanically has been evolved the familiar weaving of the aquatic nest. The Nest of the Stickleback {pasterosteus). — From Thomas Bolton. § 8. Incubation. — The physiological sacrifice of the female birds does not 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 tribe takes the duty usually upon himself. 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 observed to be an increased circulation in the skin of the 252 THE EVOLUTION OF SEX. abdominal region at this time, it has been argued that the bird merely sits to cool itself ! This view has been supported by the cruel experiment of singeing off the feathers from the same region in a cock, which then sat to cool the irritated surface. Yet the increased circulation may also be viewed as increased by the sitting ; in any case, the patience and solicitude of the brooding, and the subsequent diligence in feeding- the hatched young, are obviously the expression of genuine parental affec- tion. The female Surinam Toad, with young ones on its back. — From Leunis. 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 simple and possibly periodic structure, arising from aninsinking 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 SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 253 precociously after a very short uterine life, are sheltered in similar, but more developed, pouches of the skin, withiri which the teats open. In 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 The female Notoirema marsupta-ium,—3.i\ amphibian, with eggs in a dorsal sac, which is shown partly uncovered. — From Carus Sterhe, after Giinther. on the matter. Thus in the Surinam toad (Pipa), the male spreads the ova on the female's- back, a sort of erysipelas sets in, 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 Nototrema the eggs are stored in dorsal pouches. Nor are the males without their share in the task of parentage. In the obstetric frog {Alytes obsteiricans), the male helps to remove the eggs from the female, twists them in strings round his hmd legs, and buries himself in the water till the tadpoles escape and 254 THE EVOLUTION OF SEX. relieve him of his burden. In Rhinoderma darwinii, the croak- ing sacs, which were previously used for amatory calling, become enlarged as cradles for the young. The Sea-horse {Hippocatnpits guttulatus). — From the Atlas of the Naples Aquarium. 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 pva The female of the ' ' Paper Nautilus i^A rgonauta argo), with its brood-chamher. — After Leunis. 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- SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 255 cence is smoothed away. In Solenostoma (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 stickle- back, 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. § 9. Nemesis of Reproduction. — We have already shown how reproduction in its origin is linked to death. The primitive 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 true statement, as far as history furnishes an answer, is not that animals reproduce because they have to die, but that they die because they have to reproduce. 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 Magosphara, a protozoon which just as it had formed for itself a multicellular body broke up into the component 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 orthonectids, which Van Beneden has classed as Mesozoa, between the single- celled and the stable many-celled animals. Here the mature female forms numerous germ-cells, and terminates her individual life by bursting. The germs are liberated, the mother animal 2S6 THE EVOLUTION OF SEX. has been sacrificed in reproduction. "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. This is approached, but suggestively avoided, in a genus of capitellid sea-worms {Clitomastus). The whole organism is A figure of cell division suggesting the internal disruptions and.re- arrangements of the nucleus (a) and protoplasm. — From Rauber. not sacrificed, but only an abdominal portion of the body. This is in fact one of the keynotes to reproductive differentia- tion, — the sacrifice is lessened, and the fatality thus warded off. But again, we find in some threadworms or nematodes (e.g., Ascaris dadyluris) 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 SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 257 maternal body-cavity through a prolapsus uteri of the sacrificed mother. In the precocious reproduction of some midge larvae {Chironomus, &c.), the production of young is fatal through successive generations. Both Weismann and Goette, though with different interpreta- tions, note how many insects (locusts, butterflies, ephemerids, &c.) die a few hours after the production of ova. The exhaustion 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 ^ Orthonectids, showing the rupture of tlie female in liberating the germs.— From Goette, after Julin. some spiders normally die after fertilising 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- R 258 THE EVOLUTION OF SEX. 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 reproductive 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. §10. 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 incon- sistent with the apparent paradox, that local death was the beginning of reproduction. Allowing, then, that multicellular 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 physiologically complete in them- selves, and have at least very great, if not unlimited, powers of self-recuperation. They leave off where higher animal life begins, that is to say, in a unicellular state. 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 Y 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 immortal. In Weismann's own words, " Natural death occurs only ainong 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 SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 259 an unending series of individuals, each as old as the species itself, each with the power of living on indefinitely, ever divid- ing but never dyin^." Ray Lankester puts the matter tersely, "It results from the constitution of the protozoon body as a single cell, and its method of multiplication by fission, that death has no place as a natural recurrent phenomena 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 (see fig. p. 193), endure desiccation with successful patience, which is rewarded by a rejuvenescence 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 mother psyche is really extinguished when she divides into two, intrudes a conception 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, 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 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. Weismarin is, however, willing to admit the possibility, that in the suctorial Acinetae, 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 waste, portions are used up and may be 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 26o THE EVOLtFTION OF SEX. 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 youth 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 begins 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, conjugation has not been shown to occur in many Protozoa. 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 immortahty, that in being without the bondage of a " body " 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 incurs. 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 redivide for a season, the life eventually runs itself out. SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 261 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 following 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 ovum. This divides, and its daughter-cells divide and redivide. The relation between reproductive cells and the body. The continuous chain of dotted cells at first represents a succession of Protozoa ; further on, it represents the ova from which the *' bodies" (undotted) are produced. At each geiieration, a spermatozoon fertilising the liberated ovum is also indicated. 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 protozoon 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 262 THE EVOLUTION OF SEX. 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 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 continuously growing branch. Thus although death take inexorable grasp of the individual, the continuance of the life is still in a deep sense unaffected ; the reproductive elements have already claimed their protozoon 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. 263 SUMMARY. 