Hate QfoUcgp of Agncuiture At (dai-neU Iniueraitg Stljata, jr. g. Slibratii Cornell University Library QH 316.N4 1914 General biology; a book of outlines and p 3 1924 003 043 704 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/cu31924003043704 GENERAL BIOLOGY A Book of Outlines and Practical Studies for the General Student By JAMES G. NEEDHAM, Ph.D. PROFESSOR OF LIMNOLOGY AND GENERAL BIOLOGY IN CORNELL UNIVERSITY „ WITH 64 PRACTICAL STUDIES 287 TEXT FIGURES, AND 9 PORTRAITS SIXTH EDITION ITHACA, N. Y. THE COMSTOCK PUBLISHING CO. 1914 L-L- Copyright, 1910 BY CoMSTOCK Publishing Co. TO THE MEMORY OF JOHN GORE MY TEACHER IN THE DISTRICT SCHOOL, WHO FIRST TAUGHT ME THE USE OF A PRINCIPLE AS A TOOL OF THE MIND. PREFACE. This book offers a series of practical studies of biological phenomena for the guidance of the general student. It is not a formal text, and not at all a treatise, but only a guide intended to assist the student in acquiring for himself some real knowledge of living nature. It differs chiefly from other books intended for the use of college classes in the wider range of studies it offers, some important phases of biology having hitherto been dismissed with mere didactic instruction. Morphology has dominated — often monopo- lized — college work in biology in the past; doubtless, be- cause it was first reduced to pedagogic form, and made available for laboratory instruction. A more equable treatment is here attempted, in the hope of leading the student to a practical acquaintance with elementary phenomena in the whole broad field. The generation of biologists which began its studies with Huxley and Martin's pioneer laboratory manual has wit- nessed a marvelous expansion of biological knowledge. Departments have sprung up, and teachers as well as prac- titioners have specialized, and courses have multiplied amazingly. Yet I am persuaded that the reasons given by Huxley and Martin for offering a general course are as valid today as they were in 1868. Indeed I am inclined to think that some added reasons have grown out of the increasing applications of biological knowledge to the practical affairs of life. The conditions of our living make ever increasing demands for knowledge of life phenomena, and some com- prehension of biological principles is fast becoming a part of the common intelligence. V VI GENERAL BIOLOGY We are organisms; and out of that fact grow the funda- mental relations that general biology bears to a whole wide range of special sciences, the threshold of which may, I hope, be reached by those who follow the course here outlined. After Chapter I, which is introductory, the studies of chap- ter II should lead up to physiology* algology, mycology, bacteriology, protozoology, etc., those of chapter III, to morphology, comparative anatomy, embryology, palaeon- tology and special botany and zoology ; those of chapter IV, to c3rtology and eugenics; those of chapter V, to ecology, and limnology; those of chapter VI, to pathology, experi- mental biology, etc.; those of chapter VII, to neurology, psychology, sociology and ethnology. And in the broader sense of these terms many more special sciences are included. In the preparation of this course I have had in mind the needs of the great majority of college students, who may hardly spend more "than a year in 'cnis subject. Certainly no other subject touches their lives at so many important points. What will best serve their needs ? has been the ques- tion constantly before me; not. What has been taught hitherto? Ecological and evolutionary phenomena are just as available for practical studies as are morphological types, and I have introduced them freely, although not without pangs of regret for the good things of former courses that had to be omitted to make room. I have reduced to a minimum the directions for the laboratory study of morphological types, for excellent outlines are everywhere available for work of this sort; and I have given a larger place to outlines for field work and experimental studies. I have arranged the subject-matter to suit the seasons of the college year. I have included more than a year's work in order that selections might be made. For pedagogic reasons, I have introduced at the first phenomena of some familiarity, postponing more technical matters. Mere PREFACE vii technique has no part in this course. Pacts are neither better nor worse for educational purposes because of technical difficulties that may or may not stand in the way of their acquisition ; and therefore, other things being equal I bave given preference to such observations as are most likely to be continued after the work of the college course is ended. The purpose of the introduction given for each subject is to orient the student for the work assigned — not to replace the lecture or the recitation. I have tried to tell what he should know in order to outline what he should do; and I have tried so to shape the conclusion of his work as to invite a little thinking. During the past seven years I have been seeking methods that would facilitate the handling of bodies of facts sufficiently large for satisfactory illustration of general biological principles and phenomena. Many new exercises have been tried by my classes in field and labora- tory; the ones that have proved most serviceable are included in the following pages. Herein are detailed the methods I have found most available. The materials used are of less consequence. I have used whatever lay nearest at hand, only seeking to draw my materials from a wide range of groups, in order to extend the acquaintance of the student with the face of nature. In so far as it has been necessary to touch upon theoretical questions, it has been my purpose, not to advance any biological theories but to bring the student into practical contact with the facts under- lying all the theories. The field of biology is so vast that no one can claim expert knowledge in any considerable portion of it. It is very probable, therefore, that in covering so much ground in even so elementary a manner, I have made some mistakes. I can only hope that they may not be of such nature as to seriously mislead or confuse the student and that I may viii GENERAL BIOLOGY have the further aid of generous colleagues toward their early elimination. Many of my colleagues and former pupils have helped me with valuable suggestions and I would be glad if there were space to thank them all; and I cannot refrain from making mention of the special help that has been given me by Pro- fessors J. H. Comstock, W. A. Riley, G. F. Atkinson, B. M. Duggar, B. F. Kingsbury, I. M. Bentley, A. Hunter, R. H. McKee and Drs. A. H. Wright and W. A. Hilton on the part of the proofs that they have seen. Others of my colleagues have generously loaned me valuable portraits, concurring in my belief, that it would be worth while to introduce the faces of at least a few of the great pioneers of biology unobtrusively into the students' intellectual environment. This book exists for the sake of the practical studies con- tained in it. Mere attendance on a lecture course does not amount to much; for in biology, as in other subjects, it is only those who handle the raw materials and ,build up with them, who can ever really comprehend the superstructure. James G. Needham. CONTENTS. CHAPTER I. THE INTERDEPENDENCE OF ORGANISMS. I. The Relations Between Flowers and Insects, p. 7. i. Theadaptationof flowers to insect visitation, p. II. 2. The adaptation of insects to flower visitation, p. 17. How to know the orders of flower insects, p. 24. 3. The relative fit- ness of the different visitors to one kind of flower, p. 26. 4. The relative fltness of the different flowers visited by one kind of insect to profit by its visitation, p. 28. 5. Precise adaptation between flowers aiid insects, leading to mutual dependence, p. 29. 6. Specialization miscarried, p. 32. II. Galls, p. 35. Animal galls, p. 38. The animals that produce galls, p. 42. Key to the commoner insect larvae and mites found in galls, p. 43. III. The Relations Between Ants and Aphids, p. 47. The chance feeding by ants on the honey dew offered by aphids, p. 48 ; The habitual guarding of aphid colonies by ants, p. 49. The domestication of aphids by ants, p. 50. PRACTICAL EXERCISES. Study I. Flowers adapted to insect visitation, p. 14. Study 2. Insects adapted to visiting flowers, p. 24. Study 3. The relative fitness of the different visitors to one kind of flower, p. 26. Study 4. An examination of all the flowers visited by some common insect, p. 28. ^ Study 5. A case of precise adaptation, p. 32. Study 6. A study of common galls, p. 46. Study 7. Observations on ants and aphids, p. 54. CHAPTER II. THE SIMPLER ORGANISMS. I. SomeTypicalAlg^, p. 56. The cell, p. 61. The form of the plant body in common algae, p. 64. II. Some typical protozoans, p. 68. X GENERAL BIOLOGY IIL The Life Process in Plant and Animal Cell, p. 82. Mat- ter, p. 82. Energy, p. 83. Protoplasm, p. 88. IV. Some Intermediate AND Undifferentiated Forms, p. 91. i. Plants that lack chlorophyl, p. 92. Molds and other fungi, p. 95. Bacteria, p. 97. 2. The slime molds, p. loi. 3. The flagellates, p. 104. IV. Reproduction Among the Simpler Organisms, p. 109. Cell division, p. 109. Sexual reproduction, p. 112. PRACTICAL EXERCISES. Study 8. The cell of Spirogyra and the protoplasm of Nitella, p. 60. Study p. Observations on cell form and growth habit in algoe, p. (J/. Study 10. The stricture and activities of ParamcBcium, p. 'j2. Study II. The specialized cell bodies of Stentor and Vorticella, p. 7(5. ■ Study 12. Observations on cultures of yeast and molds, p. g6. Study I J. A few observations on bacteria, p. 100. Study 14. Observations on slime molds, p. 103. Study IS- A comparative examination of common -flagellates, p. loj. Study 16. Observations on reproduction among the simpler organisms, p. 115. CHAPTER III. ORGANIC EVOLUTION. I. The Plant Series, p. 118. Bryophytes, p. 118. Alternation of generations, p. 124. Pteridophytes, p. 128. Spermato- phytes, p. 142. II. The Animal Series, p. 156. The hydra, p. 157. The earth worm, p. 163. The salamander, p. 179. Development, p. 193. Types of nurture, p. 214. The life process, p. 217. Common features of development in plants and animals, p. 218. Systematic classification, p. 221. II. General Evolutionary Phenomena as Illustrated in Briefer Series of Organisms, p. 222. i. Divergence and convergence of development, p. 2 2 2 . Homologies and analogies p. 223. The veins in the wings of insects, p. 225. The serial homology of the higher crustaceans, p. 230. Phylogeny, p. 236. Convergence,' p. 243. 2. Progressive and regressive development, p. 245.- Palaeontology, p. 246. The persistence of the unspecialized, p. 250. Regressive development, p. 251. 3 . The correspondence between ontogeny and phylogeny, p; 255. Why evolutionary series? p. 264. CONTENTS xj III. The Processes op Evolution; Attempted Explanations, p. 266. Natural selection, p. 266. Variation, p. 267. Mutation, p. 273. The struggle for existence, p. 276. Arti- ficial selection, p. 279. Orthogenesis, p. 281. Segregation, p. 283. The interaction of external and internal forces, p. 286. PRACTICAL EXERCISES. Study 77. An examination of bryophyte characters, p. 127. Study 18. Fern development, p. 140. Study ig. The general structure of the fern sporophyte, p. 141. Study 20. A comparison of developmental features of other pterido- phytes, p. 141. Study 21. Spermatophyte structure, p. 154. Study 22. Spermatophyte development, p. 1^4. Study 23. Observations on the structure of the hydra, p. 162. Study 24. The general structure of the earth worm, p. 178. Study 25. The cellular structure of the earth worm., p. lyg. Study 26. The internal organs of an am.phibian, p. 206. Study 27. The structures of the body wall in an amphibian, p. 20J. Study 28. The cellular structure of an amphibian, pp. 208. Study 2g. The early developm.ent pf an amphibian, p. 2og. Study JO. Determination of homologies in three series of closely allied insects, p. 22g. Study 51. Observations on plasticity of form and persistence of type in Malacostraca, p. 2jj. Study 32. An attempt at interpreting a possible phylogeny, p. 238. Study 3j. A comparison of convergent species, p. 243. Study 34. The ontogeny of organs in the frog or salamander, p. 261. Study 55. Fluctuating num,erical variations, p. 2^2. Study 36. The struggle for existence among seedlings, p. 2^8, CHAPTER IV. INHERITANCE. I. The Visible Mechanism op Heredity, p. 289. The history of the germ cells, p. 296. Fertilization and maturation, p. 299. Parthenogenesis, p. 204. II. The Observable Results op Inheritance. Types of in- heritance, p. 308. Alternative inheritance, p. 310. III. Nature and Nurture, p. 318, Inheritance of acquired characters, p. 318. The meaning of nurture, p. 321. xii GENERAL BIOLOGY PRACTICAL EXERCISES. Study 57. Observations on cell divisions and on maturation of sex cells, P-305- Study j8. Observations on parthenogenesis, p. 306. Study 5p. Observations on the relation between fecundity and nurture, P- 325- CHAPTER V. THE LIFE CYCLE. I. Alternation op Generations, p. 330. II. Special Methods of Asexual Reproduction, p. 331. III. Change of Form With Alternation of Hosts, p. 340. IV. Metamorphosis, p. 342. The transformations of insects, p. 343. Internal metamorphosis, p. 347. V. Artificial Division and Combination of Organisms, p. 353. Regeneration, p. 353. Grafting, p. 360. PRACTICAL EXERCISES. Study 40. Observations on asexual reproductive methods, p. 557. Study 41. External m.etamorphosis in insects, p. 346. Study 42. Observations on internal metamorphosis, p. 351. Study 43. Experiments with regeneration in planarians, p. 360, Study 44. Grafting practice with plants, p. 363. CHAPTER VI. THE ADJUSTMENT OF ORGANISMS TO ENVIRONMENT, I. Adjustment in Place and Time, p. 369. 1. Local distribu- tion of green plants 2. Hibernation and aestivation, p. 376.. 3. Local distribution of animals, p. 378. 4. Pond life, p. 385- II. Adjustment in Manner of Life, p.' 390. i. Symbiosis, p. 390. 2. Parasitism, p. 396. 3. Pollen distribution, p. 400. III. Adjustment IN Form and Appearance, p. 404. i. The re- adaptation of insects to aquatic life, p. 407. 2. Phylogenetic adaptation in diving beetles, p. 415. 3. Animal coloration, p. 422. PRACTICAL EXERCISES. Study 4^. Woodland plant society, p. 373. Study 46. Observations on the dessication and resuscitation of rotifers, P- 378. CONTENTS xifi Study 41 . The local resident terrestrial vertebrate fauna, p. 383. Study 48. A laboratory examination of typical pond animals, p. 386. Study 4g. A field study of pond animals, p. 388. Study 50. The relations of fungus and alga in the lichen, p. 3Q4. Study 5J. A comparative examination of a series of adult parasites, of a single order, p. 400. Study 52. Pollen production as affected by its mode of distribution, p. 402. Study 5_j. The principal types of gills found in aquatic insects, p. 410. Sttidy 54. The comparative development of respiratory apparatus in aquatic inesct larvae, p. 413. Study S5' A comparison of the structure of ground beetle and diving beetle, p. 41^. Study 56. A comparative study of the size and activities of diving beetles, p. 418. Study 57. Field observations on diving beetles, p. 420. Study 's8. The adaptive structures of diving beetles, p. 421. Study §p. Examples from the local fauna of the principal types of animal coloration, p. 432. CHAPTER VII. THE RESPONSIVE LIFE OF ORGANISMS. Introduction, p. 434. I. Animal Activities, p. 437. i. Some typical sensory phe- nomena of the Protozoza,^. ^^'j. Organs of out-reach, p. 438. Some reactions of Paramoecium, p. 439. 2. Some general features of the sensory mechanism, in the Metazoa, p. 441. Intercommunication without nerves, p. 442. Sense organs p. 444. Nerve and muscle, p. 448. The reflex arc, p. 450. Control circuits, p. 453. CephaUsation, p. 455. A mechanism for adaptation of the individual, p. 4S7- Re- lations between parts and functions in the vertebrates p. 460. 3. Some typical sensory phenomena of the Metazoa, p. 469. Automatic unvarying activities, p. 469. Respon- ses automatically varying, p. 469. Sequences of automatic activities, p. 473. Learning by experience, p. 479- II. The Responsive Life of Man, p. 485. 1. The natural history of mun, p. 48^. Distinguishing human character- istics, p. 486. Language, p. 489. Tool using, p. 490- Use offire, p. 491. 2. Unwritten human history, p. 492. Ar- xiv GENERAL BIOLOGY chaeology, p. 49:8. Ethnology, p. 494. Ontogeny, p. 498. 3. The social organism, p. 500. Animism, p. sog. Social integration, p. 507. Social conduct, p. 509. PRACTICAL EXERCISES. ' Study 60. Demonstration of functions of some of the principal parts of the. nervous system in the frog, p. 456. Study 61. Observations on certain activities of caterpillars, p-. 472. Study 62. The case-building instincts of caddis worms, p. 477. Study 6 J. Experiments with trial and error in chicks, p. 481. Study 64. Survivals of animism in our own times, p. 504. APPENDIX. Preliminary outline and instructions, p. 513. Lenses, lighting, etc., p. 513. Stage mounts, p. 518. Dissecting, p. 519. Draw- ings, p. 520. .1. Materials for the practical exercises, p. 520. -3. Supplementary studies. Plancton, p. 525. Instincts of the tent cater- pillar, p. 527. PRACTICAL EXERCISES (Added in the 6th Edition) 44a. An examination of lake plancton, p. 527. 62a. Observations on a series of instinctive activities in the tent caterpillar, p. 529. INDEX. Pages 535-546. PORTRAITS: Schultze, p. 89; Pasteur, p. 93; Von Boer, p. 174; Linnaeus, p. 220; Agassiz, p. 224; Darwin, p. 277; Leeuwenhoek, p. 298; Mendel, p. 309; Aristotle, p. 470. GENERAL BIOLOGY CHAPTER I. INTERDEPENDENCE OF ORGANISMS The primary demand of individual livelihood is for food. Getting a living is the first business of life, and food is the basis of a living; for the body derives both its substance and its energy from its food. The gathering of the food for the living world is mainly the work of green plants. These derive carbon from the air and mineral matters from the soil, and build them up into living substance, clothing the earth with verdure and storing up food materials that make animal life possible.- Green plants constitute in themselves by far the greater part of the living substance that is in the earth, and support other forms of life out of the excess of their product over what is necessary to maintain their own growth and reproduction. The primary food of animals is plants and plant products. Animals consume a small part of living plants, a much larger part of plant products (fruits, tubers, wood, etc.) and nearly the whole of plant remains. They use this plant material for building their own bodies and supplying their energy, and excrete it again as simple mineral com- pounds. Thus they rapidly restore to the soil plant food materials which might otherwise remain long locked up in the bodies of dead plants. Thus the world's available supply of food material is kept in circulation; and thus, green plants and animals are complemental, each preparing food for the other. That so large a part of living vegetation escapes being eaten is due to the fact that animals, primarily herbivorous. 4 GENERAL BIOLOGY have become carnivorous, and have taken to eating each other. The carnivores prevent overproduction of herbivores, and are themselves held in check by parasites within their own ranks. Herbivores and carnivores, parasites and scavengers are everywhere; for they fulfill permanent functions of animal society. The need of shelter is another large factor in determining the habits of animals, for few can afford to live in the open, and most are so limited that they must find food and shelter in the same haunts. For both food and shelter animals are dependent primarily upon plants and secondarily upon each other, and the relations that have come to exist between them are so intricate they may fairly be compared to a web with its threads all interwoven. Interdependence. — The weak are dependent on a few, the strong upon many. The sturdy oak in the woods seems very independent in comparison with the vine that hangs upon its branches or the green mould lodged in a crevice of its bark. But from leaf to root it is beset by enemies and aided by friends.* There are caterpillars feeding upon and within its leaves. On its twigs are aphids sucking the sap out, and within them are beetles boring. - Other beetles and caterpillars live in bark and sapwood and heartwood of its trunk, and other aphids attack its roots. But about its roots there are friendly earthworms work- ing in the soil, mixing it and making it porous; and moulds, assisting in the preparation of its food. Neighboring trees shade its trunk from the scorching rays of the summer sun, and woodpeckers, nuthatches and warblers search its bark and leaves for hidden insect enemies. There are hosts of parasites also, individually insignificant, but collectively, its greatest safeguards, that work wholesale *In Packard's Forest Insects there are listed 442 species of insects affecting American oaks, and 20 additional that are found in their dead stumps. INTERDEPENDENCE OF ORGANISMS 5 destruction upon any of its enemies that may become excessively abundant; and even the squirrels that greedily gather its acorns to eat, distribute some of them in just the way to insure another generation of oaks. Moreover, this complex relation began at its birth, and will continue until it is "resolved to earth again." Weevils devour its acorns; cutworms and lusty smothering weeds imperil its infancy; and the trampling and browsing of quadrupeds are a great menace to its early youth. The storm that scars it, or the disease that weakens it makes the opportunity for attack by beetles or molds that are harmless in its health. When it is dead, its corpse is riddled by borers and softened by molds and speedily reduced to dust. And of the host of friends and enemies with which it has come in contact, each has its own friends and enemies, ready to help or to devour. There is no living thing that either lives or dies unto itself alone. Let anyone who would see for himself the complexity of the web of life, study some common plant or animal, observing all the other plants and animals affecting it, and their inter-relationships ; or, let him examine the home of some social animal, and find all the inmates of different species, and learn how they manage to live together. There is no plant or animal, no flower or fruit, no nest or burrow, no carcass or log, no product whatsoever of living nature, that will not show a community of life with re- lations infinitely varied and complex. To see how much we ourselves are continually dependent on the organic life of the world, we need only examine the food on our table, the furnishings of our house or the materials of our ward- robe ; however simple these departments of our living may be, each will attest that many kinds of plants and animals from many parts of the earth are tributary to it. GENERAL BIOLOGY Balance in Nature. — To the careful observer the face of nature changes little from decade to decade. There are giants and weaklings in every natural community, but every species is strong enough to keep on living. There are shifts of place, but rarely is one lost in the shifting. Casualties may devastate a valley or a hiU slope, but, left to itself, it is soon repopulated. And there is order and progress in the shifting. The fungi growing on this stump (fig. i) and the beetles boring in- side it, are not the same species that will feed there when it is half rotted; nor is any one of these the same that will mix its disintegrating fragments with the soil. There w^U be other stumps with sound wood in them waiting for the descend- ents of those now at work on this one. The conditions for the life of all are fairly constant, and all are capable of making such shifts as these conditions demand. The balance is maintained by limitations of food and shelter and increase of enemies, serving to prevent the undue multiplication of any species. Man is the only disturber of the natural balance of any consequence. He plows under the mixed population of the prairie and gives the soil all over to com. He finds the Fig. 1. Oak stump in an early stage of decay; shelf fungi on the bark. INTERDEPENDENCE OF ORGANISMS 7 potato a wild, straggling, solitary weed in the hills and propagates it and covers whole acres with it. Thus he disturbs the balance, for the enemies of the com and potato, with food supply enormously increased, multiply and spread as never before; and to maintain his artificial conditions man must continually put forth his hand, to assist his chosen species and to stay their enemies. A few species (such as the bison and the auk) he may by persistent slaughter exterminate; a few (such as cockroaches and some weeds) multiply in spite of his efforts against them; but most are merely held in check, and when his efforts against them cease, they speedily reoccupy their former place. All economic procedure that deals with plants and animals is based upon the knowledge of their relations to each other. The arts that feed and clothe the human race make progress as such knowledge is advanced. It is the purpose of the studies of this chapter to'give a closer acquaintance with some common phenomena il- lustrating close interdependence and intricate vital relations between organisms. Three subjects have been selected as especially available and serviceable to this end: 1. The relations between flowers and insects. 2. Galls. 3. The relations between ants. and aphids. THE RELATIONS BETWEEn/ FLOWERS AND INSECTS. Inter-relations of mutuar advantage are excellently shown by flowers and insects, and may be studied anywhere during the flowering season. The more abundant the flowers, and the more sunshiny the weather, the better will be the opportunities. Two products of the flowers are eagerly sought by insects for food, nectar and pollen: Nectar is the sugary sap of the plant secreted by nectaries 8 GENERAL BIOLOGY that are generally located within the flower, and pollen is the product of the stamens. In return for these the in- sects serve the flowers by transferring the pollen of one flower to the stigma of another, securing cross fertiliza- tion. Fig 2, A bee (Macropis ciliata) gathering pollen from the erect stamens of a loosestrife flower {Steironema *■ ciliatum.) « Figure 2 illustrates this relation. The yellow flowers of the fringed loosestrife are not nectar-bearing but they produce abundant pollen. This is m.uch sought for by a little black bee. The bee settles upon the tops of the clus- tered and protruding stamens, as shown in the figure, and scrapes the pollen out of the pollen cavities of the five long curved anthers. In doing this it turns around several times and gets the hair of its legs and of the under surface of its body filled with closely packed pollen grains. It visits a number of flowers in succession, and is very likely to deposit upon the stigma of each one, some pollen from another flower. It must be borne in mind that the purpose of the flower is to produce seed. Seeds are ripened ovules : ovules are contained in the ovule case, which is the swollen INTERDEPENDENCE OF ORGANISMS ^^S CT and hollow base of the pistil (see fig. 3). This central organ is the most important part of the flower and all the other parts are merely accessory to its seed pro- ducing function. A slender style rising from the top of the ovule case serves to hold aloft the stigma in a proper position for the reception of pollen. The stigma is the moist, uncovered spot on the tip, where the tissues of the interior are exposed. Pollen grains lodge there, and each one sends out along, hollow and excessively slender tubular process (the pollen tube), which grows down- ward through the tissues of the style like a root through soil, until it reaches an ovule. Fertilization consists in the fusion of part of the substance that passes down the pollen tube with that contained in the ovule, and will be studied in a subsequent chapter. Suffice it to say here, that fertilization is necessary for the development of seed; and that, in the case of most of the fiowers that are visited by insects, it is necessary that pollen be brought to a flower from the flowers of another plant of the same species. This is cross-pollination. In the loosestrife there are five stamens arranged in a whorl around the pistil. Each consists of a large curved pollen-bearing anther supported on a long, erect filament. The other, accessory parts of the flower (petals, sepals, etc.) while wholly unnecessary to seed production, are often of great aid in securing cross-pollination, being often wonder- fully adapted to suit the convenience and to secure the aid of insects. Pig. 3. The essential or- gans of the loosestrife flower, p, pollen-bear- ing anther; /, filament, bearing glandular hairs; o, ovule case; si/, style; 5ig, stigma; /, the bases of the fila- ments of other stamens; the dotted lines indicate the position of style and stigma at the open- ing of the flower. 10 GENERAL BIOLOGY See how the loosestrife flower is adapted to small pollen eating bees. Its stamens stand erect with anthers curving inward. The trough-like pollen cavities of the anthers, opening upward, expose their stores to the insect standing on the top. So great is the excess of pollen production over actual needs that the little the bee wastefuUy and unwit- tingly scatters over the stigma is enough for setting the seed. This store of choice food the flower reserves for its proper visitors — chiefly for this little bee. Large bees would have great difficulty in collecting pollen from flowers that hang on such slender stalks. Wingless insects, like ants, which, if gathering pollen, could run only from flower to flower upon the same plant, and which would thus be poor agents in cross-pollination, are rigidly excluded. Should they be able to run out along the slender flower stalk, and round the fringed border of the corolla and get inside it, they would still findbetween themselves and the pollen overhead a barrier of glandular hairs bearing an acrid and offensive se- cretion with which they would choose to avoid contact. This flower has a simple and very common device for ^__ preventing self poUin- ation. Its anthers ma- ture in advance of its stigma. When the flower opens the stigma is turned aside (in the position indicated by the dotted lines in fig. 3) , but later, usually when its own pollen is removed, the stigma is lifted up into the proper position for receiving that brought by some late visitor from another flower. This simple illustration of the more general phenomena will serve to introduce the following studies of the subject. Fig. 4. The flowers of a willow (Salix dis- color.) T, a single pistillate flower, removed from its cluster; x, its covering scale; n, nectary, 5, section of its ovule case-; i, clusters {catkins) of staminate flowers; M, single staminate flowers removed from the cluster. INTERDEPENDENCE OF ORGANISMS II Fig. 5. Diagram of the flower of a buttercup {Ranunculus), m, petal; n, nectary on the base of the lowermost petal; o, a se- pal \p, a central group of separate pistils; q, a mature, and r, an immature anther. I. The adaptations of flowers to visitation by insects. Most flowers that profit by insect visitation are composed of the four whorls of organs seen in the loosestrife flower. Two of these sorts of organs, the stamens and the pistils are essential to seed production : the other two sorts, petals and sepals (the floral envelopes or perianth) are merely accessory: they are often highly serviceable, being adapted in manifold ways to secure the visi- tation of proper insects. These may be wholly absent: and stamens and pistils may be de- veloped in different flowers, or even upon different plants, as in the willows (fig. 4) . The type to which most insect-visited flowers conform finds a simple expression in such a flower as that of the but- tercup (flg. s). There are many separate pistils and sta- mens: petals and sepals are separate also, and alternate in position: all the parts of these whorls are inserted on a common receptacle at a common level : the nectar, secreted under a little scale upon the base of each petal, is quite ex- posed and readily accessible to almost all visitors ; and the color is nearly uniformly yellow. These characters are variously modified in adaptation to insect visitors: a) The flowers may become more showily colored and more attractive to the eye. They may be specially marked with darker or lighter streaks or blotches about the entrance, as if to guide their visitors to the right place. In the iris (fig. 6) there are three separate GENERAL BIOLOGY entrances, each with its own guide streaks : and at the center of the flower where there is no entrance there is a convergence of lines that often deceives ill-adapted visitors in quest of the nectar. Not all the markings upon flowers are thus significant: doubtless the deposition of pig- ment follows struc- tural lines and results from physiological causes, and may often be wholly unrelated to the exigencies of pol- len transference. But there is no mistaking the meaning of the general fact that flowers adapted to insect visitation are showy, while flowers whose pollen is distributed by wind are generally inconspicuous; or, the fact that humming- "bird flowers are scar- let; or, that night blooming flowers are oftenmost white : or, that the points of en- trance for visitors are conspicuously mark- ed. b) The nectar ex- hales a great_ variety of attractive scents, and the nectaries are sequestered in various ways beyond the reach of ill-adapted visitors — Fig. 6. Top view of the flower of a wild iris {Iris versicolor). Fig. form 7. Diagrams of forms of corollas, a, bell- b, funnel-form; c, tubular; d, spurred; e, two-lipped; a, b, c are radial; d «nd ^ position , stigma or anther ) [ movements. First ready for fertilization, anther or stigma. The student will use this table for recording his observa- tions on the ten or more species of flowers selected, which should include the following floral types: 1. A simple open solitary flower. 2. A tubular or bellshaped, loosely clustered flower. 3. A spurred or saccate flower. 4. A strongly bilateral mint flower. 5. A papilionaceous flower. 6. An umbelliferous flower. 7 . A malvaceous flower. 8. A composite flower (see fig. 236.) Interpretation of the table. — The student should write out the principal conclusions that can be drawn from the facts included in the completed table. In doing this he should consider the facts of each column by themselves, and after- wards, looking for correlated characters, he should compare the columns together. For example, he will be able to see in the several columns what forms of flowers cluster and o: corolla , what colors, guide-marks, scents, what rain guards etc. prevail: but it is only by carefully comparing columni together he will learn which of the flowers show fewes' adaptations to insect visitors, which of the tubular anc which of the bilateral flowers show most adaptations, anc INTERDEPENDENCE OF ORGANISMS 17 whether there exists any correlations between bilaterality, position of the flower in the cluster, arrangement of the stamens, etc. 2. The adaptation of insects to flower visitation. In the body of an in- head thorax abdomen sect,there are three prin- cipal divisions: head, thorax and abdomen. The head bears eyes, antennae and mouth- parts, the latter con- sisting of upper and lower lips, with two pairs of jaws working horizontally between them. Fig. 10. insect, thorax ; Diagram of the external parts of an a, antennae; e, eye; oc, ocelli; /, pro- //, mesothorax; ///, metathorax; and M/z, fore and hind wings; Z,, fore, middle and hind legs; segments of the abdomen. 2, 3, 4, etc., The thorax is di- vided into three horny rings or seg- ments, each of which bears a pair of legs, and the hindmost two bear each a pair of wings. The abdo- men consists of a variable number of segments. The accompany- ing diagram (fig. 10) will serve to repre- sent the arrange- ment of parts for insects in general. Fig. II. Mouthparts of grasshopper and beetle, a, face view of grasshopper (Melanoplus femur-rubrum) showing at /, labrum ; b, labium of same ; c, mandi- ble of same; d, maxilla of same; e, mandible of soldier beetle (Chauliognath-us scuiellaris) ; /, maxilla of same, showing pollen brushes. i8 GENERAL BIOLOGY Since insects visit flowers for food, naturally, it is the parts of their bodies that serve for collecting and carrying the nectar or pollen that are most modified for flower visitation. It is their feeding apparatus, therefore, that most merits our present attention. The nature of the remarkable changes that have fitted insect mouthparts for nectar-gathering will best be understood after comparison with the simple biting mouthparts of a grasshopper. These are shown in fig. ii. The upper lip or labrum is a simple transverse membranous flap covering the mouth above. The lower lip or labium is a compound, appendage-bearing flap covering the mouth below. Between the two are two pairs of jaws that swing in and out laterally, and that are toothed on their opposed tips; but one of each pair is shown in the figure. The upper pair (mandibles) lie directly beneath the labrum; each mandible is simple and strongly toothed. The lower pair (maxillae) lie directly below the mandibles, between them and the labium. Each maxilla consists of two basal pieces {cardo and stipes) and three terminal appendages; the innermost, the lacinia, is simple, and toothed internally; the next, the galea, is two jointed and closely fits over the back of the lacinia; and the third, the palpus, is five jointed and is sensitive at its tip. The labium is a compound organ made of a pair of appendages similar to the maxillae, fused to- gether during their development on the middle line. The fused cardines constitute the submentum, the fused stipites, the mentum, and the three terminal parts are easily recog- nizable, although the lacinia is greatly reduced in size, the galea greatty expanded, and the palpus but three jointed. Of insects with this simple type of mouthparts, only a few, chiefly beetles, have taken to flower visiting; and these show more or less of narrowing of the front of the head, adapting it for entering corollas, and alteration of the tip of the lacinia to brushes of stiff pollen-, or nectar-gathering INTERDEPENDENCE OF ORGANISMS 19 hairs, in place of the usual teeth. This is shown in our figure for a pollen-eating soldier beetle (Chauliognaihus scutellaris) which swarms upon goldenrod flowers in autumn (fig. 11.) Proboscides — Most nectar-eating insects have mouthparts prolonged and combined into some sort of a sucking pro- boscis, with which they are better able to reach sequestered nectaries. In general it may be said that the proboscides are of three types : 1. The hinged and retractile type, variously developed in bees and files. 2. The coiled type, characteristic of butterfiies and moths. 3. The jointed and rigid type, characteristic of bugs (Hemiptera) . The first of these types is well illustrated by the common honey bee, in which the proboscis is made out of maxillae and labium. Labrum and mandibles are much as in the grasshopper: the labrum is narrower, and the mandibles are not toothed at the tip, but scoop-like, adapting them for moulding wax. But the maxillae and the labium are exces- sively elongated, hollowed out in- ternally and closely applied together to form a sucking tube, the anterior part of the alimentary canal being Pig. 12. Diagtam of head of ^ . , . „ , . honey bee (Apis meiufica, at the Same time modified to torm a from the side, a, antenna; 6, eye; c, labrum; d.mandi- suckmg Organ and nectar rcservoir. ble; ^, maxilla; r, its cardo; ^ . . , s, its stipes and ^, its palpus; The rcsultant proboscis IS slung /, labium, p, its palpus. , , , , , j ,1 beneath the head upon the car- dines of the maxillae (fig. 12 r) , and provided with muscles which readily extend or retract it. At the tip of the stipes is another hinge, which allows the long terminal portion to be folded backward under the head when not in use. This terminal composite joint is hollow, and from its tip pro- 20 GENERAL BIOLOGY jects a long, slender hairy tongue, that is itself retractile, and that bears a minute membranous nectar-lapping lobe at its tip. These parts seem at first very unlike labium and maxilla of the grasshopper, but it is not difficult by separating them, and examining them carefully to recognize their identity. In the accompanying figure (fig. 13) the parts are all indicated by name; and the proboscis of a short-tongued bee is similarly drawn and lettered to make their recognition easier. It will be ob- served that in the honey bee the long, tubular terminal joint of the proboscis is composed of the hollowed out laciniae of the maxillae and basal segments of the labial palpi, closely applied together. In the flies (Diptera) labrum and mandibles are rudimentary, the rudi- ments of the maxillae are intimately combined with the highly specialized labium to form the proboscis, which is hollow, retractile beneath the head, its terminal joint folding downward, much as in the bees: but at its tip, instead of the hairy, decurving, pro- trusible tongue, there is often developed a pair of up-folding, opposible flaps {labellae) with corrugated inner sur- faces (fig. 14). Since investigators are not wholly agreed as to the identity of parts in the fly labium, it will be suffi- cient if the student note its length, its folding and extension, the action of its labellae, and other characters that have to do with pollen and nectar gathering. Fig. 13. Comparative diagrams of probos- cis of long-tongued and short-tongued bees. Upper figure, the honey bee i^Apis). Lower fig- ure (Halictus, from Dr. W. A. Riley) ; o, labrum; b, man- dibles; c, maxillae; d, labium; p, pal- pus. INTERDEPENDENCE OF ORGANISMS 21 Fig. T4. Diagram of head and pro- boscis of a syrphus fly {Rhingia nasica). a, antenna; b, eye: c, proboscis, with parts outspread; h, its hinge ; /, its labellae. The coiled proboscis of moths and butterflies is th6( most specialized of all, and limits its possessors to feeding on liquids. So slender as to be filiform, coiling compactly like a watch spring beneath the head, and extending when un- rolled to a length sometimes exceeding the length of the body, it is adapted for reach- ing the nectar in the deepest corollas, and for entering the narrowest passageways. Moreover, it is most unique in structure in that it con- sists of the laciniae of the two maxillae only, these be- ing elongated, channelled within and closely applied together to form a tube. The only other mouthparts that are well developed in the commoner butterflies and moths are the labial palpi, which project forward from beneath the head, and be- tween which the proboscis coils itself up when at rest. So greatly have the mouth- parts been modified that the identity of them in their present condition would not be recognized by a beginner ; the accompanying diagram (fig 15), of a speci- men cleaned of the scales which densely cover the ves- tigial organs, indicates all the parts by name. The jointed proboscis of the Hemiptera is relatively unimportant in nectar feeding. It is rather adapted for Fig. is. Diagrams of head and mouthparts of a butterfly. a, side view of head, with proboscis partly uncoiled; b, oblique view of face, denuded of scales ; I, labrum ; md, mandible ;^, rudi- mentary palpus of maxilla; x, proboscis, composed of con- joined laciniee of maxillae: i, labium, with the large ter- minal joint of the proximal palpus re- moved. 22 GENERAL BIOLOGY piercing the tissues of plants and sucking out the sap. Only incidentally is it used for gathering nectar. Its very position and direction show it to be unadapted to probing flowers. It consists of two pairs of lancet-like organs, the modified mandibles and maxillae, enveloped by the sheathing lower lip, which is practically destitute of palpi, and distinctly jointed: the labrum is rudimentary. The accom- panying diagram shows the parts as they appear when somewhat separated (fig. 1 6). Vesture. — The parts thus far considered have to do with getting food. We will next consider that which has to do with the distributing of pollen, the vesture, or hairy covering The horny shell of the insect's body if bare would carry little pollen, but the brushes of hairs with which it is usually clothed carry pollen excellently and serve well for im- planting some of it on the surface of the stigma. Fig. 18. Pollen gathering hairs of the honey bee. Fig. 16. Diagram of head and proboscis of a bug (Pentato- nta). a, antenna; 6, eye; c, labrum; a, lancet-like man- dibles; e, maxillse; /, the jointed en- sheathing labium . of the body Fig. 17. Side view of abdomen of a bee(Afocro^i5), show- ing ventral pollen brushes. INTERDEPENDENCE OF ORGANISMS 23 Fig. 19. Hind leg of bee, ex, coxa; tr, trochanter;/, femur J, tibia; i, 2, 3, 4, s, segments of the tarsus; i, carrying the "pollen combs. It is a part of the fitness of things that these brushes are usually best de- veloped on the top of the thorax, the under surface of the abdomen (fig. 17) and the outer faces of the legs — ^the places of most frequent contact with anthers and stigmas: but special tufts of hair or scales are occasionally found in unusual places, serving the needs of some partic- ular flower. The hairs of many bees and syrphus flies bear numerous mi- croscopic lateral branches and hold pol- len grains the more securely in the angles of the branchlets (fig. 18). The hairs may gather of themselves sufficient pol- len to be worthy of consideration as food: but the pollen must then be gathered up and massed together, and for this purpose "pollen combs" (fig. 19) are developed upon the inner face of the enormously enlarged basal joint of the hind torsus of bees, and a "pollen basket" is developed on the outside of each hind leg. Other parts. — ^The modifications of other parts of the in- sect, antennae, wings and legs, have to do chiefly with ac- commodating it to entering corollas. Obviously the but- terfly shown in figure 20 could not enter, and does not need to enter bodily into a flower. The bee will again illustrate by what means the antennae have been made reversible, the legs, closely appHcable to the sides of the body, and the wings, close-folding upon the back; the whole insect compacted together, and admirably fitted for getting into, and for getting out again from, the tight places on the road to the nectar in speciaUzed corollas. 24 GENERAL BIOLOGY How to know the orders of flower insects. — But five orders* of insects are com- monly found upon flowers. The membersof these orders may readily be recognized by the following single distinctive characters : The Diptera alone have but two wings. The Lepidoptera alone have the wings covered with dust-like scales that rub off between the thumb and finger: likewise, a coiled proboscis. The Coleoptera alone have the fore wings {elytra) meet- ing in a straight line down the middle of the back, not over- lapping. The Hemiptera alone have a jointed proboscis directed backward between the fore legs. The Hymenoptera alone have a sting; likewise, they lack all the preceding characters: the small hind wings being usually attached to the margin of the fore wings by a series of booklets, the beginner may overlook them at first. Study 2. Insects adapted to visiting flowers. Apparatus needed : A cyanide bottle, an air net and a lens. Materials needed: Ten or more species of insects, to be gathered from flowers by the student, who should observe Fig. 20 Butterfly {Colias philodice) on a clover head. ♦Omitting from consideration the minute but ever present thrips (order Physopoda) found hidden within the flowers — insects usually less than a millimeter long, with straight bodies and veinless wings, of slight importance in this connection. INTERDEPENDENCE OF ORGANISMS 25 the while what each insect is doing, in order to be able to interpret the meaning of its peculiarities of structure. The advantage of possessing elbowed and reversible antennae, for example, can only be appreciated after seeing a bee force an entrance into a closed corolla, such as that of Linaria (fig. 168). The insects should then be studied in the laboratory, not too hastily, and while still fresh. If allowed to become brittle through drying, they may be relaxed again by plac- ing in a moist atmosphere (as, under a bell jar with a wet sponge) for a few hours. They should include the following types: 1. A long-tongued bee (bumblebee). 2. A short-tongued bee. 3. A wasp. 4. A fly (two winged) . 5. A beetle. 6. A bug. 7. A butterfly or moth. The record of observations should be made in a table prepared with the following column headings (ab- breviated as desired) : Name of the insect. Order to which it belongs. Flowers on which it was taken. Seeking pollen or nectar. Proboscis < , _,, I length Pollen-gathering parts. Antennae — ^length, form and position. Position of wings when at rest. Relative size and weight (as compared with the others of the table). 26 GENERAL BIOLOGY 3- The relative fitness of the different visitors to one kind of flower. It will have been observed in the course of the field studies hitherto outlined, that not all the visitors to one kind of flower are equally proficient in obtaining its stores or in transferring its pollen: also, that the manner of visitation is very different in different insects. The butterfly perching atop of a phlox corolla and probing the deep tube only with its long proboscis (fig. 2 1) could not exchange places with the bee that plunges bodily into the chelone flower (fig. 22): it would ^'°i,^'- Diagram of a butter- meet with difficulties like those of fly on Phlox, and ot the posi- the"cOToiia\ube'"™^ '''**'™ ^^^ stork of the fable, attempting to dine with the wolf. A more careful study of this matter will show that the pollination of a flower may be well effected by in- sects that operate in very different ways. The following study of all the visitors to one kind of flower is in- tended to reveal the actual rela- tions existing between a flower and its visitors, and the relative fitness of these visitors. Clearly this fit- ness consists in two things: i) ability to get the food store the flower offers, and 2) ability to transfer pollen from anther to stigma. Study 3. All the visitors to some common flower. Apparatus needed: insect net, cyanide bottle, lens and note book: use chiefly the two last mentioned. Fig. 22. The flower of turtle heads {Chelone glabra) , and its visitor (worker Bombus). INTERDEPENDENCE OF ORGANISMS 27 First, select a flower that is abundant, and that has pol- len and nectar so exposed as to be accessible to a consider- able variety of visitors. Before beginning to observe the visitors study the structure of the flov/er itself , as to i) the position of the pollen and nectar stores, 2) the passageway to the nectar and its guards, 3) the position of anthers and stigmas in relation to this passage at different stages of flowering, and if clustered, 4) the form of cluster as likely to affect the convenience of big or little visitors. The field work of the following outline must of necessity be individual: it cannot be done in a crowd: the student should work quite alone so as to avoid having his observa- tions interrupted by the movement of companions. He should wear quiet colors, and approach the insects cau- tiously, avoiding quick motions : thus it will be quite possible to observe many cf them at work under a lens. Some degree of warmth and sunshine and dryness of the weather will also be necessary to success. The record of observations. — From a study of as many kinds of insect visitors as can conveniently be found, fill out a table prepared with the following column headings (abbreviating as desired) : 1. Name of the insect. 2. Order to which it belongs. 3. Seeking pollen or nectar. 4. Alights where. 5. Enters how far. 6. Touches stigma or anther first. 7. Carries pollen how. 8. Visits how many flowers in succession without inter- vening long flight. 9. Visits how many flowers per minute. 28 GENERAL BIOLOGY lo. Well or ill-adapted for visiting and pollinating this flower. 4. The relative fitness of the different flowers visited by one kind of insect to profit by its visitation. A more careful study should now be made of the relations existing between one kind of insect and the many kinds of flowers it visits. This is a study of the relative fitness of the several flowers to avail themselves of its services as an agent of pollen distribution. Clearly fitness in this case consists in i) offering the insect an accessible food supply to win its visits, and 2) having anther and stigma located aright for proper pollen transference. Sttidy 4. All the fiowers visited by some common insect. Apparatus needed and general directions, as for preceding study. An insect should be selected that is abundant, that is an active flower visitor; it should have a rather long proboscis in order that it may have access to flowers of con- siderable variety. It should be carefully examined, before the proper work of this outline is undertaken, as to i) its nec- tar-gathering parts, particularly as to the length and posi- tion of its prcjboscis; 2) the position and structure of its pollen brushes ; and 3) its size and weight, and 4) the position of its appendages when at rest. The record of observations. — In the field one should ex- amine freshly blooming clumps of as many kinds of flowers as possible, first seeing whether the insect selected for study is visiting them, and if so, watching it carefully and quietly until the points given below as table headings have been determined: 1. Name of flower. 2. Furnishes pollen or nectar. INTERDEPENDENCE OF. ORGANISMS 29 3. Offers what alighting place. 4. Is entered how far. 5. Stigma or anther touched first. 6. Pollen carried how. 7 . Number of flowers visited in succession without inter- vening long flight. 8. Number of flowers visited per minute. 9. Well or ill-adapted for pollination by this insect. 