1. Sexual maturity generally occurs towards the limit of growth, is marked by liberation of reproductive elements and by secondary charac- teristics, due to the reaction of the reproductive function on the general system. Precocious maturity may be due to constitutional or environ- mental conditions, and has been of much importance in the evolution of flowering plants. 2. Menstruation is interpreted as a means of getting rid of the anabolic surplus of the female in absence of its foetal consumption. 3. Sexual union, at first very passive and random, becomes active and definite with the gradual evolution of sex and secondary sexual organs. 4. 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. 5. 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. 6. Lactation is interpreted as an anabolic overflow. 7. 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. 8. Incubation, reaching a climax in birds, is paralleled in many other classes. 9. 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 probably caused it. 10. 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 biblio- graphy win 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, " Uber den Ursprung des Todes," Hamburg and Leipzig, 1883; and A. Weismann, " Ueber die Dauer des Lebens," Jena, 1882 ; " Ueber Leben und Tod," Jena, 1884 ; E. Maupas, " Archives de Zoologie experimentale," 1888. CHAPTER XIX. Psychological and Ethical Aspects. § I. Common Ground between Animals and Men. — Hitherto we have been justifying the orthodoxy of an anatomical training, 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 treat- ment of sex and reproduction is, however, obviously incom- plete. It wduld be rejected with scorn in reference to human life ; it must be equally rejected in regard to the higher animals, which, taken together, exhibit the 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 sub- lime," we accept the conclusion of Darwin, followed by Romanes, and others, that all other emotions which we ourselves experience, are likewise recognisable in less perfect, or sometimes more perfect, 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 proto- plasmic hunger. In multicellular animals, the liberation of sex-elements is at first very passive. It concerns the individual alone. Fertilisa- tion is a random matter; and though sex exists, sexual attraction does not. PSYCHOLOGICAL AND ETHICAL ASPECTS. 265 A grade higher, true sexual union begins to appear. But at first this simply occurs between any male and any available female. The union is physiological, not psychological ; 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 im- portant 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 Aieuckus, where the two sexes pursue their somewhat disinterested labours together. The male and female of another lamellicorn beetle {Lcthrus cephalotes) inhabit the same cavity, and the virtuous matron is said greatly to resent the intrusion of another male. As degene- rate offshoots from the path of psychic progress, or as illustra- tions 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 crustaceans where the positions are reversed (see figs. pp. 17 and 71). Among the cold-blooded fishes, the batries 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. It is indeed only in sexual and reproductive relations that the amphibians 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 266 THE EVOLUTION OF SEX. 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. Not only is there often partnership, co- operation, and evident affection beyond the hmits of the breeding periods, but there are abundant illustrations of a high standard of morality, of all the familiar sexual crimes of man- kind, and of every shade of flirtation, courtship, jealousy, and the like. 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" which contains an overflowing wealth of instances. § 3. Sexua,l 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 bathos 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 deliberate choice which not a few females 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 psychology too has been too much influenced by our intellectual superiority ; but while this, no doubt, has its correspondingly increased possibilities of emo- tional range, it does not necessarily imply a corresponding PSYCHOLOGICAL AND ETHICAL ASPECTS. 267 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 beside the man what the child-musician is to the ordinary dulness of our daily toil and thought ? The fact to be insisted upon is this, that the vague sexual attraction of the lowest organisms has been evolved into a definite reproductive 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 recog- nising 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 avei-age of the race. Whereas, admitting the theory of evolu- tion, we are not only entitled to the hope, but logically com- pelled 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 inental. The differences may be ex- 1 aggerated or lessened, but to obliterate them it would be necessary to have all the evolution over again on a new basis. I What was decided among the prehistoric Protozoa cannot be I 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 biological considera- tions 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. 268 THE EVOLUTION OF SEX. Even a recent discussion, which is professedly from the bio- logical point of view, that of Mr Romanes, sorely disappoints us in this regard. 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 inatter 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 laissezfaire, 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 discussion of the question, but our book would be left, as biological books for the most part are, without point, and its essential thesis useless, if we did not, in conclusion, 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 standpoint, the way in which it becomes possible relatively to affiliate the most varied standpoints. We shall not so readily abuse the poor savage, who lies idle in the sun for days after 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 PSYCHOLOGICAL AND ETHICAL ASPECTS. 269 life of incessant struggle with nature and his fellows for food and for life involves upon him, and the consequent necessity of correspondingly utilising every opportunity of repose to recruit and eke out the short and precarious life so indispen- sable to wife and weans, we shall see that this crude domestic economy is the best, the most moral, and the most kindly attain- able 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 earn- ings, 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 fre- quent, but these have been exaggerated. The absolute ratifi- cation 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, how- ever, 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 pre- valent, and still more towards reconstituting that complex and sympathetic co-operation between the differentiated sexes in and around which all progress past or future must depend. Instead of men and women merely labouring 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 270^ THE EVOLUTION OF SEX. 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. Pro- duction 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 failed to realise itself. 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. But to dispute whether males or females are the higher, is like disputing the relative superiority of animals or 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 sea-horse, the ob- stetric frog, many male birds, are certainly maternal; while a few females 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, ai^ stable. The males, or, to return to the terms of our thesis, the more katabohc organisms, are more variable, and therefore, as Brooks has especially emphasised, are very frequently the leaders in evolutionary progress, while the more anabolic females tend rather to preserve the constancy and integrity of the species ; thus, in a word, the general heredity is perpetuated primarily by the female, while variations are introduced by the male. Yet along paths where the reproductive sacrifice was one of the determinants of progress, we shall see later that they must have the credit of leading the way. The more active males, with a consequently 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 PSYCHOLOGICAL AND ETHICAL ASPECTS. 27 1 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 constitutional 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 predominant katabolism. That men should have greater cerebral variability and there-' fore more originality, while women have greater stability and therefore more " common sense," are facts both consistent with the general theory of sex and verifiable in common experience. The woman, conserving the effects of past variations, has what may be called the greater integrating intelligence ; the man, in- troducing new variations, is stronger in differentiation. The feminine passivity is expressed in greater patience, more open- mindedness, greater appreciation of subtle details, and con- sequently what we call more rapid intuition. The masculine activity lends a greater power of maximum effort, of scientific insight, or cerebral experiment with impressions, and is associated with an unobservant or impatient disregard of minute details, but with a stronger grasp of generalities. Man thinks more, women feels more. He discovers more, but remembers less ; she is more receptive, and less forgetful. § 5. The Loiie 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 established in her affections. So in normal cases, there naturally remains an alloy which prevents us from regarding even maternal care as alto- gether disinterested. In all such cases, our interpretations risk an undue materialism on the one hand, and an undue transcend- entahsm on the other; and while our modern temper may 272 THE EVOLUTION OF SEX. 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, A Sea-cucumber, or Holothurian (C-ucuvtaria crocca), with numerous young attached to the skin. — From Carus Sterne, after " Challenger" Narrative. PSYCHOLOGICAL AND ETHICAL ASPECTS. 2^^ 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 often occurs, as we have already noticed, a close association between mother and offspring. Even in some coelenterates, worms, and echinoderms, the offspring cling about the mother animals, and may be protected in various kinds of brood-chambers. In A Male " Sea-spider," or Pycnogonid, carrying the ova. — After Cams Stern^, some lowly crustaceans, the young may return to the shell- cavity of the mother after hatching, and even after they have undergone a moulting. 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 s 274 THE EVOLUTION OF SEX. 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, like Gammarus, swim along with their young ones, like a hen among her chickens. Some cuttlefishes take pains in keeping their egg clusters clean and safe ; while even the headless fresh-water mussel retains her young, when there is no fish present to which they may attach themselves. In fishes. , Egg-Clusters of a species of Cuttlefish.- From Von Hayek. it must be allowed that the care, if at all evident, is usually paternal ; in amphibians, it is rare ; in reptiles, somewhat more marked. In birds and mammals, however, parental care is general, and unquestionably grows into love for offspring. § 6. The Habits of the Cuckoo. — As animals exhibit the analogues of the human virtues, it is not surprising to find the occurrence of parallel vices. Those of much magnitude, such as parental negligence or cruelty, are however rare, for the conditions of life are too simple to admit of such developed PSYCHOLOGICAL AND ETHICAL ASPECTS. 275 evils as in human society, while the crimes of sexuality are also lessened by the limitations of definite breeding seasons. With- out exposing the details of the crime list, it will be instructive, as a concrete illustration, to discuss at some length the parasitic instinct of the cuckoo. Every schoolboy knows that the female cuckoo shirks the brooding sacrifice usually associated with bird maternity. But though as the Scriptures say, • somewhat too severely, of the ostrich, "she is hardened against her young ones, as though they were not hers," she is not " deprived of wisdom ; " by an elaborate and well-executed trick she foists her several eggs, at intervals of a few days, into the nests of various birds, which are usually insectivorous and suited for the upbringing of the intruder. The foster-parents, all unconscious of being fooled, hatch the cuckoo egg among their own. The nestling grows rapidly, and is a dog in the manger by birth. Greedy and jealous, he (the pronoun is oftenest correct) soon asserts his monopoly of nest and food and care, by the summary evic- tion of the rightful tenants, whether they be still passive in ovo or more awkwardly assertive as nestlings. The result is the success of the stronger. Of this habit there are various explanations, but the pre- valent one regards it as only a special case of a universal method which favours selfishness. Jenner was the first to emphasise what he regarded as obvious advantages of the trick. The bird has but a short time to stay in its breeding area, and much to do in that short time. " Nature," lie said, " has a call upon it to produce a numerous progeny," and as it is at the same time advantageous to migrate early, the gain of leaving the eggs to a succession of other birds to incubate is manifest. Darwin supposed the habit to crop up as a mere fortuitous variation, as it occasionally does in the normally nesting American cuckoo. The result was an advantage to the parent, and also to the offspring ; the former got away sooner, the latter were better cared for. Those that learned the trick prospered, those that did not were eliminated ; and so, in virtue of its natural or unnatural success, the device passed from being exceptional to become universal, became in fact an inherited specific instinct. Commenting upon this, Romanes, in a surely somewhat sanguine passage, says : " We have here a sufficiently probable explanation of the raison d'Hre of this curious instinct ; and whether it is the true reason, or the only 276 THE EVOLUTION OF SEX. reason, we are justified in setting down the instinct to the creating influence of natural selection." But against the supposition that a mere freak has been fostered by selection into a habit, it must be noticed that the trick, to be successful, must be played w^ith some care. It is hardly on a par with the casual use made by a partridge of a pheasant's nest, or by a gull of an eider duck's. Again, the advantages to the parent, apart from that of trouble saved, are somewhat dubious. Food, Macgilivray says, remains abundant, and the climate which does not injure the young for two months longer could hardly incommode the parents. Nor is the case improved outside the British area. To suppose, on the other hand, that the advantage to the young has formed the utilitarian basis, is involved in difficulties. We cannot suppose that the mother bird had or has a careful forethought of the best for her offspring in sending them out to nurse. Nor is it easy to see how the comfort of fostered youth will remain as an impulse to the adult to do the like for her young in turn. The difficulty as to the inheritance of such a freak, especially with the preponderant majority of males, is certainly appreciable. The common difficulty of the combination of happy circumstances required to ensure incipient success is unusually great ; the young bird has its part to play as well as the parent ; the habit is not generic, yet obtains in related genera, and also in the widely separated starling-like cow-birds. A truer view of the habit is that which considers it as a deliberate expression of the whole constitution of the bird. (i.) The general character of the cuckoo is very significant. Brehni describes it as a "discontented, ill-conditioned, pas- sionate, in short decidedly unamiable bird." "The note itself, and the manner in which it is emitted, are typical of the bird's habits and character. The same abruptness, .insatiability, eagerness, the same rage, are noticeable in its whole conduct." The cuckoos are notoriously unsociable, even in migration individualistic. They jealously guard their territorial " pre- serves," and verify in many ways the old myth that they are sparrow-hawks in disguise. The parasitic habit is consonant with their general character. (2.) The species consists predominantly of males. The preponderance is probably about five to one, though one observer makes it five times greater. In so male a species, it is not surprising to find degenerate maternal instincts. PSYCHOLOGICAL AND ETHICAL ASPECTS. 