5. Precise adaptation between flowers and insects, leading to mutual dependence. It will have been noticed ere this that the flowers which are very irregular or have closed corollas, or secrete their nectar at the bottom of deep and narrow tubes or spurs, have fewer visitors than those that are open and regular. Only those insects which have long proboscides, or which are endowed with special ability at forcing passageways can obtain their stores. Flowers thus specialized receive the usual good and ills of specialization; they enjoy especially efficient aid when their proper visitors are abun- dant and lack it when these are scarce. It is in the relations existing between these most highly specialized flowers and their few guests that one sees the most remarkable phe- nomena of fitness and learns the extent and the precision of mutual adaptation. Such adaptations are peculiar and special, and no general outline can be given for their study: instead, an example will be detailed and a few suggestions offered, and not a formal outline. For example, let us consider the pollination of the marsh weed commonly known as turtleheads (JOhelone glabra). Its rather large white flowers are arranged in four vertical rows at the top of the leafy stem. They are strongly bilateral and have abundant pollen and nectar, so guarded JO GENERAL BIOLOGY by a nearly closed corolla entrance and by internal barri- cades of spines, as to be accessible to only one kind of visitors — small worker bumblebees. A side view of a single flower is shown in fig. 22 a. The narrowly three-lobed, pro- jecting lower lip offers an alighting place for the bees and the reflexed edges of the corolla mouth offer them footholds. A swollen palate upon the lower lip blockades the entrance against other insects, but under the weight of the worker bumblebee this is depressed sufficiently to allow the head to be thrust into the median groove that divides the '"palate," and thereafter, a little pulling and pushing effects an entrance. The queen bumblebee sometimes tries to enter, but can only get her head inside: she is too big. Other insects that are small enough are too light to ' 'tip the beam," or, entering, are barred from the nectar by a mat of bristling hairs on the floor of the corolla and by dense fringes of spines on the stamens: so that the worker bumblebees have a monopoly. These bees serve the flower well. They exhibit a maximum of efficiency in effecting cross pollination. The stigma of the flower projects slightly beneath the tip of the upper lip ■of the corolla. The pollen is carried by the bee in a great ■quantity amid the hairs on the top of its prothorax. It will be easily understood that in forcing an entrance through the narrow passage, this pollen mass is pushed hard against the stigma. A single visit is sufficient for complete pollination of a flower. The chief means whereby the flower reserves its sweets for proper visitors are not seen from the outside : and these are its most peculiar and special devices, such as are taken least into account in the preceding studies of this chapter. So let us look within. Looking at the flower from below we can see within the narrow corolla mouth the anthers stand- ing close behind the tip of the pistil and close under the upper INTERDEPENDENCE OF ORGANISMS 31 lip (fig. 23a). Figure 236 represents the stamens and pistil inside view with the corolla cut away. The stamens are re- duced to two pairs and a hairy rudiment of the fifth. The ar- row in the figure indicates the position of the bumblebee when, it is inside feeding, its body be- tween the paired stamens, its long proboscis reaching down- ward to the nectaries in the bot- tom. Only the bee in action could explain the purpose of some of the peculiarities of these stamens. They are laterally flattened so that they will easily bend aside. Their planes are set aslant at an angle opening forward, so that the bee may easily crowd between them. The conspicuous bend forward in their middle portion, being convex toward the entrance, is set in opposition to the pushing of the bee so that they may not be crowded backward out of place. Now turning the stamens so as to see them from the front, as in fig. 23c, we observe that the space between them is much narrower than the bee's body. Separating them a little with our forceps, as at fig. 2^d we observe that the anthers, held together by matted hairs above, rotate upon their stalks, separate below, exposing their pollen cavities, out of which a shower of dry pollen falls. Thus it is the bee gets dusted on the back. One may demonstrate this by thrusting a pencil of the thickness of the bees body into the flower and getting a deposit of pollen upon the end of it. Smaller Fig. 23. Diagrams illustrating the structure and mechanism of the turtle heads flower, a, anthers; p, pistil; r, a rudi- mentary fifth stamen. Other things explained in the text. 32 GENERAL BIOLOGY insects would not be large enough, and weaker ones would not be strong enough to swing open the heavy doors of the pollen cupboard: so this flower reserves its pollen as well as its nectar for its special guests and affords us a good example of mutual fitness and exclu- siveness. Study 5. A case of precise adaptation. This is an individual study, to be undertaken only when condi- tions are right — proper flowers abundant, warm and sunshiny weather, etc. A highly special- ized flower with its nectar not easily accessible to flower visitors should be selected, and it should abound in freshly blooming clumps ; for the visits to such flow- ers are often few and far between. Fig. 24. Diagrams illustrating the structure of the ordinary flower of the violet (yiola cucullata). a, a front view of the flower with the tip of the saccate petal cut away, showing the blockade of hairs around and above the stigma; b, lateral view with petals and sepals in part re- moved; c, the same more en- larged, and with the lateral stamens removed ; d, one of the spurred stamens; drop- lets -of nectar on its outer side shown at o, and pollen cavities, at p. The Record of this study may well consist of a few drawings to illustrate the structure of the flower and the details of the en- trance, of the feeding, and of pollen transference by its visitor, with copious explanations there- to. 6. Specialization miscarried. Among our showy flowers are a few possessing the charac- teristics which elsewhere we find associated with insect aid in pollen distribution, and which are never or rarely visited INTERDEPENDENCE OF ORGANISMS 33 by insects. Such an one is the common blue violet (Viola cucullata). Other species of violets are commonly visited by bees ; and this one is apparently finely adapted for such visitation. Yet the bees rarely visit it, and the showy flowers, being incapable of self-pollination, produce no seed. The accompanying figures show the structure of the flower. It is strongly bilateral, with a saccate lower petal en- veloping two spurred stamens : it is blue, with pretty ' 'guide marks" about the entrance: it secretes a little nectar, and exhales a slight perfume : its entrance is blockaded against improper visitors, but it is narrowed and curved conven- iently to admit the proboscis of a bee standing head downward upon its front. Furthermore, it is well adapted to profit by the bee's visits. A proboscis plying between the spurs of the two Fig 25. Tip of pistil lower stamens would dislodge the dry of the violet as seen . "^ from the front, pollen from the anthers, and catch it as showing pollen 7 , „ , . , , - pushedinto thehoi- it falls, and Carry it out, and when prob- ing the sac of the next flower visited, would deposit it on the stigma: for the stigmatic surface is contained in the hollow of the pistil tip, turned toward the entrance (fig. 25); the lower edge of it would scrape up pollen from an entering proboscis, but would only evade pollen that was being withdrawn. What better device could be imagined for securing cross pollination? The trouble with the mechanism is that it no longer works. The bee stays away. Did it visit the flowers, it would transfer their pollen perfectly and they would be very fertile. This anyone may demonstrate by transfer- ring the pollen with a tooth pick and watching the result in seeds produced. The failure seem-S to lie farther back in 34 GENERAL BIOLOGY the physiology of the plant ; it secretes but a little nectar — that little on the outside of the spurs — not enough to run down into the sac where the bee's proboscis can reach it. There is, however, a small bee-fly {Bombylius major) that is able to get the nectar which hangs in minute droplets on the outside of the spurs (fig. 26). It is often seen poising be- fore a flower, making an ob- lique thrust at each side of the entrance, push- ing its excessive- ly slender proboscis, not down the proper middle passage- way at all, but between the spur and the wall of the sac. Thus, it touches neither stigma nor pollen, and gets the nectar without doing the flower any service in return. But even if this, our commonest violet, has been deserted by its proper visitors, and left to the comradeship of nectar thieves, if its fine adaptations have become useless and its pretty flowers are left to waste their diminished 'sweetness on the desert air,' the plant has not been without resource: after the showy flowers of spring cease to appear, it devel- ops at the surface of the soil minute self-fertilizing (clistogamous) flowers, which shun the light, never rise up into view and never open, but which are abundantly fertile, and are produced all summer long* (fig. 27). Fig. 26. A beefly {Bombylius major) visiting the violet flower. *These clistogamous flowers will be examined in Study 51. INTERDEPENDENCE OF ORGANISMS 35 These brief studies of the relations between flowers and insects should have made it apparent that we have with us all grades of association from the most casual contact to mutual dependence, and that we have all grades of fitness on botn sides : further that while the adaptations are often wonderfully intricate and fit, they rarely work perfectly, and may even wholly miscarry. And while gratified in observing that they often work with delightful precision, Fig. 27. Flowers and fruit of the violet. The ordinary blue flower and a seed capsule (from a hand pollinated fiower) shown above; a row of clistogamus flowers shown below; the lowest one in full bloom. we should not overlook the fact that the simpler plans suffice for the maintenance of the species that are less specialized. II. GALLS. Galls are abnormal growths of plant tissues occasioned by stimuli external to the plant itself. The stimuli are furnished by a great variety of insects, by a few parasitic 36 GENERAL BIOLOGY fungi, and by a number of other less important and less common agencies. All parts of the plant are subject to these malformations. As the raking of a wire against a tree trunk that is swayed back and forth by the wind causes great ridges to grow upon the sides of the trunk, so the gnawing or sucking of an insect in the growing tissue of the plant causes a gall to grow. Not all irritations to plant tissues cause such over- growths, but only such as are applied while the tissue is rapidly developing. There are, for example, a number of moth larvae that work in the stems of goldenrods: those whose attack is made before the stem tissues are fully formed cause galls; the others are merely stem borers. Like- wise, in oak leaves the little fly larvae that attack them in the bud cause galls; the later ones make only leaf mines. The stimulus might be the same, but the period of response on the part of the plant being overpast, there is no gall formation. Overgrowth of the plant tissue is, therefore, the criterion of a gall. So generous is the response of the plant in the pro- duction of tissue that serves for both food and shelter, that the habit of attacking young tissues has been biologi- cally profitable. Hence there is developed a large fauna especially and exclusively adapted for exciting galls and living in them — a very favorable subject for the study of interrelations. . We will confine our study here to those malformations that are caused by insects and mites, notwithstanding that there are some common and conspicuous galls, like the one on the sumach top shown in figure 28, made by fungi. This one belongs to that general class of galls popularly known as "witches' brooms": other common fungus galls appear as knots and swellings upon the trunk or the branches of trees : all consist of more or less solid tissue INTERDEPENDENCE OF ORGANISMS 37 and are readily distinguishable from animal galls which contain distinct cavities for the occupancy of the gall makers.* Fig. 28. A lungus gal] of the "witches broom" type on the smooth sumac (Rhus glabra .) *While we commonly speak of the gall insect or fungus as a "gall maker," we are not unmindful that it merely furnishes the stim.ulus to overgrowth on the part of the plant itself. 38 GENERAL BIOLOGY Animal galls. — Animal galls are less diffuse. Under the stimulus of the attack of the insect in feeding, the tissue grows rapidly, producing more food: moreover, around the point of attack it grows and shuts in and covers andv protects the gall maker. Furthermore, it continues to grow and shape itself into symmetry, its final form often resembling a fruit. More remarkable still, it often de- velops unpalatable substances (such as tannin) in its walls and sharp spines upon its surface, and thus protects its enemy the gall maker, from being eaten. Most animal galls are small, but a few of them, such as the aphid gall of the cottonwood shown in figure 29, grow large enough to become when numer- ous, a feature of the winter land- scape. This one is formed not about a single aphid, but about an aphid colony; and its irregu- larity is doubtless due in part to the grouping of individuals in the attacking aphid flock. The commoner forms of ani- mal galls are these : ffelted open mantle scroll gall pocket gall fluted gall covering gall rFlG. 29. Winter aspect of aphid galls on a cottonwoo'd tree. closed [simple [nucleated. The differences between these forms are indicated in the following diagram, (fig. 30). INTERDEPENDENCE OF ORGANISMS 39 The primary distinction between open and closed galls lies in their mode of origin. In the open gall the attack is made from the outside, while in the closed gall the insect enters the tissue bodily and feeds inside: ordinarily, it Fig. 30. Diagram of typical form of galls, a, felted; b, scroll; c, fluted; d, pocket; e, covering;/, simple closed;] g, nucleated, a to ^ are open galls; /and g, closed. enters in the egg stage, the egg being inserted through a puncture in the epidermis. In the open gall the insect may be covered and inclosed by the overgrowing tissue, but when inside the gall it is still outside the leaf substance, and in feeding, stailds upon and punctures the epidermis with its piercing mouthparts. Felted galls (fig. 3 1) represent a low degree of gall develop- ment. They occur mostly upon leaves, and are as a rule made by mites. They usually consist of a slight sacculation of the part of the leaf blade that is subject to attack, and the malformation is mainly confined to the epidermal cells, which develop a wonderful growth of robust plant hairs that are twisted and matted together like felt, whence the name. The mites clamber around and feed between the bases of these plant hairs. Mantle galls represent a better development Fig. 31. A felted gall (a, cross-section) from the leaf of button-bush {Cephalan- thus occidentaLis) and the mite (6) which causes it. of leaf cover for the gall maker: the cavity is deeper 40 GENERAL BIOLOGY and more completely inclosed, and, usually, not felted within: the walls often rise and shape them- selves with marked symmetry and even beauty. The four names given in the table as types of mantle galls are but convenient designations of the more typical forms which Fig. 32. Stem, leaf and flower galls, o, a nucleated gall on the twigs of white oak (Quercus alba), b, a mantle gall on the leaves of witch-hazel (Hama- melis virginiana); c, simple closed galls on the flowers of goldenrod (Solidago nemoralis). often inter-grade or combine together in a single gall. The scroll gall is formed by the uproUing of the leaf margin: the fluted gall, by the furrowing of the blade (chiefly along veins) in elongate grooves. The pocket gall and the cover- ing gall although much alike in appearance are most unlike in fact, being diametrically opposite in their manner of INTERDEPENDENCE OF ORGANISMS 41 growth. Atypical pocket gall is shown on the witch hazel leaf in fig. 32. It is formed by the descent of the tissue attacked to form a pocket upon' the leaf blade: the attacking insect is carried into the pocket, which usually dilates, and forms a spacious chamber. The covering gall, on the contrary, rises up around the point of attack, and covers the insect over, leaving only a small aperture at the top. The removal of a pocket gall leaves a hole through the leaf: the removal of a covering gall leaves only a superficial scar. Closed galls, as already stated, re- sult from internal attack: the cavities they contain lie wholly within the plant substance. They likewise differ among themselves in the degree of their development. The simpler ones (fig. 32c) have thin walls, of the ord- inary tissues of the part bearing them. The nucleated galls (fig. 32 a) show often a high degree of differen- tiation of parts. There are often three well defined layers in their walls : an inner (when mature) very hard layer forming the "nucleus" whose cavity contains the gall maker, an intermediate softer and more or less spongy layer, and an outer hard layer, often protected with spines and hairs and ornamented with beautiful colors. The stone-like nucleus in the middle and the form and color of the exterior greatly enhance the superficial resemblance of the gall to a fruit.* *It is to be noted in passing, that the gall when fruit-like almost invariably resembles the fruit of some kind of plant other than the one that bears it. Fig. 33. Compound gall on the root of wild let- tuce {Lactuca spf) 42 GENERAL BIOLOGY As to their distribution upon the plant, galls are solitary, (as in fig. 32a) clustered (as in fig. 32c-), or compound (as in fig- 33) • they are called compound when they contain separate cavities surrounded by confluent walls. The animals that produce galls. — ^With a few unimportant exceptions the animals that cause galls to grow belong to a single family of mites and to five orders of insects, Hemip- tera, Coleoptera, Lepidoptera, Diptera and Hymenoptera. The mites are very minute four- or eight- legged creatures without distinction of head and thorax (fig. 31^). They live amid the growth of matted hairs that fills the cavity of felted galls. Hemipterous gall makers are aphids, psyllids, etc., and they generally live within mantle galls. Coleopterous and Lepidopterous gall mak- ers are beetle and moth larvae respectively. They are but a few stray members of large families that are not much addicted as a whole to the gall making habit: but these few make comparatively large closed galls, some of which are sure to be encountered in the following field study. Dipterous gall makers mainly are gall gnats (Cecidomyiidae), with a few scattering representatives of other families. Cecidomyiid galls are very common, and of the utmost diversity of structure and appearance. The larvae within them are often very small, but they are distinguishable by the possession on the under side of the first segment behind the head of the so called "breast bone," a flat, brown homy piece that pro- jects forward toward the mouth and is often notched at its tip (fig. 34). Pig. 34 Dia- gram of a gall midge larva (family Ceci- domyiidcE of Diptera). m, the so-called "breastbone;" n, respiratory apertures. INTERDEPENDENCE OF ORGANISMS 43 Hymenopterous gall makers belong, with a few exceptions to two families, Tenthredinidae, saw flies, and Cynipidae, gall wasps. Sawfly larvae make rather simple closed galls, which they abandon when grown, to find some other place of transformation. Gall wasps are gall makers par excel- lence. They cause the most perfect 'nucleated galls : as a family they are most completely adapted to the gall making habit.* The tenants found in the course of the following study occupying the galls collected, may be identified by the stu- dent himself. For the adults, of which few, if any, will be found, use the keys of any good manual of entomology. Pupae if found may easily be reared. Place them uninjured in a glass jar, add a wet sponge, or bunch of cotton to pre- vent drying up, and tie netting (preferably fine swiss) over the top of the jar, and let them stand till they emerge as adult insects. Larvae, which will generally be found, may be identified as follows; Key to the commoner insect larvae and mites found in galls. A. Body short and thick: legs rather long. B. Head fused with body and not distinct; legs 2 or 4 pairs Acarina, Mites. BE. With distinct head: legs 3 pairs (Hemiptera). C. Wing pads present, projecting at right angles with the body: no cornicles on abdomen. .Psyllidae. CC. Wing pads absent, or if present, laid lengthwise of the body: cornicles often present (see fig. 39) .Aphidae AA. Body cylindric, worm-like: legs minute or none. B. With 3 pairs of minute legs under the thoracic seg- ments. *It is to be observed that there is not a single family of insects whose members are all gall makers: Cynipidae comes nearest. 44 GENERAL BIOLOGY C. With a brown shield covering the prothorax above : body armed with stiff bristles Lepidoptera, moth larvae. CC. Without a brown prothoracic shield. D. With rudimentary legs (pro-legs) underneath some of the abdominal segments Tenthredinidae, sawfly larvae. DD. With no abdominal prolegs Coleoptera, beetle larvae. BB. Legless. C. With a distinct head segment : body arcuate, white. D. Body segments deeply wrinkled: head brown: skin dull white Coleoptera, Family Curculionidae, weevils. DD. Body segments smooth, shining, head mostly white Cynipidae, gall wasp larvae. CC. With the head segment greatly reduced, very minute or wanting: body straight. . . .Diptera. ,D. With the ventral piece shown in fig. 34 Color often red or yellow. . . Cecidomyidae, gall gnats. DD. Without this structure. Color white Other dipterous larvae. Despite the food, cover, and defense, provided by the plant for the gall maker, the fact must not be lost sight of that the creature i s the plant's enemy. The young bur-oak shown in figure 3 5 gives evidence of this. Cynipid galls, growing too thickly have killed the ter- minal shoot, and the lateral shoots are taking up the growth. Such positive injury from galls is rarely seen, how- ever, for the gall makers are kept in check by hosts of very efficient parasites. The student following the field work outlined below will be sure to come upon some of these parasites, and it may be with some difficulty that he will distinguish which is parasite and which is gall maker in INTERDEPENDENCE OF ORGANISMS 45 some cases. The parasites are all Hyinenoptera, with larval form very like that of Cynipid larvae (see key). Such larvae found in galls that are made by insects of other orders may of course be set down at once as parasites. In cynipid galls, which will give the trouble, thess sug g e s - tions may help: The Cynipid lar- va generally quite fills the central calvity of its gall ; the par- asitic larva is usually consider- ably smaller: the cynipid larva is very strongly arcuate with- in the cavity; the parasiticlarva is generally not so strongly bent. The gall when grown offers often a place of shelter and sometimes a place of development to other insects besides the one that caused it to grow. Thus new interrelations are brought about. Some of these are well shown by the cone gall of the willow (fig. 36), whose fleshy scales when green furnish forage for the burrowing larvae of several species of moths and sawfiies, and when dry furnish shelter and a place of incubation for meadow-grasshopper eggs. Guest gall-flies, also, develop between the outer scales, Fig. 35. Clustered galls on a young bur-oak. Ob- , serve that the central shoot is not putting forth leaves {Quercits macrocarpa.) / 46 GENERAL BIOLOGY often in great numbers : and each of these species has its inevitable train of parasites. All these forms together constitute a miniature animal society, dependent on the overgrowth of willow tissue that re- sults from the attack of the gall midge. Study 6. A study of common galls. Apparatus needed: A scalpel, or knife, a lens, and a basket, bag, or very capacious pockets. Collect afield a large number of galls, bringing into the laboratory enough to fairly represent each kind found. Search such trees as oaks, hickories, lindens, hackberries and wil- lows; such shrubs as sumach, roses, witchhazels and dogwoods and such herbs as goldenrods, ox-eyes, and touch-me-nots. The record of observations. — Select a dozen or more species that represent best the general phenomena outlined in the preceding pages, and write down their characters in a table prepared with the following column headings: Name of plant. Part of plant affected. Position of gall on this part (upper or lower surface of leaf, etc) . Gall type. Aggregation, solitary, clustered, or compound. Cavity of gall (shape, close fitting, etc.). External coat, armature, etc. Special structural features, if any. Defences against foraging animals. Fig. 36. — Diagram illus- trating the distribu- tion of the inhabitants of the cone gall of the willow: a, the gall maker, h, moth larva. c, sawfly larva. rf, meadow - grasshopper eggs. e, guest gall- midge larvae. The Gall INTERDEPENDENCE OF ORGANISMS 47 The Insect Order to which it belongs. Family. Solitary or gregarious. Stage found. Parasites or hyper-parasites. Inquilines. Summarize the results of the preceding study in a table of the orders of the gall makers, prepared with the following column headings. Order (of insects, or mites) Mouth parts (biting or sucking) Habits (solitary or gregarious). .Gall type. Then state any relation appearing i) between type of mouthparts and type of gall, and 2) between order of insect and type of gall. III. THE RELATIONS BETWEEN ANTS AND APHIDS. Aphids are familiar plant pests which infest our fields and gardens. They are minute Hemiptera, possessed of a slen- der proboscis, with which they puncture soft plant tissues and suck out the sap. Some aphids, which attack develop- ing plant tissues, will already have been found in the cavities of the galls to which they give rise. All are gre- garious in habits, mainly because their great reproductive capacity is coupled with poor power of locomotion. Genera- tion after generation they are wingless : but when the time for their wide dispersal is at hand, a winged generation appears, which flies freely in search of new locations. Autumn is the time of dispersal of most species, because of the general failure of food supply at that time, and the necessity of relocation for winter: but the failure or un- favorable alteration of food supply may occasion the pto- duction of a winged generation at any time. 48 GENERAL BIOLOGY Individually aphids are insignificant, but collectively their drain upon the plant may be very serious. Each aphis is an animated sap pump. It sits quietly on bark or leaf, with its proboscis immersed in the green tissues, and pumps by the hour, scarcely changing its place or moving by more than an occasional sweep of its long antennae. Its food consisting of sap, contains considerable sugar — much more indeed than the creature is able to assimilate. This excess of sugar is discharged from time to time, along with the other rejectamenta and excreta of the body, in fluid drops of "honey dew." Honey dew is very sweet and palatable. It is gathered from the leaves where it falls by ants, bees, wasps and other animals. Bees store it as honey, and although it is not the best of honey still it is not unwholesome, and men eat it gladly. When aphids are abundant on growing trees honey dew is often secreted in large quantities. A sudden jarring of an aphid covered bough may cause such a sudden and sim.ultaneous discharge by the aphids that the honey dew will fall in a shower of fine spray. It often covers the lower boughs of trees and the bushes beneath them, with a shiny, sticky, sweet coating. That ants have a "sweet tooth" everyone knows from observations in his own pantry or lunch basket. They like honey dew, and from gathering it at large, they have passed to gathering it at its source — from the aphids themselves. The relations between the two that find their simplest ex- pression in chance visits by ants to aphid colonies, become much more intimate when ants begin to guard and care for the aphid flocks, to build shelters for them, or to share their own homes and fortunes with them. These relations may be grouped in three categories : I. The chance feeding by ants on the honey dew offered by aphids. — This is hardly more than accidental associa- INTERDEPENDENCE OF ORGANISMS 49 FtG. 37. — Aphid colony on a leaf of Ceanothus, attended by ants seeking honey dew. h, a larva of ^ a syrphus fly, feeding on a wingless aphid. *', a wing:ed aphid, j, an ant patting an aphid with its antennae, k, the empty skin of an aphid that has been parasitized. tion. It may be recognized in an aphid colony that is attended by one kind of an ant on one day, by another I kind on another day, and is part of the time unattend- ed. 2. The habitual guarding of aphid colonies by ants, safeguarding their own supply of honey dew. — ^This is the commonest type of association, and the one easiest to observe. I n summer or autumn, on many a curled dock or thistle or dog- wood bush, wherever ants are seen gathered together upon the green foliage, there one may expect to find on closer inspection, an abundance of aphids as well. And if one approach quietly and watch carefully he may see the ants moving about among the aphid herd, fondling them with their antennae, patting or stroking an individual here and there, and obtaining sometimes as a response, the extrusion of a drop of honey dew, which is lapped up as soon as it appears. The ants will often be seen to drive away intruders — chiefly winged parasitic . insects, which seek to lay their eggs upon the bodies of the aphids. They will even rush at an intruding finger, and attack it fiercely, though ineffectively, with their jaws. Yet, though they show great dash and courage in dealing with any parasitic syrphus fly or ichneumon that ventures too near the flock, they show a sad lack of insight in allowing the egg, when one has been successfully laid by 5° GENERAL BIOLOGY the parasite, to remain where placed, and the fly larva, when hatched, to feed openly (fig. 37h) upon the aphids. That their guardianship is often eluded may be seen on close inspection of almost any aphid flock. 3. The domestication of the aphids by ants. — This covers at least two distinct sorts of activities on the part of the ants : i) the building of shelters and enclosures about the aphid Fig. 38. Aphis shed on twig of dogwood; photo of a specimen in the Cornell University collection. flocks, and 2) the safeguarding of the development of individ- ual aphids and the establishment- of aphid colonies. These are two well recognized functions of all animal hus- bandry. Ant sheds are built usually near or on the ground about compact colonies of aphids (or other honey dew secreting INTERDEPENDENCE OF ORGANISMS 51 Hemiptera) so as to completely enclose the flock. With but a few small openings left through the walls for entrance and exit, the guarding of the flock is easier and the security of the flock is greater. The aphids no longer "run the range," but are kept in folds. Excesses of heat and cold are less felt, and the great injury from exposure in rainy weather is largely avoided. The ants probably reap the usual rewards of good husbandry in the larger and more constant secretion of honey dew. The sheds are of two sorts as regards the materials of which they are made : i) earthen sheds, made of sand grains, etc., stuck together with wet clay, and 2) felted sheds made of interwoven bits of shredded plant tissues. Both sorts are often placed about the stems of bushes (fig. 38) and supported on branches or leaf stalks. Finally, there is a permanent association, with the ants exercising care and control over the aphids in all stages of their development. This is complete domestication. The best known case of it is that of the little brown ant of the fields and the corn-root aphis. This subterranean aphid lives on the roots of Indian com, where these roots traverse the branching passageways of the nests of the ants. It is a hapless creature (fig. 39), quite incapable of uncovering corn roots for itself, or even of finding them if uncovered: so, the ants excavate the soil, making lateral foraging chambers communicating with their nest, and carry the aphids in and place them on the roots. There the aphids feed and secrete honey dew through the season, and in the fall, there they lay their eggs. The following account is quoted from a report on corn insects by Professor Forbes, to whom our knowledge of this relation is chiefly due : "These eggs, which are yellow when first deposited, but soon become shining black, and turn green just before hatching, are at first scattered here and there, as it 52 GENERAL BIOLOGY happens, but are finally gathered together by the ants for the winter in little heaps, and stored in their galleries, or sometimes in little chambers made by widening a gallery as if for storage purposes. If a nest is disturbed, the ants will commonly seize the aphid eggs, often several at a grasp, and carry them away. In winter they are often taken to the deepest parts of the nest. . . as if for some par- tial protection against frost: but on bright days in spring they are brought up, sometimes, within half an inch or less of the surface, some- times even scattered about in the sunshine, andcarried back again at Pig. 39. Com root aphis night — a practice probably to be (Aphis tnaidiradicis), wing- j j_ i r i . . less female x 14 (from undcrstood as a mcaus of hastenmg Forbes.) The two black it, ■ -u x i.- ti .11 processes at the rear are tneir hatchmg. i havc repeatedly seen these ants in confinement with a little mass of aphid eggs, turn the eggs about one by one with their mandibles, licking each carefully as if to Fig. 40. Com root aphis, winged female x 16 (from Forbes) INTERDEPENDENCE OP ORGANISMS 53 clean the surface. These anxious cares are of course ex- plained by the use the ants make of the root louse [aphid], whose excreted fluids they lap up greedily as soon as the young lice begin to feed. "That the young of the first generation are helped by the ants to a favorable position on the roots of the plants they infest is quite beyond ques- tion. . . We have repeat- edly performed the experiment of starting colonies of ants on the hills of com in the in- sectary, and exposing root lice from the field to their attention and in every such instance, if the colony was well established the helpless insects, have been seized by the ants, often almost instantly, and conveyed under ground, where we would later find them feeding on the roots of the com. "I need hardly say that the relations above described be- tween the corn-root aphis and these ants continue without cessation throughout the year." Thus sequestered from parasites, and guarded by the ants and cared for at every turn, this long unknown aphid has flourished inordinately, and has become throughout the great "com belt" a serious pest. It is another illustration of man's influence in disturbing the natural balance. Corn fields have replaced the native prairies and woodlands over wide areas, and have offered opportunities for almost un- limited increase in numbers of com insects that were doubt- less but sparingly distributed before. Fig. 41. Small brown ant (Lasius niger alienus) that domesticates the com root aphis; worker, x 8 (from Forbes). 54 GENERAL BIOLOG^i Study y. Observdtions on ants and aphids. It is not possible to give a hard and fast outline for tht study of these phenomena : for, though widespread, they are not equally available at all times and everywhere. It should be possible to find anjrwhere in summer a nurnber of colonies of aphids with ants in attendance, on such plants as curled dock, milkweed, thistle, dogwood, etc. Ants are easily seen when running about over green vegetation, and almost always there will be found flocks of aphids (or of other honey dew secreting hemiptera with which the ants have similar relations) as the occasion for their assembling. Such an association being found, the apparatus needed will be a low power magnifier (such as a reading glass is ex- cellent) and a note book, and the things to be observed are : i) The ordinary behavior of the ants toward the flock. 2) The gentleness of the ants toward individual aphids: the stroking and patting of them first with the antennae and coming closer, with the palpi. 3) The lapping up of the honey dew when an aphid responds by ejecting it. 4) The ferocity of the ants toward intruders : this may be tested with one's own finger. 5) The stupid indifference of the ants toward the eggs and larvae of the parasites. 6) The general inactivity and helplessness of the aphids. , 7) The prevalence of (parchment skinned) parasitized individuals. If aphid sheds can be found, their materials and con- struction should be noted, their doors, their braces, and their shape as adapted for giving a maximum amount of foraging surface with a minimum of construction. Some advantages to both ants and aphids can readily be seen to accrue from them. INTERDEPENDENCE OF ORGANISMS 55 Root aphids can usually be found in any corn field where burrows of the little brown ant are common. The bur- rows are easily seen after a rain, when the ants open them up and toss out upon the surface annular mounds of little pellets of earth. The aphids will be found by exposing some of the com roots where they traverse lateral passage- ways ramifying outward from the ants' nest. The ants will usually promptly demonstrate their care-taking function irf the premises, by seizing any aphids that may be shaTien oS'- from the roots and carrying them into their nests. An ac- count of a cage that may be used for rearing such mixed' colonies in the laboratory will be found in the appendix.' The record of this study may well consist of brief notes on the things observed. CHAPTER II. THE SIMPLER ORGANISMS. To understand the complex phenomena of life we must seek their simpler expressions. The relations between the higher and more familiar forms of life are very intricate. The bodies of such plants and animals as we have been ob- serving are highly organized — composed of many parts having special functions. How shall we learn what are the primary parts and functions of living things ? It will help us to distinguish essentials if our first quest be made of organisms lowly Fig. 42. Closteriura lunula, c, cytoplasm; «, nucleus; w, cell among bearing protoplasm; Structure. The simpler plants vacuole,^""' ' "' and auimals Hvc in the Water. We have already learned that the main gatherers of food material for the living world are green plants. The simplest green plants are the algae ; so with these we will begin. SOME TYPICAL ALGAE. When we learn to recognize them we can hardly look into the water anjrwhere without seeing algae. They float in green masses upon the surface ; they hang in graceful drapery of verdure on submerged branches; they drip in globules of gelatine from twigs that are lifted out of the water: they rise from the bottom; they lie amid the silt ; they trail across the rocks that are swept by the cata- ract; they cling to wave-beaten piers and boulders; they are free-swimming, and come in our water supply ; and they grow and flourish in the bottle of clear water that is long left THE SIMPLER ORGANISMS 57 standing on the window sill. It is not hard to find them in great variety of size and form, and in great beauty and delicacy of organization. Closterium (fig. 42) is a very pretty simple alga that is commonly found in the bottom sediment of fresh water ponds. Although very small, its bright green color and crescentic form make it easily recognizable. If we gently lift up from the pond bottom some sticks that have long lain undisturbed, and shake into a white plate filled with water the silt that covers them, spreading it out in a thin layer, we may usually find Closterium scattered about over the plate. It is visible to the unaided eye, and is easily recognized with a pocket lens. It is easily reared indoors in a cool, well-lighted place in a jar of pond water sup- plied with some mud from the pond bottom, and this is the best way to get a large supply. Enough for class study may usually be obtained by mounting the scrapings of silt from submerged leaves, that have lain long in clear, well- lighted water. A few specimens transferred to a slide and examined with a microscope present at once to the eye some important characteristics of green plants. The crescentic plant body is seen to be encased in a transparent capsule, the cell wall, with a green substance filling the greater part of both ends of the crescent, leaving a transparent, clear band across the middle. In this clear band on closer inspection there is seen a slightly granular substance of such transparency it is at first easily overlooked, and in the centre of it is a round body of slightly denser consistency. Although so inconspicu- ous, it is well to fix attention at once upon these latter structures, for they represent the essentials of living struc- ture. The granular mass is protoplasm and the round body within it and forming part of it is the nucleus. The green substance filling and obscuring the protoplasm at the S8 GENERAL BIOLOGY sides is chlorophyl, and the transparent capsule inclosing the whole is the cell wall. The whole plant thus enveloped is a single cell. Well down in the angle toward each end of the crescent will be noticed also a round droplet of watery fluid called a vacuole, in which, under high magnification may be seen suspended some minute crystals in continuous (Brownian) movement. If from a freshly growing Closterium culture a number of individuals be mounted and examined, they will be found to differ considerably in size and in appearance at the trans- parent middle crossband where the nucleus lies. Some of the larger ones will show a broader clear area there, or an indentation of the cell wall at each side, or a constriction e.xtending entirely across the cell, cutting it more or less deeply into two parts, as indicated in figure 43. Closely examined, this process will be seen to be initiated by the division of the nucleus into two parts, one of which passes to each side of the cross band and into the edge of the chlorophyl. The deepening t^V;T;;;r\\i constriction thus divides the mass of proto- ^^^^^^^T)r^\\ plasm, and forms two smaller cells out of one //[ZlT^^^!^^ large one. Each of the smaller ones; before /^"^"■^'"--^X the separation, is lacking in the crescentic '' ^ symmetry of the grown plant, the newly Fig. 43. Divi- formed end being blunter, lacking chlorophyl terium;succes- and vacuole, and having the cell wall thin and not symmetrical with the other end. Reflecting on the few readily observable details of this ap- parently simple process whereby new plants are produced, it is obvious at once that certain of the structures seen are more essential than others. It is the protoplasm that pas- ses on unchanged from mother cell to daughter cells — ^both the general protoplasm of the cell-body (cytoplasm) and THE SIMPLER ORGANISMS 59 the nucleus. About the new end a new cell wall is formed, andintheprotoplasmof that end new chlorophyl develops- it is for the sake of the protoplasm that these other parts exist. The normal structure is regained, during the period of growth which ensues. Little is directly observable except the increase in size of the plant. The two processes of growth and reproduction so simply shown in Closterium, are characteristic ' of all living organ- isms, and are their most distinctive phenomena. Many algae consist, like Closterium, of cells existing singly, while many others consist of numbers of cells ag- gregated together to form a more complicated plant body. But whether the plant cell exist alone and apart, or whether it live in contact or in combination with other cells, its parts are usually those seen in the cell of Closterium: 1. Protoplasm fc3rtoplasm | usually "the physical basis of life" | and nucleus J inclosed by, 2. The cell wall, an investing capsule of transparent cel- lulose which envelops, besides the protoplasm, certain diverse substances of greater or less importance that may collectively be designated as: 3. Inclusions, the more important of which are a) the cell sap; a watery fluid which fills all the spaces (vacuoles) unoccupied by the more solid parts and is the medium of exchange of food and waste materials. b) chlorophyl, the greenish substance above noted, in the presence of which occur carbon reduction, and the storage of energy of the sun's rays (to be dis- cussed under a subsequent heading) , and c) secretions, excretions, reserve stores of starch ana other food materials, precipitations of mineral crys- tals, (such as oxalate of lime), from the saturated solutions of the cell sap, etc. 6o GENERAL BIOLOGY The two studies which immediately follow are intended to give i) an acquaintance with the appearance of the living part of plant substance, and 2) some knowledge at first hand of the diversity of form of cells and of the manner of their combination to- gether into a plant body among the algae. Study 8. The cell of Spirogyra and the protoplasm of Nitella. Materials needed — a supply of fresh Spirogyra and Nitella in clean water. Apparatus needed — ^the usual labora- tory equipment of simple and compound microscopes, small tools, glassware and reagents. The student should first examine Spir- ogyra in mass, as it lies in the water, and then lift out a small tuft of its long filaments forexamination in water upon a white plate. He will there note their length and their unbranched condition. Examining them with a simple lens, he will be able to distinguish clearly the spiral bands of green that wind about each filament on the inside and make Spirogyra easy of recognition among other algae of similar manner of growth (fig. 44). If he then mount a few filaments upon a slide, placing a cover-glass upon a favorable portion, and filling up ' the space beneath the cover glass with water, he may with advantage apply the compound microscope to the ex- amination of them.* Placing the slide thus prepared Fig. 44. Spirogyra. bit of a filament containing nine cells; &, a single cell, more highly magni- fied; c, cytoplasm; «, nucleus; p, pyre- noids, in the cnloro- phyl band. *If the student be not familiar with the use of the compound microscope, let him at this point pursue the supplemental study outlined in the opening pages of the appendix, for which Spirogyra is appropriate material. THE SIMPLER ORGANISMS 6i upon the stage, and examining the delicate filaments with low power of the microscope, he will at once observe that they are not all alike: different species of Spirogyra often grow together, but the filaments of a single species differ: some are of a richer green, with the chlorophyl bands adjusted closer together about the inner walls of the fibre. Let him select for study a filament with the green bands as far apart as possible (so that the internal parts may not be hidden) and examine it as to the arrangement of its parts. The cell. — It will be at once apparent that the plant body is composed of elongate cylindric cells placed together end to end : the filaraent is a simple linear aggregate of cells. Look- ing at a single cell, it will be seen to have a rather thick cell wall squarely cut at the ends. The chlorophyl is.restricted to the spiral band, which is not continuous from cell to cell, and which varies considerably in appearance and in number of turns in the cells of different filaments. Focusing upon the upper surface of the cell, the chlorophyl band will be seen most clearly — a beautiful wavy band of green, marked with a narrow median ridge, and studded here and there along the course of this ridge with round bodies containing the pyrenoids. Focusing downward, the band appears below, in- clined in the opposite direction, and less clear because of the parts now intervening. Focusing upon the axis of the cell, and looking between the green bands for the more funda- mental parts, there will be seen (and, readily, when one be- gins to see) at the center of each cell a mass of protoplasm containing the nucleus. As compared with the size of the cell, the amount of cytoplasm is small. It consists in: i) the central mass containing the nucleus, 2) slender strands radiating outward therefrom to various parts of the cell, but chiefly to the pyrenoids, and 3) a thin film next the cell wall. This last fits the cell wall so closely and is so trans- parent it is hard to see. It may be drawn into view by 62 GENERAL BIOLOGY osmotic pressure, if one only replace the water beneath the cover glass with some denser liquid, such as dilute glycerine, or 5% salt solution. This outer film will then be seen to shrink away from the cell wall, and if the shrinkage con- tinues, to collapse altogether; but if replaced quickly in pure water, it is soon restored to its original condition; clearly the larger part of the cell is occupied with the watery cell sap, easily withdrawn or replaced. Thus the main features of structure may be seen in the living cell. But the relations of some of the more delicate parts may be made rnore clear by the two following experi- ments. If a drop of iodine solution be placed upon the fibres upon the slide, it will stain the protoplasm yellowish brown, making the peripheral parts of it more apparent. It will also stain the minute starch granules that lie about the edges of the pyrenoids dark blue or blackish. If a few fresh green filaments be placed in strong alcohol, the chlorophyl will be dissolved out by the alcohol (more rapidly with the aid of heat) and the protoplasmic matrix in which the chlorophyl is held will be apparent. Mtella. — In order to get a large enough single mass of pure protoplasm to see without' lenses and to handle, it is necessary to find cells much larger than the ordinary ones, that shall contain it. The common stonewort, Nitella, is an alga with some very large (multinucleate) cells, from which the proto- plasmic content is easily removable, and may^ well be used for a first direct observation on protoplasm. Nitella grows upon submerged limestone rocks in permanent water. It is one of the most highly organized of the algae. It is attached at its base, bears branches arranged Fig. 45 ella. tip Nit- a, the of branch; h, a bit of the same some- what mag- nified: « . node ; i, i, intemodes- THE SIMPLER ORGANISMS 63 in whorls along its stem, grows apically from terminal buds, and has more of the aspect of familiar plants of other groups than any of the algae studied hitherto. An examination of its structure (fig. 45) will reveal that its stems and its branches are alike made up of alternating nodes and internodes, the nodes consisting of a ring of short, closely packed cells, the internodes consisting each of a single very large and long cell. The branches arise from the nodes. The internodes are wholly exposed. It is these very large intemodal cells, with their consider- able quantity of contained protoplasm that we will study- Since they are wholly exposed to view and have more or less transparent walls, it will be well to observe first the movements of the living protoplasm as seen under low power of the microscope. A fresh green spray may be plucked from the top of the stem, placed upon one slide and held flat under' another laid upon it, and thus placed upon the stage for observation. Focusing upon the upper sur- face of an intemodal cell, just beneath the roughness of the cell wall will be seen the numerous oval green chlorophyl bodies. At a slightly lower level, by looking intently for a minute, there may be seen the streaming protoplasm, which, though itself transparent, contains minute granules, by the movement of which it may be recognized. These granules will be observed to have a slow, flowing or gliding motion-, and they may be traced in a definite path of circulation round about the wall of the cell. A comparison of different intemodal cells will show that the streaming movement varies in rapidity in different ones and is much more clearly seen in some cells than in others.* *In case Nitella be not obtainable, the closely allied Chara (Fig. 48) may be used for the foregoing study: but for observation of the streaming protoplasm, single intemodal cells will usually be found only at the tips of the leaves. 64 GENERAL BIOLOGY The protoplasm may be removed from an internodal cell by snipping off one end of it with scissors (after it has been wiped dry) and squeezing the contents out upon a slide. The largest available cells should be selected, for even then the drop of protoplasm obtained is a minute one. Still it is large enough to see and to handle. One may lift it on the point of a needle, and test its viscosity. One may see it with the microscope, wholly uncovered. And if, in looking at it, there is little to be seen, there is enough to reflect upon in the fact that this inert and apparently well-nigh structure- less mass is the essential living part of every living thing, much the same in all, and, despite appearances, the builder of all the array of organic life. It is this substance that in the long aeons of the past has reclaimed the earth, and clpthed it with verdure and peopled it. The record of the foregoing study may well consist in drawings of some of the things seen, such as: A few filaments of Spirogyra, showing their common features and the individual differences between them. A single Spirogyra cell showing all the parts in detail. A bit of the chlorophyl band, highly magnified, show- ing its form, the median ridge upon it, the py- renoids, and starch granules. A cell treated with dilute glycerine, showing the shrunken protoplasmic capsule withdrawn from the cell wall. A diagram of the internodal cell of Nitella, showing the direction of the protoplasmic current. THE FORM OF THE PLANT BODY IN COMMON ALGAE. Some hints of the diversity of form in algae will have been gained from the study of Closterium, Spirogyra and Nitella — ^the first, unicellular; the second, a linear aggre- gate, its cells all alike; and the third, a branching, well-in- tegrated body of cells of very different sizes, with terminal buds and apical growth. THE SIMPLER ORGANISMS 6S The form of the plant body is much influenced by the manner of cell division. When the cells separate completely at division, the plant remains per- manently unicellular. When elongate cells divide transverse- ly, and remain attached, the linear aggregate results: when they divide lengthwise, such rafts as those of Scenodesmus (fig. 46) and of many diatoms result. When the planes of di- vision of the cells of a linear ag- gregate become oblique, cutting off frointhe cells prolonged api- cal angles, the filaments become branched as in Cladophora (fig, 47). When no division planes are formed, only the nucleus, but not the cytoplasm dividing, overgrown multinucleate cells are formed. One such type, that is enormously overgrown in long irregular interlacing fibres, is Vaucheria, the green felt — an alga that is found abundantly on wet soil in greenhouses. One observes in studying the algae that the transition from unicellular to multicellular forms is very gradual. First, there are multitudes of single, fig. 47. ciadophoia completely independent cells. Then f/a^tip^from'^thi there are those algae that consist I^^Tmenr"" Fig. 46. Miscellaneous algae, further illustrating types of cell form and arrangement, a, Clathrocystis, actively divid- ing; b, Scenodesmus acutus; c, Scenodesmus caudatus; d, Sele- nastrum; e, Hydrodictyon, /, Cosmarium; g, Staurastrum; h, Euastnim, 66 GENERAL BIOLOGY of practically independent cells that merely hang together. Then there are those that show some differentiation of parts, and some mutual relations between them. Scenodesmus cattdaius (fig. 46c) shows a very moderate beginning of differentiation in the modified form of the two end cells. Then we have a differentia- tion between base and- apex, the one end tak- ing up the duty of se- curing attachment, the other providing for growth as in Cladophora (fig. 47). Finally, w. have in Chara, a solid Fig. 48. Chara. a small branch; b. piece of the stem containing a node and part of two internodes, the lower one hav- ing the cortical cells spread apart from the central cell; o, ovary (archegonium) ; s, spermary (antheridium) ; c, the mature ovary more enlarged, showing the egg cell within; d.e.and/, successive stages in the development of the ovary; g, the mature spermary in section; h, a pair of sper- matic filaments; i, a bit of one of the filaments more magnified to show the sperms developing within the cells; j, a single sperm Set free. aggregate of greatly differentiated cells. Chara, like Nitella, is made up of a succession of nodes and internodes, but in the former there is one central cell completely surrounded laterally by a layer of slenderer cells (fig. 48). Thus the central cell is completely inclosed and removed from the source of supply of food and air ; and it is rendered dependent on its neighbors for its living. And in Chara and in many other algae there is a high degree of division of labor, certain cells of the plant body being set apart to serve the reproductive process, while others perform the nutritive functions. The purpose of the following study is to observe in a variety of representative algae the phenomena of cell aggre- gation and of cell differentiation. Incidentally there should THE SIMPLER ORGANISMS 67 be seen something of the place algae occupy in the world, something of their diversity of form and size, something of their exquisite beauty and delicacy of organiza- tion, and the principal differences between them in the manner of their chlorophyl distribution. Study g. Observations on cell form and growth habit in alga^. The materials needed are : i . A few of the larger, more typical green algae (such asNostoc,Cladophora,Hydrodic- tyon, Vaucheria and Chara) in water. t I Fig. 49. Some water-supply diatoms, i, Navicula; j, Cocconema; k, Asterionella; /, Tabellaria; m, Fragilaria. 2. Some submerged or floating leaves of aquatic plants, from which may be scraped a variety of diatoms and des- mids. The student will get these for study by mounting, and examining the scrapings upon a slide. Stalked diatoms may usually be found upon the filaments of the larger algae, such as Cladophora. 