277 (3.) Reproduction and nutrition, we have seen, vary in- versely. The love-impulses wane before those of hunger. Now there is no doubt that even among greedy birds the cuckoos hold a very high rank. They are remarkably in- satiable, hungry, gluttonous. Even the anatomical conditions asserted by some to be important, the swollen low-set stomach, may have their influence in the cuckoo, which has certain other peculiarities, though the same conditions may be overcome in other birds which remain perfectly natural. It might almost be suggested, that the habit of feeding so largely as cuckoos do on hairy caterpillars, whose indigestible hairs form a fretwork in the gizzard, may also have its irritant, gizzard-fretting, dyspeptic influence. But the main point is, that in a bird with so strong nutritive impulses, it is little wonder tiie reproductive emotions are degenerate. There is too much hunger and gluttony for the higher development of love. (4.) The reproductive relations of the sexes are at a lower level than polygamy, or rather polyandry. The males and females do not pair in the strict sense, there is no keeping company, though the males are said to be passionate during the breeding season. Nor is the female in its adult state externally distinguishable from the male. (5.) The reproductive organs of both sexes are very small for the size of the bird. There is said to be a diminished blood supply. Little wonder then that the reproductive emo- tions are in degree slightly developed. The sluggish parturition, at intervals of six to eight days, is also striking and significant. (6.) The eggs are remarkably small. While the adult cuckoo is some four times the size of an adult skylark, the eggs are about the same size. The American cuckoo, which is only' occasionally parasitic, lays full-sized eggs. It is true that the size of an egg is not always proportionate to the size of the bird ; but it is reasonable to believe, that when a bird for con- stitutional conditions seems to require all it can for itself, then it will have less to spare for its reproductive sacrifice To say that the small size of the cuckoo's egg is "an adaptation in order to deceive the small birds," seems to strain the natural selection theory to the breaking point. (7.) It has been usual in discussing beginnings to take some cue from the young stages. It is noteworthy, in this light, to emphasise the jealous cruelty of the young form, — a fit pro- phecy of the adult character. In the restlessness of rapid 278 THE EVOLUTION OF SEX. growth, the nestling expresses the constitution of the species in its selfish monopolising greed and insatiable appetite. Obser- vations are recorded of the persistence of the cruel disposition into adolescence, though it usually wanes with the anatomical peculiarity of the back, not very long after birth. The young form at any rate exhibits the essential character of the species. (8.) Some corroboration is obtamed from the character of the American cuckoo. There seems no doubt that it is occa- sionally parasitic, and it is interesting to note that observers speak of its unnaturally careless indifference for the fate of its young. The character in fact is less markedly evil ; the occa- sional parasitism is just as intelligible as the occasional " rever- sion " of our cuckoo to ancestral habits, even in some cases to apparent affection for the young. (9.) In the cow-birds, again, where the habit occurs in different species in different degrees of perfection (if the term be admissible), the character is strikingly immoral. In one species {Molothrus cadius), a nest rviay be simply stolen, or the rightful nestlings may be thrown out, or actual parasitism may occur as an exception. In M. canariensis, the eggs may be dropped on the bare ground, or fifteen to twenty from different parents may be lazily and of course fatally huddled together in one nest. Two cuckoo eggs are sometimes found in one nest. In M. pecoris, which is polygamous, the crime has been evolved, and the habit is that of our cuckoo, one egg being laid in each foster-nest. The important point is the general immorality and reproductive carelessness, which in one species finds expression in an organised device. Co7iclusioH. — The general character of the birds — the un- social life, the selfish cruelty of the nestlings, and the lazy para- sitic habit — have a common basis in the constitution. The insatiable appetite, the small size of the reproductive organs, the smallness of the eggs, the sluggish parturition, the rapid growth of the young, the great preponderance of males, the absence of true pairing, the degeneration of maternal affection, are all correlated, and largely explicable, in terms of the funda- mental contrast between nutrition and reproduction, between hunger and love. Similar unnatifral or immoral instincts in other birds, in mammals, and even in the lower animals, are expheable in similar terms. The cuckoo's habit is a natural outcrop of the general character or constitution, only one expression of a dominant diathesis. PSYCHOLOGICAL AND ETHICAL ASPECTS. 279 In his recent important work on the " Origin of Species," Professor Einier maintains a similar view. He briefly criticises the Darwinian explanation, which appears to him to postulate too many happy combinations. He maintains that the ancestral cuckoo acted deliberately in the trick, and some of this delibe- rateness of device may still persist. The explanation of the unnatural habit is to be found in the bird's whole character and mode of life. In this connection Eimer emphasises (a) the vagabond, restless habit ; (d) the looseness of the sex relations, strong in passion, weak in love ; (c) the irregular and gluttonous nutrition considered in relation to reproductive stimulus; (d) the slow laying of the eggs, itself dependent upon nutrition, and pointing to physiological conditions which modify even the deeply-rooted impulse and instinct to brood ; {e) the degenera- tion of social instincts, and the preponderance of the egoistic. §7. Egoism and Altruism. — ^The optimism which finds in animal life only " one hymn of love " is inaccurate, like the pessimism which sees throughout nothing but selfishness. Littrd, 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 attrac- tions 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 estabhshed. 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 dis- tinguishable at the outset, the primitive hunger and love become the starting-points of divergent lines of egoistic and altruistic emotion and activity. The dilferentiation of separate sexes; the production of 28o THE EVOLUTION OF SEX. offspring which remain associated with the parents ; the occur- rence of genuine pairing beyond the Hmits of the sexual period ; the establishment of distinct families, with unmistakable affec- tion between parents, offspring, and relatives ; and lastly, the occurrence of animal varieties wider than the family, — mark important steps in the evolution of both egoism and altruism. Ideal unity. ^ ■ ^■ society. family. offspring. mates. N V R Protoplasmic identity. Diagrammatic Representation of the Relations between Nutritive, Self-Maintainmg, or Egoistic, and Reproductive, Species- Regarding, or Altruistic Activities. The diagram sums up the important facts. There are two divergent lines of emotional and practical activity, — hunger, PSYCHOLOGICAL AND ETHICAL ASPECTS. 251 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 action and reaction between the two complementary functions, the mingling becoming more and more intricate. Sexual attrac- tion ceases to be wholly selfish ; hunger may be overcome by love ; love of mates is enhanced by love for offspring ; love for off- spring broadens out into love of kind. Finally, the ideal before us is a more harmonious blending of the two streams. 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 means of sexual attraction rise from the crude and physical to the subtle and psychical, with the growth of love. 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. Except in a few precociously tender animals, genuine love for offspring is only ismphatic in birds and mammals, where the reproductive sacrifice of the mother has also been increased. 6. The cuckoo illustrates the evolution of a criminal habit, mainly due to constitutional conditions. 7. 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 turn- ing-points, and ought to blend more and more in one. LITERATURE. See works on Sexual Selection cited at Chap. I. ElMER, G. H. T. — Die Entstehung der Arten auf Grund von-Vererben Erworbener Eigenschaften nach den Gesetzen Organischen Wachsens. Jena, 1888. BiJCHNEU, L. — Liebe und Liebesleben in der Thierwelt. Berlin, 1879. * Roi.PH, W. n.—Op.-cit. Romanes, G. J. — Animal Intelligence. Internal. Sci. Series. Fourth edition, 1886 ; and Mental Evolution in Animals, by the same. Thomson, J. A. —A Theory of the Parasitic Habit of the Cuckoo. Proc. Roy. Phys. Soc. Edin. 1888. See also Carus Sterne's most admirable of general natural history books — Werden und Vergehen. Third edition. Berlin, 1886. Ploss. — Das Weib in der Natur und Vblkerkunde. Second edition. Leipzig, 1887. Mantegazza, p. — Die Physiologic der Liebe; Die Hygiene der Liebe ; Anthropologisch-Kullurhistorische Sludien liber die Geschlechtsver- haltnisse des Menschen. Jena. 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 reaUty 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 miUions, — 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 Austraha. 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 284 THE EVOLUTION OF 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 ihis, 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 poputeion" 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-Darwinian 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 with which its individuals LAWS OF MULTIPLICATION. 285 establish a moving equilibrium, sooner or later overthrown in death. To prevent extinction, the organism meets these environing actions in two distinct ways, — (l) 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 prese,rvative 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 iii 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 286 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 prion principle is thus seen to be the obverse of the other. And if we group under the term individuation all those race-preservalive 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, — (l) 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 la* 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. 287 is slill 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. mm. 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 p^ species of Onion answered in course of the preceding argument, is, How is with asexual vege- the ratio between individuation and genesis established in tative bulbils_ («) each special case? and the answer is. By natural selec- j^ong the flowers 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, 288 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 -/q =? ■j'v. 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 Man. — 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. 289 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 290 THE EVOLUTION OF SEX. effected upon the conception of Maltlius* 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. Mai thus. 1798. Increase of population tends to outrun that of subsistence. But meets checks : A. Positive. B. Preventive. To avoid A, adopt B. III. Darwin. ■ 1859. Do. Hence struggle for existence : A. Natural selection. B. Artificial selection. Leading to evolution. Laisses -/aire, i.e., on ac- 1 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 o 1 u- tion of species. Do. \Indimduate.\ 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 httle 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. 291 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-faire, upon which Darwin and Spencer alike take their stand, not only almost ignores tlie 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 se'ek, — it lies in the generalisation above estab- Let the perpendiculars above the "•' " ■ • -.- . — lineABdenotelhe increasing degree of total individuation of a series of forms i, 2, 3, 4, S, 6 (say Worm, Fish, Frog, Bird, Man, Elephant), and similarly let the perpendicu- lars to C I) represent the rate of multiplication of the same forms ; the curves joining these two series of points respectively illustrate by tlieir inverted symmetry the inverse ratio -of individuation and genesis. 1 A / / B C \ \ s s n lished; yet it is remarkable that Mr Spencer, after not only estabhshing 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- tiorj. This holds true of all species, yet most fully of man, since that modification of psychical activities in which his 292 THE EVOLUTION OF SEX. evolution essentially lies, is par 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 beheve on the subject. The vague feeling that control of fertilisation is " interfering with nature," in some utterly unwarrantable fashion, cannot be LAWS OF MULTIPLICATION. 293 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 mortahty " (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 adults 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 2 94 THE EVOLUTION OF SEX. the details of the various methods, we must refer to the Malthusian Hterature ; 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. (d.) 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 emjiloyed 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 the medical objections to both these methods of artificial check, it is answered (a) that it may in many cases be necessary tft 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 ; (6) that it is hardly a fair argument as yd to urge that the proposed checks of neo-Malihusianism 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. 295 of the present rapid rate of increase ; the possibihty 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 aVnong the poorer menibers 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 nriust 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 ri'ormal 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 aiT:iong 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 296 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 gynaecologists 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-i 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. 297 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 must 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 mainl)', as an intellectual construction, but as a discipline of life; and we need 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 unhmited 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, just as we are be- ginning to see the cure of 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, — thatthorough 298 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 encyclopsedia of medicine, or conveniently the recent careful monograph of P. Miiller {Die Unfruchtbarkdt der Ehe, Stuttgart, 1885), will furnish bibliographical details. LAWS OF MULTIPLICATION. 299 SUMMARY. 1. 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 ihat man's future evolution must continue mainly in the direction of psychical development, and pre- dicts with the iricrease 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 wilh sterility. LITERATURE. Mal'IHUS. — Theory of Population. 1806. Si'ENCKlt. — 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 demonstration ' of the fact that evolution is a modal explanation of the origin of organisms, and, on the other, the deeper problem of tlie real mechanism of the process. The former, the empirical fact of evolution, may be said to have been virtually demonstrated, soon after the middle of this century, by the labours of Spencer, Darwin, Wallace, Hseckel, and others; the latter — the real aetiology of organisms, the "how" of the 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 ta 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, altered climate, food, and other elements of the environment, were external factors in internal change, whether for progress or for degeneration. . 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 really living conception of nature, he urged the general conception of evolution, and THE REPRODUCTIVE FACTOR IN EVOLUTION. 30 1 emphasised the organism's inherent power of self-improvement, the moulding influence of' new needs, desires, and exertions, and the indirect action of the environment 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. Evolution seemed to him to be due to the inter- action of two fates, — an internal progressive power of life; and the external force of circumstances, encountered in the twofold struggle with the inanimate environment and with living competitors. 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 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 302 THE EVOLUTION OF SEX. 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 is clearly recognised. 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 prolong- ing the period of uterine life in favourable nutritive conditions; 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 suggestiveness, especially in relation to the evolution of mammals. Apart from his common-sense vie\y 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, Geoffroy and Isidore 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 pro- portion of carbonic acid in the atmosphere, which had deter- mined 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. THE REPRODUCTIVE FACTOR IN EVOLUTION. 