3. Strainings from the water tap, yielding diatoms (fig. 49) and other algae that are common in the water-supply, obtained by tying a sac of fine silk bolting cloth over the tap and letting the water run slowly through for an hour or less. The study should consist in the examination by the stu- dent of these different algae, one by one, observing and recording the points outlined above. He will find it desirable to familiarize himself somewhat with the princi- 68 GENERAL BIOLOGY pal groups of algae by reference to any good text book of botany. The identifications of unusual forms that may be found will be facilitated by the use of the plates in such works as Wolle's Fresh Water Algae, Wood's Fresh Water Algae of North America West's British Fresh Water Algae, and keys in such works as Lampert's Das Leben der Binnengewasser and Stokes' Analytical Key to the Genera and Species of the Fresh Water Algae and Desmidiae of the Uni- ted States. The record of the results of this study may be pre- served in a few simple out- line drawings, showing for the larger forms, a diagram of the manner of growth, and a drawing showing the cell form and the distribution of chlorophyl. Nucleus, protoplasm, cytoplasm, and other internal parts may be taken for granted, and need not be sought out nor repre- sented in this record. SOME TYPICAL PROTOZOANS. The simplest animals are the Protozoans. In a much greater proportion than in the algae, the cells exist singly. Like the unicellular algae they consist of few parts, and such of those parts as they have in common are found in every cell — nucleus, cytoplasm, inclusions, etc. Amoeba (fig. 51) is one of the simplest of animals. We call it an animal because it moves about freely and feeds on other organisms ; but at first sight it seems wholly lacking Fig. 50. Micrasterias (after Carpenter.) AtoF, successive stages in cell formation. THE SIMPLER ORGANISMS 69 in the usual features of animal life. It has no legs, nor even muscles, for moving, no mouth for eating, no nerves for feel- ing, no organs whatever for any purpose. Since the amoeba lives an essentially animal life without these parts, a careful study of it may enable us to discover what are the essentials of animal existence. Probably the easiest of the amoebas to obtain for study is the small species that develops in a hay infusion. If a quantity of dry hay be put into a jar of water and left stand- ing uncovered where not exposed to the direct rays of the sun, soon the soluble organic matter in the hay is dissolved by the water. In the course of a day or two the bacteria that feed on this solution, form a soft jelly-like substance which gathers in a film upon the surface of the water in the jar. In the course of about three days amoebas begin to appear commonly in the jelly layer, moving about therein and feeding on the bacteria. In another day or two they generally reach their maximum of abundance ; but they may continue much longer, if the conditions of their living be maintained. They_are too small to be seen with the unaided eye, and hence, must be mounted upon a slide and looked for with low power of the compound microscope. Since they inhabit the under part of the surface layer of bacterial jelly, they are best obtained free from it on the slide, by lifting a little patch ;of the jelly upon the slightly separated tips of a for- ceps, dabbing it down several times on a slide, thus shaking off the drop of adherent water and the amoebas with it, and then throwing the mass of jelly away. Even thus, so much of the jelly may have fallen into the drop, that one will have to look about the thin edges of it to find a clear field for observation of the animals. It is very important that the temperature of the animals be not lowered during the process of mounting them or of 7© GENERAL BIOLOGY observing them later; else, the following observations will not be possible : for if they be cooled, they will contract into a heap, and remain inactive and scarcely recognizable. Therefore, the air of the room in which they are studied, and the slide and cover and stage of the microscope must not be cooler than the water fro^m which they are taken. In a few moments after their transfer to the slide (the drop being properly covered, and the space beneath the coverglass entirely filled with water) the amoebas .should be- gin to creep around freely iipoii5the sur- face of the glass. Althou'gh.yepr' minute they will be recognized even under low power by their form (see fig. 51) and especially by their slowly changing out- lines. The details of internal struc- ture in a single animal are not to be ob- served except with high magnification, and a sufficient cutting down of the light to allow the more transparent parts of the animal to come into view. When found and properly lighted, it will be easy to recognize in the body a granular cen- tral mass of protoplasm, a clearer exterior layer, with definite, though slowly changing outline that never shows sharp angles, but only rounded lobes. The granular internal portion of the body of the animal , is spoken of as the endosarc. Within it are to be seen i) the round and uniformrly translucent nucleus: 2) the- very clear contractile vacuole, which disappears at intervals,, and which usually shows a tinge of pinkish color, and 3) ingested food particles, usually aggregated more or less into round food balls which may be seen moving about in the endosarc. In these food balls the forms of some of the bacteria more recently eaten may usually be recognized^^ Fig. 51. Amoeba, a, an active individ- ual; ps, pseudopo- dium ; n, nucleus ; /, f ood : V, vacuole ; 6, diagrammatic representation of division. THE SIMPLER ORGANISMS 71 Different individuals will differ much in clearness of these parts, according as they have recently eaten much or little. The very clear external protoplasmic layer is generally known as ectosarc. It shows no definite • cell wall, but only an enveloping film; hardly more of this than may be accounted for by surface tension of the body fluids. It has in amoeba developed no external skeleton, but remains so clear that we may here observe nearly simple naked protoplg,sm, unobscured by an3rthing, and in action. The movements seen here are quite different from the simple streamin^^naotion in a single current of the intemodal cell of Nitella; they are of a higher order of complexity. A portion of the body substance may be pushed out in any direction, but always in the direction of locomotion, forming Fig. 52. Amoeba. Diagram illustrating in 7, 2, 3, food intake; in ^, J, 6, removal of indigestible residue \ s, a. food organism ; p, the same occupying a food vacuole, recentlv engulfed; q, the same partly digested; r, residue of same, discharged. the broadly rounded lobes called pseudopodia: first the ectosarc pushes out, and then the granular endosarc streams forward into it. Pushing out forward, and pulling up from the rear is the process^^ of locomotion, and it is dependent solely upon the contractility of protoplasm. By this same power feeding is accomplished. Two pseudopodia (fig. 52 1-3) encircle a suitable bit of food, and press it into the interior of the body, where, engulfed by the protoplasm, it is digested: any indigestible residue is gotten rid of by the reverse process — the protoplasm flows away from it and leaves it behind (fig. 52 4-6). These activities imply volition of some sort or degree, for there appears to be some selection of food and some spontaneity of movement : changes of direction, the taking 72 GENERAL BIOLOGY of a circuitous course in avoidance of an obstruction, etc., indicate this, and since there is only protoplasm present and responsible for the actions, it follows that sensibility, or at least, the capacity for responding to external stimuli, is another property of protoplasm. We tap the slide sharply and the amoebas contract, .drawing in their pseudo- podia, but soon, after a different interval in different indi- viduals, they resume activity again. The larger amoebas live in the sediment on the bottom of ponds and ditches, in the slime on the submerged leaves of aquatic plants, etc., and while much more difficult to obtain in sufficient numbers for class use, they are, on account of their size, much more favorable for studies of some of the phenomena outlined above. One gets specimens for study by mounting the slime upon a slide and searching till amoebas are found. Since these commonly devour diatoms and desmids, which show their characteristic colors for a time after being engulfed, the process of digestion (as ev- idenced by the disappearance of the normal plant structures) is in these species more easily observed. Study 10. The structure ami activities of Paramoecium. Materials needed: A hay infusion a week or more old in which the surface layer of the bacterial jelly is breaking up, being largely consumed, and in which Paramoecium has appeared in large numbers: or, an old infusion that has been kept going by occasional feeding with com meal. The paramoecia will be found about the edges of the culture close to the surface. They are large enough to be seen as minute oblong white specks in rapid motion. They must be present in sufficient numbers for one to get a number in a drop of water taken up by a pipette, and if not so numerous, they should be concentrated, by some such sort of filtering apparatus as that described in the appendix. There will be needed also, besides the usual labora- THE SIMPLER ORGANISMS 73 tory apparatus and reagents, a little dry carmine, and a 2% solution of gelatine, or its equivalent. The student should perform the work of the following outline : 1. Obtain a drop of water containing paramoecia upon a slide. Examine it uncovered with a simple lens to make sure that the animals desired are present. A little trash from the jar included in the drop will be of assistance in pre- venting the cover glass from coming down too close and crush- ing the animals. Numerous smaller, but similar infusorians are likely to be associated with Paramoecium, and often the phenomena of division are more commonly found among these. 2. Before applying the cover glass, survey the contents of the drop with low power of the compound microscope, and, by moving the slide, follow some of the paramoecia as they go swimming about. Observe the spiral course of the swimming, and the resultant rapid motion directly forward. Observe also the habit of the animal when it meets an ob- struction : note the slight backward motion before the turn- ing aside. 3. Apply the coverglass, with plenty of water under it, so that there will still be room for swimming. Find a place where a paramoecium is repeatedly meeting with obstruc- tion to his swimming, and observe what relation the direc- tion of his turning aside bears to the position of the oblique groove {oral groove) down one side of the anterior end of the body. Observe also, the rolling motion of the body in swimming, and determine what relation the position of the mouth bears to the axis of the spiral couise in which the animal swims. 4. Withdraw some of the water from under the cover- glass with ablotting paper strip held at the edge, so as to confine some of the animals in close quarters. Find a place 74 GENERAL BIOLOGY where several may be observed together, the movements of all somewhat restricted. Note the perfect definiteness of anterior and posterior ends of the body. Note the general pliancy of the body, best seen when turning sharp corners.etc. 5. Observe the presence of a sharply defined layer of ectosarc, thickly covered over its outer surface with minute transparent hair-like processes called cilia in constant rapid motion. To the lashings of these cilia, the movements of the animal are due. The cilia are invisible in too strong light, and also when in rapid motion; in the latter case the scattering of such minute particles as come near them will testify to their presence and activity. A few longer cilia at the hinder end of the body, seem to serve as a sort of steer- ing apparatus or rudder, and probably assist in keeping to a true course in swimming. 6. Observe the position and relations of the oral groove, its length, its oblique position, and the funnel-shaped depres sion in which its posterior end terminates. Observe the peristome surrounding the mouth, bearing a continuous line of larger and stouter cilia, which, besides their participation in the spiral swimming, drive food particles into the funnel- shaped mouth opening. 7. Mount a small drop of clean water containing para- moecia in a large drop of gelatine solution upon a slide, cover and study the action of the cilia. The movements are restrained more or less by the gelatine, and, with proper lighting, should be easily observed. 8. In a quiescent but living specimen observe the large vacuole near either end of the body. Watch it long enough to observe its contraction and disappearance, and the formation of the circle of radiating clefts through the sur- rounding protoplasm. Consider the part such movements may play in keeping the contents of the cell in circulation. " 9. Observe also the nuclei nearer the centre of the body. The nucleus is differentiated into two parts; a large oval or THE SIMPLER ORGANISMS 75 oblong meganucleus and a little round micronucleus close beside it. The former at least should be visible in the live animal. 10. Mount another drop of clear water containing paramoecia, first adding to it a little finely powdered car- mine, stirring the carmine through the drop. Cover, remove excess of water, find a little group of paramoecia in some more or less restricted area, among trash, or at the edge of the cover, and watch them eat the carmine. It is wholly indigestible and may be followed in its entire course through their bodies. Unless the mounting, stirring, covering and finding be done with unusual celerity, bright red food-balls of carmine will already be seen within the protoplasm of the animals when first looked at. Other food-balls may be seen forming in the neck of the funnel-shaped rudimentary esophagus that leads inward from the mouth, and those first formed may be seen in their course of circulation round the body, and may in a little while be followed through their entire circuit. 11. Mount another drop containing paramoecia, adding thereto a little methyl green or iodine. Cover and study carefully the details of cellular structure : a) Ectosarc and endosarc. b) The peristome and its fringing cilia, and the esophagus. c) The cilia of the body in general and of the posterior end in particular. d) The stinging threads which the reagent used caused to be thrown out from the ectosarc. These will be seen among the cilia along the sides of the body, and will be distinguishable therefrom by their irregularity and unevenness of length, and by their different mode of attachment to the ectosarc. e) Meganucleus and micronucleus. , f) Vacuoles and food-balls. 76 GENERAL BIOLOGY 12. In a fresh mount containing a large number of indivi- duals study the division of Paramoecium (or, if more abun- dantly evidenced, use any other available, infusorian). Observe the division of both mega- and micro-nucleus, and the subsequent division of the protoplasm. Observe the fate of the oral groove. The different stages may often be found simultaneously in different dividing individuals. The record of this study may well consist in : 1. A diagram to illustrate the spiral course of swimming of Paramoecium. 2. A diagram to illustrate the movement by which an obstruction is avoided. Indicate plainly oral and aboral sides. 3. A detailed drawing of a single animal, showing all its normal structures. This should be begun with the begin- ning of the study, and details added as they are worked out. 4. A series of outline drawings illustrating the progress of division. Study II. The specialized cell-body of Sientor and Vorticella. It ■\yill now be well to study a few of the higher protozoans, illustrating the great degree of dif- ferentiation of parts and of specialization that may occur in the single free-living cell. For this purpose two com- mon protozoan inhabi- tants of fresh water ponds (fig. 53) are sel- lected, Stentor and Vorticalla. Fig. 53. Three common infusorians. A, Paramoecium; n, nucleus; u, v^ vacuoles; /. food-ball at the bottom of the rudimen- tary esophagus; ^,i>eristome. C, Stentor; I, lorica. F, Vorticella; s, extended; t, contracted. THE SIMPLER ORGANISMS 77 Stentor. — This is a large protozoan that is often found adherent to submerged twigs and leaves, and that is usually obtained by placing the trash from a pond in jars of water and letting it stand a few hours. The stentors, large enough to be seen with the unaided eye, and to be cer- tainly recognized with a pocket lens, will be found extended in the form of a trumpet, the narrow basal end attached to the twigs, etc., or suspended beneath the surface film. If a twig bearing stentors attached be transferred to a slide, covered, and allowed abundance of room and plenty of water beneath the cover, the stentors will soon be ready for observation, and for the work of the following outline: 1. Make a preliminary survey of the contents of the mount, finding: a) . Stentors extended and trumpet shaped (whence their name) , and attached by their slender bases to some support. b) Others contracted into globular or club-shaped form. If possessing a gelatinous cup-shaped receptacle about their bases of the sort known as a lorica (fig. 53 C, ^), these will be more or less with- drawn into it. c) Others detached, more or less contracted, aild lying free or swimming about in the water with something of the spiral rolling motion of Paramoecium. These may have been detached in mounting; however, Stentor may voluntarily make a change of base. 2. Find a little group that may be brought into the field with the lowest power of the microscope, and take time to study their actions: a) While watching a fully extended animal through the microscope, tap or jar the slide sharply and see it contract : continue watching until it is again fully extended. 78 GENERAL BIOLOGY b) Observe the action of the fringe of strong cilia {peri- stome) surrounding the rim of the trumpet, and try to see objects free in the water driven by these cilia into the mouth. If not well seen this may be demon- strated, as for Paramoecium, by adding a little finely pulverized carmine to the water. 3. Using an eyepiece of higher magnification, study the extended stentor, observing: a) The lorica, if present; note its shape, appearance, and consistency. b) The disc-like attachment of the foot. c) The long tapering body, covered with minute cilia, d) The flaring distal end, with its encircling peristome, involute at one, end to surround the mouth. Com- pare with the peristome and mouth of Paramoecium. 4. Within the body observe in a specimen having the mouth uppermost: a) The short esophagus ending blindly in the endosarc. b) Food-balls moved about in the endosarc. c) An elongate, moniliform meganucleus, and a micro- nucleus close beside it. The latter is usually hard to see in the living specimen, but may be demonstrated with iodine as in Paramoecium. d) A large contractile vacuole, of varying proportions. e) Fine nearly parallel lines extending from foot to disc in the ectosarc (myonemes) . 5. Observe the ordinary reproduction of the animal by division of the single cell into two ; note the plane of the divi- sion, and the relation it bears to foot, disc, peristome and meganucleus. The Record of the study of the stentor may well consist in: I. A sketch in simple outlines of a little group of stentors in various posi^ons. .2. The details of structure of a single animal. THE SIMPLER ORGANISMS 79 3. The phenomena of division, in a series of outline sketches. Vorticella. — This protozoan will usually be found associa- ted with Stentor and specimens for study are readilyj ob- tained by the same means. The individuals are smaller, and singly are difficult to see ; but they commonly occur in groups, and a little cluster of them about a twig, contracting so strongly as to almost disappear when touched, will be easily recognized. Vorticellas when abundant appear to the unaided eye as a whitish fringe about the edges of submerged twigs. The student should obtain upon a slide a small bit of rootlet or other solid support with vorticellas attached, should mount and cover this, filling up with water all the space beneath the cover, and then should perform the work of the following outline : 1 . Survey the mount for : a) Single vorticellas contracting and extending their stalks. b) Little groups of individuals, attached to the rootlet separately. c) Detached heads, broken off from the stalks and swimming free. 2. Study the actions of the vorticellas, observing: a) The contraction and subsequent extension of the stalk. b) The closing and opening of the peristome. c) The action of the cilia and its effects on free particles in the water. d) The action of the vorticellas toward one another when touched, and toward other free swimming organisms which happen to come into contact with them. 3. Study the differentiation of parts in the body of Vorti- cella, noting: 8o GENERAL BIOLOGY a) The complete differentiation of the body into bell- shaped "head" and contractile stalk. What is the distribution of ectosarc and endosarc in each ? b) The great development of the peristome, and the restriction of the cilia thereto. Note the size of the cilia, and the contractility of the ridges that bear them. c) The band of contractile substance, a highly de- veloped myoneme, extending in an open spiral down the stalk. Observe its position in extended and in contracted specimens. 4. In the body of the cell, observe the usual internal Structures: a) A curved, often horse-shoe-shaped meganucleus near the middle of the body, and a micronucleus lying close beside it. If the latter be not visible in the living specimen it may be demonstrated later with iodine, as in Paramoecium. b) A clear contractile vacuole, near the nucleus, appear- ing and disappearing. c) Food-balls, moved about within the endosarc. The taking of food and the formation of these balls at the end of the rudimentary esophagus, may be demonstrated by feeding with carmine, as in Paramoecium. 5. By surveying a large cluster of vorticellas, a number are likely to be seen in process of division. In such observe the plane of division, and its relation to nucleus, peristome and stalk. The record of the study of Vorticella may well consist in: 1. A drawing in outline of a little group in various positions. 2. The details of structure of a single cell much enlarged. THE SIMPLER ORGANISMS 8i 3. An outline drawing illustrating the manner of dividing. Colonial Vorticellidae. — In a number of protozoans allied to Vorticella, the two cells resulting from a division do not entirely separate, but both remain attached basally to the common stalk, each later prolonging the attachment into a stalk of its own. Successive divi- sions thus give rise to colonies. Such colonies are likely to be found asso- ciated with Vorticella, and should be compared therewith. When the colonies are large they are easily distinguished with the unaided eye from clusters of Vorticella by their height, due to their elevation on a common stalk. One of the com- monest of these is Epistylis, dia- grammatically shown in the accom- panying figure (fig. 54). This differs from Vorticella in that the stalk is not contractile, lacking the myoneme: myonemes are re- stricted to the base of the elong- ated "head" which, becomes trans- versely wrinkled when contracted, v%howrng"the 'elongated and tothe pcristomc which bccomcs Fig. 54. Epistylis umbellarius. a , a portion of a colony ; x and y, successive divisions pro- ducing conjugants of reduced size; z, conjugation between one of these reduced cells and a cell of normal size, b, a single individual in lateral view, showing the elongated dia|r''aTof""the^top°rf ihe enrolled, as in Vorticella. peristome, showing its spiral arrangement ; d, a normal mdividuai, contracted. ^j^^ individual stalks are In Charchesium, however, individual stalks are con- tractile and in Zoothamnium, the common stalk of the colony also, in-so-much that when Zootharanium contracts, the main stalk and all its branches acting synchron- ously, all the bodies are suddenly brought down into a 82 GENERAL BIOLOGY round, berry-like heap. These three genera include all ou common allies of Vorticella of colonial habit. It is not to be overlooked, while studying protozoan that even in these forms, there is a foreshadowing of thi principal organs of the higher animals. The long esophagus of Epistylis is prototype of the alimentary canal; the con tractile vacuole, forerunner of a sort of rudimentary circula tory apparatus; and the myonemes constitute a sort of elemental muscular system. THE LIFE PROCESS IN PLANT AND ANIMAL CELL. We have seen that in many algae and in most protozoans the cell is an independent organism: all functions of plant or animal are performed by it. Even when such cp are grouped together to form a larger organism, their v is for the most part a loose one, and their physiologic independence is little impaired. To the cell, then, we touf go to learn what are plant and animal functions, and ho\ they are performed. How does the cell live and grow? This is a hard question, answered as yet only in part. The answer so far as avail- able is best stated in terms of matter and energy. Matter. — The bodies of living beings are composed of a few chemical elements, such as are common in soil and water everywhere. This is readily determinable by chemi- cal analysis. In all living substance there are nine chemical elements constantly occurring, three others (the three last named below) that are nearly always present, and a number of others occur here and there. The twelve are : Carbon C Sulphur . . . . S Magnesium . . Mg Hydrogen . . . H Phosphorus . P Sodium .... Na Oxygen O Potassium. . . K Chlorine .... CI Nitrogen . . . . N Iron Fe Calcium Ca THE SIMPLER ORGANISMS 83 The four in the first column, carbon, hydrogen, oxygen and nitrogen constitute over 99 per cent of the living sub- ,stance, the others being present in very small amounts. In nature these elements are found everywhere in the crust of the earth, combined as simple mineral salts, which, being more or less soluble, are found also in the waters of the earth. That these salts will maintain the life of the green plant cell may readily be determined by supplying them to it as food. The commonly used food solution for green plants has the following composition: Distilled water (H^O) 1,000. grams Potassium nitrate (KNO3) 1. " Sodium chloride (NaCl) 0.5 Calcium sulphate (CaSO J 0.5 " Magnesium sulphate (MgSo^) 0.5 " Potassium phosphate (K^HPOJ .. 0.5 " Ferrous sulphate (Fe^SO^) a trace Here we have all of the twelve elements listed above ex- cept carbon, and this the green plant obtains from the car- bon dioxide (CO J of the air, either direct, if it be a terrestrial plant, or dissolved in the water, if it be aquatic. On such a solution of the simplest mineral compounds green plants thrive. These elements are recombined in the living body into compounds of very much greater variety and complex- ity, the more important of which fall into two great classes, according as they possess or lack nitrogen in their composi- tion: I. Carbohydrates and fats: non-nitrogenous compounds, containing carbon, oxygen and hydrogen, but no nitrogen. II. Proteins : nitrogenous compounds of great complexity. These substances, formed in, and constituting the bodies of plants, are the primary food of animals. Energy. — ^The forces that operate upon living bodies are those that operate upon the non-living: gravitation, heat, 84 % GENERAL BIOLOGY light, electricity, magnetism, mechanical energy, molecular energy (cohesion, adhesion, attraction of molecules) and chemical energy (chemical affinity, the attraction of atoms). In the living world, as elsewhere, energy may not be de- stroyed, but may be endlessly transformed.* The primary source of energy for living beings is the sun's rays. The radiant energy of the sun, acting on the chloro- phyl-bearing protoplasm of the green plant cell, effects the cleavage of carbon-dioxide into its two constituent ele- ments, carbon and oxygen. Then ensues the synthesis of the liberated carbon with water to form sugar, which may be transformed into starch, and stored in the tissues. The chemical statement of the reaction (a statement of the shift of the elements only, that tells nothing of the enormous consumption of energy involved) is, in its simplest form, as follows: Carbon dioxide Water Fruit sugarf Oxygen 6 CO, + 6 H,0 = C,H,,0, + 60, This equation expresses graphically the primary syn- thesis of inorganic materials to form an organic compound. *Energy may be either active (kinetic) or latent (potential.) Kinetic mechanical energy is that of a clock spring, moving by the release of tension the works of the clock : it is potential when the spring is wound up, before the pendulum is started swinging. Or it is that of a pile driver hammer falling and delivering a stroke : it was potential when the hammer was lifted and ready to be let fall. Kinetic chemical energy is that of coal burning in an engine, moving the piston: it was potential in the coal. It is that of powder exploding in a gun : it was potential before the cap was struck. Energy was used to wind the clock, to lift the hammer, to combine the unwilling elements of the powder- — it disappeared: it was rendered latent or potential. tThe simple sugars differ from starch (QH^Oj) mainly in that they contain relatively more water. The complex sugars differ in being multiples of the simple sugar lacking one molecule of water for each molecule of the simple sugar taken (ordinary cane sugar, C„H,jO„) . Through the series of carbohydrates (sugars, starches, etc.,) carbon is combined with hydrogen and oxygen, the two latter retaining the ratio they have in water. The formula of the series, (C.H„Os)„. THE SIMPLER ORGANISMS I 85 In order to understand the energy involved it is necessary to take into account the attraction of the atoms. Carbon and oxygen have strong mutual affinity, and combine to- gether in carbon dioxide to produce a very stable compound. In the above reaction the carbon is separated from the oxygen, and this requires the expenditure of energy — ^the energy of the sun. In overcoming the strong af&nity of carbon and oxygen for each other, this energy disappears, being rendered potential in the separated atoms: it will reappear in like amount whenever these reunite. It is readily measurable in terms of heat. The heat produced by com- bining twelve grams of carbon with thirty-two grams of oxygen (an ounce and a half of these two elements) is suffi- cient to raise the temperature of a kilogram (over two pounds) of water from the freezing to the boiling point, and in the separation of like quantities of these elementa whether in the electric furnace or in the green leaf, a like amount of energy is rendered potential. It is easy to demonstrate that starch is formed by chlorophyl-bearing protoplasm only in the presence of sun- light. It is not difficult by proper chemical means to determine the composition of the sugar or starch formed, but it is impossible to follow its formation by direct obser- vation: hence it must be borne in mind, the above equation is a theoretical explanation, based on knowledge of the behavior of the chemical elements, of the nature of the com- pounds in the food and in the praducts of the plant, and of the observable phenomena of its nutrition. If so great difficulties attend the explanation of the first step in the synthesis of organic substances, it will be readily appreciated why the succeeding steps involving the manufacture of proteins, are little understood. A purely theoretical ex- planation of the production of asparagin, one of the simplest of organic nitrogen compounds, of wide distribution in green plants is that of the following equation : 86 GENERAL BIOLOGY Glucose + Potassium = Asparagin + Potassium + Water + Oxy- nitrate oxalate gen CfiH^.O, + 2KNO3 = C.NgN.O, + K,C,0, + 2H,0 + 3O The mineral nitrates, sulphates, phosphates, etc., enter into succeeding combinations. Few proteins have been successfully analyzed ; but it is well known that many of them are of exceedingly complex structure. Their molecules are composed of a very large number of atoms, in loose combination. As the size of the molecule increases, the stability decreases, as bricks incline to topple when piled too high. A sample analysis of the molecule of a familiar protein, hmmoglobin, from the blood, gives results corresponding to the following formula: C6coH,6oN,s,Fe.S3 0,„. The reverse process, whereby these cpmplex and unstable compounds are broken up again into simpler ones with the liberation of their energy for the use of the body, is even less understood in its details : it is chiefly known by its results. The end products of metabolism in animals are water, carbon dioxide, urea (CH^ N^O) uric acid (CjH^N^ O^) and such other simple nitrogen compounds as ammonia, adenin, xanthin, creatin, etc. ; and in plants they are the same ex- cept that the nitrogen liberated by proteid dissimilation is recombined and does not appear as waste. The accompany- ing crude diagram is an attempt to represent graphically the relations the more important of these compounds bear to each other in income and outgo of matter for organisms. It is as the map of a country as yet but little explored. a s o ft p OJ P4 "O w ■y H o < ■a ^ c o JO u cS U .Box- o o a, -g •rH o !>, 'Td -4J ■d Xi a (D -1 1 a 8 cl + 'S.g iins in+a carbo cin 1 m 2 1- iJ enzymes and secretions ■proi ucle in irote rote mu a "3 ^ A G , II O > t3 L evi Albu Glob lerve n, etc. si fi. ^•s" 1 ol E in 1 to §143 -a " 2 O a) ■d 'H o '•B « , o ca O o CO en - (U <; 88 - GENERAL BIOLOGY While the analysis of the processes involved in the metab- olism of the living substance is difficult, and details are somewhat uncertain and only the beginning and the end steps have hitherto been traced, there is no doubt whatever about a number of the main facts : 1. That the organic life of the world is supported on water, carbon dioxide and simple mineral salts, gathered and assimilated by the green plant cell. 2. That these mineral substances are of simple composi- tion, are composed of but few elements united strongly, and that they are very stable, and devoid of potential energy. 3. That the non-nitrogenous substances first combined under the power of the sun's rays, are compounds of a higher order of complexity of less stability and of much more potential energy. 4. That the nitrogenous substances (proteins) are of great diversity and of exceeding great complexity of struc- ture, very unstable, and of very high potential energy. 5. That protoplasm is a complex substance (not a single chemical compound) , probably a mixture or combination of various proteins, water, etc., so unstable it is impossible of analysis for, to analyze it kills it, and death initiates changes altering its composition. 6. That the primary source of energy is the sun, drawn upon by the green plants first ; the supply for other organ- isms is the potential chemical energy of manufactured carbo- hydrates, proteins, and of free oxygen. Protoplasm, the physical basis of life, the living part of every living thing, and essentially the same in its general properties and functions in all, possesses in green plants the capacity for developing 'chlorophyl, through the agency of which the energy of the sun can be utilized in effecting such analysis of simple mineral compounds and such synthesis of more complex organic compounds as result in the storing THE SIMPLER ORGANISMS 89 up of a large amount of energy. Then the living substance acts as a chemical engine, using the energy of these same organic compounds, and in that use, reducing them again to simpler ones. Here as elsewhere, neither matter nor energy MAX SCHULTZE (182S-1874) "The father of modern biology." Physiologist; histologist; who first showed that protoplasm is the common basis of plant and animal life. is created or destroyed, but both are endlessly transformed. The living body is constantly changing. It is only the con- stancy of the stream of income and outgo that allows it to present a semblance of an abiding presence. It is like the chemical engine in that it uses fuel — the food — whose 90 GENERAL BIOLOGY transformation into gases and ash liberates energy for its work. It is unlike the engine in that, far frombeing a mere contrivance of chambers in which transformations and reac- tions may occur, it is itself changed constantly, formed and reformed, regularly gathered from and returned to the stream. The dissimilation process (katabolism) , whereby the complex organic compounds are broken up into simpler ones, with the liberation of their energy for use, has not hitherto been traced step by step in detail: indeed, it is even less understood than the assimilative. Its results are well enough knoAvn: the end products are simple com- pounds, CO J, HjO, and nitrogen compounds not wholly reduced to the grade of composition they had when first taken up from the water (and therefore, a little energy that they still retain is lost to the body) . Their energy has re- appeared in various forms, mechanical movement, bodily heat, luminescence, etc. From the chemical side it therefore appears that assimila- tion (anabolism) is the process of separating chemical affinities and of storing up chemical energy in complex compounds, and that dissimilation (katabolism) is the pro- cess of reuniting affinities in stable compounds with the liberation of energy for use. Plant and animal differ typically in the nature of their intake and output of matter and energy, and the main features of that difference are expressed graphically in the diagram at the top of the following page. In this table the facts are of necessity stated broadly. For example oxygen is given off by the green plant only in the light, and among animal foods organic and inor- ganic materials are set down together, The latter consti- tutes a very small part of animal food, never-the-less the diagram should aid in forming a definite conception of the fundamental nutritive relations of plants and animals. THE SIMPLER ORGANISMS 91 Income ^ CO, from g the air In Mineral << salts in g splution Outgo Free oxygen Income Free oxygen Proteins Carbohy- drates and fats in food H,0 & salts . Outgo CO. Urea and other nitrogen compounds The Green Plant Cell The Animal Cell Radia >< energy the St W Chemi ^ energ « of fri oxyge nt of m cal T ;e n Heat, move- ments of pro- . toplasm, &c. Chiefly dis- appears in syntheses of organic compounds, becoming potential Potential chemical energy of the food and of free oxygen Movements, heat, &c. (A little is lost in nitrogen waste) III. SOME INTERMEDIATE AND UNDIFFERENTIATED FORMS. The typical algae and protozoa studied thus far, conform to our general notion of plant and animal, derived from contact with the higher, familiar forms of life. The green color of the plant and the free movement and foraging habit of the animal seem at first to mark out naturally two distinct groups ; among the higher forms there is no diffi- culty about distinguishing between plant and animal. It is easy to tell a dove from a daffodil ; it is not hard to tell a green alga from a free swimming gray protozoan ; but there are among the lower organisms some that do not clearly show even the broad distinctions of the preceding diagram, and some that so combine the characters of the two groups that one may not say with assurance whether they are plants or animals. 92 GENERAL BIOLOGY We will first consider a large and important ecological group of organisms that we recognize as plants although they do not contain chlorophyl, and they do require much the same food as animals; after that, two other groups with characters so intermediate that they are discussed in text books of both botany and zoology at the present day. I. PLANTS THAT LACK CHLOROPHYL. The most important common characteristic of the large ecological group of organisms we now come to consider, is physiological: lacking chlorophyl, they have abandoned the primary plant function of gathering food materials directly from the inorganic world. They must have organic food. They can derive no energy from the sun, and they thrive often quite as well without sunlight. They use the same foodstuffs as animals; yet in structure and growth- habit they are plants very miich Uke green species of parallel development. Yeasts. — These are unicellular chlorophylless plants of the group of fungi. Isolated cells have, save for their gray- color, much the appearance of single cells of protococcoid algae. They have cellulose in their walls: their protoplasm is somewhat more granular, contains minute fat droplets and is without a trace of chlorophyl. The process of cell miiltiplication is peculiar. It is called budding (or gemmation). Mi- nute processes are pushed out from the side of the cell, and these grow up gradually to full stature, adhering for a time to the parent Pig. 55. Yeast, a, a single cell 11 /^r^ „ xi_ 11 j j showing nucleus (dark colored), CCll. Ulten tne nCW Cell StartS two vacuoles, and numerous f ., r *• 1., r 'j • r 11 fat droplets; 6, clusters of grow- budS 01 itS OWn before it IS fuUy ftor«!oe)!'^fa°vario''us^stagef of grown itself. Thus whilc grow- ^rdSfoAr'^?hin"ea'?h™e1 ing quictly the cells come to be '^^'^' assembled in little clusters or families of cells (torulcB), as shown in fig. 55. THE SIMPLER ORGANISMS ^3 A food solution for yeast that bears the name of the great biologist, whose fame rests in part on discoveries he made of the part yeasts play in fermentation, is the following: Pasteur's Solution. Wate/H O 83-76 % Cane Sugar C ,H, O,, iS-oo Ammonium tartrate (NH^) ^C^H^O^ .... i.oo Potassium phosphate K^PO^ 0.20 Calcium phosphate Ca^ (PO^) ^ 0.02 Magnesium sulphate MgSO^ 0.02 LOUIS PASTEUR (1822-1895) "One of the most conspicuous figures of the nineteenth century." Pioneer student of fermentation, of disease germs, etc. His services to his country and to humanity are commemorated by Pasteur institutes i<^r the treatment of infectious diseases throughout the civihzed 94 GENERAL BIOLOGY If this formula be studied it will be discerned that the chemical compounds of the food of yeast are intermediate ia kind between those of animals and those of green plants. Some of the same mineral salts are used by both green and colorless plants. The nitrogen is obtained from a somewhat more complex compound in the latter. Only the sugar is properly an aniinal food. Proteins such as animals require are wholly lacking. It will be noted that there is carbon in the formula aside from the sugar : the yeast will live, indeed, in this solution if the sugar be omitted but its growth will then be very slow. It will be noted also that the sugar is present in very great excess of the need of the yeast for carbon. The yeast plant contains a sugar ferment. It utilizes only about one per cent of the sugar, and decomposes the remainder into carbon dioxide and alcohol. The re- action of the fermentative decomposition may be expressed as follows: Carbon Sugar Alcohol dioxide CfiH.p, = 2C,H,0 + 2CO, It is the production of these two by-products that makes yeast commercially important. Yeast produces the same reaction in the sugars of cider and wines, and in the meta- morphosed starches of the cereal grains, that are chiefly used in commerce in the production of alcohol. The carbon dioxide is also utilized in the making of bread. Yeast is mixed with the dough, and, fermenting in it, evolves the carbon dioxide gas, which "raises" it, making it porous, and improving its digestibility and flavor. If a little fresh yeast be sown in a bottle of Pasteur's solution (or even in a 15% sugar solution made with tap water, which will be likely to contain enough of the mineral salts for considerable growth), and kept in a moderately warm place, within twenty-four hours abundant growth will be e\idonced by the increasing turbidity of the liquid, THE SIMPLER ORGANISMS 95 and by the taste of the alcohol in it and by the odor of the escaping carbon dioxide* arising from it. It may be demonstrated by examination of a drop of the fluid with the microscope. Molds and other fungi. — ^These are chlorophylless plants of different organization. They parallel the filamentous algae in their structure. The common black mold Mucor, is a much branched, vacuolated and multinucleate cell, of a form recalling the green felt (Vaucheria) . Penicillium (figure 56) consists of branching filaments recalling in their form those of Cladophora. Molds live for the most part on a more or less solid substratum of organic matter and repro- duce vegetatively by means of spores that are distributed through the air. Therefore, they have differentiated into two parts: the mycelium, the part immersed in the sub- stratum, and concerned with gathering food, a tangle of slender root-like fila- ments ; and slender aerial sporophores that rise from the my- celium at time of fruit- ing and bear the spores. Many molds feed upon the bodies of plants and animals, living and dead, and upon ma- terials extracted there- from, obtaining both their carbon and their nitrogen Fig. 56. Penicillium. a, a little tuft of the mould, as it appears, growing on the sur- face of a nutrient medium ; &, a bit of the same, magnified; 5, the original spore; m, mycelial filaments; h, sporophores, with spore clusters ; c, one of the spore clusters. *A simple chemical test of the presence of COj in the escaping gas may be made by thrusting a glass rod with a drop of lime water suspended on it into the mouth of the culture bottle. The calcium oxide (CaO) of which lime water is a solution, readily unites with free carbon dioxide to form a white precipitate of calcium carbonate CaCOj (CaO-t-C02 = CaC03) which may be seen to form in the drop. 96 GENERAL BIOLOGY from organic compounds. A few of them make galls upon green plants (fig. 28). Many more (known as rusts, blights, mildews, etc.) are destructive pests of green plants. But most Of them are saprophytes, and assist in the circulation of food materials in the earth by hasten- ing the decomposition of the bodies of dead plants. The fruiting stages of the higher fungi are aggregates or integrates of filaments, that rise collectively from mycelia, and fashion together parts of various forms: spheres in the puffballs, with the spores borne inside: low cup-shaped receptacles in some of the disc fungi, or Ascomycetes (fig. 57), with the spores contained in cylindric spore sacs (asci) in a fruiting layer (hymenium) in the bottom of the cup : umbrella shaped caps in mush- rooms, with the spores borne on the vertical surfaces of ra- diating lamellae underneath the cap. Study 12. Observations on cultures of yeast and molds. Materials needed: A good yeast culture ^n Pasteur's solution. Several plate cultures of molds of differ- ent ages on gelatine (directions for making plate cultures will be found in any good laboratory, manual of mycol- ogy or of bacteriology). Young mycelia of Mucor, in which streaming of protoplasm may be observed. One to three day old cul- tures of Penicillium, in which the germination of the spores may be observed. Old Peni- cillium cultures, in which the spore clusters may be studied. Study in yeast, i) the evidences of alcoholic fermen- tation and of the formation of carbon dioxide in the culture jar. 2) The details Fig. 57. A disc fungus (Peziza?). the aerial part of the fungus, with a quarter section cut out to show fe, the hymeneum. k, a bit of the hymeneum showing, a, ascus, con- taining spores; «, sterile paraphyses, and m. sub-hymenial tissue. I, a bit of the involucre surrounding the hymenium.' THE SIMPLER ORGANISMS 97 of Structure of the single yeast cell. 3) Budding and the' aggregation of the yeast cells together into torulae. Study in the molds: i) The differentiation into my- celium and sporophores, 2) The type of branching, with ab- sence of cell divisions in Mucor. 3) The streaming of the protoplasm in filaments of Mucor. 4) The germination of the spores and the beginning of mycelia in Penicillium. 5) The development of the spore clusters and of the arrange- ment of the spores in Penicillium, and in any other fruiting molds that may be available. The record of this study may consist in simple outline drawings, and notes on the things observed. Bacteria. — These are the smallest of the chlorophylless plants — indeed, they are the smallest of living organisms. They feed upon much the same materials as do other fungi, and while present nearly everywhere, they are sure to abound wherever there are moist organic substances in which they can multiply. Under favorable conditions bacteria increase in numbers with extraordinary rapidity. Their laethod of increase is already familiar — ^growth in size, followed by cell division. A division may recur every half hour, and at this rate something like 17,000,000 individuals might appear as the offspring of a single one in the course of twenty-four hours. Obviously, such a rate could not long be maintained for want of food. Their reproductive capacity, together with the readiness with which they may be distributed, give them an important place in the economy of nature. They are nature 's chief agency of decomposition and decay. They play a large role in restoring spent organic materials to circulation. Certain bacteria at times develop spores. Usually but a single spore is produced in each cell, the protoplasm of which develops a resistant spore coat within the old cell wall (fig. sSd). The spores are not injured by drying, and 98 GENERAL BIOLOGY may be heated even above the temperature of boiling water without being killed. They are readily distributed everywhere by currents of air. Bacteria serve their disintegrating function quite without regard to human interests, — spoiling foods, or rotting the compost heap to enrich the soil; souring milk, or ripening cheese ; disintegrating living tissues in disease, or aiding the processes of digestion, etc. Although the study of bacteria has been possible only during the brief period that has elapsed since the invention of good microscopes, their effects have always been known, and many empirical methods have been used for combating their growth. They do not thrive in acid solutions, hence acids have long been used as a means of preserving foods, as in the process of pickling meats, fruits, etc. They do not thrive in heavy solutions of sugar, and hence jellies and preserves are composed of large propor- tions of sugar. They require 2 5 per cent or more of water in their food substances for normal growth, and hence the reduction of the proportion of water present by the drying of meats and fruits has long been practiced: also, the use of salt to make such water as is present unavailable. Then there are many substances whose presence is inimical to their growth, which we now know as antiseptics, and which are the mainstay of modem surgery (bichloride of mercury, etc.), but of old, wounds were forefended against bacteria by the pouring in of oils (such as turpen- tine) and of alcoholic solutions (strong wines) . All these methods, applied without knowledge of the causes of the evils they sought to cure, have been vastly improved with the development of bacteriology. New processes of treatment by antiseptics, by sterilization, etc., have been developed and old ones have been improved. The bacteriologist has invented transparent culture media, containing suitable food. He sterilizes his culture THE SIMPLER ORGANISMS 99 media even as housewives have always sterilized fruit for canning, sealing while hot ; but he may allow time for the germination of any spores that are present and then may sterilize again; thus the spores, as well as the active cells of bacteria are killed. This is his method of clearing the field. Then he sows in his culture media the sort of bacteria he wishes to study, and observes their hab- its and manner of growth. In order to see bacteria, rather high powers of the compound microscope are required, and even with the best instru- ments little of internal structure is vis- ible in them. There are three form- types commonly found among them : a) The spherical coccus type, b) the rod like bacillus type, and c) the spirillum type (fig. 58). Under each of these form types many different species occur, which may differ in size and proportions, in manner of grouping, in mode of cell division, etc.; or, different species may appear quite alike to the eye, and may be distinguishable only by their manner of growth in culture media. By proper staining methods some of them show locomotor fiagella, that are quite invis- ible unstained (fig. 58 6 and e). Certain soil bacteria, of very great im- portance to agriculture, cause minute galls (known as tubercles) to grow upon the roots of clover and other leguminous plants. These are important because they are able to derive their nitrogen Fig. 58. Bacteria a, form types • j, coc- cus, ij bacillus; w, spirillum types, b, these forms stained, some showing fiag- ella, others, none. c, types of division; ij, ordinary cell divi- sion; w and X, simul- taneous division of longer filaments in- to a number of cells. d, spore formation in different forms, e, two species of bac- teria of the bacillus type, showing differ- ences of appearance, both stained and un- st ained; y> the -typhoid bacillus; z, the bacillus of Asia- tic cholera. lOO GENERAL BIOLOGY directly from the air. They supply nitrogen to the clover, and thus repay the debt imposed by the parasitic life. They enable the host plants to grow upon soils that are poor in nitrogen, and by their- decomposition they leave such soils richer than they found them. Within the galls, or tubercles, these bacteria grow ■larger than other forms, the cells becoming irregularly rod- shaped, x-shaped, y-shaped, etc. Hence they are easily recognizable with the microscope. Upon examination of the large tubercle we ordinarily find them filling the in- terior. Upon the dissolution of their bodies, their nitrogen content is added to the soil, either directly, during the growing season, or indirectly through the intermediary agency of the clover. Study I J. A few observations on bacteria. Materials needed: A hay infusion a few days old; some grow ing clover, or other leguminous plant ,bearing root tuber- cles: a stock of sterilized culture dishes ready for sowing. Mount a little bit of bacterial jelly from the surface of the hay infusion, and survey it for bacteria of the three form- types. Look also for species of any type that may be dis- tinguishable by size, cell proportions, etc. Clean some root tubercles, split open; mount scrapings from their interior and study the bacteria in them. Make a few cultures on plates of agar as follows: 1. Seal one sterilized plate without opening, for a check. 2. Touch all your fingertips to the surface of the agar in a second plate, cover again, and set aside to incubate. 3. Wash the hands carefully and wipe dry on a clean towel, and repeat. 4. Capture a live fly, preferably from a dusty window; put it inside a culture plate and let it walk about a little, over the surface of the gelatine to distribute bacteria from its feet ; remove the fly and set the plate aside to incubate. THE SIMPLER ORGANISMS loi Watch the development in the several plates and com- pare results. The record of this study may consist of notes and diagrams of the things observed. THE SLIME MOLDS. These are organisms of mixed character. In certain phases of their existence they exhibit animal functions; in other phaiges, only plant functions. In textbooks of zoology they are called Mycetozoa ; in most texts of botany, Myxomycetes. Slime molds live during the greater part of their life as spreading masses of naked protoplasm, which slowly creep about through the tissues of rotten logs, stumps, leaves, etc., like giant amoebas. They are soft and slimy to the touch, and are of a consistency about like that of the white of an egg. Their prevailing tints are yellow, brown, ecru or purplish, or almost any color except green. They are usually small, but with plenty of food and moisture, a single Plasmodium often grows to be a .foot across. They shun the light and are always to be looked for in sequestered places. During nearly the whole of the growing season, from early summer until late autumn, they may be found in deep mossy woods, and in shaded places by permanent springs. On damp, muggy days following warm summer showers the plasmodia may be found, outspread upon the surfaces from which they draw their nourishment. They are saprophjrtes. They feed on the dissolved organic substances of decaying stems and leaves. They are always found associated with fungi of similar habits, but unlike the fungi they may also take solid food, engulfing it as does an amoeba by surrounding it with outflowing protoplasm. Each Plasmodium is a single multinucleate mass of protoplasm, without separating cell walls. The nuclei \JJiiN JCyJ\.Xilj JJJ.\Ji^V-/\J X divide and become very numerous, but there is no dis- tinction of cells. A Plasmodium may become divided by the flowing apart of its mass in divergent direc- tions, or two Plasmodia may meet and wholly coal- esce. They possess little individuality. Dry weather checks the growth of the plasmodia and often initiates the reproductive phase of the life of the slime molds, in which the^r re- semble plants. The plas- modia then abandon the darkness and creep out upon the exposed surfaces of the log or stump, or a little way up the stems of nearby plants. They develop cell walls about all their nuclei and these walls are compos- ed of a characteristically vegetable substance, cellu- lose. Their most elevated portions develop sporangia of various and often beauti- ful forms. These contain multitudes of spores. This maturing process takes place very quickly — a few days or even hours may be sufficient ; it is to be sought on the bright and sunshiny days that follow summer showers. The spores are scattered with the bursting of the spor- angia at maturity. In some of the commoner slime molds (fig. 59), they are assisted in making their exit by the movements of certain spirally twisted threads (capillitial threads: collectively the capillitium) which occur in the Fig. 59. stage. Slime molds in spore bearing a, Trichia; h, Stemonitis. THE SIMPLER ORGANISMS 103 sporangia with them. These threads, formed from residual shreds of the Plasmodium, are very hydroscopic, and when they dry out, twist and turn vigorously, scattering the spores. When favoring wind or water bears a spore to a favorable place for germination, it bursts its cell wall and there creeps out therefrom a minute, naked amoeboid cell, which moves about for a time by means of pseudopodia. Then it develops a long lash at one end of the body (fig. 60) with the aid of which it swims for a season. Then it settles down, in company with others of like kind, and with the others fuses into a plasmodium of minute size, which has only to absorb food and grow to attain to the size and character of that with which we started. Thus we see that from the time of germination of the spore ^ -^ \ Fig. 60. Reproduction in slime molds, a, elater; 6, spores: c, one germinating spore and three amoeboid cells escaped from other spores ; d, the same cells a little later when free swimming; e, convergence of these cells to form a Plasmo- dium; /, a small Plasmodium. until the plasmodium is mature, the slime mold exhibits the free locomotion and the feeding habits of the animal, while thereafter it. develops cellulose cell walls and pro- duces spores like a plant. Nature has not always estab- lished hard and fast boundaries, even between her major groups of organism. Study 14. Observations on slime molds. Materials needed: living plasmodia, and mature spor- angia of any common species. Both may be brought into the laboratory on pieces of moist wood. The plasmodia, if broken into fragments with the wood, and placed on slides under a darkened bell jar, will in a few hours creep off the 104 GENERAL BIOLOGY wood on to the slides, and, being thus well spread out,- and freed from dirt, will show the streaming movements of protoplasm beautifully. One may be fixed on the slide with strong alcohol and stained with safranin to demon- strate the many nuclei. If an inclined slide be placed against the edge of a Plasmodium and a gentle current of water made to run down the slide, the Plasmodium will creep up the slide in opposition to the current. Plasmodia may be grown from spores at any season, by sowing the spores upon a proper nutrient surface and keep- ing them moist and under a darkened bell jar. The things that may most profitably be studied are : i) In the living plasmodium, its movements, its struc- ture, its engulfing of solid bits of food (such as mushroom fragments), its protoplasmic currents and its reactions to stimuli. 