3O3 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 :yane in disuse on the other; to a third, organisms were seen under the hammers of external forces and circumstances, beir(g con- tinuously welded in more and more perfectly adapted forms. The organism, its function, and its environment, on each of the three factors in the problem 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 principal 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 i 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- tionists becaitie 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 304 THE EVOLUTION OF SEX. 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- ever 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 rjotice, that in his recent valuable work, 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," and as such 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." Granted, but does not the author see, that if the origin of characters so important as those often possessed by males is to be ascribed to internal constitution rather than to external selection, the origin of this, that, and the other "set of characters will next be explained in the same way, as the heretics are in fact now doing. In pulling down the theory of sexual selection in favour of that of natural selection, Mr Wallace has really handed over Mr Darwin's elaborate outwork to the enemy, who will not fail to see its value for a new assault. Before we conclude this necessary historical sketch, we must however refer to the subject of debate recently 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 functional or environmental conditions might be THE REPRODUCTIVE FACTOR IN EVOLUTION. 305 transmitted as a legacy to the offspring. According to Weis- mann, and not a few others independent of and dependent on him, this has been a delusion. Not only is positive proof of such transmission of acquired cha.ra.cteTS, i.e., other than those of constitutional, 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 Weis- mann and his school so far from close or dependent, that there is a great probability against any " somatic " dint or modification Two adjacent animal cells, showing communications through adjacent intercellular substance; also the protoplasmic network, and the nucleus.— After Pfitzner . directly affecting the reproductive elements,— that is to say, affecting 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.), do lead such a charmed physiological life within the organism that they are unaffected directly by changes in the other parts of the. body, then an optimism of heredity is demonstrable. How far we believe it from being so cannot be here discussed, but the conse- quences of Weismann's conclusion to the general theory of evolution must be re-emphasised. If individually acquired characters are of importance only to the individual body, thoy 3o6 THE EVOLUTION OF SEX. are obviously of no account 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, &c. The ground is left clear for natural selectionists, and the struggle for existence acting on variations thus becomes the exclusive factor in the mechanism of evolution. But what then starts these variations which natural selection eliminates or fosters, as the case may be ? Weismann's answer is clear and definite, the , intermingling of the sexual elements in fertilisation is the sole fountain of variation ; a view which certainly accents the )" Reproductive Factor in Evolution," though it seems to us hardly to conform with the author's previously expounded opinion, that the action of the sperm upon the ovum is quanti- tative rather than qualitative. But, even if none but constitu- tional or germinal variations 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 results of sexual interminglings, but are rather the direct and necessary results of "laws of growth," of "constitu- tional tendencies," or of the precise chemical nature of the protoplasmic metabohsm in the organisms in question. If constitutional variations occur along a few definite lines, as Eimer, Geddes, and others have shown to be true in certain cases, then we can understand the origin, though not perhaps the distribution, of species apart from any long process of selec- tion, for which indeed, if variations be stictly 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 product of environmental hammering, least of all as the survivor from a crowd of unsuc- cessful 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. THE REPRODUCTIVE FACTOR IN EVOLUTION. 307 {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 just seen, Weismann finds in the intermingling of two "germ- plasmas," which is the essence of fertilisation, the sole origin of variations of any account in the evolution of the species. Whether this be consistent with Weismann's theory of fertilisa- tion or not is matter for debate, but there is no doubt that his emphasis on the evolutionary value of sexual reproduction is a most important contribution to the general theory. _ In some- what marked contrast is the view recently advocated by Hat- schek, who sees in the intermingling essential to fertilisation a counteractive of idiosyncracies, a means of controlling and checking disadvantageous individual peculiarities. The two ■ positions are not antagonistic, but rather complementary to one another. {b.) No impartial student of Darwinism can fail to admit, that in the " struggle for existence " stress is laid upon the nutritive and self-maintaining functions and strivings, while the reproductive and species-maintaining activities are regarded as of secondary importance. One cannot forget, indeed, how much Darwin insisted upon the role of " sexual selection ; " yet it has been already shown that this recognition of the repro- ductive factor was, after all, very external ; 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 mecha- nism which he emphasised, have been seriously impugned by W^allace in an attack which reacts strongly upon the critic's own position. {c.) 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 variation 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, with- out mutual intercrossing, or by independent variation." "i'he reproductive system is very apt to vary, — why, he does not say ; the consequence might readily be, that among the 3o8 THE EVOI-UTION OF SEX. progeny of a parent stock some were fertile inter se, but infer- tile 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 again there is recognition of the reproductive factor in evolution ; but how far, and in what cases species have so originated, is obviously a ques- tion which would involve discussion of each individual instance. (d.) Worthy of reiteration is the suggestion of Robert Cham.bers, crudely illustrated as it may have been, that environmental influences acted with special power upon the generative system, and that the prolongation of gestation was a maternal sacrifice which brought its own reward in the higher evolution of the offspring. Miss Buckley, along a similar line of thought, 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 emo- tional links between mother and offspring. In emphasising the progressive value of prolonged infancy, especially in the evolu- tion of the emotions, Fiske has also recognised the importance of the reproductive factor. § 3. Further 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-re- garding as only secondary. Butalongmany lines of research, such as those indicated in the preceding paragraph, 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 concentrated 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- play 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 THE REPRODUCTIVE FACTOR IN EVOLUTION. 309 bodies, but the processes of nutritive differentiation, and other individual developments, are secondary, 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 (see pp. 201, 211). Or if we take in particular the origin of the flower, which all botanists agree in regarding as a shortened branch, the natural selectionist explanation (did the theory trouble itself with such questions) would seem to be, that the flower had arisen by selection from the two other alternatives of lengthened and unshortened axes. But this is at once excluded by the physiological explanation that shortening of the axis was inevit- able, since the expense of the reproductive functions necessarily checks the vegetative ones, for it is evident that we cannot speak of Jif/^rfw;z where the imaginable alternatives -are physi- cally impossible. So too the shortening of the inflorescence from raceme to spike or flowerhead, 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 continuous subordination of the reproductive carpellary leaf, to the varia- tions 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 lorig ago pointed out by Spencer, and admits of detailed elabora- tion 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 31° THE EVOLUTION OF SEX. 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 due to Formation of the Gastriila. — From Hseckel. 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 diminishingly 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 of any THE REPRODUCTIVE FACTOR IN EVOLUTION. 3ri prevision of future advantage in remaining clubbed together in co-operation, nor indeed credited with much primitive altruism in so doing. None the less is it clear, that this greatest of morphological steps was directly due, not to any struggle, but rather to an organic sociality, or at any rate to a process which is not interpretable in terms of individual advantage. 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 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 "genitocoel," and that it only secondarilyacquired nutritive significance. It would be indeed striking if this irhportant 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, that 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 increase 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. 312 THE EVOLUTION OF SEX. 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 associative factor upon which the victory depends, as economist and biologist have too long misled each other into doing. For An Opossum {^Didelphys dorsigera carrying its young on its back. — From Carus Sterne 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 THE REPRODUCTIVE FACTOR IN EVOLUTION. 313 entire theory of organic evolution. Leaving this to other papers, but specially to a future work, 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 orgapism, 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 selection bulking as practically all-important in the foreground, to which even sexual selection was a mere harmonious corollary. 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 — j. the action and reaction — of vegetative and reproductive pro- j cesses in alternate preponderance, which seems to us of I fundamental importance, since to this the general rhythm of 1 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 temperament. 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 rever- sal through environment or other causes, and limitation or extinction through the agency of natural selection, which, 1 fl 314 THE EVOLUTION OF SEX. 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, their enhancement or diminution by natural selection, what it 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 philosophers are disputing about the government of the world. Hunger and Love are performing the task." THE REPRODUCTIVE FACTOR IN EVOLUTION. 315 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 fevi' naturalists. 3. Further indications of the importance in evolution of reproductive and species-regarding, as opposed to the nutritive and self-maintaining LITERATURE. See articles by the writers in " Chambers's Encyclopjedia," especially Biology, Botany, Environment, Evolution, and. minor articles, such as CcELENTERATES, FLOWER, Fruit, &c. ; also " Encyclopjedia 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. — Op. cit. INDEX. Age of parents influencing sex, 34, 35 AlgEe, reproduction in, 126, 127 Allantois, 249 Aloe, 243 Alternation of generations, 200-215, 229, 230 Alternation of generations in plants, 201, 210, 211 Altruism, development of, 279-281 Amnion, 249 Amphibians, parental care of, 253, 254 ; amatory emotions of, 265 Anabolism, see Protoplasm Angiostomum, 71, 208 Animalculists, 83, 84, 157, 158 Anther cells, crystals in, 131 Aphides, 45, 46, 169, 172, 173, 175, 225, 283 Apospory, 206 Archoplasm, 98 Argonauta, 246, 247 Arthropods, hermaphroditism among, 72 Ascaris, ovum of, 145, 146 Asexual reproduction, 188-199 Aura seminalis, 158, 159 Aurelia, alternation of generations, 202, 203 Baer, Von, discovery of mammalian ovum, 86 Balbiani on isolation of sex-cells, 92 ; conjugation of Protozoa, 149 Balfour on polar bodies, 106 ; on parthenogenesis, 180 Barry, Martin, I42 Bary, De, on fertilisation, 1 60 Bees, sex characters, 42 ; partial I parthenogenesis, 172 ; reproduc- tion in, 185, 186 Beneden, Van, on origin of sex- cells, 91 ; on polar bodies, 106 ; on fertilisation, 145 Bichat's analysis, 86 Bilharzia, 71, 265 Bird of Paradise, 4 Birds, sexual characters, 6, 7 ; sexual selection in, 11 ; eggs, 103 ; love among, 266, 267 Blackcock, 7 Blochmann on polar bodies, 105, 180 Body versus reproductive cells, 93, 260-262 Bonellia, 20 Boveri on archoplasm, 98 ; on fer- tilisation, 145-148, 160 Brood-chambers, 255 Brooding, 63 Brooks on sexual selection, 10-12; theory of sex, 118, 132; on fer- tilisation, i66 BUchner on love among animals, 266 Biitschli on extrusion of polar glo- bules, 106 Buffon, 300 Butterflies, 28, 46 Canestrini, theory of sex, 34 Carnoy on fertilisation, 145, 146 Castration, effects of, 23 Cecidomyia, 243 Cell-cycle, 118 Cell-division, 221-223, 233, 234, 256 3i8 INDEX. Cell theory, 86 Chara, reproductive organs of, 131 Chironomus, separation of sex-cells in, 92, 173 Chambers, Robert, on prolonged gestation, 302, 308 Claspers, 62, 247 Coccus insects, 17, 20 Coelebogyne, 175 Coelenterates, reproductive organs, 59, 60 ; hermaphroditism, 70 ; origin of sex-cells, 91 ; asexual reproduction, 193-195 ; alterna- tion of generations, 209 Colour, animal, 23, 24 Colour, floral, 309 Comparative vigour, theory of, 36 Conjugation, 151, 152, 161 Conjugation, multiple, 151 Continuity of cells, 305 Continuity of germ-plasma, see Weismann Copulation, 245-247 Co-operation, 311 Crime among animals, 275. See Cuckoo Crustaceans, sex in, 46 ; partheno- genesis in, 174 Cuckoo, habits of, 274-279 Cupid's dart, 247 Cutleria, reproduction in, 128 Cuttlefishes, organs of, 62 ; fertilis- ation in, 62 ; egg-clusters of, 274 Darwin on sexual selection, 8-14, 25, 27, 29 ; on determination of sex, 38 ; on cross and self ferti- lisation, 138; on population ques- tion, 290, 291 ; natural selec- tion, 303, 304 Darwin, Erasmus, 300, 301 Darwinism, 312-314 Death, origin of, 255, 260, 261 Dichogamy, 73 Diplozoon, 246, 247 Division of labour, 193 Drones, nature of, 19 Ducts of reproductive organs, 60 Diising on the proportions of the sexes, 38 ; on sex-determination, 47 Echinodermata, hermaphroditism among, 72 Eotocarpus, reproduction in, 128 Edible bird's nest, 250 Egg-cell, see Ovum Egg, chemistry of the, 104 Egg-envelopes, 102 Egg-laying organs, 62 Egg-shell, 103, 104 Egoism, 279-281 Eimer on humble bees, 44 ; on cuckoo, 279 ; on evolution, 306 Embryology, 82-86, 90-92, 97-105, 112-114 Encystation, 193, 259 Engelmann on dimorphism in Pro- tozoa, 129 Environment, influence of, 235, 236; transmission of acquired char- acters, 304, 306 Epigenesis, 82-86 Ethical aspects of nutrition and re- production, 279-281 " Evolution" in old sense, 82-84 Evolution, theories of, 300-305 ; re- productive factor in, 300-315; essential problems of, 313, 314 Fallopian tube, 242 Females, 1-50 passim ; passivity, 17; size, 21-; preponderant ana- bolism, 26 ; psychical characters of, 267-271 Fern, alternation of generations, 205 P'ertilisation, time of, 34 ; in plants, 138; details of, 144-14S ; nuclear union in, 145 ; duality of, 148 ; in Protozoa, 149 ; origin of, 150- 153 ; theory of, 157- 168 ; uses of, 163-167 ; a source of variation, 301 Fish, sex differences in, 6, 30 ; parental care of, 254, 255 ; ama- tory emotions of, 265 Flower, position of, 226 ; origin of, 309- Fluke, alternation of generations, 204, 207 Follicular cells, 103 Free-martin, 37 Fungi, parthenogenesis in, 176 INDEX. 