2) In its fruiting phase, the form and structure and group- ings of the sporangia, the spores and their structure, and the capillitial threads or other sterile parts in the sporan- gium. 3) In the development of the spores, the first amoeboid stage, the later free-swimming stage, and the fusion of cells to form minute new plasmodia (all of which may be seen in drop cultures, made as directed in the appendix). The record of this study may consist of sketches and diagrams of the things observed. THE FLAGELLATES. Unlike the slime-molds, the flagellates are minute organisms having considerable definiteness of body struc- tures, yet they have not clearly and uniformly differen- tiated plant and animal characteristics. Hence these also, or at least a considerable part of them, are treated in books on both botany and zoology; in the former being ranked THE SIMPLER ORGANISMS 105 with the protococcoid algae, in the latter with the masti- gophorous protozoa. Euglena is a common flagellate that will serve for intro- duction to the group. It abounds in sluggish waters, and if a quantity of trash and bottom silt be placed in a large glass jar and allowed to stand awhile, Eu- F1G.61. Euglena. «, nucleus; glena will usuallv be found swim- w, mouth; cv, contractile . . , . vacuoiewith pigment fleck, i>, mmg m numbers at the surface beside it;;?, flagellum. . i- ij_ tj: on the side next the light. If abundant it will be very evident by its bright green color. It may form a green film on the surface visible to the unaided eye. If a drop from this film be mounted for the microscope and examined one sees as soon as he finds the organisms that they exhibit the bright green chlorophyl color of the algae along with the active swimming movements of very lively protozoans. The swimming is rapid, and at first it may be difficult to keep a single individual in the field of observation. It is jerky, too ; not the regular and orderly progression of a ciliate, but quick movements from side to side, due to the lashing of the long flagellum at the anterior end of the body (see fig. 61). In an individual that .has settled to creeping about on the slide one may observe the form of the body — oval, blunt in front and pointed at the rear, showing a transparent ectosarc, and an endosarc filled more or less completely with green chlorophyl, and containing near the front end a pigment fleck of more or less orange color. The flag- ellum may be seen to be as long as, or loTiger than the body; It may be broken off, however, and if present it is so transparent it can only be seen in very favorable light, or sometimes, only after staining. Beside its base is a cleft' — a rudimentary mouth — a receptacle for solid food io6 GENERAL BIOLOGY Fig. 62. Two shell- bearing flagellates ; c, Ceratium, and v, Peridinium. — another animal character. In the midst, more or less hidden by the chlorophyl and by engulfed foods, is the nu- cleus. Reproduction is by fission, which, also may be ob- served in favorable specimens. Ceratium. is a free-swimming flagellate which secretes a spinous shell that is probably a protection against the attack of some of the predaceous animals of its environment (fig. 62). Dinobryon is a colonial flagellate which develops an urn-shaped membranous shell .open at the anterior end: two flagella of un- equal length project from the opening; the chlorophyl is distributed (fig. 63^) in two elongate tracts within the body, and is somewhat obscured, as in many other flagellates, by a yellowish pigment. Upon division, one of the resulting daughter cells slips out to the rim of the um-shaped shell, and secretes for itself C-S^jit a new shell of like form, attached at the •'-''' base within the margin of the old one. Repeated divisions thus give rise to branched colonies. These go swimming about in the water in a most absurd fashion — a contradiction to all the mechanics of submarine navigation — the open end forward, as it must needs be, owing to the position of mouth and flagella. Gonium is a colonial flagellate of very different form — i6-celled when the col- ony is grown, in a flat raft, four central cells destitute of flagella, and twelve Fig. 63. A colonial flagellate, Dinobry- on. c, a colony; \ 1 1 y J^. W and in consequence the thick dorsal wall of the body stands in marked contrast with the thin and soft ventral wall. This axis is extended posterior- ly into the tail, and expanded anteriorly to form part of the skull, which is the skeleton of the head, and which may readily be felt with the fingers through the soft skin. Two pairs of appendages are quite characteristic of vertebrates. The close correspondence be- tween fore and hind limb will be obvious even in the living specimen. Both have a sup- porting girdle of bone embed- ded in the side walls of "the body and more or less firmly attached to the axial skeleton. Upper arm, fore arm, wrist and hand, in the fore limb, correspond to thigh, shank, ankle and foot, respectively, of the hind limb. The divisions between these joints may readily be determined by flexing them between the fingers. Were not this internal jointed skeleton, with its numerous bones united by strong ligaments and moved upon each other by the over lying muscles so familiar to us, its mechanical fitness would be most impressive. Another small part of the skeleton, located in the ventral wall in the region of the throat, is the hyoid apparatus (fig. 112). This is mainly cartilaginous, only the part that is stippled in the figure being bony. The anterior fork sup- ports the base of the tongue ; the postero-lateral arms curve Fig. 111. Diagram of a vertebrate skeleton, x, skull; y, fore limb; z, hind limb; i, 2, 3, in front, clavicle, coracoid, and scap- ula, composing the shoulder girdle; /, s, J, behind, ilium, is- chium and pubis, composing the hip girdle. l82 GENERAL BIOLOGY Upward about the sides of the neck. These maybe felt with the fingers beneath the skin of the throat, or moved about under the skin by moving the tongue with a for- ceps. This part of the skeleton, though small and weak, is of great historical im- portance. These paired cartilaginous arches are landmarks of vertebrate history: to their consid- eration we shall have occasion to re- turn later. The eyes of the salamander are prominent and shining and they both wink at once at long intervals. If one of them be touched gently, it will be withdrawn completely into its orbital cavity; thus it gets out of harm's way. Once in a while the salamander may be seen to gulp down a mouthful of air. It does not inhale; to get air down it has to swallow. The air-swallowing process will often be most clearly seen after the specimen has been handled and put down again. On the under side of the neck the pulse beat may be seen. On the body there is a mid-dorsal groove extending from the rear of the head to the base of the tail, and there is a series of costal grooves between fore and hind legs traversing the sides of the body vertically. These latter are the exter- nal evidence of that segmentation of the body that will be found later in the vertebrae, spinal nerves and ganglia, and Fig 112. The branchial skeleton from the throat of' the salamander, h, the hyoid arch; I and 2, latet al arms of the first and second branchial arches ; i, isolated basal piece corres^jonding to the missing branchial arches. ORGANIC EVOLUTION 183 muscle segments. A number of similar grooves may be seen on the sides of the tail, especially at its base. The surface of the skin is covered with the very minute openings of pores from the large skin glands within it. These pores are visible with a lens. These glands pour out the secretion which keeps the skin moist and enables the salamander to get its oxygen, as the worm does, in a large part by direct absorption; It depends far less on its lungs for air than do the higher vertebrates. Some vestiges of its early aquatic life are preserved in the rudimentary webbing between the bases of the toes, and in the flattening of the tail, which is still put through superfluous sculling motions when the salamander tries to run on land. At the tip of the snout a pair of small nostrils will be seen, each with a blackish valve-like flap attached to its hind margin within, and if the mouth be held open widely, the in- ner openings from these nostrils may be seen on its roof at the rear of the palate. On the floor of the mouth lies a fleshy tongue, attached along its middle line, its edges lying free; at the rear of the mouth, the pharynx, with its walls con- verging to the esophagus, penetrated by abundant minute blood vessels, which give to it something of the character of a respiratory organ. In the lungless salamanders this organ is better developed to serve that function. On the floor of the pharynx is the glottis, the gateway to the lungs, a narrow longitudinal slit with closely appressed cartilagi- nous lips. Very minute and numerous teeth may be found on the edges of the jaw by scraping it with a finger-nail or with a needle, and two patches of palatine teeth may be found farther back in the roof of the mouth. Internal features. — Upon looking inside the body of the salamander it is at once apparent that the main general features of structure that were found to characterize the earthworm, are repeated here. There is a compound- 1 84 GENERAL BIOLOGY tubular body, a tube within a tube, and a coelom or body cavity between ; the inner tube is the alimentary canal and the outer one is the body wall as before, but the alimen- tary canal differs in two important particulars; it is not straight, but greatly coiled and twisted ; and it is not simple, but bears conspicuous appendages. And the body wall differs conspicuously in that it bears a differentiated head, and is extended laterally into limbs and backward into a tail. On comparing the internal organs there are strong con- trasts. The central part of the circulatory system is not a a Pig. 113. Diagrams illustrating the plan of body in worm and in salamander (b) in cross section; e, enteron; c, coelom; n, central nervous apparatus; v, central circula- tory apparatus. long pulsating tube lying on the dorsal side, but a heart, lying upon the ventral side. The central part of the ner- vous system does not lie within the coelom on the ventral side, but in the body wall upon the dorsal side, The nephridia are not scattered segmentally in single pairs the whole length "of the body, but are aggregated into special paired organs, the kidneys, and in the salamander there are special respiratory organs the lungs, and special supporting structures, the bones. Thefoodtube, alimentary canal, or ewferoM, is differentiated into parts the anterior of which bear the same names as parts of like function in the earth worm: mouth, pharjmx ORGANIC EVOLUTION 185 and esophagus, and these are succeeded by stomach, small intestine, large intestine and cloaca. All these, together with the appendages, are indicated in the accom- panying diagram (fig. 114). Such differentiation of parts bespeaks many separate localized functions along the course of the enteron ; and such indeed there are, but we are here concerned with form changes and can note only the more important functions as bound up with the principal organs. The stomach has become a sharply delimited organ for the reception of food, capacious, distensible, suited to the exigencies of irregular food supply. Its thick muscular walls are filled with small gastric glands, whose secretion initiates digestion. The churning movements of the walls Fig. 1 14. Diagram of the enteron of the salamander, with its principal appendages, o, mouth; f, pharynx; e, eso- phagus; d, stomach; i, small intestine; j large intes- tine; c.cloaca; a, anus; &, urinary bladder ;g, gallbladder on m, liver; H, pancreas; I, lung. aid in the comminution of the food and in the mixing of it with the gastric secretion. At the outlet of the stomach is a guarded passageway called the pylorus, through which the food passes, when reduced to a more or less fluid condition. The small intestine is a narrow passageway (greatly abbreviated in the diagram) , well adapted to the slow pas- sage of the food, to the completion of its digestion „And to the extraction from it of assimilable materials. It is long and tortuous. Its walls are covered internally with folds and processes {villi) which greatly increase the surface in con- tact with the passing stream. These secure the better mix- ing of food with the digestive secretions of the liver and the pancreas, and the completer absorption of it after digestion. 1 86 GENERAL BIOLOGY The principal appendages of the enteron. — ^The lung is V here a new feature. As a respiratory sac appended to the alimentary canal, it is peculiar to vertebrates. Among -■-terrestrial animals, most vertebrates are giants, for whom direct absorption of oxygen through the skin would be quite inadequate. Herein is seen the advantage of the lung, which maintains inside the body extensive surfaces that are thin-skinned and always moist. The liver is the largest gland in the body, a lobed organ of mottled brownish color, its pointed left lobe partially cover- ing the stomach (in the resupinated position in which the salamander is opened) . Its secretion is collected in a bluish- green sac — ^the gall cyst, which the right lobe overlies. The cyst is connected by a slender bile duct with the small intestine near the stomach. Compression upon the gall cyst will usually demonstrate that the bile duct opens at this point, by driving the greenish bile down its length, mak- ing the duct visible. 'The pancreas is an elongated thin flat fatty-looking organ, that lies in the loop formed by the junction of the stomach and small intestine and is covered by the liver except at its posterior end where it touches the intestine. The urinary bladder is the hindmost appendage of the alimentary canal. It is a thin, crumpled sac that lies upon the ventral surface of the large intestine, and opens into the cloaca. Its connection with the excretory system will be discussed later. The thin sheet of membrane in which these digestive organs are slung from the dorsal side of the body wall and through which pass numerous branch- ing incoming and outgoing blood vessels, is the mesentery. The elongate oval reddish body suspended in the mesen- tery behind the stomach is the spleen. The lungs are the foremost appendages of the alimentary canal. They spring from the ventral side of the pharynx at the glottis, whose location has already been noted, ORGANIC EVOLUTION 187 by a slender hollow stalk-like trachea, which divides into two bronchial tubes, joining the right and left lungs. By passing a pointed glass tube into the glottis and inflating the lungs, their size, their constituent air cells and the com- municating blood vessels in their wall may be clearly seen. The circulatory system has for its central organ a heart of three chambers, two aur- icles and a ventricle (fig. ,.115). The ventricle has thick muscular walls, and is the chief propelling agent of the blood cur- rent. It drives the blood forward through the ar- terial trunk, and out- ward through the arches, as indicated in the ac- companying diagram. The outward current is called the arterial, the in- ward, the venous circula- tion. The carotid arch carries blood anteriorly to the head, the pulmo-cutan- eous inwardly to 'the lungs and externally to the skin (whence its name), and the aortic arch carries the greatest supply posteriorly and to peripheral parts of the body, and distributes vessels through the mesentery to the internal organs. The return currents reach the heart separately, entering by the two auricles. That entering the left auricle is returned from the lungs through the pulmonary veins. That entering the right auricle (by way of the venous sinus, a Fig. lis. Diagram of the Amphibian heart, and principal bldod vessels. a, right auricle: b, left auricle; c, ventricle; t, arterial trunk; e, lung; d, liver; /, carotid arch; g, aortic arch; h, pulmo-cutaneous arch, with i, its cutaneous, and j, its pul- monary branch; k, pulmonary vein, with the base of the corresponding vein from the missing lung shown at /, m, the right precaval vem; n, postcava; o, anterior abdominal vein and p, portal vein. i88 GENERAL BIOLOGY vestibule attached to the auricle) is returned from the front by the precava, and from the rear by the postcava. The postcava is the largest bloodvessel coming from the rear. Into the liver the blood returns by two main channels, _ an anterior abdominal vein that traverses the mid-ventral line of the body wall and jumps across the short intervening space of the coelom to enter the liver on its ventral side, and a portal vein tliat comes from the stomach and succeeding portions of the alimentary canal. The special organs of excretion in the salamander are the kidneys, a pair of chocolate-colored bodies lying closely applied to the dorsal wall in the posterior end of the body cavity, broader and thicker behind, and tapering to a slender point in front. From their postero-extemal angles a pair of very short ducts connects with the cloaca, entering just opposite the mouth of the urinary bladder, into which the discharge of their urine passes for temporary storage. A large vein enters each kidney from the rear, breaks up into fine branchlets, and is reformed on the opposite internal side, where, by confluence of emerging branchlets, the postcava is formed. The reproductive organs lie in the midst of the body cavity, a single pair just ventral to the pointed anterior ends of the kidneys, and they bear usually a considerable development of fat in the form of yellowish finger-like processes (fig. ii6.). The salamander being unisexual, they are spermaries (testes) in the male and ovaries in the female. The spermaries are oval yellowish bodies, which discharge their sperms through a number of fine ducts that penetrate the substance of the kidney and emerge on the opposite side to join the ureter, and thence reach the cloaca. The ovaries are large mem- branous, crumpled organs in whose walls the eggs may be seen developing, opaque and white at first, acquiring ORGANIC EVOLUTION 189 Fig. 116. Diagram of the relations of renal and repro- ductive organs in amphi- bians, male above, female below, k, k, kidneys; «, «, ureters; cl, cloaca; s, s, sper- maries (testes). Arrows in- dicate the course of the spenn ducts through the kidneys to join the ureter; a, a, fat body; o, o, ovaries; d, d, oviducts; /, /, their funnel shaped openings into the coelom; t, t, the dilata- tion(uterus) at the lower end of each. blackish pigment as they increase in size, studding the transparent membrane. The eggs are shed frona the ovaries into the body cavity and the ducts by which they reach the exterior are not connected to the ovaries at all. The oviducts are long sinuous tubules extending the whole length of the body cavity near the mid-dorsal line, opening by a V-shaped slit at the an- terior end that is situated be- tween the esoph- agus and the shoulder, and into which the eggs find their way, aided by the lining cilia. As the eggs pass down the tube a gelatinous secretion is added to them by cells along the way, and they find temporary storage in a sac- culation (uterus) at the lower end of the duct just before it enters the cloaca. Nervous system. — As already noted, the central part of the nervous system in the salamander, as in vertebrates generally, is lodged in the body wall upon the dorsal side. It consists of a hollow, but thick walled tube of nervous matter, differentiated into two principal parts: a consider- able enlargement, the brain, is lodged in the cranial cavity of the skull, and a long spinal cord occupies the channel formed by the annular vertebrae. The branches it bears, and by which it maintains communication with peripheral parts of the body are paired nerves, which it gives off igo GENERAL BIOLOGY throughout its length. Those nerves issuing through openings in the base of the cranial cavity of the skull are called cranial nerves, and those issuing from the inter- spaces between the vertebrae are called spinal nerves. The nervous apparatus of the body is composed of nerve cells and their processes. Where the bodies of the cells predominate, as in the center of the cord and in the sur- face layer of the fore part of the brain, they give the nervous tissue a pale grayish cast ; and where the fibres predominate, the tissue appears white (the so-called "gray matter" and "white matter" of the nerve centers) . We have seen a very simple sort of differentiation of nerve cells with processes in the hydra (fig. loi f). And inthe earthworm (fig. 109) we have found them very highly differentiated. But inthe vertebrates the processes from nerve ceils are often very much longer and the interrelations between them often much more complex. Each spinal nerve consists of a bundle of these long processes or fibres, inclosed in a com- mon sheath. Spinal nerves arise in pairs between the vertebrae, as already noted, each by two roots (which are also bundles of fibres), and out upon the dorsal root, just before its confiuence with the ventral to form the completed nerve, there occurs a little isolated cluster of nerve cells: that is, a ganglion. There are other nerve cells in the organs of special sense, and at the termini of sensory nerves all over the surface of the body. The apparent branching of the nerves is due to the division of the bundle of fibres into lesser bundles, and finally into single fibres that take different courses to their appropriate endings. The fibres themselves are continuous, and extend from cells in the cord or in ganglia, to other ganglia or to peripheral parts of the body. They are individual lines of nervous communication; they separate as do tele- ORGANIC EVOLUTION 191 graph lines in passing outward from the commercial centres to the remoter districts. Within the coelom of vertebrates there are other ganglia, in part arranged in pairs segmentally and connected with the ventral roots of the spinal nerves, as shown in figure 117, and in part variously disposed in the walls of the internal organs of the coelom, whose funct- ions they control and coordinate. These are connected with each other by nerve fibres. They to- gether constitute the so-called sympathetic system. The fun- damental nutritive processes of the body, that are performed involuntarily, and that are es- sential to keep life going, are unconsciously controlled through the sympathetic system. Prac- tically all the involuntary mus- cles of the body, those of the skin, as well as those of the viscera, are controlled through nerve fibres that take their origin from the cells of sympathetic ganglia. There is another, more direct line of communication between the organs of the coelom and the brain. One pair of cranial nerves (called the vagi; sing, vagus) which descends through the neck into the coelom, sends branches also to the lungs, the heart, the stomach and part of the intestine. The movements of the involuntary muscles are com- paratively simple and uniform, but those of the volun- tary muscles of the body wall and limbs are infinitely Fig. 117. Diagram illustrating the relation of the neural tube to the ganglia c, is a cross- section of the body showing the sympathetic ganglia in the coelom; 6, is a cross- section of the cord and adja- cent ganglia; showing roughly the location of the groups of nerve cells. 192 GENERAL BIOLOGY varied and complex, and are ever effected in new combina- tions. Hence there is a correspondingly large proportion of the regulative cells of the body, the nerve cells, located in the neural tube. The most significant new feature of cell grouping found among vertebrates is the aggregation of nerve cells at the forward end of the neural tube to form the brain. The cord widens on entering the skull into the medulla. On its dorsal side a thin roofed Fig. 118. Diagrams of the brain of the salamander, 'w, dorsal view; x, ventral view; 3), lateral view and jr, dia- gram of the continuous internal cavity in dorsal or ventral .view; a, olfactory lobe; b, cerebral hemi- spheres* c, pineal body; d, thalamencephalon; ^, optic lobes; /.cerebellum; g, medulla, h, spinal cord; *, in- fundibulum; y, hypophysis; fe,lateral ventricles; 2, third ventricle; m, optic ventricles; n, fourthFventricle. V-shaped slit, called the fourth ventricle, exposes the cen- tral cavity that extends in fact throughout its length. A transverse ridge of nervous tissue at the front of the fourth ventricle upon the dorsal side is the cerebellum. A pair of rounded swellings just in front of the cerebellum are the optic lobes. The pair of large oblong lobes at the front are the cerebral hemispheres. These and other parts exter- nally visible may be located by reference to figure ii8. Their relations to each other will be considered when their development is studied (p. 198). ORGANIC EVOLUTION 193 Development. — The way of access to intelligent compre- hension of the cellular structure of the salamander lies through the study of its development. The salamander begins life as a single cell, the result of fusion of egg and sperm. It is a very large cell, because distended with yolk and enveloped by a thick gelatinous envelope; but the protoplasmic part, the living part of it, is very small. The protoplasm is not equally distributed through the egg, but is more abundant on the upper pig- mented side. Therefore, the division planes in cleavage start on the upper side, and division is somewhat retarded below by the impeding yolk mass. The cell divides into two cells along a meridional plane and the two divide into four by another meridional plane at right angles to the first; then the four divide into eight by a plane parallel to the equator of the sphere, at right angles to both former planes, not at the equator, but a little nearer the darker upper pole, where it divides the protoplasm more equally. Successive meridional planes, and planes parallel to the equator mark the following divisions into i6-cell, 32-cell, etc., stages which, however, are not traceable | farther because of the retardation of division on the lower side, and because, of the planes getting ajog at the joints. The result is clearly a blastula, as before — a hollow sphere of cells of small size. The slipping inward from the surface of some of its cells results in its being more than one layer in thickness over part of the upper side, and the retardation of division owing to excess of yolk on the lower side throws the segmen- tation cavity above the middle of the sphere. All these facts are indicated in the accompanying figures. Then gastrulation takes place in a manner that is yet more aberrant. An ingrowth from the outer wall gives rise to endoderm surrounding an arch-enteron as before ; but the ingrowth, impeded by yolk does not result in a widely open 194 GENERAL BIOLOGY blastopore, but, instead, a narrow crescentic slit, the edges of the blastopore being pressed together as shown in figure iigi. Then the edges of the crescent extend downward until they meet below a little circular patch of protruding yolk, the yolk plug. Except upon the upper side where lies the direct entrance to the archenteron, they cut but a shallow circular Fig. 119. Early development __ „„- - , 4-, 8-, i6-, 3 2-cell and later segmentation or cleavage stages; i, j, gas- trulation stages; i, shows the crescentic blastopore and ;, the fully formed yolk plug;'fe to «, formation of the neural tube. groove upon the surface of the yolk. If one conceive of his own head as the sphere of the gastrula salamander, his closed mouth the blastopore, and the corners of his mouth pulled down until they join beneath his chin, he will get a clear conception of the relation of these parts. Figure 121 shows in longitudinal sections the formation of the gastrula, ORGANIC EVOLUTION 195 and the formation and subsequent withdrawal from the surface of the yolk plug. The segmentation cavity is reduced in size as the endodermal sac increases in depth. Then the mesoderm appears in the space between ecto- derm and endoderm at the blastopore on the upper side, and spreads outward and downward in a thin sheet of tissue, and begins to split to form a coelom. Up to this point in development it will be observed that nothing has appeared to suggest a vertebrate animal. In the one celled stage the embryo is more like a protozoan in structural type ; in the blastula stage it has the hollow spherical form of volvox; in the gastrula stage it is more like hydra in plan. As soon as it has acquired a compound tubular body through the Fig. 120. Sections of salamander eggs in a meridional plane, 8-cell. 32-ceU and later segmentation stages. development and splitting of a mesodermal layer, it is more like a worm, and not until the appearance of a central nervous axis upon the dorsal side is there a single structure present that can be pointed out as distinctively characteristic of a backboned animal. Thus the series of embryonic forms assumed by the salamander in its development shows a rough correspondence to the series of adult forms we have been studying. Furthermore, when vertebrate characters appear they are very generalized indeed, and the parts in formation look no more like the adult salamander than like other vertebrate animals. Vertebrate characters. — Several distinctively vertebrate characters appear now in different parts of the body almost 196 GENERAL BIOLOGY e=i simultaneously. The first to appear externally is the nervous axis upon the dorsal side and this and the gill slits and the notochord are es- pecially worthy of our consid- eration. Beside the blastopore an elevation appears which rises in two parallel ridges called the neural folds. These folds at their outgrowing ends are confluent in a loop which marks the head end of the salamander. These folds grow rapidly, and by their increase in length project from the sur- face of the egg at both ends, spoiling its spherical symme- try. The withdrawal of the yolk plug is accompanied by the outpushing of the tail be- yond the end of the neural folds. The folds come togeth- er, as indicated in the accom- panying figures, the wider an- terior end becoming the brain, the remainder, the spinal cord. This then sinks into the in- terior as indicated in the cross-section diagrams of fig- " ure 122. Meanwhile the archenteron is elongating in a parallel direction, and a fold from its dorsal wall is cutting off the notochord — ^the most ancient supporting structure of verte- brates — a larval organ in most of them at the present day. 60 s, « 01 •s.,- ^°- C .. S2 •OH a-s c .- . a ORGANIC EVOLUTION 197 The archenteron is converted into an alimentary canal by a process somewhat differing from that followed in the worm ; it is destined to occupy a reversed position ; thg nvA blastopore becomes the anus, and the R 1 1 mouth is formed at the opposite nl I end by an ingrowth from the ecto- derm that meets the front end of the archenteron and fuses and then opens a passage through. The anterior end of the archenteron becomes dor- sally flattened and laterally ex- panded into a pharynx, from whose walls sacculations of endoderm grow outward to meet the ectoderm, and then cleave apart on vertical lines, opening gill clefts on the side of the neck. The pillars of tissue left standing between these clefts become the gill arches. By these simple processes, are laid down the main lines of vertebrate structure. Endodermal differentiation. — ^The embryonic endoderm becomes epithelium, as before, and is the lining layer of the alimentary canal and of its appendages. It is for the most part a single layer of cells not remarkably modi- fied in form, differing in length according to the extent of their compression. In the pharynx they are short-cylindric and ciliated on their free ends (figure 134). In the intestine they are much more compressed and slender, and certain of their number are differentiated as goblet-shaped mucus- secreting cells. In the stomach where the wall is made up of multitudinous pit-like depressions called the gastric Fig. 122. Diagram of the formation of neuron, no- tocord and coelom. The ectoderm is white, the endoderm is solid black and the mesoderm is crosslined; c c, notocord; c M, pronephric duct. 198 GENERAL BIOLOGY glands, they are differentiated into bulky cellS' that secrete the digestive fluids at the bottom, mucus-secreting cells along the sides of the pits and protective cells about the mouths of the pits where they come in contact with the food. In the lung, where the extension of the walls is very great, the cells are spread out flat and very thin to cover them. This thinness favors the diffusion of gases between the blood and the air, and is characteristic of respiratory epithelium. The liver arises very early (fig. 123) as a wide sacculation of the archen- teron near its anterior end on the ventral side. This outgrowth becomes r e- peatedly branched, and the resulting glandular blind tubules become con- voluted, the basal connec- FiG. 123. Diagram of a longitudinal sec- fif,^ rpmninc: nc n <;1pnrlpr tion of a young embryo, b, neural tube; "O" remains aS a SlCnoer e.enteron, the black spur from the upper pn-nnpf+incr +iihp tVip hilp side represents the free part of the noto- Connecting tUDC, tnC Dlie cord. The ventral outgrowth from the f1i,„f anrl tVip rlicitnl nnrtinn front end will form the liver. QUCt, ano tne QlSXai pOTOOn, acquiring through meso- dermal additions a system of blood vessels, becomes differ- entiated into the lobes of the liver. A dilatation on the bile duct becomes the gall cyst (fig. 114). The pancreas arises later but by a simpler and somewhat parallel development arrives at its adult estate, when it consists of a mass of glandular-walled communicating tubes, which secrete the most important single digestive fluid of the body. The uri- nary bladder arises as a similar but simpler sacculation at the posterior end. Development of the neural tube.— We have already seen that the central nervous axis develops into a tube by the ORGANIC EVOLUTION 199 Fig. 124. A young salamander larva, show- ing gill slits is), b, blastopore: ^, ^, St fore-, mid- and hind-brain, respectively. closure together of two folds about a neural groove, and that the tube thus formed then sinks into the body wall. The diagrams of figure 122 show how it is over- grown, first by the ectoderm and later by mesoderm, and re- moved from the surface. The original neural groove thus closed be- comes the central canal of the spinal cord. It is lined with a little bit of the epidermal layer of the ectoderm, carried in from the surface. A small ridge of nerve cells that arises each side of the tube dorsally (fig. 122c) becomes divided with growth into pieces corresponding in pairs to the body segments ; and when later, nerves grow out as processes from the cells, these pieces become located upon the dorsal roots of the spinal nerves and become the ganglia (fig. 117) hitherto noticed. Quite early in its development the axis becomes feebly marked off into three successive tracts, which correspond to the fore brain, mid brain and hind brain of the adult sala- mander (figs. 124 and 125). The fore brain becomes bilobed by a dilatation on either side of the median plane, and by Fig. 125. Older salamander larva, showing gills, r, s, 3, fore-, mid- and hind- brain. 200 GENERAL BIOLOGY Fig. 126. Older salamander larva, showing further development of the gills. a, anus; b, gills; c, opercular fold; d, nostril; e, mouth; /, /, caudal fin; growth of the two lobes forward and extension of the ex- panded canal into them, the hollow cerebral hemispheres are formed; the small olfactory lobes grow forward from beiieath their anterior ends. At the rear of the hemispheres on the middorsal line a small process grows upward to become the pineal body — a vestigial structure, correspond- ing to the nervous apparatus of a median eye, that is functional in some lizards. A hollow downgrowth on the midventral line develops the infundibulum. This connects with the pituitary body, developed in the roof of the mouth. Paired upgrowths from the lateral wall of the midbrain become the optic lobes of the brain. The central cavity extends into each of these, the expansions of it within the optic lobes being known as optic ventricles. An axial dila- FiG. 127. Older larva of the spotted salarriander, with legs developed. tation of the central canal (fig. ii8 z) is known as the third ventricle. From the front end of the third of the primary brain divisions the cerebellum arises as a transverse solid upgrowing ridge of tissue upon the [dorsal side. Just behind this lies the fourth ventricle, as already noted (fig. iiSw). It appears from above as a triangular dilatation ORGANIC EVOLUTION 201 of the central cavity of the medulla, but thinly covered upon the dorsal side. The elongate brain of the sala- mander, with its parts outspread almost like a diagram, is very simple in comparison with that of the higher vertebrates (fig. 128). In birds the cerebellum and the optic lobes are relatively larger and in mammals, and especially in man, the cerebral hemispheres attain their maximum development. Thus a simple tube of undifferen- tiated nerve cells becomes moulded Fig. 128. Brains a, of pigeon i^to a brain. By localized out- "°'pinea'i"°bod'y.T^he™: growths of its walls all the principal ™'elSa,'vfeo?^"iob^: external features ot its torm are P, optic lobe. wrought out. The subsequent de- velopment of fibres from all these masses of cells is a matter far too intricate for us to attempt to follow here. A few of the more salient features of the ultimate distribu- tion of these fibres will be considered in chapter VII. The development of the primary circulation. — In the midst of the mesoderm, tube-like clefts appear, which, extending and becoming confluent, develop into the blood vessels. The most important of these appears as a cleft of sigmoid curvature in the region of the throat, and by the processes diagrammatically represented in figure 129, it becomes enlarged, strongly flexed, and divided into com- partments, it develops muscular walls, and becomes the heart. It becomes two chambered by the differentiation of an auricle and a ventricle, the former being carried to the front of the latter by the flexion undergone during 202 GENERAL BIOLOGY development. The passage leading forward from the ven- tricle, destined to become the arterial trunk, becomes con- joined with other paired passages in the mesoderm of the throat leading to the gills, which be- come the branchial arches. Corre- sponding vessels develop upon the dorsal side and become conjoined with the great dorsal aorta, leading rearward (see figures 130^, and 131). About the time these vessels are first marked out, gills develop upon the gill arches externally (fig. 126) "^^eiopmen^'ofth^ifr^ri and become traversed by a system of l7?h/priSlitYvecirftof capillary vessels, which are at this ik^M^t^'oi^at stage the connecting link between the compiSli,:'or4I' Ik" (dorsal and ventral vessels just men- ^^:*LXriket.:ven- Moned. The circulation of the blood *"'=^®- through these transparent external gills is easily observed with the microscope, and it is a beautiful sight. This simple type of circulation is essentially fish-like (fig. 130a;). There are no lungs as yet, and hence there is no pulmonary circulation. The heart is but two chambered. All the blood passes forward from the ventricle through the gills, to be returned rearward through the dorsal aorta. The aortic arches are four, and at first essentially alike. Development of pulmonary circulation. — Lungs develop as already noted by outgrowth from the ventral pharyngeal wall, and blood vessels to supply them arise from the fourth branchial arch and extend rearward to penetrate their walls. Return channels are developed, joining the lungs directly with the heart. When these vessels become functional, a considerable part of the blood is diverted from the gills to the lungs. This, however, is of late occurrence, being an ORGANIC EVOLUTION 203 accompaniment of the shift from aquatic to terrestrial life. Before it happens the gills begin to atrophy and new chan- nels begin to be developed. A carotid artery springs from the anterior side of the foremost branchial vessel on each side, to carry blood from the heart into the head. A short cut is developed between the dorsal and ventral portions of the branchial vessels of the second pair, and these become the aortic arches of the adult salamander, forming a con- FiG. 130, Diagram of types of circulatory apparatus in vertebrates x, a lung fish (Ceratodus) ; y, a frog and z, a mammal. (For the sake of clear- ness the auricle is ti^med backward, straightening out the sigmoid flexure.) ^> 2> J, 4x tile four branchial arches, becoming, in y and z; /, the carotid; 2, the aortic, and 4, the pulmonary arches; a, auricle; v, ventricle; /, lung; c, cava; r, the single root of the dorsal aorta in mammals. tinuous uninterrupted channel from the heart to the dorsal aorta, which ultimately becomes the main channel of the circulation. Thus we see that in metamorphosis the fourth arch that springs from the arterial trunk becomes the pulmo- cutaneous artery; the first arc h becomes the carotid , the . notocord. ORGANIC EVOLUTION 207 and actions. Freshly anesthetized specimens for gross dissection. Study the internal organs in the order outlined in the preceding pages ; (if fuller outlines are desired, they may be found in laboratory manuals of vertebrate zoology or of general zoology, nearly all of which deal mainly with com- parative anatomy) . Trace the alimentary canal and note its differentiation into parts. Identify its appendages and find their channels of communication with the enteron. Inflate the lungs and note the relation between air tubes, air cells and blood vessels. Identify the cham- bers of the heart and trace the main channels of circu- lation. Trace the ureter from the kidneys to the mouth of the urinary bladder. Compare spermaries and ovaries in male and female specimens and trace the ducts for the exit of the sex cells. Find the paired spinal nerves issuing from the spinal column and lying against the roof of the body wall, and find the paired sympathetic ganglia in the coelom attached by commissures, one pair to the roots of each pair of spinal nerves; look also for nerves extending from these ganglia to other sympathetic ganglia in the walls of stomach, heart or lungs. The record of this study may consist in drawings and diagrams of the form and relations of the principal organs studied. Study 27. The structures of the body wall of an amphibian. Materials needed: The specimens from the preceding study, if preserved in two per cent, formalin after the removal of the internal organs. Wash free from the formalin in running water before using. Also skeletons disarticu- lated, and a few mounted ones for comparison. Also, some forms with cartilaginous crania, such as sharks, or large bull frog tadpoles for the easy examination of the brain. 2o8 GENERAL BIOLOGY Compare the bones with the diagram of the vertebrate skeleton as shown on page i8i and then make a diagram of the skeleton of this amphibian. The skeleton of fore and hind feet will be easily observed if the skin be stripped off, the outside muscles cut off, and the undisturbed skeletal parts then cleared in dilute glycerine. The relations of bone, muscles, nerves and skin will also be seen in the mak- ing of the preparations. To the examination of the main external features of the brain should be devoted the major part of the time alloted to this study. In a shark or a large tadpole the roof and the coverings of the brain are readily cut away, and the parts shown in figure 1 18 will be easily made out. The record of this study may consist of diagrams of skeleton and brain. Study 28. The cellular structure of an amphibian. Materials needed : A living or freshly anesthetized male specimen ; slide mounts of prepared fragments of the ovary showing eggs and egg follicles; sections of the stomach wall, the intestine, the lung, the slcin, the kidney, the spinal cord, etc. The careful study of sections is time consuming, especially for a beginner, and little of it can be done in a single period. To expedite their examination, a number of them may be shown as microscopic projections. The student should at least examine and draw a few living cells from the fresh specimen, such as red and white corpuscles from the blood, living sperm cells, ciliated epithelium scraped from the roof of the mouth, etc. In the sections from the enteron and its appendages, the types of epithelium sho\\'n in figure 134 should be identified, and the subjacent muscle layers, the interpenetrating blood vessels and the covering peritoneal layer of endothelium should be seen. In the skin section ORGANIC EVOLUTION 209 the thick fibrous dermis should be seen overlaid by the epi- dermis of several cell layers, and invaded by the large mucus glands that depend from and open through the epi- dermis. The parts of kidney and spinal cord may perhaps be identified by comparison with figures 131(7 and 11 76, re- spectively. 0000 « Fig. 134. Diagrams of types of epithelium, a, ciliated epithelium of the pharynx; b, isolated cells of the same; c, a gastric gland from the stomach wall; m, its mouth; n, the contact layer at the surface: o, the middle layer o' mucus-secreting cells; ^, the gland cells of the deepest part (pepsinogen secreting); d, a single villus from the wall of the intestine; q. goblet cells (mucus-secreting); r, a group of replacement cells (center of cell increase) ; e, a single goblet cell; j, a bit of the wall of the lung showing the thin respiratory- epithelium (stippled); s, s, capillaries containing blood corpuscles. The record of this study may consist in a few drawings and a larger number of diagrams of the things studied. Study 2g, The early development of an amphibian. Materials needed : Egg masses of various stages of development, preserved in two per cent, formalin; wash out formalin before using. 2IO GENERAL BIOLOGY Study and diagram segmentation and gastrulation stages and the main features of formation and closure of the neural groove and the development of gill slits and gills as seen in external views of the specimens. The labor of drawing wiE be lightened if uniform circles be drawn mechanically for the earlier stages, and cut out forms be used for outlines of the later ones; or, if printed or otherwise duplicated outline figures be furnished. This may be supplemented by microscopic projection of egg sections. The record of the study will consist in the series of dia- grams made. The salamander, a typical vertebrate. — Because of its primitive structure, the salamander serves well for intro- duction to the study of the vertebrates. The parts we have found in it we would find in all the others, only modi- fied in form. As.it develops, so do the others, in the main; in all, the principal organs are laid down in like manner and have like relations to the germ layers, and to each other. Neural tube, notocord and gill arches are formed in all. A two chambered heart and a fish-like cir- culation develop first, and a pronephros, before the true kidney; but some develop much farther than the sala- mander, and along very peculiar lines. The salamander ends its improvement Of circulatory apparatus with two aortic arches doing precisely the same work; but in the higher groups of vertebrates, birds and mammals, we find one of these arches has been done away with and the other one does the work alone; the right one has been retained in birds, the left one in mammals (fig. 1302). A further improvement in birds and mammals is found in the four chambered heart, which with better de- veloped lungs makes possible a complete double circu- lation of the blood (fig. 13s), all the blood passing through ORGANIC EVOLUTION 211 the lungs on each circuit of the body. All the blood thus gathers oxygen on each round. Hence, these are the warm blooded animals: these alone are capable of sus- tained activities in cold climates. The purpose of circulatory apparatus is to get the food to the points where it is needed for use, and to get the waste to points whence it can be removed from the body. When the animal body is small and no part of it is far remov- ed from food supply, there is little need of circulatory apparatus. The amoeba may feed at any point of its body. In the hydra, the food cavity, extending to the tips of the hollow ten- tacles and out into the buds, is not far removed from any cell. Even in so large an animal as a flat worm the food cavity may, by means of exten- sive ramifications, reach nearly every part. Biit in all the higher terrestrial forms of animals, the part of the body wherein food elaboration occurs is small, and the greater part of the body is remote from food supply, and circula- tion of the food is therefore necessary. Likewise, the more the nephridia become localized in the body, the more necessary becomes circulatory apparatus, with definite blood channels leading to them from every part of the body. But there is circulation before there are blood vessels. We have seen it in Paramoecium; many of the lower multicellular animals also lack blood vessels. There are body fluids occupying the interstices between the cell layers and bathing all the tissues internally. These are the media through which the internal exchange of food and waste Pig. 13S. Diagram of double circulation (from Verworn). 212 GENERAL BIOLOGY materials is effected. These supply the aquatic environ- ment that is necessary for the maintenance of cell life: for cell life, in the beginning aqua- tic, is in an important sense aqua- tic still, even in terrestrial organ- isms. Living protoplasm is a semi- fluid substance, and metabolism is compatible only with a liquid state. The fluids of the body are moved about, (that is, circulated) in part by the movements of the tissues which they bathe, and in the higher organisms appear special organs of propulsion. Blood vessels at first appear as short open contractile tubes, that communicate freely with the coelom, and that merely serve to keep the blood irregularly moving. When, as in the higher vertebrates they have become completely closed channels, capable of retaining the differentiated red corpuscles and carrying them, about the body in continuous procession, they are still supplemented by that inter- cellular circulation that is due to the contraction of the muscles and movements of the organs. The need of this propulsion of body fluids by body movements is convincingly evidenced in ourselves by the benefit of physical exercise (even though performed by proxy, as in massage), and conversely, by the stagnation induced by too exclusively sedentary habits. Fig. 136. Diagram of the two main channels by which food enters the general circulation in mammals. e, intestine with villi, V, u, in its walls r a, right auricle of the heart, m, post cava; n, precava; o, thoracic lymph duct; ^, pancreas: g, pancreatic duct; r, epatic vein; 5, portal vein; t, bile duct from i, liver: arrows indicate the course of secretions en- tering the intestine, and of the absorbed food de- parting therefrom. ORGANIC EVOLUTION 213 There are more or less definite channels {lymph vessels) developed in all the higher vertebrates, for the circulation of the body fluids aside from the blood vessels. These, in our foregoing hasty survey, we have left out of account. But there is one such vessel {the thoracic duel) of very great importance in mammals ; for by it the greater part of the food enters the general circulation, in the manner diagram- matically indicated in figure 136. Aquatic and aerial respiration. — In water, the supply of free oxygen is that contained in the air which the water has absorbed. The simpler organisms, being small, readily obtain a supply by direct absorption through the surface of the body. Increase of size, however, disturbs the ratio between volume and surface in the body. As compensation for the excessive increase of volume, absorbing surfaces are increased by the outgrowth of gills : and then mechanical arrangements for bringing more water into contact with the gills follow. The gills are lodged in respiratory chambers through which a constant stream of fresh water is main- tained, but still the amount of oxygen available is much more limited than in free air. There are no warm blooded animals except air breathers. In the open air, the oxygen supply is inexhaustible : but air absorbing surfaces, such as are adequate for aquatic respiration, cannot endure exposure to dry air. Some land animals like the earthworm, living in moist places, are able to breathe through the skin, by keeping it moistened with mucus secretion; but if a worm be exposed to a dry atmosphere it quickly dies of evaporation. The respiratory process, being essentially aquatic, requires moist thin-skinned surfaces for the intake of oxygen, and in organisms that live in dry atmosphere these can only be maintained inside the body; hence, the lungs, reached by long tortuous mucus-moistened passageways and main_ 214 GENERAL BIOLOGY taining deep cavities next the respiratory epithelium, where a zone of moisture-laden residual air serves as a medium of exchange and as a buffer to the air waves from the outside. The amphibians make it easy to understand the transition from aquatic to terrestrial life in vertebrates. It would have been hard to imagine all the changes necessary for fitting a fish -like aquatic vertebrate for life on land, but in a salamander these changes, some of which would certainly surpass imagining, are all wrought out in a little while before our eyes; they go forward without a hitch, and most significant of all, they go forward in similar manner, in all the higher terrestrial vertebrates, whether they are to live any part of their lives in the water or not. As in the salamander, so in vertebrates generally, the sexes are separate and ttue sexual reproduction is universal. But there is very great diversity among them as to mode of nurture of young, and some of the differences are of profound significance. The lancelet (fig. 132) lays minute eggs containing very little yolk; these segment and gas- trulate typically, and the embryos hatch when they reach the gastrula stage, and thereafter shift for themselves, receiving no further parental nurture. But all the domi- nant groups of vertebrates make better provision for the development of their offspring, and do not turn them adrift in so immature and feeble and defenseless a con- dition. T3rpes of nurture. — There are two main types of nurture for the young of vertebrates, i) The storing of additional food supply in the form of yolk in the eggs. We have found a considerable store of yolk in the eggs of salamander; this process reaches its maximum development in the relatively huge eggs of birds. 2) The nurture of the young by means of embryonic membranes. This reaches its maximum development in ORGANIC EVOLUTION 215 mammals, and it has many features of unique interest and significance, but we can here consider only a few of its more general aspects. We have already seen in the salamander that a dilata- tion of the oviduct near its lower end (the uterus) serves for the temporary storage of the ripe eggs, just before their extrusion. In the higher mammals the two oviducts (called also Fallopian tubes) become confiuent at this portion of their length into a single uterus, in which the eggs on leaving the ovaries find lodgment. Being fertilized internally they remain here, and undergo seg- mentation and other early developmental changes while Ijdng against the uterine wall. Almost as soon as the primary germ layers are established the ectoderm of the ventral wall rises up about the embryo in a circular fold all about its body and over its back; the edges of the fold come together and fuse and enclose the embryo (or fatus) under a double canopy of thin membrane called the amnion (fig. 1376). Almost simultaneously a food absorbing organ called the allantois develops for at- tachment of the embryo to the wall of the uterus. This springs from the endoderm near the posterior end of the archenteron. It grows out as a hollow membranous fold posteriorly and then dorsally between the wider folds of the amnion; there is developed within the allantois a complete set of embryonic blood vessels, the principal ones being an allantoic artery that springs from the great dorsal aorta, and an allantoic vein that returns the blood to the post cava. The allantois and the outer layer of the amnion become fused together, and attached to the uterine wall in a series of minute interlocking processes {villi), the whole complex attachment layer being known as the placenta. The processes on the wall of the uterus become permeated by a dense network of capillaries developed from the blood 2l6 GENERAL BIOLOGY Fig. 137. Diagram of nurture -of young through embryomc membranes, a, a young embryo with embryonic membranes beginning as folds or outgrowths of ectoderm and endoderm; b, an older embryo with the folds extending over the back to inclose the embryo; c, an older embryo with the placental attachment to the uterine wall established; d, diagram of the channels of food intake and waste removal in the embryo; w, wall of the uterus ; o, o, folds of the ectoderm which when confluent form the outer and inner amnion; t, t, p fold of the endoderm, outgrowing to form the allantois* q, the vestigial yolk sac; r, the amniotic cavi sac, in which the embryo floats; s, gill slits; , «, umbilicus; v, v, fore and hind leg buds; w, the circulation through the placenta; m, indicating the course of the blood of the mother parallel to «, that of the embryo ; g^ gill circulation of the embryo; fe, heart; i, dorsal aorta, /, post cava: k, allantoic artery; 2, allantoic vein ORGANIC EVOLUTION" 217 vessels of the mother. This becomes the source of food supply for the embryo during its long prenatal existence. In the corresponding villi of the membranes of the embryo copious capillary blood vessels are developed as a part of the embryonic circulation: these are its food taking organs. The process of nutrition is one of exchange of blood content between mother and offspring by diffusion through the thin walls of the villi. It is quite comparable to the exchange of gases which takes place in the gills or lungs. Both food (in solution) and oxygen are withdrawn from the passing cur- rents of the mother's blood, and into the same currents are discharged the carbon dioxide and all other waste from the body of the embryo until its birth. The body of the embryo immersed within the amniotic sac, closes to a narrow opening (the umbilicus) on the ventral side of the abdomen and the closure elongates into a long stalk-like umbilical cord through whose vessels nourishment is drawn from the placenta until embryonic growth is ended. The embryo then hangs on the cord, like a ripened fruit upon its stalk. At birth the stalk is severed, and the feeding organs of the embryo, full formed and functional, are called into action. Such are the means by which the maximum of provision for development of young is attained in mammals, and to these there is added the development of milk from the mammary glands as a further food supply for infancy. What a vast difference exists in bodily equipment between a new born mammal and the microscopic gastrula of a lancelet ! The life process in the salamander and in other verte- brates, is not very different from that in the worm. Indeed, it is much the same in its essentials in all animals, the differ- ences occurring in the ways and means whereby these are accomplished. The essential processes are compre- 2l8 GENERAL BIOLOGY hended in the word metabolism, and their relation to the accessory phenomena are indicated in the following table: METABOLISM Food intake Digestion Circulation of food from the alimentary canal of oxygen from the lungs "Assimilation {Ana- bolism) Dissimilation {Katabolism) Circulation of CO, and H^O to the lungs of H,0 and N waste to kidneys Discharge of waste Accessory processes, mechanical and chemical The essential pro- cesses : the work of every cell Accessory processes, mechanical and chemical Common features of organization in plants and animals: 1. Protoplasm is the "physical basis of life," and in nearly all organisms it is definitely organized into cells. 