319 Gall-wasps, parthenogenesis in, 175 Galton, contrast between body and sex cells, 93 ; on fertilisation, 163 Gastrula, 85, 310, 311 Gegenbaur on origin of yolk gland, 6l ; on hermaphroditisifl, 78 Gemmules of fresh-water sponge, 209 Genesis, 285 Gentry on sex in moths, 46 Germiparity, 33, 66 Germ-plasma, see Weismann Gestation, length of, 24.7 ; influence of prolonged, 302 Girou on sex, 34, 47 Goette on origin of death, 234 ; on nemesis of reproduction, 255 Gonads, or essential reproductive organs, 57-64 Graaf, 83 Graafian follicle, 242 Growth, 219-230 Gruber on reproduction of Protozoa, 228 Gyrodactylus, 207, 243 HjEckel on continuity of repro- duction, 93 ; on Protomyxa, 120 ; on alternation of generations, 214; on Magosphsera, 255 Haller, 83, 84. Hamm, discovery of spermatozoon, 109 Harvey, 82 Hatschek on fertilisation, 167 ; on nutrition and reproduction, 227 Hectocotylus, 62, 246 Hensen, theory of sex, 34 ; on fer- tilisation, 163 Heredity, see Weismann ; in alter- nating generations, 210, 211 ; acquired characters, 304-306 Hermaphroditism, 65-79 ; embry- onic, 65 ; casual, 66 ; partial, 67; normal, 70 ; degrees of, 72 ; con- ditions of, 77 ; origin of, 78 Hertwig on hermaphroditism, 68 ; on fertilisation, 159 Heyer on sex in plants, 47. Hofacker and Sadler's law, 34, 36 Hofacker on determination of sex, 34 Hunger and love, 279-281 Hybridisation, 153-155 Hydra, 59, 189, 225 Hydractinia, 193 Hydroids, alternation of genera- tions, 201, 202 Immortality, organic, 258-262 Incubation, 251-255 Individuation, 285-287, 291, 292 Insects, sex characters of, 5, II, 16-19 ; determination of sex in, 42-46 ; hermaphroditism of, 69- 70; parthenogenesis in, 174, 175 Isophagy, 161 jEEger on continuity of germ-proto- plasm, 93 Katabolism, see Protoplasm Knight, experiments on water- melons, 48, 49 Kolliker on origin of spermatozoa, no Lactation, 249, 250 Lamarck, 301 Laulani^ on embryonic hermaph- roditism, 33, 66 Leeuwenhoek, 83, no, 158 Limit of growth, 220-224 Liverwort, 227 Love, 264-281 Love for offspring, 271-274. Luciola, 25 Magosphsera, 255 Males, 1-50 passim; variability, 12, 13; activity, 17; size, 20; pigmy, 17, 20, 75, 77; colour, 23 ; predominant katabolism, 26 ; complemental, 75, 77 ; psy- chical characters, 304 Male-cell, see Spermatozoon Malpighi, 83 Malthus, 284, 290, 291 Malthusianism, 290-292 Mammals, love among,^ 266, 267 Mammary glands, 249, 250 Mantegazza on sexual characters, 10, 14 Marsupials, 252, 253 320 INDEX. Maternal sacrifice, 256, 257 Maupas on conjugation and multi- plication of infusorians, 149, 150, 228, 229, 235, 236 ; on uses of fertilisation, 163-166; on theory of organic immortality, 260 Mayflies, 257, 258 Meehan, investigations on sex of plants, 47, 49 ; on cross-fertilisa- tion, 75 Menstruation, 244, 245 Mesozoa, liberation of germs in, 60, 2SS Metabolism, see Protoplasm Micropyle, 103 Microstomum, 195, 225 Miescher on the milt of salmon, Migration of sex-cells, 60 Milk, 250 Minot (5n polar globules, 106 ; theory of genoblasts, 118, 132; on parthenogenesis, 180, 181 Mivart on sexual selection, 13, 14 Molluscs, hermaphroditism among, 72 Molothrus, 278 Moonwort, 226, 227 Moss, alternation of generations, 201, 205, 206 Myzostomata, hermaphroditism of, 77 Natural selection, a check on diver- gence of sexes, 30 ; scope of, 303, 307, 311, 313, 314 Nematodes, alternation of sexual generations in, 208 Neo-Malthusianism, 292-298. Notodelphys, 253 Nototrema, 253 Nucleus, behaviour in fertilisation, 14s ; essential elements of, 240 Nussbaum, 94, 160 Nutrition, influence on sex, 41 Nutrition versus reproduction, 223-225 Nutrition of young, 248, 249 Obstetric frog, 253, 254 Organs of reproduction, 57"64 Orthonectids, bursting of female, 257 Oviparous animals, 248 Oviparous mammals, 248 Ovipositors, 62 Ovists, 83, 84, 157 Ovo-viviparous animals, 248 Ovulation, 242, 243 Ovum, 97-108; envelopes of, 103; maturation of, 104-107; libera- tion of, 242, 243 Ovum theory, 82 Ovifen on somatic and reproductive cells, 93 ; on residual spermatic force, 212 Psedogenesis or juvenile partheno- genesis, 173 Pairing, early forms of, 265 Paper nautilus, brood-shell of female, 254, ,255 ParamsEcium, conjugation of, 149 Parental care, 270-274, 312 Parental sacrifice, 255, 256 Parthenogenesis, 169-187 Parturition, 247, 248 Penis, 61, 247 Pfliiger on polyspermy, 34 ; on sex in tadpoles, 41 Pigments, expressions , of disruptive processes, 24 Pigeon's milk, 250 Placenta, 248 Planarians, multiplication of, 229 Plants, determination of sex in, 47, 48 ; origin of sex among, 127 ; conjugation in, 143 ; fertilisation in, 140, 159 ; parthenogenesis in, 175, 176; asexual reproduction of, 191, 192 Plasmodium, 150, 151 Platner on sex-cells in snail, 125 Ploss on embryonic hermaphrodi- tism, 33, 66 Polar globules, 105, 106, 179-185 Pollen-grain, 229, 230 Polyspermy, 34 Polystomum, 243 Population question, 289-298 Precocious reproduction, 207, 243, 244 Preformation, theory of, 83, 84 Primary sexual characters, 3 Protococcus, 127 INDEX. 321 Protomyxa, 150 Protoplasm, metabolism of, 86, 87, 122, 123 ; in fertilisation, 159, 160 ; mechanics of, 223 ; in rela- tion to cell-division, 223, 224 Protozoa contrasted with Metazoa, 57, 88, 89; illustrating cell- phases, 119-121; conjugation of, 148-151 ; asexual multiplication, 192 ; immortality of, 90, 166, 258, 261 ; alternation of genera- tions in, 208 Puberty, 241, 242 Purkinje, 86 Pycnogonid, male with young, 273 Rate of increase, 283, 284 Rate of reproduction, 283-289 Rauber on somatic and reproduc- tive cells, 93 Regeneration, 189, 190 Reproduction, processes of, 137- 217 ; theory of, 221-263 ! origin of, 232, 233 ; in relation to en- vironment, 234-236 ; nemesis of, 234. 25s Reproductive cells, 81-132 passim ; in relation to the "body," 261 Reptiles, amatory emotions of, 265, 266 Richarz, theory of sex, 37 Rhinoderma, 254 Rhythms of life, 224, 225 Rolph on "sex characters, 25 ; on sex in wasps, 44 ; on crustaceans, 46 ; theory of sex, 117; on sex-cells, 125, 133 ; on fertilisation, 160, 161 ; on parthenogenesis, 181 Romanes on cuckoo, 275 ; on physiological selection, 307, 308 Rotifers, sexes of, 20 ; partheno- genesis in, 174 ; male, 257 Sabatier, theory of polarities, 106 Sacrifice, 311 Sachs, origin of fertilisation, 153; nature of fertilisation, 160 Sadler on determination of sex, 34 Salensky on primitive Metazoa, 3 1 1 Schizogenes, multiplication of, 232 Schlechter on sex in horses, 49 Schleiden, 86 Schultze, hypothesis of male and female ova, 34 Schwann, 86 Sea-anemones, asexual multiplica- tion, 188, 189 Sea-cucumber and offspring, 272 Sea-horse, parental care of male, 254, 255 Secondary sexual characters, 3-8, 22-24 Self-fertilisation, 73 Seminal vesicles, 241 Sex, determination of, 32-54 ; theory of, 117-133; origin of, 126 Sexes, characters of, 16-25; different habits, 16-19; sizes of, 19-22; intellectual and emotional differ- ences, 267-271 Sex elements, 81-95 > early separa- tion of, 91 ; compared with Pro- tozoa, 119 Sexual attraction, 266, 267 Sexual maturation, 241, 242 Sexual organs and tissues, 57-63 Sexual reproduction, 137-159 Sexual selection, 8-14, 307 Sexual union, 245-247 Siebold, Von, experiments on wasps, 44, parthenogenesis, 170 Silkmoth, parthenogenesis of, 169 Simon on fertilisation, 161 Siphonophora, 194 Snail, reproductive specialisation in the, 62 Spencer, theory of growth, 220-224 ; laws of multiplication, 284-289 ; population question, 288-292 ; factors of evolution, 304 Spermatogenesis, theory of, 113 . Spermatozoon, 109-115; discovery of, 109 ; structure, 1 10 ; forms, III; physiology of, III ; resem- blance to pollen, 114 ; chemistry of, 115 ; influence of, 301 Spiculum amoris, 62 Sponge, reproductive cells, 58, 59 > hermaphroditism, 70 ; asexual reproduction, 188, 189, 192, 193 ; alternation of generations in Spon- gilla, 208, 209 Spores, 205-214 322 INDEX. Sprengel on fertilisation in flowers, 138 Starfish, reproduction of parts, 197 Starkweather's law of sex, 36, 37 Statistics on male and female births, 35 Steenstrup on alternation of genera- tions, 200 Stickleback, habits of, 24; secretion pf, 250, 251 Sterility, 298 Strasburger on fertilisation, 159, 160 ; on parthenogenesis, 181 Summer eggs, 174 Surinam toad, 63, 252, 253 Sutton on embryonic hermaphro- ditism, 33, 66 Syllids, asexual multiplication, 197 Tadpoles, 41, 248 Tapeworm, life-history of, 209, 210 Temperance, 297, 298 Temperature, influence of, on sex, 48, 49 Thury, theory of sex, 34 Tiger-lily, 226 Treviranus, 301 ; on influence of sperm, 166, 307 Tunicates, asexual multiplication, 198 ; alternation of generations, 203 Twins, 210 ; sex of, 39 Ulothrix, reproduction in, 127 Variation, 13, 166, 167, 301, 306 Vines on reproduction in plants, 126, 127 Viviparous animals, 248 Vorticella, reproduction of, 129, 149, 150 Volvox, 58, 128-130 Wallace, sexual selection, 10-14; natural selection, 29, 303, 304 Ward, Marshall, on parasitic fungi, 229 Water-fleas, parthenogenesis in, 174 Weeping willows, 188 Weismann on fertilisation, 161, 162 ; on polar bodies, 105, 107 ; on use of fertilisation, 164 ; on parthenogenetic ova, 181-185 ! hydroids, 91, 210,211; alternation of generations, 214; continuity of germ-plasma, 94, 95, 239-241; on organic immortality, 258-261 ; inheritance of acquired characters, 51, 304-306 ; variation, 306 Whelk, cannibalism of young, 249 Winter eggs, 174 Wolff, reassertion of epigenesis, 85 Women, subjection, rights, func- tions of, 267-271 Worms, hermaphroditism in, JI ; asexual reproduction, 195- 197 ; al- ternation of generations, 209 Yolk glands, 61 Yolk-sac, 249 Yung on sex in tadpoles, 41 Zacharias on male and female ele- ments, 1 14; on asexual multipli- cation, 225 Zona pellucida, 103 Zona radiata, 103 Printed by Walter Scott, Fellhi£, Neiucastle-on-Tyne. 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