2. Cells, therefore, are the units of organic structure. 3 . The method of increase is by cell division. 4. Every organism begins life as a single cell. 5. Aggregates of cells may form individuals of a higher order, with the various parts of the cell complex fitted toge- ther in a state of mutual dependence. 6. Two processes are therefore involved in the making of such organisms: 1) cell division and 2) cell differentiation. ORGANIC EVOLUTION 219 7. ThejfiltingLpf a jjart of thccells- to-perfnrm spficial functions follows the universal law. of specializatiQP., . that special fitness fpr one thing, involves limitations ,itL,reapfisJ; to other thing's., 8. Th e primary differen tiatio n in all the higher organ- isms is that into^g erm plasm and bodyplasm, the former appearing in cells of two complemerital sorts, eggs and sperms.., 9. Increa se in size of the cell complex necessitates -sup ^ portin g structures and circulatory pppargjviif. but these parts in the di fferent plant and anima l groups are exceed - ingly different in structure, 10. Exposure to the air in terrestrial organisms necessi- tates the removal of the organs for intake of oxygen from the surface of the body and the development of epidermal layers to withstand evaporation. Besides these matters of general organization, there are many other things pertaining to the functions of organ- isms, to the phenomena of growth and metabolism, to the finer structures of protoplasm and to the behavior of its parts in reproduction, that are common to plants and animals. A few of the better known (cytological) phenom- ena of the behavior of nuclear parts in reproduction, will be briefly noticed in the next chapter. The simpler organisms best illustrate the common feat- ures of plant and animal organization. The forms we have been considering in Chapter ITI illustrate rather a few of the main lines of divergence : but beneath their diversity lie the common features just stated. All living things are com^BQse4-Of-QIJ:6. kip d of substanc e, t hat is organized into equivalent structural units, t hat .increases by one method ,.fif_growth, and that reproduces, successive generations from a common starting point. "" """^ 220 GENERAL BIOLOGY The principal groups of organisms. — In the foregoing studies we have had before us representatives of a few of the larger groups of plants and animals. We have not time to LINNAEUS . (1707-1778) A great pioneer systematist ; founder of the binomial system of nomenclature; author of Systema Naturse etc. develop a system of classification, or even to enumerate all the groups, but a tabular statement of a few of the larger and more important groups is given on the following page: ORGANIC EVOLUTION 221 Plants I. Thal lophytes, algae and fungi. II. Br3rap]xyt£a,Jiverworts and mosses 1. Hepaticae, liverworts 2. Musci, mosses. III. Pteridophytes, the ferns and their allies 1. Filicinas, the true ferns and the water-ferns (Marsilia, etc) . 2. Equisetinag, the horse-tails 3 . Lycopodinae, the club mosses, etc. IV. S permatopluz tes . The seed plants 1. Gymnospermae, plants with naked seed; conifers etc. 2. Angiospermae, plants with seeds developing in closed vessels a) Monocotyledons b) Dicotyledons. Animals I. Protozoa, one-celled animals. II. Metazoa, many celled animals. 1. Porifera, sponges 2. Coelenterata, polyps, jelly-fishes, etc. 3. Vermes (in broad sense) segmented and unsegmented worms, rotifers, bryozoans, etc. 4. Mollusca, clams, snails, squids, etc. 5. Echinodermata, star-fishes, sea-urchins, holothurians, etc. 6. Arthropoda, insects, spiders, crustaceans, etc. 7. Tunicata , tunicates 8. Vertebrata, backboned animals. a) Pisces, fishes b) Amphibia, frogs, salamanders, etc. c) Reptilia, lizards, snakes, turtles, etc. d) Aves, birds e) Mammalia, mammals. 222 GENERAL BIOLOGY II. GENERAL EVOLUTIONARY PHENOMENA AS ILLUSTRATED IN BRIEFER SERIES OF ORGANISMS. In the foregoing studies we have given brief consideration to a very few plants and animals, selected to illustrate the two main lines of organic development, corresponding to the plant and animal "kingdoms"; but the wide gaps between the types studied have left far too much to be bridged in imagination. Hydra and earthworm, or liver- wort and fern, stand so far apart in point of structure that it is difficult to conceive of all the forms intervening. Let us now compare together some forms that a.re more alike in order to see, if possible, the nature of the relations organisms bear to each other. In so doing our attention will be given to tjrpical organic phenomena, rather than to typical organ- isms. These will be grouped for convenience under three heads: 1. Divergence and convergence of development. 2. Progressive and regressive development. 3 . The correspondence between ontogeny and phylogeny. I. Divergence and convergence of development. Whatever our views of relationship, the series in which we arrange organisms are based on the likenesses and differ- ences we find to exist among them. This is classification. We associate organisms together under group names because, being so numerous and so diverse, it is only thus that our minds can deal with them. Qassifi gation furnishe s the handlesj)y which j^e move all our intellectual lugi^pgp- We base oimgroupings on^what-we know.of the or ganism s^. O ur system of classification is , t jierefor e, lia ble to chang e with, every advance_of_kii o wledge ._ The earliest groupings of animals were very simple and obvious; "creeping things," "flying things," "fishes of the waters," etc. How recently, indeed, have bats ceased to be grouped with the ORGANIC EVOLUTION 223 birds, and whales with the fishes. That the very many diflEerent sorts of things living in the water were for a long time merely fishes, is witnessed by the common names they still bear: shell-fish, crayfish, jelly-fish, cuttle-fish, etc. Such classification was based on the recognition of the most superficial characters only. Generally the more funda- mental characters are the less obvious ones, and are found in internal organs, and in developmental phenomena. The earliest anatomi cal classifjca tip n nf.land a.-nimala , based on the number of feet — bipeds, quadrupeds, hexapods, octopods, decapods, centipedes and millipedes — was vastly improved when the bipeds and quadrupeds and fishes got together on the basis of the common possession of a spinaLcolumn as the group Vertebrataj_ and all the others were dissociated therefrom as Invertebrata. But the development of embryological knowledge in a later period showed that there are characters more fimdamental than the vertebrae; that certain of the invertebrates possess in common with all the vertebrates, phar3mgeal ^gilLclfifts a.nd-a. notocord; hence Cordata replaces Vertebrata as the more comprehensive group name. Homologies and analogies. — Our judgment of the like- nesses between organisms, or between the parts of a single orgaiiism.,_i&based on that essential identity of parts that we ca ll homolog y*. Two organs are. homologous when com- posed of like parts in similar relatinns , each to each . Thus , the hand of a man (fig. 271) an d the fore foot of a sala- mander~(figr~272) ar e homologou s, since they are com- posed of the same parts put together in essentially the *A few exceptional organisms, like certain bacteria, are so simple in structure that differences in their bodily organization are hardly discoverable : and their recognition depends m part at least on their manner of growth in culture media, and in the nature of the by^^products of their activity. 324 GENERAL BIOLOGY same way. On the other hand, when the Ukeness is super- ficial only, and not fundamental; when it is likeness in function or in superficial appearance, it is called analogy. Thus the wing of a bird and the wing of a butterfly are LOUIS AGASSIZ (1S07-1873) A great teacher of zoology, who did much to promote the development of science in North America, analogous organs, for though agreeing in form and function they are totally dift'erent in structure, and have no com- ponent parts that we can recognize as identical. Homology is , therefore , the ordinary criterion by which we judge of the relationship of organisms. In the neck of ORGANIC EVOLUTION 225 nearly all ma mmals there are seven ce.rvical yeteb rae._ whether the neck be long as a giraffe's or short as a mole's. The foremost is the atlas vertebra, and bears up the skull; the second is the axis vertebra, about which the atlas swings ; the other five, although less differentiated, are equally constant in position and relations, and we can not doubt but tha t-thase seven are i dentical. The fore l imbsof vertebrates are suffic iently unlike ^ in sujBerficial appearance : we know them as legs in most quadrupeds, as flippers in sealg, as wings. in birds and bats, and as arms in ourselves; but when we examine their structure we -find they are built on a common plan (fig. iii), a nd therefore , homologous.. The recognition of homologies oftgnjcallsjor the, utmost care in _ comparison of organs and for discrimiliatingljudgSlg£t_of a lugh_order. It was a dictum of the elder Agassiz that the education of a naturalist consists in learning how to compare. These^ is besid e this corre spondence of parts between diff^en t organism s, a similar correspondence between pat^ _repeate3_ja_a— siagle..4iEgaiiisnUa. This is ca lled serial homolog y. It is well represen ted in the repeti- ti pn of p arts, segment by_segment in the earthworm. The student in this course has already had in Chapter I, a little practice in identifying homologous parts; first, in flowers (pistils, stamens, corolla, etc.), and later in the parts of the body of insects. A special study of this matter is given here with material more available for critical examina- tion. The veins in the wings of insects. The veins that constitute the supporting frame work of an insect wing may bear the following names and designa- tions: Costa (C) Subcosta (Sc) Radius (R) Media (M) Cubitus (Cu) Anal veins (A) 226 GENERAL BIOLOGY The order in which they are named is that of their arrange- ment from front to rear. Branches of veins are conveniently designated by numerals added in like order to the abbrevia- tion for the vein (as So, and Sc^ for the two branches of the subcostal vein). But there is one large branch so distinc- tively formed that it has received a special name, the radial sector (Rs) . All these veins and their usual mode of branch- ing are shown in solid lines in the accompanying diagram of a typical wing. In dotted lines are shown the cross veins of most frequent occurrence. Two of these toward the base of the wing the humeral cross vein (h) and the arciilus (ar) have received special names; the others are named in ^*'^ tu, Cu, M, ^ Fig. 138. Diagram of the venation of an insect wing. accordance with the positions they occupy in relation to the veins. The radial cross vein (r) and the median (w) occupy the principal forks of the radial and median veins respectively, and radio-median {r-m) and medio-cubital iyn-cii) connect the veins whose names they bear. ■These, then, are the materials with which we have to deal in the following exercise. While they appear simple and distinct enough in the diagram, a glance at the three series of wing figures shows that it is not at once easy to - be certain as to their identity. For : ORGANIC EVOLUTION 227 First, nature has not made the cross veins visibly to differ from the bases of branches, and the angulated and transverse base of a branch may look like a cross vein. It will help in settling their identity to note carefully the type Pig. 139. The venation of the wings of a series of craneflies. /, Limnophila; 2, Cylindrotoma; 3, Liogma; 4, Anisomera; j, Ctenophora; 6, DoUchopeza; 7, Acyphona; 5, Ula; 9, Moogoma; 10, Oropeza; //, Erioptera. of branching of the veins. The radial sector springs from the posterior side of the radial stem, and is typically twice forked, as is also the median vein, while the subcostal and cubital veins are but once forked. 228 GENERAL BIOLOGY Second, there are fewer veins in most of the wings figured than in the diagram. Veins may disappear through fusion of two or more branches into one, or, more rarely, by atro- phy. Fusions may occur between branches: a) from the tips approximated on the wing margin, proximally to the forks ; b) from the forks distally to the wing margin, or c) by Fig. 140. Venation of the wings of various flies (order Diptera). a, Rhyphus; 6, Conops; c, Erax; d, Dixa; c, Xylophagus;/, Thereva; g.Eristalis; fe. Stratiomyia, All from Comstock. the elimination of a cross vein through confluence of branches of adjacent veins, and subsequent fusion distally to the wing margin. Various stages of progress in all the methods of disappearance of branches will be found in the wings figured herewith. ORGANIC EVOLUTION 229 Study JO. Determination of homologies of wing veins in three series of closely allied insects. Materials needed: Enlarged prints of the wings figured herewith (or of any other series, showing like phenomena), and a single mounted wing of the common cranefly, Tipula, With which to begin. Fig. 141. Venation of the wings of a series of Psocids. /, Thyrsopsocus ; 2, Dictyopsocus ; 3, Taeniostigma; 4, Epipsocus; j, Pt ilopsocus ;_ 6, Myopsocus; 7. Psocus ; 8, Peripsocus ; 9, Polypsocus ; 10, Calopsocus ; //, Cascilius. 230 GENERAL BIOLOGY First, draw the wing of Tipula, carefully, to see the nature of the material under consideration; for the others, to save time, use the figures, which are reasonably accurate. Then begin with the cranefly wings series. Carefully label the veins in each wing with the proper abbreviation at base and apex; do this lightly in pencil, subject to later correction. Mark fusions of branches with the plus sign between the numerals of the branches conjoined. Determine homologies carefully. Follow each main vein stem outward and see when and how often it forks. The proof of correctness will consist in having all parts of the typical wing present or accounted for. Omit to name a vein or branch only when it is considered to have disappeared by atrophy; in this series and the next following, veins M4 and 3d A may be so treated. Note particularly that the cross veins are all in their proper places, or accounted for. When correctly interpreted the series will be consistent and harmonious, and the correctness of it will be obvious. Finish the work by coloring the veins alternately in two different colors, and making the cross veins a third color. Repeat, with the second series of miscellaneous fly wings. Repeat with the third series, of psocid wings, (fig. 141) noting here in the beginning that median and cubital veins are fused together in all members of the series from near the base outward well across the wing. The record of this study will consist in the one drawing and in the coloring and lettering of the veins on the prints, and these are to be preserved as material to be used in a subsequent study. The serial homology of the higher crustaceans. Serial homology is characteristic of the group of the higher Crustacea known as the sub-class Malacostraca,'and this group well illustrates how a single plan of structure may run ORGANIC EVOLUTION 231 t1;irniig1i g RpQeg^ of forms of the utmost diversity in appea r- a nce, and h ow parts essentially alike may be ada EtedJbjQjtlie most divers e ends. T he Malacos tracan body, be it an amphipod^ an isopod,.,^, decapodr^or what not — is composed of a series of twenty* segments, each of which is essentially of the skeletal plan^ shown in the diagram (fig. 142), except that appendages of the foremost segment are typ_ically_mibranched and the hindmost segment (the telson) is rudimentary and bears no appendages at all. Some of these segments may become fused together and consolidated on the dorsal side, only the appendages and ventral margins remaining free. This may occur at either end of the body, but it occurs constantly in the five front _ segments; these b^' Pig. 142. Diagram of a cross fusiou forming the head. The ap- ' a ?ypkai body-legmen? of pcndagcs of thcsc five Segments ap^endalST^t' tSpo- always consist of two pairs of fnd6poiie^'"^°^^^'- '"• antennae at the fro nt, one pair of mandibles .'be side the mout h, and two pairs o f maxilla e following the rn andibles^ These parts and their functions will readily be understood because of their likeness to the parts bearing the same names in the insects already studied. Immediately following the maxillae are one or more pairs of maxillipeds, . likf wjpp. dirppted for- ward bene athjthe mouth to assist in the manipulation of -t.lip fpod Then follow legs and swimmerets in more or less variety, the terminal joints of some of the legs being modi- fied in many cases into highly specialized grasping organs called chelifejis. and the swimmerets being frequently *This is not counting a vestigial segment in the head region, that is discoverable only during embryonic life, and with which we have here no concern. 232 GENERAL BIOLOGY modified to serve reproductive or respiratory functions. The eight segments following the head constitute the thorax and the seven last segments (counting the rudimentary 20th segment) , the abdomen. The typical crustacean appendage consists of a single solid basal piece (basipodite) and two jointed branches arising therefrom, one on the outer side {exo- podite) and one on the inner (endopo- dite). This typical structure is best shown by the swim- merets of the abdo- men. Crustaceans being primitively free-swimming aquatic animals, it is their swimming appendages that are least altered by adaptation. The legs are the stoutest of the appendages, and these offer but one branch arising from the basal piece, and that composed of a re- duced niunber of highly differentia- ted segments. A comparison of a leg with the last maxilli- pede in the crawfish will show which appendage! has been lost and which preserved and specialized. The best clues Fig. 143. A common crawfish. (Cambarus). ORGANIC EVOLUTION 233 to interpretation of homologies in any appendage are likely to be found in other adjacent appendages, which , because of prox- imity, have been subject to some- what similar influences. Study ji. Observations on' plas- ticity of form and persistence of type in Malacostraca. Materials needed: Specimens preserved in formalin of represen- tatives of at least three orders of Malacostraca, Cambarus (fig. 143), Asellus (fig. 144), and Gammarus (fig. 145): if such marine forms as Mysis (fig. 1580) and Squilla and any of the crabs are available, all- the better. Also, a few females of each type, bearing eggs, and a few live specimens for use in de- termining the functions of the appendages. Also, slide mounts of such appendages as are too small to be readily examined in place, or easily removed. Observe the living specimens, noting especially the different uses to which the appendages are put in locomotion. Demonstrate the very special water-propelling function of the "gill scoop" that is appended to the outside of the second maxilla in the crawfish, by holding the point of a copying pencil in the water beside Fig. 144. AseUus aquaticus. (after Sars). a, dorsal view; 0, ventral view of abdomen of female; x^ last segment of thorax ; y, appendage of abbreviated [ first abdomi- nal segment _ (the second segment is without appen- dages in the female) ; z, gill cover Iflpercvium). 234 GENERAL BIOLOGY Table of Malacostracan Appendages KINDS OF APPENDAGES On Segments In DECAPOD In AMPHIPOD In ISOPOD Etc. I ex: Antennae 2 (( 3 Mandibles. 4 etc 5 6 7 8 9 lO II 12 13 14 ^S 16 17 18 19 20 Bracket together the segments that are consolidated upon the dorsal side. When different in the two sexes divide the space with a diagonal line and write characters of male and female in separately. the base of a hind leg of a living specimen until it dis- solves a little, and watching for the colored water to appear at the front of the animal when expelled from the gill cham- ber. The passageway through the gill chamber from the rear and outward at the front may be looked up later in a dead specimen. Examine gill-covers and gills of Asellus in action by turning a living specimen over on its back and watching them under a lens. Note their texture and form, and ORGANIC EVOLUTION '^SS their typically paired arrangement. The gills of Gammar- us are appended to the bases of the thoracic legs on the inner side. Study the segmentation of the body and examine the appendages in series, carefully, in the several types, with the aid of the mounted slides where necessary, and fill out a table of homologies prepared as indicated on the preceding page. Then make out a table of functions for the appendages of the several types, as indicated below, basing it first of all on what you have observed of the uses of the several organs while studying the living specimens. Legitimate inferences as to functions, may be drawn from the form and location of appendages. Table of functions of malacostracan appendages. 'V o ■^^ t>. jj & y bo E R 'S. E I o .S.e " § 'a PS •p.ti O 1 Cambarus Gammarus Asellus Etc. ♦Specify functions in foot notes. Indicate segments by number only (i to 20), as in preceding table. Specify characters of male and of female separately, where they differ. 236 GENERAL BIOLOGY The record of this study will consist in the two completed tables just outlined, together with a few brief statements as to the relative uniformity or divergence of the appen- dages of particular segments or particular regions of the body, with possible reasons therefor. Divergent development has already been illustrated by both the major and the minor series of forms that we have been consider- ing. Indeed, in all these, but es- pecially in the two main series, the divergence is greater than has been specifically pointed out; for the lower types in each series represent in themselves the termini of their own lines of development, and not mere passing stages to higher forms. The table of classification on page 221 is but a statement of the main lines of divergence. Phylogeny.— The forms of a single line of descent consti- tute a race, or a phylum. T he study o lph yla is called phylor ^eny,, A common device for expressing graphically one's con- ception of phylogenyis the so-called "genealogic tree." The generalized forms are placed near the base of the tree, the specialised forms, out at the tips of the longest branches, and the intermediates are arranged, according to one's con- ception of relationship , somewhere in between. The student who has done the work of the last two practical studies will Fig. 145. Gammarus fasciatus (after Paulmier). ORGANIC EVOLUTION 237 lerstand that the tracing out of natural phyla, even with andant material , is a matter of great difficulty, and that len forms are insufficient and relationships not clear it admits of great diversity of opinion, and makes errors of interpretation easy. The divergence of development stated in the systematic table on page 221 may be more graphically presented to the mind if the groups contained therein be arranged in such a diagram as is shown in figure 146. Such graphic represen- tati^s of the possible course of evolution have been much used in the past, in spite of their purely hypothetical character; and although less commonly employed now, still they are an excellent aid to the mind in grasping the idea of genetic relationships. Fig. 146. A genealoeic tr ea: — a-graphic mode of illustrating possible relation- ship between organisms. 238 GENERAL BIOLOGY Study j2. An attempt at interpreting a possible phytogeny. Materials needed : The completed drawings from study 30, with homologies fully determined and verified. Construct a genealogic tree for each of the three series, that shall show a possible genetic relationship (based only upon the data furnished by the venation of the figures). Assume that the wing of figure 138 is primitive. Pick out the form most like it to go near the foot of each tree. Single out in each series the different ways in which the type has been modified, and make as many principal branches as there are different kinds of divergence. Pick out the most specialized forms for the tips of the longest branches. Arrange the others in position in accordance with their degrees of divergence, and let the branching and the length of the twigs represent this. Derive no form directly from any other that is in any respect more specialized. Compare all wings in each series together with respect to each charac- ter, the divergence of the tips of the subcosta, the fusion of the tips of the first fork of the media, etc. Remember that each species is the end of its own special line of development, and place each at the end of a twig. The record of this study will consist in three genealogic trees (which may be combined into one) , drawn without any superfluous branches and with all the forms figured (including Tipula, drawn) located thereon. It need, perhaps, be stated concerning genealogic trees, that they generally err in being more explicit than the known facts warrant. The figure of a tree does not present a good likeness of evolution as it lies before us at the present time, because the branches of the tree are conjoined in per- fectly definite relations. Lines of development are in fact traceable backward only a little way, and are then lost in ob- scurity. The' liverwort shownin figure 147 presents a truer ORGANIC EVOLUTION 239 picture of evolution as we see it now. Some of the main branches are clearly conjoined; others stand in doubtful relationships. The ultimate origin of all of them is obscure, for many of the older parts have perished. There is a general divergence of the tips, but there is also convergence, and even crossing. But there are enough long stretches of unbroken growth to leave no doubt as to the general course ■v ;; -" : ' "-'H: -i,i^:^^'L.''''>-'^^-^"--'V ■:■"■'■ ^^■^s'K ■^■^.^■.•^i^---- : :;■ ^^;-fw-»:-- l*^- '•'-^..^, -:■:■„: ■ ■ .■■■ •• ■. ■" 1 .." ■■■■ ' ■ ., - 1 Fig. 147. A leafy liverwort. of progress, and there is enough convergence of all lines backward to indicate that all the branches may have sprung ultimately from a common source. Group radiation. — Perhaps the most striking of the phenomena of divergent development is that which has been called adaptive radiation. This name serves to designate that tendency seen in the members of all the larger groups of organisms to become adapted to different natural func- 240 GENERAL BIOLOGY tions, and to take on structural peculiarities suited thereto. The phylogenetic lines radiate outward, as it were, from common structural type, into forms adapted to herbiv- ^-^"'^ orous or car- — -— dom in a nt ^\^^_^ _^„*--t— ^^r-*^ N, major divi- *»^ •■ {'^ ' Progress in regres- sion. — There is an im- portant sense in which all regressive develop- ment spells progress. .„ One must take into ac- count the whole man- ner of life of the organ- ism to comprehend this. . Even the most abject J parasite, losing all or- _^ % V %,illBr^ S^T^^ for independent "y^v^^ \^ NnSV^E. existence, is advancing " in its own peculiar way of getting on in the world. There is also a sense in which regressive development is to be considered a part of the normal life of an individual. As nutritive and reproductive functions come successively into dominence in the lifetime of every organism, so a retrograde development of nutritive organs may begin with the taking up of the labor of reproduc- tion. This is well illustrated by the common rag weed. The leaves shown in figure 161 were developed at differ- ent periods of the life of a single plant. They are divid- ed into two series, which parallel the wax and wane of vegetative vigor in the plant. The second series is the one Fig. 161. Leaves of the ragweed (Ambrosia ariemiscEfolia) . a, cotyledons; 6 to e, leaves successively formed in youth, w, the mature leaf form; n to s, the dimin- ishing series of leaves successively formed during the period of seed production; z, a fruiting tip. 264 GENERAL BIOLOGY of interest here ; it begins with the maximum development in size and complexity of leaves at sexual maturity, and, passing through a diminishing series, ends with cessation of leaf production when all the energies of the plant are given over to the ripening of its seeds. Why evolutionary series? — It has long been the custom of naturalists to arrange organisms in series; such arrange- ment facilitates dealing with large numbers. The compara- tive anatomists of the first half of the 19th century, who did so much to advance biological knowledge, believed in special creation, and in the fixity of the species. They determined homologies with great conscientiousness and arranged organisms in natural groups; but for them, homology meant likeness in structure merely, and not kinship, and their groups were "natural" in the sense that like had been associated with like in them. The organisms of a series were no more related to each other than a series of one type of vessels made by the same potter. Why then do we con- sider that natural grouping signifies blood relationship? Why are the series we arrange evolutionary series to us? It is because evolution alone affords a consistent and satisfactory explanation of the facts now. known con- cerning the structure, the development and the past history of organisms. The student who has done the work hitherto outlined will have felt this explanation. But perhaps it may not be amiss to briefly indicate at this point a fewclT'^ses of facts that speak especially for evolution, and that seem to stand in the way of any other explanation: 1. The plasticity of species under domestication, and 2. The intergradation of species in nature. Both these phenomena are well enough known 00 every observing person and each shows that species are not fixed and immutable. The individuals of a species may, there- fore, be arranged in a series with its extremes having very ORGANIC EVOLUTION 265 different appearance, and the differences between them may sometimes be correlated with their geographic dis- tribution, and sometimes not. 3. The close adherence to structural type in the mem- bers of a single group that is modified for great diversity of habit and environment ; and, conversely 4. The superficial similarity wrought in different struc- tural types, when they are modified to a common mode of existence. 5. Correlations of structure ; when one part of any type is modified for a different sort of life, other parts are modified in harmony therewith. The foot a of figure 148 is never associated with the beak h, or with any other beak in the series, except with beak of the type a. This is the sort of concordance that makes the interpretations of fossil frag- ments possible. 6. Vestigial structures; why should these exist at all, except they be ancestral? 7. The tendency of all embryos to recapitulate group characters. Why should such a tendency exist, but for age-long heredity? The palasontologic record is exceedingly fragmentary, and especially lacking in the more simple forms, that" would be most significant to us. The phylogenetic record is broken by the absence of connecting forms between the groups, existing organisms being only the twigs of branches that are often widely separated. The ontogenetic record is perver- ted by marked departures from the original course of development. But, notwithstanding these difficulties, which are so great as to make it easy to err in the interpreta- tion of nature's genealogies, the evidence of descent is thoroughly convincing. It is the more so because of the way in which each of the partial records supplements and corroborates the others, and it is certainly significant that 266 GENERAL BIOLOGY the developmental lines traceable backward through both ontogeny and phylogeny are all convergent. They point to a common origin in the remote past, and to "descent with modification." III. THE PROCESSES OF EVOLUTION; ATTEMPTED EXPLANATIONS. Facts, such as have been before us in the preceding studies have satisfied biologists generally that evolution has been the method of nature; but the theories that have been advanced in explanation of the processes whereby evolution has been wrought out, have not met with so general accept- ance. Yet, if evolution has had a past, it will have a future; and that future is of importance to us, because it must include the destiny of all races, including our own. Nothing could be of more practical importance to us than that we should understand the conditions of evolutionary progress, especially if these conditions should prove amen- able to our control. Many explanations have been offered, and some of them appear in part really to explain. All of them are under scrutiny at the present time. Investigations are in progress to determine their validity. It is well to reserve judgment, but it is also well to know the main features of the current explanations; for such knowledge is part of the common intelligence. Some of the more important explanations will, therefore, be outlined briefly here and in the next chapter. Natural selection. — The first explanation to receive any general approval (or even to attract much notice) was that of Charles Darwin. He observed how breeders, by selecting and isolating new forms as they arise in domesticated ani- mals and plants, are able to establish new varieties or races. He saw them producing perfectly definite results; ORGANIC EVOLUTION 267 horses bred for draft or for speed ; peas selected for color of flower or for palatability of seed, etc. And in his mind's eye he saw nature producing like results by the removal of the unfit and the preservation of those best suited to her conditions. So, he called the process natural selection. The theory of natural selection is based on four facts: i) Organisms vary; 2) In every species more young are produced than can possibly survive;' 3) Offspring tend to b c Fig. 162. Three sassafras leaves from the same tree. resemble parents ; and 4) There exists competition between the members of the earth's population. Inheritance will be considered in the next chapter. Let us here examine the other three classes of facts severally. Variation. — Animals and plants vary. No two persons look alike, nor do the individuals of any species, on suffi- ciently close acquaintance. The careful shepherd knows his sheep as individuals, and it is only to the casual observer 268 GENERAL BIOLOGY that they look alike. The robins on the lawn may be known personally by any one who will take the trouble to note personal characteristics. Nature abounds in little refinements of structure, such as we see in the raised lines traversing the cuticle of our finger tips. These lines are never exactly alike in any two per- sons. So distinct are these differences that finger prints are now-a-days a well recognized aid to the identification of - criminals. No two leaves on any tree are exactly alike; indeed those on the same tree may exhibit differences that are very marked (fig. 162). Fluctuating variations. The differences between the individuals of a species extend to every personal character- istic: stature, strength, activity, temperament, etc., but they are usually slight , and fiuct- FiG. 163. A six-spined seed uatc about a mean that of the rag weed. , ■, i expresses the normal con- dition for the species. This may be simply illus- trated by the seeds of the common rag' weed (fig. 163) each of which bears a long apical point, sur- rounded by a circle of short spines. The normal number of these spines appears to be six, but many seeds have five or seven of these spines, and a few have even smaller or greater numbers of them. A count of the spines on 100 seeds taken at random gives the following results : No. of spines 1234 5 6 7 89 No. of times occurring i 3 7 9 25 37 25 12 i If now the seeds of each class be arranged in columns, and aline be drawn joining the tops of the columns, that line will be the curve of variation (fig. 164), a common means of expressing variations of this type. The class containing the greatest number of seeds (called the mode; the six spined class in this case) may be regarded ORGANIC EVOLUTION 269 Fig. 164. One hundred rag weed seeds arranged in classes according tp the number of their spines. The line represents their curve of variation. 270 GENERAL BIOLOGY Fig. 165, Leaves of the smooth sumac, showing variation, a, the normal odd — pinnate leaf; b, an abrupt pinnate leaf; c and d. intermediate forms (in the tabulation, such were counted for the whole number to which they most nearly approximated) ;€, an odd- pinnate leaf with a leaflet of one pair omitted (of rare and prob- ably accidental occurrence) ; /and g, leaves from the base of the fungus gall that is shown in fig. 28, page 37, showing a tendency (under the stimulus of the parasite), to be more compound. ORGANIC EVOLUTION 271 as representing the normal condition for the species. It will be observed that the variations from the normal are here a little more numerous on the side of fewer numbers of spines, but that the curve is nearly symmetrical. It is an approximation to the symmetrical mathematical curve representing the distribution of error. Chance variations fluctuate thus about the normal. A count of the ray flowers of 315 heads of the bur mari- gold, gives, when the results are plotted, a curve that is very much askew: No. of ray flowers (classes) 3 4 5 6 7 891011 No. times occurring (frequencies) 2 3819522219 o i Pig. 166. The curve of numerical variation in leaflets of the smooth sumac, 2730 leaves counted 272 GENERAL BIOLOGY The normal flowering head has eight ray flowers, and the relatively fewer variants are nearly all on the side of the lesser nmnbers. In the midst of such fluctuating variations there sometimes exists a marked tendency toward a definite structural type. Such is the tendency of the compound leaves of the smooth sumac (fig. 165) to be odd-pinnate; that is, to have one terminal unpaired leaflet, with all the other leaflets arranged in pairs. A count of the leaflets of 2 730 leaves of this species results as follows : No. of leaflets (classes) 56 7 8 9 10 11 12 No.of times occurring (frequencies) 3 9 24 n 57 20 75 30 13 14 15 16 17 18 19 20 21 22 23 24 25 26 224 n 352 80 501 106 331 35 143 14 31 .4 7 2 Here is a total of 1748 odd-pinnate and of 382 abrupt- pinnate leaves. The broken curve which these figures yield is obviously the equivalent of two similar curves for the two types of compound leaf, and the greater height of the odd-pinnate curve is the index of the tendency toward such leaf type in this species. Study jj. Fluctuating numerical variations. Select some common, organism or organ havirig parts that can readily be counted and that vary in number/ and study the variation in numbers of these parts. Let the numbers be small ones (for economy of time in counting, prefer- ably not above 20). Such things as the seed spines, ray flowers, or leaflets of a compound leaves just cited in these pages, or leg spines, wing hooks, leaf lobes, etc., etc., are everywhere available in sufficient abundance. Gather the material at random. Count at least 100 specimens and record the classes and the number of times occurring, as in the first example cited (see fig. 164). Then plot the curve of variation on a square of cross-section paper, lay- ORGANIC EVOLUTION 2-]'. ing off the classes-upon the ordinates and the frequencies upon the abscissa. Then, if this work be done by a class, let the totals of all ths indiA'idual counts be represented i n another curve, plotted in another color upon the same square; this will, on account of the greater numbers, more truly represent the normal variation for the species, and it should be a closer approxi- mation to the sym- metrica] and balanced curve of distribution of error. The record of this study will consist in : i) A drawing of a variant showing the normal condition for the species, labelled with the name, and showing clearly the parts counted and plotted. 2) The individual and collective curves of variation. Mutation. — Variations are not all so light and inconstant. Figure 167 shows a variant of the common linaria (L. vul- garis, "butter and eggs"), the ordinary flowers of which are Fig. 167. A probable mutant of Linaria (L. vulgaris), "butter-and-eggs." 274 GENERAL BIOLOGY shown in figure 1 68. Among the offspring of a single-spurred and strongly bilateral flower appeared this one plant bearing mainly five-spurred and radial flowers. Such larger variations, when they affect a number of correlated characters so as to change the aspect of the organ- ism, and when with self fertiliza- tion they are self maintaining (i. e., when they "breed true"), are known as mutations. That mutants establish a new gradg of variations is evidenced by the fact that each mutation estab- lishes a new normal, about which ordinary variations fluctuate. Mutations appear rather rarely, and under conditions that are not at present understood. Their importance as starting points in the development of new races of plants and animals is well recognized by breeders. The long and care- ful pioneer study of them by Hugo DeVries has made clear their probable importance as starting points in the evolution of new species. DeVries calls many of the mutants he has found "elementary species." Their significance will again be referred to in the chapter on inheritance. More young produced than can survive. — The species of organisms differ extraordinarily in the number of young produced, but all agree in the tendency to increase in a geometric ratio. The offspring of a single parent may number millions, or may be but few; but in either case, if all survived to reproduce. in like ratio, the earth would soon lack standing room for the progeny. In the edge of the Fig. 168. The normal flowers of Linaria. - ORGANIC EVOLUTION 275 pond a single female frog may lay, 300 eggs on a spring morning, and she may repeat the performance in successive years. If half of the succeeding generations were females, and were at maturity equally prolific, and if all should surr vive to reproduce, a simple calculation would show that in a very few years we should have more bulk of frogs than of water in the pond. Three pairs of offspring in one hundred years is' said to be the rate of reproduction of the African ele- phant—a rate phenomenally slow ; j-et even this is an increase of 300% in a century — sufficient if maintained without any losses except from old age, to cover the earth with elephants. It is by excess of births that nature provides for inevitable losses; and the excess is proportioned to the dangers to, be encountered in the race of life. A single pike may lay upwards of 80,000 eggs each season, scattering them broad- cast in shoal waters, where most of them early fall a prey to other fishes. When hatched, their ranks continue to be thinned, however, in a diminishing ratio, as they become larger and better able to take care of themselves. But if out of all these offspring there remains at maturity for every pair of old pike a single pair of young ones surviving to re- produce each season 80,000 potential offspring, this race of fishes is holding its own ; the natural balance is maintained. For more than this proportion to survive persistently would disturb that balance, by depleting the numbers of other fishes on which pike feed. A sunfish that guards its eggs until hatched, need not produce so many of them. But every species, in order to avoid extinction, must produce sufficient excess of offspring to make good the inevitable loss of life during immaturity, and the failures of adult life. Competition. — For want of food, therefore, and often indeed for want of standing room, the vast majority of organisms born into the world are foredoomed to perish before reaching maturity. Yet the method of nature is not 2 76 GENERAL BIOLOGY more harsh than that we pursue in making a flower bed. For, do we not sow the seed thickly, to insure a good stand, and then thin out rigidly after germination? Among all organisms the vast majority of offspring are swept away by casualties against which they have no power to cope; by exposure to unwonted conditions, to floods, to drouth, to ruthless enemies, to diseases, etc. Here the elimination is wholesale and indiscriminate. But casualties are more or less local and occasional, and they always leave an excess of young to be eliminated by slower methods which allow some play for the powers or merits of the individual, and, there- fore, some opportunity for competition. The struggle for existence. — The thinning out process inevitably goes forward, but it is no longer wholly indis- criminate, for individuals vary. Some are better fitted than others to meet and cope with the perils and exigencies of life. If these be physical agencies, some are better able than others to withstand excess of heat or cold or drouth ; if enemies, some are better fitted than others to combat, to escape or to elude; if competitors, some are stronger than others and better able to seize and appropriate to themselves the lion's share of the means of livelihood. If we did not thin our seedling bed, nature would thin it for us by the slower, but not less certain methods of com- petition; and a few of the seedlings of stronger growth, reaching down more deeply with their roots to the food supply in the soil and spreading out their leaves more broadly to the sunlight would prove the better able to maintain themselves. The survival of the fittest. — Herein lies the efficient prin- ciple of natural selection. The -fittest survive. Not in the face of casualties; for these sweep out of existence good, bad and indifferent alike. Not in the face of insuperable diffi- culties; the best seeds may fall where there is not sufficient ORGANIC EVOLUTION 277 depth of earth; the best may have no chance of living. And not in times and situations of piping peace and plenty, when there is a living for all, and even weaklings may reach CHARLES DARWIN (1809-1882) Prophet of evolution, whose theory of natural selection was founded by almost unexampled industry and patient endeavor; author of "The Origin of Species," the most influential book of the nineteenth century. maturity. But, casualties and disasters of station aside, and given a stress of competition keen enough to call into requisition all the powers an individual may possess, the fittest survive. They survive to perpetuate their powers in their descendents. This means evolution. 278 GENERAL BIOLOGY Fitness. — Fitness for natural selection consists in two things : _ i) Ability to' get a living and to reach maturity; this is provision for individual needs. 2) Ability to leave well equipped descendents possessed of like good qualities. This is provision for the future of the race. It is not, therefore, the superior excellence of a par- ticular organ, but the balanced excellence of the organism as a whole that is of determinative value. Good leg muscles doubtless make for speed ; but speed alone will not avail the hunted hare, if it have not 'also endurance and instincts of self preservation. "The race is not always to the swift." And all these will be of no moment whatever from the point of view of evolution if it leave no well bom descendents. For the sterile variety "carries its own death warrant." What has a chance of survival, therefore, under the most rigid natural selection, will depend on what variation of the several parts of the body appear, and in what combination. Study j6. The struggle for existence among seedlings. This study is one that requires time, a:nd observations at repeated intervals ; the struggle for existence is not a matter of laboratory periods. Seedling plots of ground, thickly sown by nature to annual weeds are always to be found in the corners of neglected gardens, by roadsides and in fence rows. Other plots in wet, shaded places by streams are overgrown annually by wild touch-me-nots (Impatiens), and in sunny places by smartweeds (Poly- gonum) . If for want of time this study be deemed unavail- able for class use, it may be carried out by anyone in his home garden. Select a plot of ground a few feet square, more or less, free from rooted perennials, in which nature has sowed the seed of annuals and where the seedlings are just beginning to ORGANIC EVOLUTION 279 crowd one another. Stake it out with markers at the corners. Count the seedlings present and record the num- ber, and note any peculiarities in their distribution. After allowing time for growth of several additional leaves and a little differentiation in size among the seedlings, count them again, this time in three classes, small, medium and large, and record the numbers. Watch now the intensification of the struggle for existence and count the survivors of the three classes at longer inter- vals through the season, and record the results. Count in the end the individuals that are able to mature seed. Tabulate the results, showing what proportion of each class fruited. Calculate the area that would have been required if all the plants that germinated from seeds had attained the mini- mum fruiting size ; if all had attained the maximum size of the species. Artificial selection. — Man selects the variants he finds among his cultivated species of animals and plants, not for the good of the species, but for his own advantage. He selects com for the starch or for the protein content of the seeds. He, selects cattle for beef or for milk production. He selects fowls for egg production or for rapidity of growth, or for form of comb and wattles (fig. 169) or for color or sheen of plumage or for feathers or spurs on the feet; and pigeons and gold fish he selects mainly for qualities that suit his fancy. In the variability of living organisms he finds resources, the value of which he is only just beginning to comprehend. But his improved varieties are all weaklings, incapable of maintaining themselves in competition with the wild races from which they are derived, and requiring to be isola- ted and cared for, in order that the values for which they are selected may be realized. High bred race horses are 28o GENERAL BIOLOGY Black Minorca f Brahma short-lived, of nervous temperament and of weak constitution. A belleflower apple is a beautiful, fragrant and luscious fruit, but the tree that bears it is quite incapable of entering into open competition with the worthless wild crab apple. Nothing could be more striking in illustration of this point than the certainty with which wild species crowd out the cultivated ones on abandoned farms. Lop-eared rabbits, and flightless ducks, and udder- encumbered cows, and small-boned, small-brained pigs, and hairless, witless, barkless and tailless dogs are all freaks, and nature will have none of them. Her own creations, while often far more curious and extraordinary than any of these, differ from them all in the one essential quality of fitness. This then, in brief, is the doctrine of natural selec- tion, as a partial explana- tion of the process of evo- lution. Heritable varia- FiG. 169. Standard varieties of chickens (after Rice). ORGANIC EVOLUTION 281 tions of whatever sort or origin, furnish the materials of progress, and the competition of life, when of eliminative severity, "selects" the fittest variants for survival, chiefly by the elimination of the less fit. Real selection involves a psychic factor; it may occur if, for example, birds select the most luscious wild cherries or other fruit, whose seeds they carry to a place favorable for growth ; or if insects select the showiest of the flowers whose pollen they distribute. Natural selection is thus seen to be an explanation of the modus operandi of those extrinsic forces that tend to make every race conform to conditions of environment. With the intrinsic forces of the living organism, it can only indirectly deal. Natural selection does not, therefore, account for the origin of anything new among organisms, but only for the preservation of such new things as are heritable, advantageous and fit. Nevertheless, it is at this day the one process of evolution whose operations are clearly set forth. Orthogenesis. — By this name we designate a racial ten- dency toward some one particular line of development : an innate tendency, uncontrolled by external conditions. Such racial development is not fortuitous, but in a single direction, straight ahead, as the name indicates. But orthogenesis is not an explanation of a process ; it is merely a name for one. The orthogenetic tendency is manifest in its incipiency when a group of organisms tends to vary strongly in the direction of some one particular structural type ; when the variations are not promiscuous (indeterminate) but show a strongly marked trend. This is illustrated by the inherent odd-pinnateness of the compound leaves of the sumacs; and equally well by the inherent abrupt -pinnateness of che leaves of the cassias (partridge pea, etc.) It is best illus- trated by the actual history of races as revealed by the long 2 82 GENERAL BIOLOGY records of palaeontology. Many definite lines of specializa- tion, manifestly independent of environing conditions, are traceable among fossils, and some of these lines of specializa- tion may be followed out to their final end. Useful struc- tures, such as in their beginnings natural selection might have favored, have been developed far beyond their optimum, and their possessors have disappeared from the earth. Famous examples are the sabre-toothed tigers and the Irish elk. The canine teeth of the sabre-toothed tiger were so over developed as to be useless, their tips projecting out- side the mouth when opened ; and the antlers of the Irish elk attained such size and weight as to be a very great encumbrance. Well developed canine teeth are manifestly advantageous for tearing prey, and all carnivorous mam- mals have them; and strong horns for meeting rivals in combat, are advantageous too, and the males of most social ruminants have them; but in both cases the good thing was overdone ; specialization far outran utility. We need not go so far afield for illustrations of develop- mental tendencies that have exceeded utilitarian demands. The studies of floral structures in Chaptei' I should have brought us into contact with numerous examples. What possible use is there for all the complicated apparatus of the milkweed or the orchis flower^ or for all the arching, scalloping, and fringing of the lips of a mint flower! Clearly the living substance has inherent powers that manifest them- selves in racial tendencies, independently of outward mold- ing forces, and that sometimes are not amenable thereto. We may perhaps conceive of orthogenesis as a manifesta- tion of a sort of developmental inertia. A genetic tendency, once set going, tends to keep going in a straight line. How it starts we do not know. Natural selection may have something to do with its survival in the beginning, but evidently cannot stop it at the point of optimum develop- ORGANIC EVOLUTION 283 ment, for we must always remember that there can be no selection of single characters; it is individuals that are selected, with whatever combination of characters they may happen to be endowed. If the fittest Irish elk had ever antlers of increasing size, the only possible curb to antler development would lie in the extermination of the line. Natural selection can affect an organ only when that organ causes such manifest unfitness in the organism as is incom- patible with the conditions of racial existence. The phenomena of orthogenesis indicate that the springs of genetic progress lie very deep and that we must look for the origin of species in the origin of variations and of develop- mental tendencies. This matter will be considered a little further in the next chapter. Segregation. — The breeder of plants or of animals isolates his choice varieties (except when propagated asexually) in order to obviate the retrogression that would inevitably result from intercrossing with inferior varieties. Biparental reproduction necessitates this. Nature also segregates her new forms more or less rigidly, and by a great variety of means, among which may be mentioned both external and internal agencies. i) Geographic barriers. — Two closely allied species, whose differentiation from one another may have been comparatively recent, are often found on opposite sides ot a mountain chain or desert, or other impassible barrier. Thus most of the fishes found on the two sides of the Isthmus of Panama are represented by two closely allied species, one on one side and the other on the other side. This is held to confirm the opinion of geologists, that the two oceans were once connected across the isthmus by open sea, the assump- tion being that time enotigh has elapsed since the emer- gence of the Isthmus, closing the passage, for the differentia- 284 GENERAL BIOLOGY tion of the species from each other and from the com- mon original stock. This is segrega- tion of the most obvious sort. Many a wide ranging species has varieties or sub-species for every distinct f aunal area with- in its range. The accompany- ing map illus- trates the geo- graphic distri- bution of the races of the common song sparrow. Whatever the means employed, nature has practiced segregation on a large scale, even isolating more or less the larger groups of organisms — ^the palms in the tropics of the world, the marsupials in the Australian region, etc., etc. This is a subject of great biological interest and importance, but it falls outside the scope of our practical studies and therefore, the stiident is referred for fuller statement to general works on the geographic distribution of plants and animals. 2) Climatic and meteorological conditions. — Tempera- ture and altitude, rainfall and winds, and other similar influences differentiate desert and plain, meadow and forest, and all the host of animal and plant forms that accom- pany them. This is so familiar a matter, that any one who Fig. 170. Diagram of the distribution of the common song sparrows of North America. Shaded areas in- dicate the range, a of the eastern song sparrow; b, of the Rocky Mountain song sparrow; c, of the gray- song sparrow; d, of Samuel's song sparrow; e, of Heer- mann's song sparrow; /, of the Oregon song sparrow, and g, of the rusty song sparrow. ORGANIC EVOLUTION 28s has travelled a few hundred miles away from home should be able to illustrate it by recalling the new form of animals and plants met with in the new environments visited. 3) Physiographic barriers. — We often find two closely allied species in one locality inhabiting haunts that are just a little different topographically. This is illustrated by two of our common dragon flies, one of which (Libellula semi- fasciata) inhabits the small brooks and the other (L. pul- FiG. 171. A common pond inhabiting dragonfly (Libellula pulchella) . chella,iig. 171) the small ponds over a considerable part of the United States. This matter will be abundantly illustrated in Chapter VI under the subject of the adjust- ment of organisms in place. 4) By ecological differences. — Two species may live even nearer to each other and yet dwell apart ; as in the case of two species of squirrels of the same locality, one of which burrows in the ground, while the other lives and nests in trees. This sort of adjustment in place also will be studied in Chapter VI. 286 GENERAL BIOLOGY 5) A species might segregate itself into two groups, if among its members there should arise marked differences as to the date of the breeding season. Those maturing early could only mate with others of like early development, and would thus be segregated, (permanently, .if this seasonal habit were heritable) from those that mature late. Differ- ences like this would be likely to be correlated with other differences, and thus two races might begin to diverge. 6) A species might be segregated into two, if two of its groups of variants should be mutually sterile. Such variants occur among cultivated species. 7) Two races are developed out of one species when the variants fall apart in two 'groups, keep together in these groups, develop a "race feeling," and refuse to interbreed. This is reported to have, occurred not infrequently when considerable numbers of degr have been kept in private parks. Birds of a feather flock together, even when the feather is distinctive only of a race or a sub-species. This is the kind of segregation known as "preferential mating.'' These are the principal means whereby nature keeps her creatures apart in sepatate strains, or in groups of higher rank; far apart if the barriers be external agencies of isola- tion, but still apart even though near together, if there be such internal agencies as prevent intercrossing. The interaction of external and internal forces. — So there have been and are still two main types of explanation of the process of evolution, typified by natural selection and orthogenesis; the one emphasizing outward conditions, the other, inner tendencies. The contemplation of the environ- ment, and of the fitness of organisms thereto, leads to the over emphasis of adaptation; the study of the spontaneous and automatic activities of the living substance, tends toward confidence in their sufficiency. The two have much too often been treated as though they were mutually ex- clusive. ORGANIC EVOLUTION 287 Direct adapta tion seems especially to explain such classes of facts as are furnished by geographic distribution, especially of island life, by parallelisms, by mimicry, by degeneration, etc. Let us illustrate by means of the parallelism of the swift and the swallow. How have these birds that are so different structurally, become so very much alike in form, in flight, and in foraging habits that it requires something of an ornithologist to distinguish between them? It is a peculiar field for bird life that they occupy. Above the ponds and lakes there hovers a teeming population of midges and other little insects excellent for food. How have these two, of all the groups of birds, become so finely and so similarly fitted to profit by it? Is it more likely that internal forces automatically produced such external like- ness built upon persistent structural unlikeness, or that a common environment, imposing common conditions, has, acting through long ages, shaped to common form and function those parts with which it came most directly in contact ? When vre note the numerous details of similarity that are coupled with convincing evidence of diverse origin, we incline to doubt that these likenesses can be wholly due to internal spontaneous developmental tendencies, just as we doubt the originality of two essays that show many points of correspondence. Surely internal forces would modify internal form, as well as external. The impress of environment appears in this that it is the outside of organ- isms that show all the special fitnesses to the environing conditions. As a distinguished American zoologist has graphically stated it, "The inside of an animal shows what it is; the outside shows where it has been." Environmental influence comes out most conspicuously where different environm.ents impose very different condi- tions; as, for example, when part of a group passes over from terrestrial to aerial or to aquatic life. Some such cases will be taken up for special study in Chapter VI. 2 88 GENERAL BIOLOGY Nature has set bounds to which all the living must con- form themselves. This is seen not alone in externals of form, but also in the very fundamentals of organization. Even the types of animal symmetry correspond to evironment. Of the three main types, spherical symmetry, like that of volvox (symmetry about a point) prevails where uniform conditions exist on all sides of an organism ; radial symme- try like that of hydra and most plants (symmetry about a line) prevails when conditions are alike at the sides of the axis of the body but differ at the two ends; and bilateral symmetry like that of the higher animals (symmetry about a plane), prevails when conditions are alike upon two sides but differ- ent above and below as well as before and behind, as they must be in all organisms that travel over the surface of solids. On the other hand, there are phenomena of divergent development, of the persistence of types through the vicissitudes of all environmental changes, of grotesqueries of form, and superfluities of structure and ornamentation, that speak most strongly for the dominance of the inner forces of life, and that negative or minimize external influences. But it is not wise to exclude the possible action of either inward or outward forces in development when we know that both are ever present. The sightless condition of the fishes that live in the underground streams of caves in total darkness has often been treated as though it were a case of pure adaptation. But when we note that other fishes belonging to the same family (Amblyopsidse) have weak eyes and incline to stay in the deeper shadows of the shores, we see that a racial tendency toward this sort of develop= ment may have favored the adaptation. Nature may have segregated the fishes best suited to cave life in the environ- ment best suited to them, and then may have gone on per- fecting the adaptation, either directly, or by perfecting the tendency, or by both means concurrently. Inherent ten- dencies and environmental influences are ever present, and development can only be the resultant of their interaction. CHAPTER IV. INHERITANCE. Nothing is more familiar than the close adherence of offspring to the specific type of their ancestry. Although variations abound, they occur within very narrow limits. The egg of a frog can produce only a frog ; never a newt, or a salamander. A hen sitting on duck's eggs can never avail to hatch anything but ducklings out of them; for there is nothing else in them. Moreover, our confident expectation that offspring will resemble not only their race, but their individual ancestors as well is expressed by the proverb, *'Like father, like son." Heredity and variation are two aspects of evolution as viewed from the standpoint of the present, heredity looking toward the past, and variation toward the future. But whether a valuable variation counts for anything or not in . racial development depends, as we have seen, upon whether it is heritable or not. Hence we must ask, whether able to answer or not,, what is the nature of the bond between the generations ? Such facts as have been accumulated bearing on this question may be briefly considered under two heads: i) the visible mechanism, and 2) th^ observable results of heredity. I. THE VISIBLE MECHANISM OF HEREDITY. The process of reproduction is one of the chief distinguish- ing phenomena of living things. We have in the preceding pages considered numerous remarkable structures and developments connected with it. But to distinguish its essentials we must now retrace our steps and consider again the simpler organisms. The yolk accumulation, the em- bryonic membranes, the^milk glands, etc., which we have 290 GENERAL BIOLOGY been considering are mere accessories of birth and being. Even the primitive vertebrates lack them all; they have only eggs and sperms, and often merely scatter these free in the water, to develop Fig. 173. Diagram of the division •cs7ifhnii+ fnrtlnpr riQ of a paramoecium (after Jennings). wltnOUt lUrtner pa- O and 6 show loss of specific charac- i-o-n + ol nr\-rt + ar-+ r\^ i-n ' tere; c, d and e show division; /, g rental COntaCt Or m- and h show re-fonnation of one of flnonr-o a n r1 -nrVi o-n the daughter cells. Huence, ano wnen we reach the simplest organisms, in some of them we find not even sex cells but only protoplasm; yet there appears to be faith- ful reproduction of parental characters ; and again we are impressed with the fact that the primary functions of life are functions of protoplasm. In chapter II we traced the origin of separate germ cells. Let us now note certain fundamental likenesses and difEerences of development with them and without them. First, there is continuity of living substance in either case. A part of the old lives on in the new. The protoplasm out of which new organisms are formed is potentially immortal. Secondly, reproduction is, to a greater or less extent, a new production in either case. Even the two daughter cells of a protozoan are not merely halves / / \ of a divided mother cell; for the materials of that \J cell have been reformed with more or less of change. OThe unicellular organism undergoes regressive change before division takes place; the specific characters are lost, to be refashioned during the y\ adolescence of the new cell (fig. 173). The process [j has been aptly likened by analogy to the dissolving of a crystal in its mother liquor, to be subsequently recrystallized out of it. On the other hand there are very considerable differences, accompanying development by means of germ cells. These INHERITANCE 291 alone have descendents living on in successive generations. Being protected within the body of a multicellular organism and having no nutritive functions to perform, they are removed from direct contact with environment, and remain unspecialized with reference thereto. The germ cells are developed from the egg along with the body cells, but are set apart therefrom, sooner or later in the course of differentia- tion. Soon the body cells invest the germ cells with a cover- ing in which they are sheltered and nourished during all their subsequent development. This general relation between germ plasm and body plasm is diagrammatically set forth in figure 174. Fig. 174. Diagram of the relation between germ plasm and body plasm. 5, body plasm (soma), egg and sperm shown below, and zygote (circle inclosing dot) beyond ; 5, 5, s, the line of succession; t, the line of descent. It may well be, therefore, that parent and offspring resemble each other because both are developed from the same stock of germ plasm. Every organism begins life as a single cell. It behooves us, therefore, to look a little more closely into the structure of the cell. Since the fertilized egg may develop into the complete individual without further parental influence, that new individual must be potentially present, and also the mechanism whereby its parts are wrought out. To the egg cell let us go, therefore, to learn further of the nature of this mechanism. Our task will be easier if we examine the minute transparent, nearly yolkless eggs of such simple marine organisms as sea urchins and starfishes, which if placed alive in sea water under the microscope will go on developing, the divisions succeeding each other in quick 292 GENERAL BIOLOGY Fig. 175. Diagram of nuclear behavior in cell division (after Wilson), a, resting stage between divisions; b, beginning of division phenomena; c and d, formation of nuclear spindle and fragmentation and splitting of chromosomes; e to i, later stages: t, centrosomes; u, nucleolus; v, spireme; w. chromo- somes; ^, ic, asters;' y, chromosomes in equatorial plate ; z, chromosoiaes separating. INHERITANCE 293 succession before our eyes. Nothing could be more convinc- ing of the wonderful refinement of structure of the living substance, or of the precision of its processes, than to watch the behavior of the nucleus in a segmenting egg. And if we supplement what we can see in life by an examination of eggs that have been fixed at different stages, and stained by the precise differential methods of histology, we may discover the chief phenomena of nuclear behavior that regularly recur at every cell division. Figure 175 is a diagram of ordinary (indirect) cell division. In the resting stage preceding division (a) the nucleus, in- closed by a nuclear membrane is seen to contain an irregu- larly disposed darker substance (deeply stained in his- tological preparations) called chromatin. This is deposited in a network of excessively fine and almost invisible threads of a substance called linin. Besides these, the watery fluid in which these lie ("nuclear sap") there is also present, more or less constantly a rounded body, the nucleolus, different in character from chromatin, as shown by its staining reactions. Outside of the nucleus but lying close to it is a minute body, the centrosome. The first sign of division appears in the division of the centrosome;- the resulting daughter centrosomes move apart along the outside of the nuclear wall. The chromatin inside that wall begins to be gathered together in a long convoluted skein called the spireme (&). Before the centrosomes reach opposite sides of the nucleus, the nuclear wall begins to be dissolved. The linin threads take up a position stretched between the two centrosomes and so form the nuclear spindle. Corresponding linin threads in the cytoplasm batome radiately arranged about the centrosomes to form the two asters. The chromatin of the spireme becomes broken into segments, that are at first irregularly disposed on the linin threads, and that later are shifted to the middle 294 GENERAL BIOLOGY of the spindle, as soon as the spindle is fully formed. These are called chromosomes. This completes the first phase (prophase) of division. Then the chromosomes that have split lengthwise, each intotwo equal parts, move apart in halves alongthelines of the spindle in two equivalent groups. The centrosome also divides. This is the second phase {metaphase) and climax oi cell division. Now there is provided the nuclear material for two daughter cells. The two succeeding phases are the reverse of the first two phases. The chromosomes move in the next phase {anaphase) of division to the ends of the spindle, and form two compact groups,which tend to coalesce more or less into a spireme, and a nuclear wall begins to be devel- oped about them and the spindle begins to disappear. Finally, {telophase of division) the chromatin becomes scattered again upon the finer mesh work of the dispersed linin threads, the cell body divides, and the resting stage with which we began, is resumed. The outcome of these processes is that each daughter nucleus receives half of the nuclear material of the mother cell. However, unequally the cell body may be divided, this process guarantees an equitable distribution of the chromosomes in cell descent. This is the ordinary indirect process of nuclear division known as mitosis (or karyokinesis) . The figures successively Fig. 176. Cell division in growing tissue (sala- mander epidermis). A number of resting nuclei, and three in process of dividing, a, spireme; b, anaphase of division, and c, late anaphase. INHERITANCE 295 formed by chromosomes, aster and spindle, are known as mitotic figures. These regularly appear at every cell divi- sion, not only in the embryo, but in almost every growing part of the body, throughout the life of the organisms. They are freely exposed to view in living transparent eggs, but in any developing tissue properly sectioned and stained, nuclei maybe seen in some of the division phases above out- lined (fig. 176). These phases follow one another in an inviolable order; each stage conditions the one that is to follow it; and together they seem admirably fitted for the equivalent distribution of those parts of the nucleus which appear most constant. What role these parts may play in inheritance it is as yet impossible to say. They are all minute, and their study is attended with very great difficulty. The centrosome is usually at the limit of vision with the best microscopes, and, hardly anything is known of its structure. The chromoso- mes are the nuclear organs most readily followed, and as we have just seen, between spireme and spireme these are scat- tered in granules on an inconstant linin meshwork, to be reintegrated at each successive division. Only their con- stituent chromatin persists in our view, and this in particles of such minuteness as to be individually unrecognizable. Yet the chromosomes, as integrates of such particles, show such constant features that we are compelled to attribute considerable importance to them. They appear and reap- pear in like number and in similar form. The number dif- fers in different groups but it is constant and characteristic for all the individuals of any given species, in all the cells of the body. The number varies from 2 in a species of round worm (Ascaris) to 168 in the crustacean Artemia, ranging in most cases between 12 and 36. The number appears to be a family characteristic in the grasshoppers, it being 2? in the shorthorned grasshoppers and 33 in the meadow grasshoppers. 296 GENERAL BIOLOGY Chromosomes exhibit marked individuality of form, dif- fering in different organisms in length, breadth, curvature, etc., but in a given species, they are fairly constant in form. In certain genera it is claimed that two species may be dis- tinguished as well by the chromosomes of a single cell* as by the external characters of the adult animal. The history of the germ cells. — Since at the beginning of embryonic life, the egg is already a new organism, charged with the potentiality to develop all the characters of the adult, we must seek the source of these characters farther back. How does the egg come into being? It traces its lineage from an antecedent egg, as we have already seen (fig. 174). That antecedent egg gives rise to both body-plasm and germ-plasm, but the latter is very early set apart from, although surrounded by, the former; walls are built up about the germ plasm (spermary or ovary walls) , by which it is protected and through which it is nourished. Thus, the germ plasm is removed from direct contact with environ- ment, and also from direct relations with the functional cells of the body, and in this isolation it develops. The primordial germ cells, thus segregated, pass through a period of rapid divisions which succeed each other in quick succession without much intervening growth, and the result of which is great increase in numbers and great reduction in size. The small germ cells thus produced are called sperma- togones or oogones, according as they develop in spermary or ovary. Then follows a growth period, without division, in which the normal size is regained, and much new cyto- plasm is formed. The differentiation of the cytoplasm into the different materials that will subsequently be devoted to the production of different parts of the embryo occurs dur^ ♦They differ ambng themselves in size and form in the single nucleus; whereforej it is ordinarily the chromosome complex that offers recognition characters, rather than single chromosonies. INHERITANCE 297 ■sperrn\ ^ egg \ / z u nil Pis. 177. Diagram of the derivation of the sex cells (after Boveri). s, the fertilized egg (zygote) ; sopi, the body plasm (soma); /, the de- velopmental period during which the germ plasm and the body plasm are indistinguishable; sp, sperm- ary;^ ov, ovary; p, primordial germ cells; M, the period of 'rapid in- crease in number and diminution in size '(the number of divisions is much greater than shown) ; v, the period of increase in size with dif- ferentiation of cytoplasm; w, the two maturation divisions; pb, polar bodies; e, egg. ing this period. Then foUo-ws a period of maturation, or ripening of the sex cells, -^hich involves t'wo successive divi- sions only, and during -which the germ cells are kno'wn as spermatocytes or oocytes. The four cells resulting from these t'wo divisions become the sex cells, eggs or sperms, but there is one marked differ- ence, indicated in the accom- panying diagram (fig. 177). In the case of spermatocjrtes, the divisions are equal, and four sperms result ; but in the case of the oocytes, the divi- sions ■while equal -with respect to nuclear parts, are very un- equal -with respect to cyto- plasm, one cell retaining nearly all of it, the others being cast out from it as the so-called polar bodies; therefore, but one functional and perfect egg results. Such are the form changes undergone by the germ cells during their development, among the higher animals in ■which they have been most carefully studied. They are not to be considered as occur- ring at one- time only and in a 29^ GENERAL BIOLOGY direct succession, for many of the primary germ cells remain undeveloped through the life of the individual organism, and in most organisms they develop in cycles, corresponding to ANTONY VAN LEEUWENHOEK (1632-1723) Pioneer microscopist and naturalist; maker of his own lenses; discoverer of capillary circulation, of sperm cells, etc. breeding periods. DiAdsion, growth and maturation may often be found side by side in a single reproductive organ. But these external phenomena are mere curiosities of cell behavior, tmtil we inquire what is going on inside the cells. INHERITANCE 299 Fig. 178. Diagram of the sepa- rate maintenance of paternal and maternal chromosomes as seen in certain hybrids. sp, sperm; o, egg; o, the form of the chromosomes of the sperm; b, the form of the chromosomes of the egg; w, fertilization about to _ take place; x, the nucleus in its succeeding resting stage; y, the reappearance and group- ing of the two sorts of chroni- '\)mes at a subsequent divi- sion; z, division of the cyto- plasm. The condition in each nucleus is diagrammatically indicated by the circles below. Fertilization and maturation. — The existence of sperm cells has been known ever since the great pioneer Dutch naturalist Leeuwen- hoek and his pupils with home made lenses found them in the seminal fluid of animals, but they were long regarded as" wild animal- cules." In 1875, Oscar Hertwig established the fact that fertiliza- tion consists in the union of one egg and one sperm only, showing that in sexual reproduction each parent contributes one cell of its own body to the formation of the young. Then it became evident that the sexes play an equal, al- though not necessarily an identical role, in hereditary transmission. This conclusion was strongly re- enforced by the important dis- covery of Van Beneden (1883), that germ cells contain but half the number of chromosomes that is normal to the body cells of their own species. It became evident, therefore, that reduction and fer- tilization are complemental pro- cesses, the one leaving each sex cell with but a half stock of chro- mosomes, ^^fi other -restoring to the fertilized egg ceil the normal number. At the same time, the sperm introduces new elements into the lineage of the egg cell; 300 GENERAL BIOLOGY the new organism must differ in composition from the old. Every organism, therefore, that is developed from a fer- tilized egg sets out in life with a material endowment that is derived from two antecedent cells. In its nuclear equip- ment there are two more or less unlike sets of chromosomes. It is probable that, by the precise mitotic method, the sub- stance of both paternal and maternal chromosomes (fig. 178) is equally divided and distributed at every cell division. We can see that this is so, when paternal and maternal chromosomes are visibly different in form, as is notably the case in certain species that may be hybridized; for in the hybrid embryos two sorts of chromosomes reappear, con- stant in number and form and grouped by themselves, in successive cell divisions. Chromosomes. — Protoplasm, the physical basis of life, is of course, the material basis of heredity. Among proto- plasmic structures, those of the nucleus maintain the great- est permanence and uniformity of behavior. The chromo- somes especially give evidence of continuing individuality of organization. What the chromosomes are we do not know. That they are chemical substances is indicated by their micro-chemical reactions; it is by means of their reactions to specific stains that we are able to bring them clearly into view. Their vital organization is complex. That they play an important role in cell division is sufficiently obvious; mitosis might well have for its object the equitable division of them among the descendent cells. That their role in sexual reproduction is likewise important is indicated by their uniform and parallel behavior in egg and sperm while C3rtoplasmic parts are undergoing the greatest differentia- tion. Naturally, the greatest speculative interest has centered about the chromosomes. They have been assumed to be the bearera of hereditary characters, and the agents of INHERITANCE 301 transmission. Imagination ~^has proceeded beyond the limits of vision, and has pictured them composed of "bio- phores," "ids" "determinants," and other hypothetical structures, capable of handing down unit characters in inheritance. The existence of these, or of any other such mechanism, is not at present capable of either proof or dis- proof; and need not detain us here. But we may note in passing that some progress has been made in relating charac- ters of the adult organism to characters of the chromosomes of the germ cells. An excellent example is furnished by the so-called "accessory" or sex-accompanying chromosome of certain Hemiptera and other arthropods. In the squash bug, for instance, in the body cells of the female there are 22 chromosomes; in the male, but 21. In the cells of this sex one chromosome exists unpaired, all the others are joined in pairs. In the maturation of' the sperm mother cells, the division that occurs without the previous splitting of chromosomes, leaves an odd chromosome in half the cells. The resultant sperm cells, odd and even in their chromosome complement, unite with the full-equipped egg cells, as indicated in the accompanying diagram, to produce new male or female organisms, according to the chromosome distribution. This accessory chromosome (fig. 179), which the female zygote only receives, is sometimes, as in the plant bug (LygcBUs) accompanied by a small mate, {y, of the figure) , which in fertilization only the male offspring receive. This is a further evidence of the connection between the accessory chromosome and the sex of the adult. ■Chromosome reduction occurs, apparently, in all the higher organisms, both plants and animals, but the attend- ant circumstances appear not always to be the same. It is everyTvhere a preliminary to fertilization. In the higher plants it occurs at the time of spore formation; and the spores and all the cells of the gametophyte phase, as well as 3° 2 GENERAL BIOLOGY eggs and sperms, contain half the number of chromosomes that are found in the cells of the sporophyte. Thus, reduc- tion and fertilization are widely separated in point of time. Two divisions of the spore mother cell with only one splitting of the chromosomes, result in these cells issuing with the half Maturing divisions of the sperm cells Sperms Eggs Actual number of somatic chromo- Zygotes somes Afiasa Fig. 179. Diagram illustrating the behavior of the "accessory," sex-accompanying chromosome in fertilization (after Wilson), For the sake of clearness, but four other chromosomes are shown, and these four diagrammatically ; accessory (;c) , solid black. number in each, just as in the &gg and sperm mother cells of the higher animals. If x represent the number of chromo- somes in the sex cells, these relations may be expressed by the formula: The higher fsperm fxnzygote (zx) The new individual (2x) fsperm fx) animals \egg (x)/ '" ^ ' ^ ' \&zi W The higher fsperrn (x) "I Zygote, Sporophyte (zx) Spores (x) Gameto- f sperm (xj plants legg {t.) S -^ i ^ i v \ / phyte{x)Xegg (x) Differentiation of the cytoplasm of the egg. — There is also definiteness of organization in the cytoplasm of the egg. Conklin has shown that in the egg of the ascidian Cynthia there are three kinds of protoplasm that are quite different INHERITANCE 3°3 Fig. 180. Cynthia showing difEerentia- tion of organ forming sub- stances in the cytoplasm, lateral views (after ConklinJ . a. unsegmented, but after fertilization; b, in the eight- cell stage. in their appearance and also in their destination in the tis- sues, i) There is a clear protoplasm that will develop into the ectoderm ; 2) there is a gray, yolk-filled protoplasm that will develop into endoderm, and 3) there is a yellow proto- plasm that will develop into mesoderm. In figure 180 these are imperfectly delimited, the yellow protoplasm being diagrammatically indicated by the heavier stippling, and the gray by the Eggs of the_asoidian intermediate stip- pling; a repre- sents the egg half an hour after the entrance of the sperm cell, but before the first division; the yellow protoplasm has taken up its position in a crescent across one side of the egg, half of it being shown in the figure. The first cleavage plane (median plane of the body to be formed later) will be in the plane of the paper, dividing these sub- stances symmetrically; 6 is a corres- ponding view in the 8-cell stage, with the polar bodies still persisting at the upper pole, and the yellow protoplasm occupying a part only of two cells of the lower hemisphere, while most of the gray protoplasm has withdrawn into the other two. The yellow protoplasm will all develop later into the early muscle seg- ments lying alongside the notqcord. Here we have definite, predetermined materials for the making of the embryo, which, though differing in kind do not correspond with cell boundaries and which are therefore, clearly unrelated as yet to chromosome behavior. Synapsis. — There is another process that is believed to intervene in the case of many of the higher animals and 304 GENERAL BIOLOGY plants at least. At the beginning of the growth period that precedes the two maturation divisions there occurs a fusion of the chromosomes within the nucleus in pairs, apparently with like paternal and gjatemal elements in each pair (these elements having, since the preceding fertilization, main- tained themselves apart, although in one nucleus). This fusion is called synapsis. It brings into more intimate asso- ciation the equivalent paternal and maternal units, appar- ently commingling their substance, and possibly merging the influences they have borne separately since the preced- ing fertilization brought them together. Parthenogenesis. — New individuals develop from eggs, and not from sperms alone, but either eggs or sperms alone seem to have the necessary nuclear equipment for the com- plete development of new individuals; the eggs alone, have the cytoplasmic equipment necessary. In many large and widely separated groups of animals, (aphids and other in- sects, daphnids and other crustaceans, rotifers, etc.), there occurs habitually the development of eggs without fertil- ization. This is called parthenogenesis (parthenos, virgin and genesis) . In the honey bee, all the drones are believed to be developed from unfertilized eggs. This phenomenon is usually an accompaniment of peculiar conditions of life, and it alternates at longer or shorter intervals with true sexual reproduction. In the vast majority of organisms, however, the addition of the sperm to the egg is a necessary stimulus that must be supplied before development proceeds. But the eggs of a number of animals that ordinarily require fertilization can be artificially stimulated to develop by temporary immer- sion in proper alkaline solutions. The sperm nucleus also totipotent. — If an egg be deprived of its nucleus, the cytoplasm dies; it has no power to develop alone. But enucleated eggs have been fertilized and caused INHERITANCE 305 to develop by the experimental addition of sperm cells (fig. 207); the sperm cell enters as it would to fertilize the egg nucleus, but instead, takes the place of that nucleus, and then development proceeds. Evidently the necessary nuclear equipment for development is present in the sperm as well as in the egg, and is duplicated in the zygote in cross fertilization. The chief facts now before us, regarding the material basis for inheritance are : 1 . The continuity of the germ plasm through the genera- tions. 2. In cell division, mitosis, a process apparently well fitted for carrying development forward along the even tenor of its way. 3. In sexual reproduction, fertilization, a process ap- parently well fitted for introducing new elements into cell lineage. 4. Preliminary to fertilization, synapsis and chromosome reduction. 5. The development of eggs without fertilization in cases of parthenogenesis. 6. The duplication of the chromosome content of the nucleus in fertilization. Study 37. Observations on cell division, and on the matur- ation of sex cells. 1 Materials needed: Prepared slides showing clearly the chief phenomena of cell division, either in growing tissues or in developing egg cells. Freshly laid and living eggs of pond snails showing polar bodies. The student, duly cautioned as to the damage wrought in tissues in section making, and expedited somewhat in his observations by the guidance of the teacher (a demonstra- tion with projection microscope will for these purposes be 30 6 GENERAL BIOLOGY most serviceable) should study the sections, identifying successive division phases, and should sketch at least the spireme, splitting chromosomes and a complete spindle with the chromosome complex upon it. Newly laid eggs of pond snails will show the formation of the polar bodies (in external aspect; not chromosome con- tent) , and as these persist to the 8-cell stage or later they may be found and sketched in outline in relative proportion to the egg to which they are attached. The record of this study may consist of notes and sketches of the principal things observed. Study j8. Observations on parthenogenesis* Materials needed: Growing plants of cabbage, turnips or lettuce (or any other green house plant that may be more convenient), infested with viviparous parthenogenetir female aphids; also, small individual plants ! growing in thumb pots, and covers for them (see appendix) . The student should isolate a newly born, (or, at least, a very young) aphid, transferring it (to avoid injury) on the point of small camel's hair brush to an uninfested lettuce plant in thumb pot, and covering it as in a cage. He should keep the plant growing and watch the reproduction of the aphid from time to time, recording progress at each observa- tion. Continue through the lifetime of a single individual at least, so that data may be available for calculating possible progeny and rate of increase for a season. Note recurrence ■of birth of young, and total absence of males during the experiment. Add to your record of this experiment at each time of observation ; trust nothing to memory. After the *This is a running experiment, covering a number of weeks at least, but it will require only a few moments observation each week after it is started. » INHERITANCE -. 307 isolated aphid begins to bear, remove the young as fast as found to the leaves of the second enclosed plant, leaving the original aphid alone, for certain determination of the num- ber of her progeny. Observe in the other cage the time of beginning of reproduction on the part of the descendent aphids. The record of this study may consist of diagrams illus- trating the method used, and a statement of observations ma/de-.' y II. THE OBSERVABLE RESULTS OF INHERITANCE. As bearing on the points just cited, we may note that many facts indicate the uniformity of development, when cell increase proceeds by regular mitotic division, and, on the other hand, that marked changes result from cross fer- tilization. This is, perhaps, most familiar to the horticul- "turist, who maintains his choice varieties of fruits by rigid adherence to asexual methods of propagating them, (cut- tings, layers, stolons, buds, etc.), well knowing that cross fertilization would introduce new characters to modify (and from his point of view, to deteriorate) his commercially valuable strains. The breeder of domesticated animals has not this advantage. He can increase his flocks only by bisexual reproduction. Hence, he must isolate his pure bred individuals in order to maintain the purity of his stock. This is the key to the efficiency of isolation, as well in nature, as in plant or animal industry. Pure breeds and hybrids. — In nature, the individuals of a species usually present great uniformity of appearance. They "breed true." But in some wild species there are diverse forms more or less constantly appearing. Some- times group-differences are correlated with habitat, as in the case of the spermophiles (ground gquirrels) of our Pacific slope, where nearly every valley has its own peculiar variety 3o8 GENERAL BIOLOGY Sometimes they are found side by side, as are the long winged and short winged crickets of the Eastern States, or, they may be related to season, as are three forms oftheAjax butterfly. Species that have been long domesticated always show greater diversity, due to man's influence in selecting and isolating the most divergent types — especially such types as natural selection would ruthlessly eliminate. This is the only way of obtaining new forms. We cannot compel nature to produce anything; we can only wait upon her, and preserve our choice of what she offers, from the swamping effects of intercrossing and from the rigors of a harsh environ- ment. When the breeder of plants or animals wishes to obtain a new strain, he breeds together forms that differ with respect to the characters of which he desires to secure a modification. This is hybridization. If all the offspring of any given variety that is bred inter se, are like in any given character, they may for all practical purposes be considered in respect to that character pure bred. Types of inheritance. — ^What the result will be when any two varieties are crossed, what characters the offspring will bear, can only be determined by trial; it will always be the same between the same two pure varieties. Observations on matters of this sort fall outside the possible scope of the practical studies of this course ; we will content ourselves, therefore, by noting in passing a few of the more general phenomena of hybridization. As to heritability of results, the offspring may be sterile, and therefore, self-annihilating. The best known example is the mule, a cross between the horse and the ass. There is no race of mules; other mules are to be obtained only by repeating the crossing of the parent species. Or, the hy- brids may be relatively stable, and breed true, as single new- formed race from the beginning. The garden sunberry, a cross between two wild inedible species of Solanum, is said INHERITANCE 309 to be an example, and there are many others among culti- vated plants. Or, they may be unstable, as in the great majority of cases, some of which will be illustrated further on. These when bred inter se give offspring of different sorts. GREGOR MENDEL (1822-1884) Pioneer student of hybridization: discoverer of the "law of dominance." The characters that the hybrids bear may be only the characters of their parents, or they may be new characters of their own, the following types of the latter being com- monly recognized : 310 GENERAL BIOLOGY i) Blended inheritance. The offspring may possess characters intermediate between those of the parents. If one parent be short and the other tall the offspring may all be of intermediate height. 2) Intensified inheritance. The offspring may be more extreme than either parent. If one parent be dark and the other light, the offspring may be darker than the dark parent. 3) Heterogeneous inheritance. The offspring may ex- hibit characters differing in kind from those of either parent. Certain races of white and buff pigeons when bred together give slate colored offspring. Possibly, more knowledge of the characters involved in such cases may show them less lawless than has been thought. Alternative inheritance. When parental characters are preserved in the hybrids, unfixed and unaltered, we have alternative inheritance — ^the type that has hitherto received most attention, and which in its behavior seems to offer the closest parallel to the behavior of the chromosomes. The offspring of the first generation exhibit the characters of one parent or of the other ; but when these hybrids are bred together, in their offspring the characters of both parents reappear. In the first generation hybrids one character appears (is dominant) , and the other disappears (is latent or recessive) . Both characters are present, but both cannot appear at the same time ; a flower cannot at the same time be fragrant and scentless. Both have been inherited, how- ever, along with or by means of the duplicate apparatus for conditioning egg development, and in succeeding genera- tions the parental characters will reappear. This type of behavior among hybrids was first studied carefully by Gregor Mendel, and is often called Mendelian inheritance. INHERITANCE 311 Mendel's great service lay in a long series of carefuJ hybridizing experiments, from which the principle was deduced that whatever the appearance of the hybrid, it pro- duces germ cells like those of its parents in approximately equal numbers, and the character of its own offspring will be determined by the way in which these germ cells are paired in fertilization. Suppose, for illustration, that D and R of the accompanying diagram (fig. 181) represent the two parents, which differ in one character only, that of color. D is black and R is white. Suppose also black to be the dominant and white the recessive color. Then the offspring in the first generation (F^) will all be black. But if they be bred together, their offspring will be both black and white ig the proportion of three black to one white. Then, if the whites be again bred together, all ©^— ^ their offspring will be white. The vSmv ^hite character which disappeared in the first hybrids, was obviously still present, and has been sorted out again. This relation between characters in the germ cells has been aptly compared to the putting together in pairs of pieces of glass of two kinds, one transparent, the ^'ilarinhe°S^°'^,'"tt Other opaque ; when placed «c"?s^"veVfa"^d FTtife fiSt together Only the opaque one is visi- brille^e^ctivSr*'™ ^^' ble,but when again separated, both are again apparent. If, as Mendel supposed, the germ cells possessing the two characters separately, are present in equal numbers in the reproductive organs of the hybrids and are combined in pairs according to the law of chance, there are but four possible combinations of them, giving three classes, as indicated on succeeding page : 312 GENERAL BIOLOGY DandR, germ cells of one hybrid parent I y^ I in chance combination with D and R , germ cells of the other hybrid parent give in the second (F^) generation, DD + 2DR + RR in the proportion, (3 black: i white) n. Otherwise stated , if any two of these hybrids be bred together there result the following possible combinations of their characters in their offspring (the characters D and R of one parent being underscored in order to distinguish, sources) : ■black D and D, uniting, give D, a pure dominant DandR, " " DR] R and D, R and R, " " R, a pure recessive — white. j^^jhybrids Of the black individuals it is obvious that there are two classes although all look alike; for one individual out of every three is, like the white, pure bred containing sex cells of one kind only, while the other two are still hybrid in character. The further results of intercrossing of like with like in successive generations is indicated by the following table : Parents. Offspring, {mated like with like.) Generation I D 1 Gen. II fiD.. (si y.-D(R)... l2D(R)^ Gen. Ill . ..D.... fiD. 2D (R) Gen. IV ....D ...D iD 2D(R) R [iR. iR. .R . iR .R R INHERITANCE 313 I Mendel's law assumes that the gametes bearing the characters of the two parents are produced in equal num- bers, and distributed at pairing in accordance with the law of chance; and this assumption is not contradicted by the known facts. And since it offers a simple mathematical basis for calculating the results of a variety of crosses, it may readily be tested, as, for example, by backcrossing a hybrid with an individual of either parent stock. If mated with the recessive stock, the result should be as follows (given, of course, as for any test, a sufficient number of offspring) : D and R, germ cells of the hybrid parent I ^ I in chanc3 combination with RandR, germ cells of the recessive parent, give' in the second (FJ generation, 2D (R)* + 2RR i. e., 50% of each color. In the more typical cases of alternative inheritance, all the foregoing proportions have been substantially realized in breeding experiments. When the parents differ in two or more characters, the hybrids bearing germ cells that bear all these characters severally, will effect new combinations of them, and forms differing from either parent will appear in the second and later generations. If we let X and Y represent the dominant and X and y the recessive phase of two characters (as, for example, eye color and hair length) there will appear in the second generation, besides the unstable hybrid forms, the stable forms XXYY , XXyy, xxYY, and xxyy. Which- ever two of these represent the combination of characters found in the parents, the other two are new combinations. The law of Mendelian inheritance, substantially as estab- *The parenthesis is thus used as a convention for indicating the recessive character. 314 GENERAL BIOLOGY lished by its founder, is represented in the accompanying table (after Castle). Number of Differences between Patents. Visibly Differ- ent Classes, each contain- ing one Pure Individual. Total Classes, Pure and Hybrid. Smallest Number of Offspring allowing one Individual to each Class. „ 2« 3« 4» ) Tested by Men- y del for peas and J found correct. I 2 3 2 4 8 3 9 27 4 16 64 4 S 6 i6 32 64 81 243 729 256 1024 4096 > Calculated. Castle has proved by breeding experiments that in guinea- pigs length, pigmentation and roughness of coat are hair characters that are separately heritable, and that in crossing they follow fairly Mendel's law. And he summarizes his observations as follows: "This experiment illustrates two important principles in heredity: First, if as regards the hair alone there exists such a variety of characters separately inheritable, how great must be the number of such characters in the body as ^ a whole, and how remote the probability that any animal will in all characters resemble any individual ancestor, pro- vided that in a considerable number of heritable characters a choice is offered between alternative conditions. Secondly, the experiment shows how a variety of new organic forms may quickly be produced by cross-breeding, leading to the combination in one race of characters previously found separately in different races. Thus, in guinea-pigs, one can obtain within two generations any desired combination of the three pairs of alternative coat-characters, if one pro- duces a sufficiently large number of individuals. INHERITANCE 315 "From what has thus far been said it would appear that in alternative inheritance characters behave as units, and, more than that, as wholly independent units, so that to fore- cast the outcome of matings is merely a matter of mathe- matics. While this is in a measure true, it is, fortunately or unfortunately, not the whole truth. In alternative inheri- tance characters do behave as units independent of one another, but the union of dominant character with recessive in a cross-bred animal is not so simple a process as putting together two pieces of glass, nor is their segregation at the formation of gametes so complete in many cases as the separation of the two glass plates. The union of maternal and paternal substance in the germ-cells of the cross-bred animal is evidently a fairly intimate one, and the segregation which they undergo when the sexual elements are formed is more like cutting apart two kinds of differently colored wax fused in adjacent layers of a common lump. Work carefully as we will, traces of one layer are almost certain to be in- cluded in the other, so that while the two strata retain their identity, each is slightly modified by their previous union in a common lump. ' 'Thus , when we cross short-haired with long-haired guinea- pigs, we get among the second-generation offspring a certain number of long-haired animals with hair less long than that of the long-haired grand-parent, or with long hair on part of the body only. "Cross-breeding, accordingly, is a two-edged sword which must be handled carefully. It can be used by the breeder to combine in one race characters found separately in different races, but care must be exercised if it is desired to keep those characters unmodified. If modification of characters is desired at the same time as new combinations, then cross- breeding becomes doubly advantageous, for it is a means of inducing variability in characters, as, for example, in the 31 6 GENERAL BIOLOGY intensity of pigmentation and in the length of hair, quite apart from the formation of new groupings of characters. Sometimes it causes a complex character to break up into simpler units, as the agouti coat of the wild guinea-pig into segregated black and yellow, or total pigmentation into a definite series of pigmented spots. In other cases it operates by bringing into activity characters which have previously been latent in one or other of the parental forms. "Now, what bearing, we may ask, have these theoretical matters on the practical work of the breeder? They show i) that a race of animals is for practical purposes a group of characters separately heritable, and 2) that the breeder who desires in any way to modify a character found in this group, or to add a new character to the group, should first consider carefully how the character in question is inherited. "If the character is alternative in heredity to some other character, cross-breeding between the two, followed by selection for pure individuals, will within two generations give the desired combination of characters in individuals which will breed true. This process of selection is simplest when the characters to be combined are recessive in nature, but individual breeding-tests become necessary when domi- nant characters are included in the combination desired. "If a character gives blending inheritance, it must be treated in a different way. Suppose, for example, that we desire to combine lop-ears in rabbits with albinism, but that our lop-eared stock consists wholly of pigmented animals. How shall we proceed? First, mate a pigmented-lop with a short-eared albino. The offspring will be pigmented half- lops. If two of these be bred together their young will all be half-lops, and about one in four of them will be albinos. Now these albino half-lops may be mated with pure pig- mented lops. The young will again all be pigmented, but will this time be three-quarter lops, and by breeding these INHERITANCE 317 togebher albino three-quarter lops may be obtained in the next generation. By continuing this process of back-cross- ing with the lop-eared stock, and selecting the albino off- spring obtained, the lop-eared character may be steadily improved in the albinos until it is practically as good as in the original lop-eared stock. The rate of improvement pos- sible can be readily calculated. The albino young will be: After 2 generations, one half lops. After 4 generations, three fourths lops. After 6 generations, seven eighths lops. After 8 generations, fifteen sixteenths lops. After 10 generations, thirty-one thirty-seconds lops, etc. This will be the result on the hypothesis that no secondary variation occurs in the lop-eared character. If, however, variation is induced by the cross-breeding, then it is possible that the desired end may be reached sooner, or that an even better lop may be obtained in the albino cross-breds, than that of the original pigmented stock. "Latent characters are an important elem.ent in practical breeding. Sometimes they greatly aid the breeder's work; sometimes they impede it. If a stock contains undesirable latent characters which are brought into activity by cross- breeding, these latent characters will have to be eliminated, or a new stock tried." Obviously, without variation no new characters are obtained by such intercrossing, but merely new combina- tions of characters that previously existed apart. But when new characters appear among the variants of a species, and especially when a number of new characters appear simultaneously as in typical cases of mutation, then inter- crossing may be the means of bringing these characters together in all sorts of combinations, some of which may be of value to man, and some of which may be -fit, and may, therefore, furnish a basis for further natural evolution. 3i8 GENERAL BIOLOGY III. NATURE AND NURTURE. The germ cells constitute the bond between the genera- tions. To the egg and the sperm we must look for sources of hereditary characters. The human species inherits as do the other organisms. Characters of various sorts "run in families;" form charac- ters, such as shape of nose, of chin, of fingers; physiological characters, such as left- handedness, baldness (in males), slenderness or corpulence, etc.; psychological characters, such as emotional or judicial bype of mind, phlegmatic or effervescent temperament, etc. But most of these are examples of complexes of characters, that must be analyzed to their component units before their manner of inheritance can be studied. To speak of infectious diseases as being hereditary is wholly inaccurate, for disease germs are not part of the body, but foreign organisms ; they can be passed on from one generation to another only by infection, and not ,by inheritance. There' may, however, exist innate physio- logical weakness that favors the infection in successive gener- ations, and infection may occur before birth as well as after. However much the young may receive of fostering parental aid in yolk, in shelter within or without the body, in nourishment by means of embryonic membranes, etc., it has already received when egg and sperm have united, its full hereditary endowment; all else is nurture. Inheritance of acquired characters. In the lifetime of the individual, the body may acquire various characters. The skin may get a coat of tan in a few days exposure to the sun. The hands become calloused with toil. The muscles strengthen with use. Dexterity results from practice, and by long effort we may acquire an education. But are any of these things which the individual may acquire during his lifetime heritable, or does the offspring start at the common level of its kind, nothing advantaged by whatever his INHERITANCE 319 individual parents may have gained ? This is an exceed- ingly important question, the answer to which must have something to do with determining our educational policy. In the long run all characters are acquired characters, if evolution be conceded. The question is, Can the peculiar conditions which cause new characters to develop in the body so affect its germ cells that these will develop the same characters in the next generation, even in absence of the con- ditions that first called them forth ? When we remember the early isolation of the germ cells, their lack of participa- tion in the work of the body, and their remoteness from con- tact with environment this seems unlikely. How, for example, could the abuse of the eyes, causing partial blind- ness in the adult, so affect the germ cells that have no eyes, as to cause them to develop in the next generation, with proper use, the same weakness? That new characters are , acquired by the individual body needs no proof; that they are at the same time acquired by its germ cells is not proved, although it has been widely believed. Mutilations of the body we know are not inherited. / The loss of an eye in one generation does not prevent its perfect development in the next.* The tails of sheep have been docked for centuries, and yet lambs continue to develop tails in apparently undiminished luxuriance. On the other hand, there are facts showing that the germ cells (or, at least, the sex organs, collectively) do affect the characters of the body of which they are an isolated part. The effects of castration (removal of the spermaries) of young animals are often very marked. The differences between a bull and a steer, for example, are very apparent in the horns and neck muscles, in voice and attitudes, in disposition, in ability to put on fat quickly, and in other *"Wooden legs do not run in families, but wooden heads do." CONKLIN. 320 GENERAL BIOLOGY characters that are equally remote from the missing sex organs of the latter. Doubtless the condition of the body does affect the germ cells also (whether well- or ill-nour- ished ; healthy or not) , but in what manner and to what extent is not readily determinable at the present day. The physical basis of racial solidarity. — ^This we know; that with all the changes of its outward conditions, human nature changes little. Civilization advances, but civiliza- tion concerns itself with methods of nurture alone ; and its gains, every individual must re-appropriate for himself. Nurture creates many artificial distinctions among men, but their nature is little altered. Good health and good spirits and normal desires for life, liberty and the pursuit of happi- ness are not the possession of any class or condition of men. Good brains are probably as equitably distributed as are good muscles, although the opportunity for their develop- ment may not be. Dymasties may rise and fall, systems come and go, but the racial currents run on serenely. Art and science are not transmitted in the germ ; the only part of our education that is inherited is the organic part of it that is common to the race. Fortunately or unfortunately, the springs of racial progress lie very deep ; and if they are not readily reached by humanitarian effort, they are at least remote from unskillful meddling. It is the common stock of germ plasm of our race that breeds our common interests and common needs, and makes it possible for us to have common schools and common law. This is the great guarantee of democracy. Racial differentiation. — Nevertheless, our common stock of germ plasm is not quite homogeneous. It has had a long history. It has developed divergent tendencies. It has lived under different environments. The spirit of one people is not that of another. That the slow methods of nature INHERITANCE 321 have wrought changes in the constitution of her segregated strains appears in this; their civihzations differ, and though civilization be nurture, the capacity for it, the impulse toward it and the genius to modify it must be inherent. And, if this is true of tribes, it is true within each tribe, on a lesser scale. "Blood does tell." To some extent at least genius does run in families,* as also do criminal tendencies, the capacity for the development of either being organic. Hitherto human society has taken little account of these springs of future character. The meaning of nurture. — Most organisms give little nur- ture to their young. They merely breed. Their innumer- able progeny are scattered broadcast in a pitiless environ- ment, and here and there, by chance, one survives. De- struction is the rule ; survival the rare exception. We have already seen in our study of the plant and animal series how the dominant organisms have made their advantages secure by better care for their offspring during development; by adding food to the egg or supplying it to the embryo, and then by adding housing and parental care. Ever, there is a lessening of the number of young produced coupled with increase in the care for them. The powers of the body are devoted less and less to starting new individuals in life and more and more to the better equipment for life of those that are produced. The lioness of the fable might well boast that though. her offspring were but one at a birth, that one was a lion; and then might well care for it as though there were no lions to spare. *I have often heard false pride of ancestry condemned, but I have not seen the true pride of ancestry explained and commended. Surely the man who is conscious that he comes of stock sound in body, able in mind, tested in achievement, and who knows that, mating with like stock and maintaining himself in health, he may hand down that heritage to his children, surely such a man may have a legitimate pride in ancestry." — K. Pearson 322 GENERAL BIOLOGY The eggs and sperms of some of the lower organisms may be mixed in a glass of sea water and watched at one sitting Fig. 1S2. Nest and eggs of the musk turtle {Eremochclys odoratits). This nest was made in an old muskrat house. Tne full complement of eggs is shown below. Photos by Hankinson and McDonald. through fertilization and the early stages of development, until they are ready to enter actively into the struggle for IXHERITANCE 323 life. A few weeks of lying in the sunwarmed marsh suffice for the hatching of the eggs of the musk turtle (fig. 182), which receive no parental care. Three weeks of persistent incubation are necessary to hatch the eggs of the common fowl, and a still longer period of maternal care after hatch- ing is needed to get the chicks well started in their careers. Fig. 183. Sandpiper (precocious) a few days old, swimming. Photo by G. C. Embody. The young of altricial birds (fig. 184) are fewer, hatch in a more helpless condition, require to have their food brought to them and put in their mouths, and receive the care of both parents for a long time. Months of pre-natal nur- ture are required for the development of the young of all the larger mammals, and after birth, other months of nursing, of 324 GENERAL BIOLOGY sheltering, of care and assistance. And to all this man adds education, which is only an extension of the original mother Fig. 184, Young marsh hawks (altricial) some days old, yet hardly able to stand. Photo by E. McDonald. function.* In the eye of the law it takes twenty-one years to make a man. Thus, human society has learned the first lesson of racial progress. *"And it was told him, Thy mother and thy brethren stand with- out, desiring to see thee. But he answered and said unto them, My mother and my brethren are these which hear the word of God, and do it." Luke 8:20.21 INHERITANCE 325 Study JQ. Observations on the relation between fecundity and nurture. Materials for this study are so diverse and ever present, that instead of a definite outline, the following suggestions of tj^ical illustrations are offered: I. In order that the enormous numbers of young pro- duced by some species may be realized, study some such thing as the number of spores produced by a flowering fern, or seeds by a cottonwood tree. Count for example in the fem the number of good spores in an average sporangium, the number of sporangia on a sorus, the number of sori on a fruiting frond, the number of fruiting fronds on an average plant, and multiply together for totals, multiplying in the end by the number of years of fruiting for the normal life of the plant ; the numbers will be sufficiently significant even though the last point be indeterminable. If done by a class, the averaging of the collective counts will give better approximation to the truth. , 2 . For observation of the reduction in numbers that goes with a little parental care, compare number of young pro- duced by some nesting fish, such as sunfish, bass or bull- head, with those produced by a pike or a carp ; for this, ripe ovaries may be taken and their content counted in part and estimated (see table on page 512). 3. The concomitants of more extended care and careful nurture may be studied by comparing the number of young and the care they receive in the precocious and altricial birds, abundant data for which will be found accumulated and ready to hand in many good bird books. The record of this study may consist of a tabular state- ment of the data obtained. The disturbance of the natural balance by conditions o^ civilized life. — The rate of reproduction established by 32 6 GENERAL BIOLOGY nature for the human species is far too high for civilized conditions. It was adequate to replace the losses by war, pestilence and famine in primitive society. But now that these agencies of death are in a measure controlled, the natural balance is disturbed. Without these checks the human population of the earth is rapidly increasing. Many wild species are being exterminated, and most of them are being reduced in numbers. For man must carry with him the few domesticated species on which his livelihood depends, and wherever he spreads, the native population of the earth must be annihilated to make room for his fields and stock pens. The pressure for room has often been felt in "congested districts" throughout human history. With the present excess of birth rate over death rate, the whole habitable earth will be one congested district soon. Every triumph of science over plague or famine or other casualty increases the pressure, so long as the excessive rate of in- crease continues. The ideal condition of society is that toward which nature points the way in the series of phenomena we have just been studying: the adjustment permitting the normal well-condi- tioned development of every individual. There are biological aspects of our civilization that are not reassuring: i)The possibilities of the germ are realized only in the in- dividual. Whatever the nature of it, only nurture can bring it to perfection ; and nurture is still largely wasted among us in broken lives. 2) The weaklings of our race under existing conditions, not only survive, but they usually survive to perpetuate their weaknesses in descendants. 3) There are processes of civilization that select the best for elimination; wars, which kill off the strong and the brave on the battlefield and leave the weak at home to breed. And economic conditions that take the brightest of INHERITANCE 327 the children of the poor from their studies and their play, and set them prematurely at grinding toil, thus hindering their normal development. 4) The excess of offspring is mainly coming from the lower ranks of human society. Those classes that are most advanced in arts and education are hardly reproducing themselves, while the earth is being over-populated by the descendants of the less progressive. That the educated classes are not taking a larger share in the building of the future race is not in itself a necessary evil, for education is not always an accompaniment of either physical or moral fitness. Indolence and self gratification and the cultivation of low desires breed degeneracy in rich and poor alike. That the population of the earth in the immediate future will be composed mainly of the sons and daughters of poor and ignorant parents is not so serious a matter as it might at first appear; for, with normal aspirations, property and edu- cation may be acquired, and the lack of these things may be due to accidents of birth and station. But the danger of qualitative degeneration lies in the rapid and as yet almost unrestricted breeding of the physically mentally and morally degenerate. The increase in population, which, if continued at the present rate would certainly bring disaster, is mainly due, strangely enough, to the progress of knowledge and to the extension of humanitarian effort. These have reduced the death rate for all classes of society, while diminishing the birth rate for only the better educated classes. It is thus the natural balance has been disturbed. In the cities the pressure for room is first manifest ; and it is here that the annihilation of the green earth and all the host of living things belonging with it is completest. Even here in times of peace and plenty all men may live in com- fort; but when the pinch of famine or disaster comes, then 328 GENERAL BIOLOGY men crowd, as do the beasts, for food and standing room. And whenever and wherever they crowd, in good times or bad, they make such use of the earth's resources as means irreparable loss, and insures that the crowding ofthefutture will be of intensified severity. The greatest problems of man's future upon the earth are connected with better breeding and better nurture. Security for the future undoubtedly lies in having more knowledge, and in making such use of it as will yield better results in racial improvement and in individual develop- ment. CHAPTER V. THE LIFE CYCLE. Among the simplest organisms, in which each cell may go on growing and dividing indefinitely, the familiar phenomena of youth, maturity, and old age, are not apparent. Every cell is a germ cell, and, therefore, ever young. But with sexual reproduction comes in the dual organism, composed of body plasm and germ plasm, only the latter continuing, the former mortal. The normal life cycle. — It is the common lot, among all organisms except the lowest, to be developed from an egg, to be supplied in infancy with food, to pass through hereditary changes of form, to grow and reach maturity to exercise for a longer or shorter period the matured powers of the body, to produce offspring, and then to grow old and die. Youth, maturity, and age follow each other in an' orderly progression that is readily definable in terms of metabolism, thus: 1. In youth the building up processes are in the ascenden- cy. Assimilation is greater than dissimilation (juvenescence) . 2. In maturity the waste and repair of the body are on a parity. Assimilation is equal to dissimilation. 3. In age the building up processes are in a state of relative decline. Assimilation is less than dissimilation (senescence) . Although we may thus state in metabolic terms the his- tory of the body, the explanation therefor must be stated in terms of reproduction. It is necessary for the organism to establish itself, before it can do much to provide for pos- terity; hence, the nutritive apparatus of the body is developed first, and growth precedes reproduction. The 33° GENERAL BIOLOGY period of adult life may be long, as in elephants, or short, as in mayflies; it may be reached by a gradual and regular development, or by a series of abrupt form changes; but when it arrives with full maturity of powers, the reproduc- tive process takes the ascendency. Primarily it is the period of making provision for descendants. Whether such provision consist merely in mating and depositing eggs, or whether in addition to this, the substance of the body be transformed into food for the young, or whether, still further, the physical powers of the body be devoted to the care of the young, or whether, finally, as in human society, with long years for individual activity, the labors of life be devoted to securing for posterity the betterment of those conditions that hinder its best development, it is all the same ; the primary concern of adult life is provision for the future of the race. The regularly progressive life cycle is sufficiently familiar and needs no further illustration. But this undergoes some remarkable changes under natural conditions, and other alterations of it may be caused artificially. Some of the more typical of these phenomena will now be con- sidered, under the following headings :■ i) Alt_ernatiqn_rf^ generaiions, 2) Specialjnethods of asexual reproduction, 3) Chajige^ of form with alternation of hosts, 4) Meta- morphosis, s) Artificial division and combination of or- ganisms. I. ALTERNATION OF GENERATIONS. This phenomenon consists, as we have already seen, in the establishment of two segregated phases within the life cycle, one sexually, and the other asexually reproducing. We have already traced the development of it in gameto- phyte and sporophyte of the higher green plants. An equally good example of it is found among animals in the group of marine hydroids. Medusa and hydranth are THE LIFE CYCLE 33-^ there the sexual and asexual phases of the life cycle respec- t i V e 1 y . The free-swimming medusa (jelly fish, fig. i8sc) produces the eggs and sperms and liberates them in the sea. These after fertilization develop into sessile hydranths (more or less similar to the common hy- dra) , which in turn develop me- dusas by various modes of asexual budding. II. SPECIAL METHODS OF ASEXUAL REPRODUCTION. Sexual reproduction results in the main in a qualitative increase in a species. Without segregation it tends toward reducing all the forms to a common level, but with segrega- tion, whether external or internal, it is a potent means of effecting species change. Asexual reproduc- tion is quantitative rather than quali- tative increase. One individual is Fig. 185. The Colonial hydroid {Bougainvilled^ a , the form of a small colony ; 6, a bit from the ' tip of one of the branches ; w, tentacles of a single hydranth ; x, y, z, stages in the develop- ment of the buds (meduSEe). c, The fully formed and free swimming medusa (jelly fish) k, the body (manubrium) with^he mouth at its tip;/, the surrounding bell; m, radial canal; «, sense organ; o, tentacle; b and c, after Allman. 332 GENERAL BIOLOGY multiplied ; the new ones growing up are separated parts of that one, and are therefore essentially like it. The parts, however, are not necessarily identical with the parent, or with each other; when separated they may develop slight differences, as in the well known phenomenon of bud variation, and such differences may be increased Fig. 186. Duckweed (Spirodela polyrhizd). The growth from a ^j- single plant in twelve days. Photo and culture by L. S. Hawkins. Note the grouping in pairs, with small lobes_ forming between the larger old ones of the dividing individuals. artificially by selection. One branch of a rose bush may develop finer flowers than any other on the bush. Cuttings of this branch may be selected for growing, and the best- flowered shoots developing from these cuttings may be again selected with some advantage. But there is no probability that these improvements would be inherited. THE LIFE CYCLE 333 "fhe special means by which individuals multiply them- selves asexually are far too numerous and diverse for us to attempt to consider them all here. Stools and stolons and runners and tubers and offsets and bulbs and a dozen kinds of detachable buds, are known to every student of plants. Indeed, many of those plants that have been able to advance into and conquer difficult environments and become dominant in them, (such as the pond-weeds on the bottom in shoal waters, and the grasses and sedges in the fire-swept prairies and marshes) increase mainly asexually, by extensions of the plant body. They still produce seeds, but they hold their ground by continuous and exclusive occupancy of it. Budding and fragmentation and other such methods are common also among the lower animals. This we have observed in the hydra. But all such increase has to do with growth as well as with reproduction. Let us here consider some more specialized reproductive parts and methods, that are more exclusively adapted to reproductive ends. Asexual reproductive cells. — When these are formed for dispersal, they are usually called spores. With ordinary spores,as they are commonly produced upon the aerial parts of plants, we have already become acquainted. These are minute resting cells, usually invested with a protective covering that resists evaporation, and that permits of their being distributed by currents of air. Among aquatic thallophytes, both algte and fungi, there occurs at intervals a breaking up of the cell contents into minute naked unicellular reproductive bodies. These are called zoospores or swarm spores. Each zoospore acquires two or four flagella (or sometimes a circlet of cilia), and, escaping out of the old cell wall, it swims about in the water. Finding a suitable situation it attaches itself and begins to develop a new plant body like that of its parent. 334 GENERAL. BIOLOGY The alga Draparnaldia, shown in figure 187, which grows attached to stones in the riffles of small clear-flow- ing permanent streams, and is easily seen trailing its long beautifully branching fila- ments in the current, is a favorable one in which to observe zoospore formation; for it will usually develop zoospores a day or two after being brought out of its native environment into the laboratory. The spores, escap- ing from the cells singly, will swim to the lighted side of the containing vessel, from the surface of which they may often be obtained in great numbers. It will be noticed that the figure of the zoospore of Drapar- naldia does not differ materially from that of certain of the gametes we had before us in Chapter II. It is highly probable that sex cells were developed out of zoospores. Both are present in certain algae, and are hardly to be dis- tinguished in form; and the differences between them almost vanish, when, as sometimes happens, the gametes develop without preliminary fusion in pairs. Multicellular reproductive bodies. — In certain of the lower fresh water animals, notably in sponges and bryo- zoans, there are special multicellular reproductive bodies called statoblasts (also known as winter buds, and gem- mules) . These are like spores only in function, and in hav- ing resistant walls which tide them over the dry, hot Fjg. 187. Draparnaldia, a, a bit of the stem, with three branches; 6, a bit of a branch that is yielding zoospores. THE LIFE CYCLE 335 season when the shoal waters in which they grow evaporate. While they are often spoken of as seed- like bodies, they are wholly unlike seeds in that they V ^ ^}^^^^.....l contain no embryo, and -m they are entirely different If '^"~- /ifM^ J^S -^ '^^ origin. During the '^^^^^ Mfe^ growing season, (spring and , -v^ss-»__^'//'////f early summer) , little groups y^^^^>^;!^^^g;^^^Vy of cells become segregated within the tissues of the parent animal (fig. i88fe), ^ (\ rf .i; and become invested there \ ^% wO^'^lf (f '^'i'Od. a common protective , \ wF^iji^ \ covering, the statoblast U. -^^ "^'ili' )/ j\ji 1^: wall, that is often of re- iui^^^^^' markably complicated and ^!e^^^ beautiful structure. When ^ the parent dies and its flesh S^ disintegrates, the stato- FiG. 188. PiumateUa a, a small colony blasts are liberated, to be growing on a submerged stick; b, a porriprl a^hniit -nri+l-i +Vio smaU part of a single branch, with CameQ aOOUt Wltn tne coSkSfinl'rLniTsft^SLItal^^^^ waters, or blown about iX1ok*\?rel?of tInttFisT/ m! ^^^h the dust of the dessi- esophagus; n, the chitinous sheath that r-ci+i^rf Vinf+nm mnrl mi- i-n shelterl an individual, (after AUman) CatCO DOttOm mUd, Or, m the case of statoblasts pro- vided with grappling hooks, such as those of Pectinatella, to be carried and distributed by aquatic animals. In the Spring, those favorably situated germinate and develop new colonies of the parent form. Statoblasts occur in groups most of whose members are marine. They are probably an adaptation of the life cycle to the conditions imposed by shoal and impermanent 33^ GENERAL BIOLOGY waters. Not all the sponges and bryozoans that live in fresh waters are known to produce statoblasts, but the more common shoal- water forms produce them in very great abun- dance (fig. 189). In early simimer the freshly grown sponges may be found by lifting and overturning boards, boughs, or almost any solid j c support that pro- jects into the water, and few if any statoblasts will be found; but in late summer, when the sponge flesh is falling away, the statoblasts will be found in patches scattered thickly over the surface as minute rounded yellow bodies about the size of small mustard seeds. These may be germinated after a resting spell, in a watchglass, the numerous amoeboid cells contained in them issuing separately from a side pore in the wall, and then soon coming together to form a delicate chimney-like tube, which is the first of the water channels of the new sponge, and out of the summit of which the water can very early be seen streaming. Doubtless it is in this same way that new sponges are started in the sloughs each spring. Many of them, doubtless, remain attached to the support where they grew, there to develope a new sponge on the old site. Fig. 189. Freshwater sponge iSpongilla ftuviatilis) disintegrating in late summer, showing the abun- dant seed-like statoblasts. THE LIFE CYCLE 337 Stiidy 40. Observations on asexual reproductive methods. Materials for this study are almost limitless in number and variety, and those mentioned below are suggested merely as types. Study and compare together a few special reproductive bodies, such as the "gemmules" of the common greenhouse liverwort, Marchantia, the frond bulbs of the bladder fern (Cystopteris bulbifera), the "bulbils" of the tige.r lily, the "sets" of the onion, the tubers of the potato, etc., etc. Study the swarm spores of Drapernaldia (fig. 187), or of Cladophora (in which they are produced in great numbers in single terminal cells), or of any of the water molds. Study and compare together the stato- blasts of such forms as Plumatella and Pectinatella among bryozoans and of Spongilla and Heteromyenia among fresh water sponges; prepared slides will prob- ably be needed for this. If some sponge Fig. 190. Poly- "^ , , , . , , , ^ embryonyin statoblasts Can be germmatcd under obser- Polygnotus . . - . -^ , .■,*■. (after Marchai). vatiou, their multicellular nature will be a, the egg; b, the same after apparent. repeated divi- - ^ , . ., sionsof its nu- The rccord of this study may consist cleus; c, the . - , ^ . same after the m notcs on and lists of the objects ex- amined, together with sketches of some of them. Polyembryony. — This is another kind of asexual method of increase, that is even more different in kind from the two preceding than they are from each other. In certain parasitic insects of the order Hymenoptera, the eggs separate em bryos from each of the parts (de- tails indicated diagramm a t i - cally in but two, and these at dif- ferent stages of progress). 338 GENERAL BIOLOGY which are laid within the soft and richly nourished larvae* of other insects, undergo a division which is rather fragmentation, than segmentation; for it results in the development not of a single embryo but of many embryos. The parts into which the nucleus divides develop separately as indicated diagrammatically in figure 190, each becoming a complete embryo, and growing later to adult estate. A significant feature of de- velopment by this method is that all the individuals de- veloped from one egg are of the same sex. Reproductive methods in general. — Sufficient illustrations have now been before us to show that there is one sexual method of reproduction, fairly uniform and consistent throughout the organic world, but that there are man- asexual methods, and that these latter are most divers' The former is uniform in its fundamentals in all kin- of environment; the latter are uniform in nothing, andthe-^ show the most significant relations to conditions of life. The unity of the organic world is hardly more manifesc in the possession of protoplasm, than in the production of gametes, and in the fusion of these in fertilization. The primary differentiation of multicellular bodies is into germ plasm and body plasm. This is even older than the differentiation between plants and animals. But the secondary sexual characters show as great diversity as do asexual reproductive phenomena: these are the after thoughts of reproduction: these are the special means adopted by special groups. How different are even sperm- aries and ovaries in the stoneworts and in the liverworts ! How lacking in common features are the reproductive organs of an earthworm and a salamander ! All these have been more recently developed, along independent lines, in accord- ance with the tendencies and in adaptation to the needs of the different groups in which they are found. THE LIFE CYCLE 339 The principal relations that the sexual and "asexual methods may bear to each other are diagrammatically indicated in figure igi. Six generations are represented in the six vertical columns. A small circle represents the egg; a dash with a tail, the sperm; a circle inclosing a dot, the zygote; the black dots are spores, and the black 7 'P Fig. 191. Diagram of types of reproduction; i^, normal sexual reproduc- tion; 2,^-parthenogeiiesiSr;j, alternation of generations; ^, polyembryony, etc.; 5, occasional-production of sex cells in a series of spore forming individuals; 6, continuous spore formation. dashes are egg fragments. The top line of figures repre- sents ordinary sexual reproduction, occurring alone, sub- stantially as previously shown in figure 174. The second line represents parthenogenesis (with only three genera- tions of females included between the bisexual generations*) . *In most parthenogenetic species, the sexual generations recur at much greater intervals. In fact, in certain species of rotifers and also fti certain gall wasps (Cynipidae) no males are known, and parthenogenetic reproduction appears to be continuous. On the other hand in one genus of gall wasps (Neuroterus) single sexual and parthenogenetic generations regularly alternate; and, strangely enough, the females of the latter differ so much in form and structure from the former that they have been described as a different genus (Spathegaster). 340 GENERAL BIOLOGY The third line represents alternation of generations, as we have studied it among the higher plants; substitute buds for the spores, and it would represent equally well alternation of hydranth and medusa in the hydroids. The fourth line represents polyembryony, as just described. It also repre- sents the conditions found in the alga Coleochete, in which the fertilized egg breaks up into eight zoospores, each of which then develops an independent bisexual plant. The fifth line represents the production of sex cells upon occasional members in a series of spore-bearing plants. This occurs in Mucor and other molds. The last line represents continuous spore formation, and entire absence of sexual reproduction — a condition that is believed to pre- vail in some of the green algas. III. CHANGE OF FORM WITH ALTERNATION OP HOSTS. Among the shifts that organisms make for a place and a living on the earth, none are more remarkable than those Pig. 192. The witch hazel pocket-gall aphid (Hormaphis hamamelidis) a young larva; b, grown larva; c, adult (after Fergande). THE LIFE CYCLE 341 of parasites from one host to another; and there are some remarkable changes of form accompanying the shifts. For example, the witch hazel aphid(fig. i92)that causes the con- ical mantle galls(shown at fig. 326) upon the leaves, and that Fig. 193. The same aphid shown in figure 192, in the form assumed after migration to the birch, a, dorsal, 6, ventral, and c, lateral views of the adult (after Pergande). grows up inside them, is of the ordinary form of the common aphids during its life within these galls (two generations). But in midsummer, when its food supply begins to be cut off by the drying up of the galls, it migrates to a new host plant. It flies through the air in search of birch trees, and finding them, settles upon the under side of the leaves to dwell there the remainder of the season. There it gives "birth to numerous young, which will grow up for three suc- cessive generations into the adult form shown in figure 193. 342 GENERAL BIOLOGY In autumn the descendants of these will grow into an adult form very like that shown in the first figure (fig. 192), and will fly back to the witch hazel, and the young of these developed upon the witch hazel will be wingless males and females, all the other generations through the year having consisted of females alone. There are other minor differ- ences, none of the seven generations of the season being exactly alike either in adult form or in developmental stages; but the two forms shown in the figures are certainly so different they would not be thought to be one and the same species by any one who did not know their life history. Other cases of change of form with alternation of host are well known ; probably they are more numerous than we now realize, because of the great difficulty of recognizing the identity of the different forms. The best known are perhaps among animals the liver-fluke of the sheep (whose host ani- mals are the sheep and the snail ; an account of it may be found in almost any general text book of zoology) ; and among plants, the wheat rust (whose host plants are wheat and barberry; an account of it may be found in almost any text -book of botany) . We will now leave these cases of heteromorphic adult organisms, which though striking are rather rare and in- consequential, and consider the far more common form- changes that occur in the life time of single individuals. IV. METAMORPHOSIS. This is the name applied to changes of form undergone after the close of embryonic life — ordinarily, after hatching from the egg. These changes may be inconsiderable, as in the earthworm, or the leech (flg. 194), but in a number of the higher groups of animals they are so great that the young of many forms were originally described as inde- pendent organisms, and given separate names. Thus THE LIFE CYCLE 343 arose the names still borne by the nauplius and zoea stages of post-embryonic development in crabs, the leptocrphalus stage of eels, etc. We have seen that there is something of a transfor- mation occurring in the sala- mander at the beginning of its adult life, and a still greater one in the frog, when gills and tail are lost, new mouth-parts are acquired and the lungs become functional. Indeed, we should not fail to recognize a sort of transfor- mation in ourselves during our earlier years, when our first set of teeth drop out and we develop another and larger one; and in other changes that occur later, in adolescence. But the most remarkable examples of meta- morphosis, as well as the most available for study, are found among insects, and these will serve us for illustration of this phenomenon. The transformations of insects. — In all of the winged insect groups there is a considerable change of form at the time of entrance upon adult life. When these changes are least, as in the grasshopper (fig. 19s), the wings are expanded and the reproductive organs, perfected; when they are greatest, every part of the body is refashioned, and the larva bears hardly any resemblance to 'Pig. 194. Leech (Clepsine) overturned, showing the brood of young protected beneath the body. 344 GENERAL BIOLOGY the adult. In the former case, there are but two stages of metamorphosis, following hatching, the nymph* (fig. 196) and the adult (imago). In the latter, there are three: Fig. 195. An adult grasshopper. larva, pupa and adult. When the differences between the larva and adult become so great that rapid change from Fig. 196. A grasshopper nymph, well grown. *Larva is a general term, covering all sorts of immature stages, each of which bears a separate designation in nearly every one of the major groups of animals: nymph is the name for one sort of larva in insects — the sort that is most easily recognized by its externally developing wings. THE LIFE CYCLE 34S one to the other is incompatible with the ordinary use of the organs, the quiescent pupal stage comes in as a transi- tion stage, a period of making over, during which the neces- sary extensive alterations of the body are effected^ The Fig. 197. The larva of undulatus). a. diving beetle (Hydroporus pupal stage is peculiar to insects, and its presence or absence within the group distinguishes between the so-called "complete" and "incomplete" metamorphosis. Fig. 198. The pupa of the same diving beetlefdrawn by Miss Helen V. William- son}. Pio. 199. The adult diving beetle (Hydroporus undu- latus). 346 GENERAL BIOLOGY Study 41. External metamorphosis in insects. Materials needed: Two selected examples illustrating the two main types of insect metamorphosis, preferably living and actively transforming in the laboratory; i) nymphs and adults of a grasshopper, a mayfly or a stonefly, and 2) larvas, pupas, and adults of mosquitoes, or meal-worms or bean weevils, or any other easily managed forms (see appendix) . Also nymphs and adults of the following in alcohol: grasshoppers, (Orthoptera) ; psocids, (Corrodentia) ; stone- flies, (Plecoptera) ; mayflies, (Ephemerida) ; dragonflies, (Odonata) ; bugs, (Hemiptera) : and larvae and adults of any Neuroptera, Trichoptera or Mecoptera, of Lepidoptera, of Coleoptera (a weevil, and a larva with legs) , of Hymenop- tera (a sawfly and a bee or ant) and of Diptera, (mosquito or cranefly or other nematocerous larva, and one of the degenerate housefly or fleshfly type). Prepare a table with the following column headings, abbreviated as desired : 1. Name of insect. 2. Order to which it belongs. 3. Relative size of head, thorax and abdomen, expressed in the ratio i :% :y. 4. Skin (thick or thin, hairy spiny or naked, etc.) 5. Eyes (well- or ill-developed, large or small). 6. Antennas (relative development) . 7. Mouth parts (adapted for biting or sucking, or vestigial) . 8. Wings (externally or internally developing). 9. Legs (relative development). 10. Peculiar parts (parts found in this larva only). 11. Lives where. 12. Eats what. > TJ THE LIFE CYCLE 347 13. Relative size of head, thorax and abdomen, expressed in the ratio, i -.x-.y. 14. Antennae (relative development.) 15. Mouthparts (adapted for biting or sucking, or atrophied) . 16. Legs, (relative development). 17. Lives where. 18. Eats what. Write the forms in this table (by groups) in the order of their departure from primitive similarity between larva and adult. Fill out the table. Then study it, and read out of it the story it contains of the divergence of larval and adult stages, and in the last two columns under both larva and adult, see how this divergence is correlated with change of manner of life. The internal metamorphosis of insects. — While there is no pupal stage in insects of incomplete metamorphosis, such as the mayfly (fig. 200) , there is a corresponding period just before transformation during which the full grown nymph is quiescent for a short time, and during which there is rapid growth of wing muscles and of other internal organs; and some pupae, like those of the mosquitoes, caddis flies and the true Neuroptera, are not wholly quiescent. But in the pupae of all the more specialized forms, besides the development of new tissues, there is going on a de- struction of old ones that are not suited to the needs of the adult and a reconstruction of their materials in new form. The pupal stage thus becomes one of peculiar helplessness in the life of the insect and it is spent in the shelter and seclusion of a pupal cell or burrow or cocoon. Larval life is abbreviated. The larva stores fat rapidly, and in relatively large quantity, postponing the final elabora- tion of it into orgaiis. And the amount of fat in its body 348 GENERAL BIOLOGY is more or less proportionate to the extent of the changes to be made during transformation. The advantage of this lies in the ability of such an insect to avail itself of a rich but transient food supply. A generation may be reared Fig. 200. Adult and nymph of the mayfly Calli- baetis skokiana (drawn by Miss Maude H. Anthony). on the fallen carcass or the ripe fruit before it is decom- posed, or in the rich endosperm of the developing seed before it has hardened. THE LIFE CYCLE 349 Pig. 201. Metamorphosis in tlie iris weevil (Mononychus vulpeculus) m, the larva at the beginning of transformation; /, leg buds and iw, wing buds, showing through the translucent skin. n,- longitudinal section of a leg bud of the larva, showing three principal divisions: /, the remains of fat cells; /, leucocytes. o, vertical section through the wing bud of the larva; k/, the point of the wing that is to be, with a shelf of ei>idermis projecting below it; c, the loosened cuticle; f. fat; vn, muscle; t, air tube (trachea). p, Cross section of a larva just before its final transformation to a pupa: a;, wing and /, leg are now exserted, and the latter .shows differentiation into femur, tibia and tarsus; fe, stomach; e, digestive epithelium; n, doublenerve cord; dv, dorsal blood vessel; /, whole fat and o, o, o, dissolving fat ; m, new muscle fibres forming. 35° GENERAL BIOLOGY This destruction of larval tissues during the pupal stage is one of the most remarkable deviations from the ordinary progressive course of the life cycle. Similar processes occur wherever larval organs are to be made over. The tadpole's tail does not drop off; that would be a waste of valuable organic materials. It is reabsorbed: i. e., it is dis- solved and transported and used again for the building of other parts. The agents of the reabsorp- tion in the tadpole are leucocytes that have turned temporarily to a diet of muscle fibres (and are called during their tis- sue-eating period, phago- cytes: the phenomenon is called phagocytosis see fig. 202). Fig. 202. Phagocytosis in the fat of the abdomen of the iris weevil. /, fat; p, phagocytes, i Some of the tissues of the insect larvae are eaten and transported by phagocytes. Others appear to be self dis- integrating; their nuclei divide extensively and become very small and then gather about themselves the reinte- grated remains of the old cytoplasm and of dissolved fat cells, and fashion them into new cell bodies, often constitut- ing organs of very different form in the adult. The process is somewhat like a return to an embryonic condition, followed by a new embryonic development, wherein the fat of the larva stands in the stead of the yolk in the egg. If at the height of metamorphosis one cut open a pupa that has developed from any of the more degenerate larvae, he finds little semblance of organs, the greater part of the body being reduced to a fluid mass which flows out at every cut. THE LIFE CYCLE 351 Not all of the body is thus destroyed, however; there are preserved little islands o regenerative cells in all the principal parts of the body from which their respective continents will be reformed. In the walls of the stomach, for example, there are grouped rings or masses of little cells, rich in protoplasm, by which the new epithelium of the new stomach will be developed. The undeveloped legs and wings exist in the larva as little buds of active cells, at- tached to the inner face of the body walls. From these legs and wings now grow out, at first beneath the larval skin, to be freed at its last moulting. About the bases of these organs and from other regen- erative cell masses in the wall itself, the new body wall is developed. Details of thesa wonderful processes may not be studied here, but there are some easily observable phe- nomena, which will help us to understand the main points. Study 42. Observations on internal metamorphosis. Materials ncv^v^ed: Living larvae and pupas of some dipterous species having red blood* ; preferably of the cone Fig. 203. Cone galls of 'the willows caused by the gall midge Rhabdophaga strobiloides. a, the pubescent gall produced on Salix discolor; 6, the crook-necked gall produced on Salix bebbiana. *The blood of insects is not red, except in a few forms, such as the so-called "blood worms", that are the larvae of midges (Chir- onomidae), and in some of the larvae of gall midges (Cecido myidas). 352 GENERAL BIOLOGY gall midge of the willow, (fig. 203) in abundance (see appen- dix). Prepared cross-sections of the thorax of old larvae showing wing and leg buds. Cross sections of the thorax of a damsel fly for comparison of fat development. The central cavity of the gall may readily be exposed by sinking the end of a knife blade or scalpel lengthwise through the end of the stem in the base of the gall, and twisting laterally, laying it open. Although, the blood is red, the grown larva will appear white, because it is filled with opaque white fat. As the dissolution of the fat pro- ceeds the red color of the blood will reappear. The progress of the pupa in metamorphosis may, therefore, from the first be gauged by the extent of the red color; later, as the end of the pupal period approaches, the black pigmentation of the adult will gradually overspread the surface, beginning with the eyes. Sketch the pupa in outline, and make several rapid copies ' of the sketch by tracing. Then color these with red and black pencils (or with water colors, if preferred) to indicate the external evidence of the internal changes. Show thus the place of beginning and the order of progression in fat solution, and later progress in pigmentation. Place a live pupa that is in the midst of metamorphosis on a hollow ground slide in a drop of normal salt solution, and split it lengthwise with fine scissors. Gently wash away the dissolved interior with a little stream of normal salt solution from a fine pipette and examine carefully the extent and the appearance of the solid organs remaining. A like treatment of a larva would disclose the wing buds and leg buds appended to the interior of the body wall. Study and diagram a section of the thorax, showing wing and leg buds ; show also the proportion of fat and solid tis- sue; compare this with cross section of thorax of a damselfiy. The record of this study will consist in the drawings and diagrams suggested above. THE LIFE CYCLE 353 V. ARTIFICIAL DIVISION AND COMBINATION OP ORGANISMS. In the arts of men, artificial division for increase of organisms asexually and artificial combination of the parts of two organisms for the purpose of temporarily combining their characters in one individual, have long been success- fully practiced. The former is known in the gardener's art as artificial propagation; the latter, as grafting. The parts of an organism that are to grow up into separate whole organisms must contain cells sufficiently undifferentiated either to be able to redevelop the missing parts, or to re- shape them out of pre-existing tissues. The parts of two organisms that are to be organically joined together must contain cells sufficiently plastic and formative to be able to effect organic union between the conjoined parts. Regeneration. — The artificial propagation of the gardener is called, when practiced by the zoologist, regeneration; and this name embodies the essential phenomena involved — the redevelopment of the missing parts of a piece of an organ- ism. The slip cut from the top of an old geranium lacks roots, and when placed in wet sand in a window box, it develops a new root system out of the undifferentiated tis- sues of its base, and does not proceed much further with leaf development until roots are established. The capacity for regenerating missing parts varies much in different organisms. It is very great in most plants, and inmany of the lower animals; but it is so poor in ourselves, that after we reach adult life we may hardly replace a patch of skin well enough to avoid a permanent scar. If the tentacle of a hydra be cut off, another promptly grows from the base of the old one. If the body be cut in two, two perfect hydras develop from the parts, a new foot being formed on the one and a new head on the other. Indeed, the body may be cut into many pieces, and each piece that 354 GENERAL BIOLOGY contains the fundamental tissues, in such relation that food can be taken, may, under favorable con- ditions, develop into a perfect hydra. A single arm broken from a starfish will regenerate the body and all the other arms. But as we ascend the scale of animal life, the power of regeneration becomes more limited as organization becomes more complex and the adjust- ment between the organs, more delicate. Herein lies one of the limitations of specialization already mentioned (page 251). Blood vessels, for example, are excellent agents of circulation when intact, but when cut, they wonderfully facilitate bleeding to death. Planarians (fig. 204) are classical subjects for regeneration experiments. They may be cut to pieces apparently without very serious inconvenience, and regenerate Fig. 204. Diagram of Hiissing parts with great readiness. fJ'o'rcTvttytnlr^ They havc no parts that can be put and central nervous entirely out of commissiou by being system in black, m, ^cMc^hl^z^^ severed. They have no blood vessel downward! '^^'^*^'^ system, nor Organs of respiration. They have a sort of brain, but it is of so little consequence that when the head containing it is cut off, another one is promptly grown. The other organs all appear equally well adapted to withstand mishaps. The extraordinary food canals, branching and ramifying all through the tissues, supply with a food receptacle even the smallest piece of the body that may readily be severed. Figure 206 shows the regeneration of the two halves of a planarian that was cut in two in the median plane of the THE LIFE CYCLE 6S5 body — a division that, as every one knows, would be in- stantly fatal to any of the higher animals. Most arthropods regenerate lost appendages readily, but slowly, the new appendage increasing in size a little at every moult. The crawfish (and many of its allies) is so provided against the loss of its legs that a special breaking place is developed across the middle of the second joint in them, a groove across the joint, and folds of membranes within it, that prevent -n i\j (^ V Fig. 205. Regeneration in Planaria (a to g after Morgan; h, after Voigt). a, a planaria that was divided as indicated along the median line of the body, b, c, d, the regeneration of the left half, that was fed. e, /, g, the regeneration of the other half that had no food, h, regeneration of pieces obliquely cleft partly free from the body: at a: a new tail and at y a new head and at z both a new tail and a new head have appeared. excess of bleeding when the leg breaks off. Specimens are collected not infrequently, having one of the big claws much smaller than the other, and in process of regeneration; a crawfish, seized by one of the big claws will sometimes automatically cast it off and escape without it. Indeed, so readily are the big claws of the related fiddler crabs cast off, that in handling the crabs one may hardly touch the claws without inviting their loss. 3S6 GENERAL BIOLOGY Normal regenera- tion tor the main- tenance of the body. — Regeneration o f lost parts is but a manifestation of the power of growth ap- plied in abnormal circumstancss. It is a very noticeable thing when, by some mishap with tools or machinery, we knock off a finger nail and have to grow a new one; but physiologi- cally it is not very different from getting our hair cut and hav- ing it grow out again. Our epidermis is con- tinually being shed from the surface and new cells are con- tinually growing up from below. An ex- cellent example of the renewal of tissues inside the body is Fig. 206. Growth of digestive epithelium in a dragonfly nymph (Gomphus). a, the alimentary canal as a whole ; g, gill chamber, / and 2, Main divisions of the intestine, x, nephridia (Malpighian tubules) p*, stomach, z, crop. b, Section of a bit of the stomach wall: k, digestive epithelium, t, longi- tudinal muscle fibres; ?w, longitudinal muscle layer; n. (as in all the following) a nest of cells for replacement of the functional epithelial cells, c, the same as b, after fasting two weeks: note the accumulation of di- gestive secretion as shown in height of functional cells. d, a dissociation preparation of part of one of the replacement cell nests. e, the discharge of the digestive secretion after feeding, o, and p, globules of discharge: the oldest of the functional cells are thus thrown oftbodily. f, the replacement of the discharged cells with new ones from the cell nests «, », n. Note the new (clear) cells crowding to the surface. THE LIFE CYCLE 357 furnished by the digestive epithelium of the dragonfly shown in figure 206. New cells are constantly being formed in little replacement centres at the base of the epithelial layer, and the old ones, charged with the digestive secretions, are thrown off at every meal, to be mixed with the food and by their action upon it to dissolve it. The tissues of the body differ much in their capacity for cell replacement; some cells like those of the lowermost layer of the epidermis, retain this capacity through life, others like nerve cells do all their dividing in embryonic life (hence, the great size to which the brain of the higher verte- brates so early attains), and have no capacity for making good cell losses. But if they have lost the power of produc- 6)*®« Fig. 207. Diagram of cell regeneration (after Morgan). a, an egg of a sea urchin that was divided as shown by the oblique line; b to f its subsequent development; g, the enucleate part of the egg; h, its fertilization by a sperm cell; i, j, k, its subsequent development, ing new cells, they retain the power of repairing the old ones. If a nerve fibre be severed, a new fibre may grow out from the cell body at the stump of the old one. It is thus that a limb regains sensitiveness after being paralyzed by the cutting of a nerve. Regeneration in cells and in embryos. — If an egg cell be divided the portion containing the nucleus may reshape itself, and go on developing quite normally, as illustrated in figure 207 b to /. And if the other part of the cytoplasm be supplied with a nucleus, as by the addition of a sperm cell of the same species (fig. 207 h) it also may develop in the ordinary way. The two cells resulting from the first division of the egg of a sea urchin may develop as indicated 3S8 GENERAL BIOLOGY in figure 208, producing two individuals of half the usual size. At first they are likely to develop as half embryos, each cell and its descendants behaving as though the other were present. Con- sequently the blastula when formed is open on one side ; but it closes and forms a normal embryo later. In most bilateral animals the first cleavage plane lies in the medium plane of the body that is to be, and doubtless, when the two cells remain together each develops its own half of the body, left or right; but the above experiment shows that either is capable of developing any part of the body. Frogs eggs, with one cell killed at the two-ceU stage, likewise develop at first half embryos, which later become whole ones. Wilson long ago showed that each of the cells of the developing lancelet, isolated at the 4-cell stage is capa- ble of forming an embryo, but at the 8-cell stage, each cell may develop emb^of L°ividtd only as far as the blastula. Apparently (SteV'^Morgan)'^^^ differentiation is slight at first, and when^twasdfvidS! "ontogeny assumes more and more the character of a mosaic work as it goes Fig. 208. The de- the two cells were isolated as at c, d, t°Tjr^7^-t forward." the same as incom- plete blastulas: /, the same later in the gastrula stage, complete but of half normal size. Some aberrancies of regeneration. — Ordinarily after mutilation, if normal conditions be maintained, regeneration tends toward the production of parts like those removed. When the head is cut off a hydra it produces a new head, and not a foot. What marked THE LIFE CYCLE 359 antero posterior polarity, for example, is shown by the in- cised planarian of figure 20 5 fe, which is producing new heads where the strips of severed tissue are directed forward, and tails where these are directed backward. But the expected does not always happen in regeneration. In at least one genus of earthworms, it any number from one to five of the front segments of the body be cut off, these will be replaced in like number; but if a dozen or twenty or any number of segments more than five be cut away from the front end, only five will be regenerated in their stead; and if more than the anterior half of the body be cut away, from the front end of the posterior piece there will develop not a new head but a new tail. Apparently, there is a limit to anterior polarity. The hydroid Antennaria regenerates a new head when the decapitated stem is kept in, the upright position, but a new foot when it is kept in inverted position. And stems of the hydroid Pennaria, which regenerate heads under ordinary circumstances, will regenerate roots if the cut ends are held against a solid support. From the severed eye-stalk of a craw- fish (or almost any other deca- pod crustacean) a new eye never develops, but on the contrary, if there be any re- generation (as there is pretty sure to be if the animal be young, and the conditions favorable for growth), it is usually a jointed appendage, more or less antenna-like, and at its best development dis- tinctly bi-ramous, that grows out in the eye's stead (fig. 209). Fig. 2 09 . Regeneration of the stalked eye of the crawfish (after Miss Steele). In 5, a simple appen- dage is regenerated in the place of the eye; r, a biramous appen- dage, that regenerated in the place of the eye of another speci- men. 36o GENERAL BIOLOGY Study 43. Experiments with regeneration in planarians. Materials needed: Plenty of living planarians, in indi- vidual dishes of clean water. This is a running experiment, requiring repeated observations at successive laboratory- periods. Cut small pieces from some of them, cut others in two in the middle, at various planes, and make diagonal clefts in others to observe polarity of the partly severed pieces. Divide the bodies of a number of the animals. They may be cut with fine and sharp scissors while creeping, fully extended, on a piece of thin wet paper; cut paper and planarian together, at single rapid but careful strokes. Excessive cutting up of the animals may be avoided by apportioning the work among several members of the class. (That need not be a serious consideration, however, since the regenerated pieces may be returned to the waters whence the whole ones were originally taken, and the tribe will have been increased by the operation) . The record of this study should consist of sketches of the animal, one for each operation, and outline drawings of the forms assumed at subsequent examinations. Grafting. — The parts of two organisms, if brought together by clean cut surfaces, with growing parts apposed, and held in close contact for a time, may grow together, and, if complemental parts be taken, they will thereafter function as a single organism. This is grafting. In the higher plants, on which it is most commonly practiced, the piece that is to represent the top of the combined plant is called the cion, the rooting piece is called the stock. These two parts are combined into one in a number of well known ways, three of which are represented in figure 210. The essential things in the practice (with such woody plants as these shown) are i) the bringing of THE LIFE CYCLE 361 the cambium or growth layer of scion and stock into close contact, and 2) protection of the cut surfaces from evapora- tion and from the weather. Such combinations of the higher plants are possible only between rather closely related forms (usually, between members of the same genus), and every species has its com- bination preferences, which can only be learned by trial. Pear cions, for example, will grow well on quince stocks, but quince cions will not thrive on the pear. Potato and tomato will thrive in either combina- tion, and when the tomato is the cion, both potatoes and tomatoes may be produced on one plant (fig. 211). At its best the union is mainly a co-adjustment of trans- portation systems, admitting of interchange of food mater- rials; each part retains its own in- dividuality, and, the results of the combination are not heritable. The objects of grafting are mainly two: I. To combine the characters of two species in one individual. Thus, in order, to add to the good qualities of certain apples the hardiness of the Siberian crab, apple cions are grown on crab stock. In order to adapt certain plums to southern soils, plum cions are grown on peach ^'SepUkt °rodJ?Sby stock. When the vineyards of France i[?pi?|to°stoc*k! "°° Fig. 210. Grafting methods |with plants, o, splice grafting; b, cleft grafting; c, bud grafting (or more commonly called budding). «,«,«, «, M, cions. V, V, V, V, stocks. The second figure of the cleft graft shows how the grafting wax is applied to cover the wound. 362 jEneral biology were being destroyed by the imported American grape phylloxera (a root infesting aphid, Phylloxera vastatrix), the situation was saved largely by grafting wine grape cions on stocks of the hardier and immune American grapes. 2. To perpetuate in the fruiting part of the combination a valuable variety; one that does not breed at all, (as for examplfc, a seedless grape or orange) or one that does not breed true. In such case the kind of stock used is of little consequence, ^:!^M).t as it is a good feeder for the cion Grafting in wtBtttk. — Such combinations of parts are not so readily made in animals. The specialized contractility of the animal body is against it. It is harder to keep the growing parts in close apposition, while being knit together. Advantage may be taken of quiescent periods in the life of the individual when stored food is available for growth, such as the early embryonic stages of frogs (fig. 212) and the pupal stages of insects, etc. Thus moth pupae of the right age carefully cut in two across the base of the abdomen, and carefully handled to prevent loss of blood have been united successfully in cross-combi- nations, the parts being sealed with paraffine while being knit together. An- tennae, and wings have been cross-grafted in similar man- ner. By uniting male bodies with female abdomens, fe- males having the appearance of males have been produced. Similarly, male and female wings and antennae have been combined upon the two sides of one individuaL Fig. 212. Diagram of grafting operation on frog larvae (after Harrison), a, the larva at suit- able age for grafting; b, the same larva, older, to which has been grafted the tail of a larva of another species. THE LIFE CYCLE 363 Objects. — ^The purpose of regeneration and grafting experi- ments on animals has been to obtain new side lights on the nature of the organism. Nature furnished the hints for the first experiments tried. The rooting of detached twigs of the crack willow might have suggested to anyone the possible rooting of cuttings. The finding of regenerating star- fishes, broken in the surf, inight have suggested regeneration experiments on animals, and if a portion of an animal's body from which the sex organs were removed, were able, as it is in fact in some cases, to reproduce the missing parts with sex organs included, then the experiment would seem to have shown that the distinction between body plasm and germ plasm is not to be too sharply drawn. Although the sex cells would normally come from the sex organs removed, they might come from new sources in the body. Study 44. Grafting practice with plants. Materials needed : Selected and over- wintered cions and rooted seedlings or other stocks for their reception; grafting, wax (see appendix) and sharp pocket knives. It wUl be worth the time of a laboratory period for the student to make with his own hands the combination of parts of two species, and later to see them growing as one. Different types of grafting may be demonstrated also, and the later matured results of previous operations. The work should be directed by someone who has had practical experience. The record of the study, should be an illustrated account of the student's own operations and observations. Reserve potentialities of the living substance. — The studies of this chapter should have been convincing of the wide range of methods by which the ends of life — ^the preserva- tion of races — are accomplished. We began this chapter by speaking of the normal course of life, which is merely the 304 GENERAL BIOLOGY more usual course and the more primitive. All the devia- tions from this course that we have been studying, have become thoroughly normalized in the races that exhibit them; the methods of development are stereotyped alike for all. Whatever the course of life, each individual of a species follows it with the most minute exactness. And yet, when something happens to block the usual course, another may be followed, as regeneration and grafting experiments most plainly show, to reach the same end. There are reserves of power for development that the ordinary circumstances of life do not draw upon ; accidents and losses reveal their existence. If a member be maimed and a por- tion of its tissues be injured beyond repair, the injured part must be removed and new tissue fashioned in its stead. Phagocytes enter a wound to clear away old materials, and the blood brings new materials to be gradually fashioned into the form of the old. This is artificial regeneration; but nature makes use of these same pathologic methods in the removal of old tissues and the building of new in meta- morphosis. That the functional activity of certain parts of organisms may be increased by selection is shown by the increased milk production of the best dairy breeds of cattle, and by the increased egg production of fowls, etc. Selection has made the dairy cow an improved machine for turning hay and ensilage into milk. But nature presents examples of the exaggerated activity of special functions yet more striking. One such has been fittingly described by Lloyd Morgan in the following words: "There is perhaps, no more wonderful instance of rapid and vigorous growth than the formation of antlers of deer. These splendid weapons and adornments are shed and renewed every year. In the spring when they are growing, they are covered by a dark skin, provided with short, fine, THE LIFE CYCLE 365 close-set hair, and technically termed 'the velvet.' If you lay your hand on a growing antler, you will feel that it is hot with the nutrient blood that is coursing beneath it. It is, too, exceedingly sensitive and tender. An army of tens of thousands of busy living cells is at work beneath that velvet surface, building the bony antlers, preparing for the battles of autumn. Each minute cell knows its work, and does it for the general good, so perfectly is the body knit into an organic whole. It takes up from the nutrient blood the special materials it requires; out of them, it elaborates the crude bone-stuff, at first soft as wax, but ere long to be as hard as stone; and then, having done its work, having added its special morsel to the fabric of the antler, it remains embedded and immured, buried beneath the bone products of its successors or descendants. No hive of bees is busier, or more replete with active life than the antler of a stag as it grows beneath the soft, warm velvet. And thus are built up in the course of a few weeks those splendid 'beams' with their 'tynes' and 'snags,' which, in the case of the wapiti, even in the confinement of our Zoological Gardens, may reach a weight of thirty-two pounds, and which in the free- dom of the Rocky Mountains, may reach such a size that a man may walk without stooping, beneath the archway made by setting up upon their points the shed antlers. When the antler has reached its full size, a circular ridge makes its appearance a short distance from the base. This is the 'burr' which divides the antler into a short 'pedicel' next the skull, and the 'beam' with its branches above. The circulation in the blood vessels of the beam now begins to languish, and the velvet dies and peels off, leaving the hard, dead bony substance exposed. Then is the time for fighting, when the stags challenge each other to single combat, while the hinds stand timidly by. But when the period of battle is over, and the wars and loves of the 366 GENERAL BIOLOGY year are past, the bone beneath the burr begins to be eaten away and absorbed, through the activity of certain large bone-eating cells, and, the base of attachment being thus weakened, the beautiful antlers are shed; the scarred sur- face skins over and heals, and only the hair-covered pedicel of the antlers is left.'' Antler development on the part of the male is no less remarkable, al- though far less im- portant, than the organic response on the part of the female that follows upon fertilization of the egg and re- sults in the produc- tion and nurture of the young. Figure 213 is intended to show how qtdck is this response to fertilization in the common spreading dogbane. If a flower fail of fertil- ization it dies, but if fertilized, the fruit which then develops from it may reach full size before the last of the flowers on the same peduncle have faded. These examples of organic activity, suddenly and inter- mittently recurring, are the results of internal (perhaps orthogenetic) tendencies. But the reserves of develop- mental power which organisms possess may be tapped by outside agencies as well. Gall insects for example, have Fig. 213. New seed pods of the spreading dog- bane (Apocynum), showing quick response to fertilization. THE LIFE CYCLE ?fi7 turned to profit their ability to call forth plant growths in excess of the normal. We have already noted how often galls are fruit-like in form (fig. 214). The cone gall of the willow (fig. 203) is not a deformed shoot, but an overgrowth (hypertrophy) of tissue superadded to the normal growth of the shoot. Organic harmony. — Whether an organism develop out of an egg under normal or under artificially altered circum- stances, whether out of a piece of a pre-existing organisms or out of pieces of two put together, if it develop at all it is pretty sure to develop with organic unity, with symmetry and proportion. Its dominant tendency is toward organic wholeness. Fig. 214. A pod-like bud gall on Pistachia (after Kerner and Olivier) show- ing response to external stimulus. CHAPTER VI. THE ADJUSTMENT OF ORGANISMS TO ENVIRONMENT. "Life is response to the order of nature." — Brooks. In this chapter we shall attempt a more careful examina- tion of the phenomena of fitness, selecting arbitrarily for the purpose, out of a world-full of examples, a few that seem fairly illustrative and typical. Plants and animals, which were primitively much alike and lived under more or less uniform conditions, have multi- plied, differentiated, specialized, and spread to every habit- able part of the globe, and have become adapted to condi- tions of utmost diversity and complexity. Fitness to meet these conditions is a necessity of existence under them. Unfitness so pronounced as not to admit of getting a living or of leaving descendants, would mean for any species speedy elimination. That all living things are adjusted to their places in the world is most obvious; how this has come about is a subject of much speculation at the present day. We may not be able to determine whether the initiative of the variable organism or the impress of environing condi- tions has been the major factor in producing the results, but we can at least see the sort of facts on which all the theories advanced in their explanation are based. As a matter of convenience we will divide our studies of this subject into three groups, according to the more prominent phenomena involved, as follows: Adjustment in place and time. Adjustment in manner of life. * Adjustment in bodily characteristics. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 369 I. ADJUSTMENT IN PLACE AND TIME. We go to the woods for squirrels, to the marsh for snipe and to the lake for fish. We do not expect to find either in the place of the other; indeed, we know they could not live if they exchanged places. If we likewise go to the beach for sand or to the mine for gold, we know that either might exist as well unchanged if put in the place of the other. The gold or the sand may have lain unchanged for ages; but squirrel and snipe and fish ha-v^e developed with their environment, and are developing still. It is not everywhere in the woods that we find squirrels. They have their own particular haunts. They like the nut- bearing trees, and shun the thorny locusts. They like cer- tain bird neighbors and dislike others. In the water we find pickerel and top-minnows feeding at the surface, cat- fishes and mud-minnows feeding on the bottom, and other fishes foraging between; different forms of life at different levels ; and likewise, passing out from deep water shoreward we find that every change of forage and shelter brings with it its own peculiar forms of life. The more closely we look into any environment the more we see of small and seques- tered species, restricted in range and peculiar in mode of life, segregated into definite and sharply delimited haunts. The physical conditions of life in the water are still simple, but with the multiplication of individuals and differentia- tion of species, by reason of the stress of competition on every hand, the biological conditions have become severe. Only a few of the stronger and larger species frequent the open water, and these only when they have attained maturity; the great majority of the lake's inhabitants dwell in some restricted sphere. The great sturgeon may roam the lake at will, but the little darters, and infant sturgeons as well, must keep to shelter. A new study, appropriate at this point in the course, has been added on page 525. 370 GENERAL BIOLOGY I. Local distribution of green plants. The distribution of plants over the larger regions of the earth is determined chiefly by physical and climatic condi- tions. We can see the effect of temperature by passing from the stunted and scanty vegetation of polar regions to Fig. 215. Engelmann's Spruce from a. sheltered valley (altitude 7600 ft.). Photo by D. M. Andrews. the luxuriant forests of the tropics; of winds (figs. 215 and 216) and altitude and drouth, by crossing mountain and desert. We can seethe effect of water and sunshine by crossing a narrow ravine, from its moist and shaded north ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 371 slope to its dry and sunny south exposure; or we can see it by walking from the north side of our house around to ^ f ' '^^ \ r P i ^ h B^S tflr ^ S »^ S^-«-v-. ^5#?l ik'.-'S' ^s*«%jf i^ Fig. 216. Engelmann's spruce from an exposed mountain side (altitu4e 10,800 ft.). Photo by D. M. Andrews. the south side. Food and water are the primary requis- ites of plants ; but green plant food is nearly everywhere present — the carbon in the air, and the other food materi- als in the soil; but water is not so uniformly distributed over the surface of the earth. So it has come about that 372 GENERAL BIOLOGY the distribution of water has largely determined the grouping of terrestrial plants into natural societies: Hydrophytes — Plants accustomed to abundant avail- able moisture. Mesophytes — Plants that live under intermediate condi- tions. Xerophytes — Plants that live where the water supply- is scanty, and that have deep roots, and many adaptations for conserving the water supply. Within each of these groups the distribution of the mem- bers in relation to each other — ^their mutual adjustment in place — is determined more largely by exposure to light than by any other single factor. Besides food, green plants must have light, to supply the energy for growth that their simple foods lack. This is especially true of the mesophyte society, with its extraordinary diversity of size and form and habitat. Be it forest, heath, or meadow, we always find it dominated by a few relatively large species of great vegeta- tive vigor. Around and between these, occup3ang the interstices, and holding what soil and sunshine they can get, are a host of lesser species, scattered, diversified and often highly specialized as to their mode of performing particular functions. It is among these that we find the most special forms of plant-body and the most special devices for secur- ing cross-pollination and seed distribution, etc. A few of these plants of the undergrowth sometimes show a sort of secondary dominance, their crowns forming imperfect foliage strata at successively lower levels. Thus in the hard-wood forests of our northern mountains there is often a top stratum of crowns of maple, beech and birch at high altitude ; a secondary stratum of the spreading tops of the hobble- bush, a few feet above the ground, and a third stratum of moss, carpeting the floor of the forest. Often in oak woods farther south, there are successively lower strata of ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 373 hazel and mandrake and moss, and in the soil there is likewise a more or less definite arrangement of the roots in strata, less easily observed because hidden, but probably as real, because supported by the stratification of the soil. The smallest herbs usually root in the top soil, the majority of shrubs in the subsoil, while many of the trees strike root far deeper. By these means the resources of both light and soil are more fully utilized and a more abundant and varied flora is maintained. The dominant plants are usually of erect habit of growth, conforming more or less closely to some of the commoner typical forms shown in the accompanying diagram (fig. 217); Pig. 217. Diagrams of growth habit in plants, a, rosette; b, scape; e, wand; d, bush, e; crown; /, climbing; g, twining; k, trailing. but growth habit varies with crowding and with the conse- quent restriction of the light. Study 45. Woodland plant society. Field study. — Select a bit of woods that has retained natural conditions, at least as natural as possible. Lay out a small area, a strip a few rods long, containing some diversity of conditions, for a detailed study of its green plant population. It must needs be a small area in order that all members may be examined. In order to determine the normal characteristics of some of the larger or rarer; species it may be necessary to extend observation of these over a wider area. 374 GENERAL BIOLOGY Study each species as to its more important ecological characters and record these characters briefly in a table prepared with the following headings : Name. Duration (annual, biennial, perennial). Increase (aside from seeds, by offsets, stolons, tubers, etc.) Social habit (solitary, commingling, copse forming, cover forming) . Growth habit (scape, rosette, wand, bush, crown. If not erect, trailing, twining, climbing or epiphyte, parasite, subterranean, etc.) Rooting in (topsoil, subsoil, deep soil, rock crevices, rotten wood, etc.) Favorite situation. Favorite exposure. Season of maximum vegetative activity (spring, early summer, late summer). Write the names in the first column by groups, as follows : r trees Spermatoph5rtes < shrubs (^ herbs Pteridophytes Bryophytes Thallophjrtes. The record. — After the table is completed (the entire green plant population of the area selected being included therein) , then write out briefly your interpretation of the facts, as to the relative dominance of each ecological characteristic, and possible reasons therefor. Adjustments in place may be further illustrated by the zoii^l distribution of aquatic seed-plants, indicated at the right in figure 224. This represents an inviolable order ; for the shoreward types are capable of shutting out the light ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 375 from those in deep water, except at depths they themselves cannot endure. Adjustments in time are indicated in the last column of the preceding table for the single season. Fig. 218. A damselfly {.Lestes uncatus). These are mutual adjustments and involve succession of periods of vegetative activity. Time adjustments that extend over long periods, and that accompany slow changes 370 GENERAL BIOLOGY U> of environment, and that result in a succession of floras, may be studied if there be available a series of ponds in the various stages of filling, or if there be burned over tracts or fallow fields in the various stages of reforestation. Sugges- tions for such studies may be found in a num- ber of modern text books of botany. The adjustment for geologic time is studied in the palasontologic record, and is the history of plant life on the earth from the beginning. 2. Hibet nation and aestivation. Corresponding to the seasonal adjustments of early and late plants, just cited, there is seasonal cessation of vital activity among animals. In our temperate climate, many warm blooded mammals, and most other resident animals, disappear on the advent of cold weather, and may be found in a dormant condition, in winter quarters. They are hiber- nating. Their temperature is barely above freezing point. Their metabolism is well nigh at a stand still. In the spring they emerge in good condition and resume their wonted activities. Nature eflEects great economy by Umiting their foraging operations to the grow- ing season. On the other hand, in the hot weather of summer, with its accompanpng drouth, when there is not enough water to maintain activity on the part of organisms that live in temporary shoals there results another resting stage that is known as cestivation. Thus, through the central United States the damselfly shown in ^v Pig. 219. The EBsti V a t i n g embryo or Lest e s , as seen through the translu- cent egg shell. ;,Ta- b r u m; «t, antenna; », mandible; o, maxilla ; p, labium; q, r, s, legs of one sid o t, abdo- men. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 37 7 figure 218, lays its eggs in the stems of bur-reed and iris, growing in temporary pools. The eggs at first develop rapidly as in other damselflies, and reach the condition shown in figure 2 19 about the time that these • pools nor- y Pig. 220. Animals that withstand dessication, x and y^ a tardigrade (Macrobiotus hufelandi) x, extended, creeping; y, in a state of apparent death, dried; x, a. lotiier {Philodina megalotrocha). Internal organs of the tardigrade shown in optic section: m, pharynx; «, salivary gland; o, stomach; p, ovary; i, ii, iii, iv, legs of one side, (z and y, after Hertwig, X, after Jennings. maUy "go dry." There they stop, and in that condition they remain until the rains of autumn refill the pools, when they resume development, hatch out and enter the water. There are many lesser organisms, notably the tardigrades and rotifers (fig. 220), so well adjusted to the exigencies of drouth they can get along and maintain themselves, living 378 GENERAL BIOLOGY in rain water spouts, and in stone urns, that are alternately drenched with showers and baked in the sun. With every sun-baking, they are almost completely dessicated, and become contracted and wrinkled into almost unrecogniz- able shapes; but with the next shower they "soak up" again, and resume normal activity. Study 46. Observations on the dessication and resuscita- tion of rotifers. Materials needed: An abundance of living rotifers, pref- erably of the genus Philodina, which is commonly found in the dried crust of the bottom in stone urns in cemeteries, etc. , and which may be cultivated in little porcelain dishes with rain water in the laboratory. For convenience of handling, cultures are best made on squares of fine-meshed filter paper laid in the hollow of the bottom of the dishes. For methods of handling,- of concentrating, of isolating the rotifers see appendix. The student should obtain specimens at one laboratory T)eriod, should isolate some of them in the bottom of a hol- low ground slide in the hollow of a piece of filter paper fitted thereto, should set this slide away uncovered to dry by evaporation, and at the next laboratory period, should examine the rotifers dry, and then should cover them with water and watch them resuscitate. The record of this study should consist of notes on and sketches of the things observed. J. Local distribution of animals. That food and shelter are the primary factors determining the distribution of animals is almost too obvious to be stated. Where to find a living and establish a home is the great question confronting every animal — even man. Terrestrial plants live where they must ; but most animals ADJUSTMENT OP ORGANISMS TO ENVIRONMENT 379 are free -to move about within certain, often narrow, limits, to find new pasture or to change their domicile. In a small society of green plants it is comparatively easy to find all the species, for they are fixed in place, and come out into the light, and into view; but so different is the case with animals, so small are many of them and so secretive and elusive of habit, that there is not an acre of the earth's sur- face of which the entire animal population is known. Even of that class of large animals to which we ourselves belong, there are many mammals living in our own immediate environs that we seldom or never see. As already stated in the opening chapter, herbivores and carnivores, parasites and scavengers are everywhere, be- cause they fulfill permanent ftinctions in natural society. The herbivores are, among animals, the pro- ducing class; all the others are consumers. The food of animals is not to be found every- where, even that of the most omnivorous species. The deer that roams the forest, crop- ping the leaves and twigs of a great variety of plants, leaves a much greater number of spe- cies untouched. The caterpillars of the gypsy moth will eat the leaves of almost every green Fia. 221. Photograph from life of a tree, but most caterpiU- young and active flag weevil larva g^j.g ^^ eat of a single {Mononychus vulpeculus). » 38o GENERAL BIOLOGY genus of plants, and many will eat only of a single species. The result of the competition of the past among animals seems to have been toward greater localization and concentration of food supply — at least for the smaller species. The flag weevil (fig. 221) eats of the seeds of the blue flag (Iris versi- color) but only of the endosperm, and of that only for a few weeks when it is newly formed. Likewise, carni- vores, parasites and scavengers all have their peculiar tastes. Fig. 222. Diagram illustrating lines of ecological specialization among terrestrial vertebrates. That these tastes may best be gratified under those condi- tions that at the same time furnish the best shelter and domicile for each species is a truly wonderful and altogether admirable feature of their adjustment. Primitive terrestrial animals, recently come up from the water, were doubtless "creeping things," with feet adapted more for . propulsion than for the support of the body (fig. no, page 180). In time their descendants were able to get up on their feet and walk. With better powers of loco- ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 381 motion they were better able to possess the land, and they multiplied in numbers and competition ensued. Super- added to the stress of competition was the direct onslaught of active enemies. Conditions became hard, and various shifts for a living were resorted to. The main lines these shifts could take were determined, however, by environ- ing conditions. There was room to run in, if speed could be attained. There was soil to hide in, if one could dig; there were trees to climb ; there was water to dive in; and if anything could fly the air offered the best of all ways of escape. So land animals differentiated, somewhat as indicated graphically in the accompanying diagram (fig. 222) into cursorial, fossorial, arboreal, aquatic and aerial groups. Size. — ^Owing to the nature of the environment; its limited quantities of food, its limited and irregularly distributed accommodations for shelter, size came early to be a determining factor in the adjustment. For the small animal, while at a disadvantage in point of strength, is at a great advantage when it comes to finding food and shelter. A flag weevil can find a life's provision in one chamber of an iris seed capsule, and leave enough seed untouched to main- tain the plant stock, while an ox may browse to the point of extermination all the herbage on half an acre of ground. The kind of differentiation of habitat possible to the larger vertebrates, was possible to terrestrial invertebrates upon a smaller scale. The runners, climbers, burrowers, etc., among the beasts of the forest have their counterparts in groups of like habits among the insects of the meadow. Moreover, among the lesser animals that climbed the tree or that went down into the burrow of the beast, there was a secondary, parallel differentiation ; so that on the trunk of the tree, and on the walls of the burrow we find small bur- rowing, running, jumping and flying forms. Indeed, 382 GENERAL BIOLOGY even on the back of the ox there are parallel phenom- ena of distribution; there are fly larvae that burrow beneath the skin, there are ticks that cling to the surface, fleas that run and jump about, and flies that take wing. Thus, the body of the large plant or animal becomes a unit of environment for a host of dependent forms. Miniature units are found in single organic products, such as the ear of Fig. 223. Young woodchuck (Ardoinys moaax) in the mouth of his burrow. Photo by T. L. Hankinson. com, the head of cabbage, or any of the larger fleshy fruits; how many inhabitants there are dwelling in each of these, and how well they are localized and adjusted in place and time, may be learned from the reports of our agricultural experiment stations. The cone gall of the willow has a con- siderable population, distributed in place as indicated in figure 36, (page 46). In cases like these the distribution is ADJUSTMENT OP ORGANISMS TO ENVIRONMENT 383 different from that on plane surfaces, as indicated above; but it is always conditioned by and always conforms to the nature of the environment. It is the terrestrial vertebrates that we know best among animals so we will next attempt a cooperative statistical compilation of facts bearing directly on the mutual adjust- ments of these. Stiidy 47. The local resident terrestrial vertebrate fauna: its ecological distribution; a compilation-study. Prepare a table, leaving a column at the left hand for group names, with the following column headings, abbre- viated as desired: Name. Inhabits (fprest, heath, meadow, marsh, shores, desert place : indicate special habitats in any of 'these). Eats what \ Animals | ^J^= (specify the kind of food eaten.) Nests \ I where ^ I when, (day, night, etc.) Special means or apparatus for getting food, where when (dates) « Constructs or takes advantage of ,Jwhat«pe^l shelter. What s'ort^f ^activifps (running, jumpinfy.%odging, burrowing, flying, diving, etc.) Escapes enemies by -! What sort of organic defense (bad odor, bad flavor, defensive armor, protective coloration, etc.) 384 GENERAL BIOLOGY Arrange the names by groups at the left hand, mam- mals, birds, reptiles and amphibians. Fill out the table as far as possible from personal knowl- edge and observation. For the balance consult reference literature, or any other source of reliable information. In a class of students this may be facilitated by division of labor. If blanks still remain they should be useful as indicating gaps in the knowledge of the local species. Such a table as the foregoing kept by the laboratory and added to, year by year, by succeeding classes as knowledge of the fauna increases, may grow into a most useful and reli- able ready-reference chart. The record. — Complete the table so far as possible and then write out briefly your own interpretation of the facts contained in it. These facts should give rise to many le|;iti- mate questions. Is there any clear relation between any systematic group and any particular habit of feeding.? of locomotion? What kind of habitat has the largest number of species in its population and why? What habits are shown by the smallest number of species and why ? Is there any clear relation between size of the animals and habitat? Between size and feeding habits? Between size and habits of locomotion? etc., etc. Animal migrations are sudden shifts of place that de- mand good powers of locomotion. When of irregular oc- currence, as is usually the case with the migration of mammals like the lemmings, and of insects like the Rocky Mountain locust, they necessitate biological readjustment in both the localities*b)etween which the migration occurs; for the natural balance is disturbed in both places: but when well established as a normal part of a mode of life, as in the regular annual migrations of birds between their summer and winter homes, the adjustment becomes per- fected, not only as adjustment in place and time, but also as adjustment between different places and seasons. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 385 4. Pond life. The pond is a well defined unit of environment. Aquatic forms of life are hemmed in by its shores. They are easy to find, easy to collect and easy to keep alive and to observe. As it is nob difficult to determine the place of each in the pond, the following study should offer opportunities for greater definiteness in field observations. The animals of the pond are in part forms that have always been aquatic, and in part land forms returned to the water. Among the latter, some have retained their terres- trial mode of breathing; some have become readapted to I. « Fig. 224. Diagram of distdbution of pond life. The right side illustrates the zonal distribution of the higher plants. 7, shore zone; 2, standingjemerg- ent aquatics; J, aquatics with floating leaves; 4, submerged aquatics; j, floating aquatics; 6, free swimming algae of the open water. The left side represents the principal features of the distribution of a nimals. r, s, t, «, are the air-breathers; v, w, x, y, and z, are the water-breathers, as per accompanying table. the water, and have respiratory apparatus of a strictly aquatic type. The problem of getting air has been a primary one determining their distribution. Collectively the animal life of the pond may be divided into two groups according to whether the air is taken free or dissolved in the water; easily recognizable ecological sub- divisions will then be those of the following table. Their places are indicated graphically in the accompanying diagram (fig. 224). 386 GENERAL BIOLOGY Forms breathing free air 1. Forms running on the surface (water skaters, etc.) 2. Forms lying on the surface (whirligig beetles, etc.) 3. Forms hanging at the surface, tipping the surface film (diving beetles, etc.) 4. Forms far below the surface, con- necting therewith by means of a long respiratory tube. (Ranatra, rat-tailed maggot). 5. Free swimming forms, (corethra, etc.) 6. Climbing and clinging forms, (mayfly, nymphs, etc.) 7. Attached forms, (hydras, bryo- . zoans, etc.) 8. Forms that walk or lie upon the bottom, (crawfish, etc.) 9. Forms that burrow in the bottom, (Ephemera, etc.) Study 48. A laboratory examination of typical pond animals. Materials needed: Plenty of living specimens of the several types of pond animals mentioned in the foregoing table ; individual beakers of water in which to examine them. First compare together representatives of the two main groups; a whirligig (Gyrinus or Dineutes) , representing the groups that breathe free air, and a Mayfly or damselfly nymph (fig. 225) representing the groups that breathe the dissolved air. Put both in a large beaker of water and watch them ; observe that the beetle carries a bubble of air at its wing-tips as it swims; its respiratory apertures are beneath its wings. Observe the cleavage of the water when Forms breathing air dissolved in water ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 387 it rises to the surface; note the water repellent surface; compare with the surface of the njonph. Compare the two as to what happens when they stop swimming; which one rises to the surface like a cork? Compare with one of the free-swimming forms in respect to this. Then examine the whirligig more carefully i) as to the differentiation of its feet; 2) the extreme specialization of its hind feet; 3) the form of its body and 4) the difEerentiation of its divided eyes, into an upper eye to look into the air, and a lower one to look down into the water ; all are express adaptations for living on the surface. Then examine the other, as to its Fig 225. The nymph of a damselfly (Ischnura veriicalis). climbing feet, the gills upon its abdomen and its protective coloring. Then compare the representatives of the groups i to 4 as to i) position in the water, and movements; 2) mode, if they have any of carrying air; 3) air repellence of the body sur- face, and 4) weight. Air carried externally can be recog- nized by its shine. Push a skater or a water-spider (or even a housefly) under water and see the layer of air enveloping its whole body. 388 GENERAL BIOLOGY Then compare together representatives of groups 5 to 9 as to i) diversity of form and habit; 2) resting position in the water. Compare together dragonfly nymphs representing groups 8 (Libellula) and 9 (Gomphus) i) as to form of body, 2) form of front of head, and 3) shape and position of feet. The record of this study may consist of brief comparative statements of the things personally observed. State briefly the characters of each type that mark its fitness for the ecological situation to which it belongs. Study 4g. A field study of the pond animals in their native haunts. A pond should be selected that has more or less shore vegetation, and banks dry enough to admit of approach with hand nets. A small pond if permanent is as good as a large one, and if no pond be available, a bay off a lake or river will offer practically the same forms. Apparatus needed: Individual dip nets, beakers and vials. A plankton towing net, a sieve net and a few pails or bowls for com- mon use will also be advan- tageous. Let the collect- ing and study be individual. Collect air breathers at the surface with a dip net ; such as are foraging or hiding down be- low may be ob- tained later. Pig, 226. Shells of fresh water snails, o, Planorbis; b, Ancylus; c, Limnea; d, Physa. (From Morse's First Book of Zoology, a pioneer American book of nature-study). ADJUSTMENT OF ORGANISMS TO ENVIRONMENT ,389 Sweep the open water with the dip net for free swimming forms. (Most of these are obtained more readily with a plankton net.) Sweep the submerged vegetation with the dip net for the climbing and clinging forms ; some members of groups 2 and 3 will thus at the same time be obtained. Scrape the bottom with the dip net for bottom forms'; scrape deeper and sift out at the surface, to get the bur- rowers; for these a sieve net is more ef&cient. Take up submerged sticks, stones, leaves, etc., from the water and examine them for attached forms (the examina- tion is very satisfactory by submersion in water in a big white bowl; bryozoan colonies (see fig. i88a) will, however, be easily seen without this submersion. Study each species as it is obtained ; put a few specimens inbo a beaker of clean water with a few clean pebbles on the . bottom and some stems at one side and watch it. Determine to which of the nine groups it belongs and write ibs ecologica characters in the proper place in a table prepared with the following column headings : Name. Stage (larva, pupa or adult, etc.) Feeding habits. Takes air how. Swimming apparatus. Clinging or climbing apparatus. Means of locomotion other than swimming. • Means of j observation of enemies escaping \ attack of enemies It should be possible to obtain: Of group I, water skaters, water spiders, springtails,etc. Of group 2, whirligig beetles. 390 GENERAL BIOLOGY Of group 3, diving beetles, water boatmen, back swim- mers, water bugs, mosquito pupse, cranefly larvas, frogs, snails, etc. Of group 4, Ranatra and rat-tailed maggot. Of group 5, Corethra, mosquito larvae, Daphnia, and a number of other micro-crustaceans. Of group 6, damselfly, mayfly and some dragonfly nymphs, amphipods, newly hatched amphibian larvae, etc.) Of group 7, hydras, vorticellidae, bryozoans (especially Plumatella), etc. Of group 8, crawfishes, dragonfly nymphs, Asellus, etc. Of group 9, Tubifex, dragonfly nymphs, small mussels, nymphs of Ephemera, etc. The record. — Find and include in the table as representa- tive an assemblage of forms as possible. Where many allied forms of closely similar habit are found, use but one example. II. ADJUSTMENT IN MANNER OF LIFE. We select for study under this heading three subjects only: i) Symbiosis: the adjustment in mode of life of two different organisms enabling them to live together in union with mutual advantage. 2) Parasitism: adjustment in mode of life between two different organisms for the benefit of the smaller and for the detriment of the larger. 3) Pollen production in flowering plants in relation to its distribution ; the adjustment of one special plant function in relation to physical and animal environment. I. SynMosis.^ Lichens are the best as well as the commonest illustrations of this phenomenon. Lichens may be gathered at any time from the trunks of trees, from stones and fences, and from ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 391 many other dry and sterile and unpromising situations. The gray encrusting species are commonest, but many forms occur. The well known "reindeer moss" is a lichen. For. the purpose of the following study, one of the gray Parme-, lias (fig. 227a) and one of the chimney lichens that grow on decaying stumps in damp woods (fig. 2276) will answer our needs. Pig. 227. Lichens, a, a common encrusting lichen, showing fruiting cup- ules; 6, a "chimney lichen," whose "chimneys" are covered with powdety white lichen buds (soredia). 392 GENERAL BIOLOGY Lichens appear as single organisms. They were long so considered. It is convenient to describe them still as single species; for they are such, for all practical purposes. But they are composite species, each consisting of a fungus and an alga, living together in structural and physiological union. The form of the combina- tion is domina- t e d by the fungus, which develops an un- derlying strat- um for attach- nient to the support, and a covering cortical layer having great capacity for resisting evaporation — of great advantage in exposed situ- ations: and in its more porous open fi b r o u s middle layer, shelters a host of algal cells. The color of the latter shows through when the lichen is wet, but the true relations of parts are best made out by cutting vertical sec- tions (fig. 229), through the thallus, and examining them Fig. 228. A strap lichen growing on a tree trunk in damp woods. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 393 Pig. 229. Vertical sec- tion through a lobe of a common lichen (Physcia stellaris), showing fruiting sur- face above, a, spores of the fungus; b, b, algal cells, held among the fungous filaments, ■which are loosely ar- ranged at c, but com- pacted together to form protective sur- face at d. (After Bessey). with a microscope. It will then be at once apparent that the body is mainly a complex of branched fungus filaments and that, the algal cells occu- pying the middle strattim, are in close union with some of these filaments, enwrapped by them, or indented by blunt protuberances from them. This union is for mutual benefit . We have already learned that a plant like this fungus, lacking chlorophyl, cannot get its carbon directly from the carbon dioxide of the air; and in such situa- tions, there is no other adequate source of supply. Through the agency of the green alga, however, and by means of its clo^e attachment to the algal cells, it gets carbon made up into assimilable form. It furnishes the alga in return shelter and protection and retains about it watery solutions containing the other materials for its food. The algal cells have room for growth and division; alga and fungus grow together, main- taining constant relations, resulting in a growth habit by which lichen species are known. The combination is an efficient one for meeting hard conditions of life in dry and sterile situations. ^li S'l ti-M^J A M\ »A " 394 GENERAL BIOLOGY Some species that live symbiotically can be cultivated apart ; but others appear to have become so fuUj' established in this manner of life that they are no longer able to live apart. There are other cases of symbiosis in different groups. We have already seen green hydras; the color is due to minute algal cells {zoochlorellce) living within the larger cells of the hydra, doubtless using there the carbon dioxide which the hydra cells excrete, and giving them back again the liberated oxygen for respiration. Attached to the roots of beech tree s are molds that do for the tree the absorbing work ordinarily performed by rhizoids, while the tree sup- plies them with carbon products. Thus here also the benefit is mutual. Study jo. The relations of fungus and alga in the lichen. Materials needed: Lichens of the two types shown in figure 227, the foliose one with spore cupules (apothecia) developed. Razor and pith for cutting sections. Place a cupule-bearing thallus between two wet pieces of pith, and cut vertical sections as thin as possible with a razor. Mount and examine a number of these and select the best for study. Mark the general arrangement and distribution of fungus and alga. Then study the fungus : i) The form of its filaments in the several layers. 2) The form of its fructification in the cupule; compare with an account of the Ascomycetes, in any good text -book of botany. Then study the alga. To do this remove the cover, tease the algal layer of a section to bits on a slide, cover again, and study the alga in the fragments. Determine the rela- tions to the algal cells of the investing fungus filaments. Look for evidences of division in the algal cells. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 395 Scrape a little of the whitish powdery substance covering the surface of the chimney lichen, and mount it on a slide in water; spread it out thin by pressure (with rotation) upon the cover glaSs, and study the dissociated fragments. These should be little groups of algal cells intertwined with fungus filaments — lichen buds (soredia): in short, minute lichens, ready for dispersal. Obviously, when the spores of the fungus upon geimination have to find and attach themselves to the proper algal cells there are some exigencies to be met that are obviated by this method of starting new plants by means of soredia. The record of this study may well consist in some diagrams and drawings of the facts demonstrated. Fig. 230. Nest of song sparrow containing three sparrow eggs and one cow bird egg. Photo kindly loaned by Professor C. H. Eigenmann. 396 GENERAL BIOLOGY 2. Parasitism. Parasitism is a relation between two species that costs the one its substance and the other its independence; the one species is called host, the other parasite. The cost to the host species may be light or severe, according to the extent of the parasitism. It is compara- tively light in such case as that of the song sparrow that hatches the cow-bird's egg. The latter is parasitic only to the ex- tent of the rearing of her brood. She deposits her egg in the nest of the sparrow (as shown in figure 230), supplanting a sparrow egg for the purpose, and leaves it there for the sparrow to hatch, and to feed through the nesting period. The cost to the host species may amount to personal discomfort merely, as in the case of many small external and internal para- sites of the larger mammals — lice, fleas, ticks, worms, etc. — or it may amount to loss of strength or even of life of many individuals. The host may be eaten by degrees by a single large parasite, as is the midge that makes the downy flower gall of goldenrod when parasitized by the braconid shown in fig. 231 ; or it may be eaten by a large number of smaller parasites, as is the caterpillar shown in fig. 232. In any case parasitism is the burden of the host species; but the manner of life of the host is little altered thereby. Such is not the case, however, with the parasite, which, according to the nature and extent of its dependence upon the host species, becomes always more or less degenerate. Fig. 231. Downy flower gall of the goldenrod. h, a gall and a flower head; t, a double gall split open, showing the pupa of the gall midge in one and the pupa of a (braconid) parasite in the other (light hand) chamber. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 397 The cowbird, relieved of the care of her young, has lost her nesting instincts. The Indian pipe (fig. 233) attached to the roots of trees whence it can draw manufactured carbon products, has lost its green color and its leaves. Sacculina, that famous illus- tration of the degen- eracy that results from the parasitic habit, living in the the perfect nurture and shelter afforded by its crab host, has lost all those structures and capacities by which we recognize its free living kindred. In general it may be said that in proportion as the conditions of living Fig. 232. A parasitized moth larva on Hue grass bcCOme simple,easy top: some of its parasites have spun their cocoons o*,^ co/mi-tq i-Ut^ beside it, others on the grass blade above; b, ''■^^ becure, cne shows an easy method of getting the adult para- nQi-aai+A r'r.moc +r\ sites from the cocoons. parasite COmeS tO lack those organs and faculties necessary to meet hard conditions, in battling with which they were developed. This loss is not the result of the parasitic habit, but of the sheltered life that goes with it. The series of insect larvae we have used to illustrate metamorphosis, excellently illustrates degeneracy also, though none of the larvee used was parasitic. It would not be difficult to select parasitic insect larvae, that would constitute parallel degeneration series. It seems clear that, as in the individual, so in the long run in the race, it is effort that builds; disuse leads to degeneration. 398 GENERAL BIOLOGY The two primal functions of feeding and reproduction not even the parasite may lose; on the contrary it often develops improved feeding apparatus and increased reproductive capacity; sacculina has done so; and the liver fluke, which is parasitic on two hosts, snail and sheep, at different stages of its existence has developed an extraordinary reproductive capacity to meet the exi- gencies of shifting from one host to the other. Parasitism may be either external or internal, temporary or permanent, at one stage, or during the whole life of either host or parasite, on the part of the female only (in its incipiency, the female seek- ing shelter for her brood) or on the part of both sexes. Parasitism is one of many possible shifts for a living. The opportunities for it have lain in the accumulation of stores of rich organic products on the part of the larger organisms. These are available only to smaller species. Hence parasitism is a prevalent habit mainly among the smaller organisms. The larger parasites offer like opportunities for smaller ones, and are themselves parasitized. The common bittern has as an external parasite the fly shown in fig. 234, living among its feathers. The fly has its own external parasites — the mites show in the figure, clustered at the joints of the legs, where Fig. 233. Indian pipe, a leafless para- sitic flowering plant. ADJUSTMfiN-T OF ORGANISMS TO ENVIRONMENT 399 thin connecting membranes offer a point of attack. Es- caping from the pressure of competition, and from the attack of enemies, a few of the smaller representatives of many groups have become parasites. In those groups of the Hymenop- tera that are most extensively addicted to the parasitic habit, primary parasites are commonly followed by secondary parasites, (hyperparasites) , and these occa- sionally by tertiary parasites, th? difference in size between ho3X and parasite, being here at a minimum. Parasites are nature's agents for regulating the natural balance . They prevent the undue increase of any species. They are them- selves self regulating; for with their own undue increase, they eliminate themselves by eliminat- ing their own food supply. In recent years the aid of parasites has been sought to stay the ravages of noxious species, like the gypsy moth. Sometimes they are imported for this purpose; in which case care is taken to leave their hyper-parasites behind. A moment's reflection upon the facts that have been before us in this course will make it clear that parasitism is by no means sharply distinguished from other phenomena of dependence of one individual upon another. It is living upon the living, plant upon plant, animal upon animal, one species upon another, that we call parasitism. That the boundary between symbiosis and parasitism is not hard and fast is shown by the case of the nematode that lives in the Fig. 234. A parasitic fly (Olfersia) that lives among the feathers of the bittern, bearing clusters of parasi- tic mites at the joints of its own body and legs. 4O0 GENERAL BIOLOGY body cavity of the earthworm (cited in chap. Ill, p. 178). Ordinarily, nematodes found in such situation are parasites, but here they are found clearing up the lumps of waste chloragogue — impedimenta to the worm — accumulated in the hinder segments, and the relation seems to be one of mutual advantage. Were the two species mutually dependent in this function, we should call it symbiosis. As it is we call it commensalism, and say that the nematode is a guest, and not a parasite. Commensalism may well have been at times a transition stage in the development of para- sitic habits. Study 57. A comparative examination of a series oj parasites of a single order. ^ Materials : As good a series of specimens for comparison as may be had in any favorable group ; flowering parasitic plants; copepods, crabs, worms, etc. Hosts may be dis- regarded. Compare together as to: Organs of feeding. Organs of reproduction. Organs of locomotion. Organs of sense perception. Compare males and females of each parasite if possible as to degree of degeneracy. Compare together young larval, and adult forms of the more completely parasitic species selected for study. The record of observations should be preserved in notes and sketches. 3. Pollen Distribution. In our study of the green plant series (Chapter III) , we have seen how the motile sperm cells of the primeval aquatic plant gradually lost their opportunity for swimming ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 401 to meet the ovum, as plants became terrestrial and grew to larger size. The distances to be traversed in order to accom- plish fertilization became greater and the route lay through the air; transportation became necessary; and it came about that the carriage of the microspore, and not of the naked sperm cell, was the plan that suc- cessfully met the difficulties of the situation. Flowering plants were sur- rounded by various means of transportation for their pollen. Two of these were of prime im- portance; the wind and winged insects. The wind had certain great advantages. It could be be depended on to blow at all seasons, night and day, and if pollen were light enough, to sift it ever5rwhere, and to deposit some of it in the right place for cross fertilization. But on the other hand, it was quite indis- criminating as to where it should blow, and very wasteful of pollen in consequence. Winged insects on their part, having a liking for the nectar of flowers, would fly from flower to flower with great precision, and if only the flower could adjust itself to profit thereby, would distribute the pollen with far less waste. But their aid was less trust- worthy, and might at any time prove inadequate; they were liable to casualties of storm and pestilence. Their very power of selection might lead them to neglect one species for others more attractive. And their aid was most needed by species of sparse distribution. Fig. 235. Black oak flowers. m, a single pistillate flower; «, a single staminate flower, be- fore the bursting of the anthers. 40 2 GENERAL BIOLOGY We have learned from the studies in Chapter I to what extent our common plants have become adapted tQ insect aid in pollen transference, and how greatly they h^vg become modified in special adaptation thereto. We are pow to study comparatively the results in pollen prod^iction of adaptations to all the various means of securing fertilization including water flotation of the pollen of submerged aqua= tics that bloom at the surface, and the automg.'tJe self pollinating acts of flowers themselves. Study 52. Pollen production as affected by its mode of distribution. Materials needed: below : Flowers of the nine sorts indicated Wind pollinated. Insect pollinated I, 1. Tree, such as oak (fig. 235), hickory, box elder or hornbeam. 2 . Herb, such as meadow rue, grass or sedge. 3. A large open solitary flower such as trillium or may -apple. 4. An open, loosely clustered flower, such as spring beauty, or buttercup. 5. A highly specialized bilateral flower, such as the woodbetonyor sweet pea. 6. A composite flower, such as the dande- lion (fig. 236). Water pollinated, f. A river weed (Potamogeton) . 8. Open, chickweed (Stellaria media) or door weed (Polygonum). 9. Clistogamous, the blue violet (Viola cucullata) . All these will be obtainable an3rwhere in spring, except, numbers 7 and 9, both of which may be used, preserved in alcohol. The clistogamous flowers of the violet may be found through the whole summer after the blue flowers have. Self pollinated ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 403 ceased to appear see figure 27, on page 35. The function of these has been discussed on page 34. Study these individually, and write their characters with which we are now concerned in a table prepared with the following column headings: Name. Sex (male, female or bisexual) .* Form of flower cluster. Number of stamens per flower. Number of pistils per flower. Number of pollen grains per stamen. Number of ovules per carpel. Ratio for the whole plant of pollen grains to ovules. The labor of making this table chiefly consists in counting the pollen grains in anthers of the nine species selected. The number of ovules will usually be found stated in the larger works on systematic botany, and these may be used for reference. Since there are some slight difficulties of manipulation to be encountered, it may be well to suggest, how to proceed. Get anthers for pollen counting from unopened buds, in order that the previous shedding of some of the pollen may not vitiate the count. Select anthers of average size, or, better, count several and average the result. Large anthers, like those of trillium, should be divided, say into eighths, and a part taken. This is easily done by placing the anther flat on a slide and pressing the edge of a scalpel into it with a rocking motion, being careful to make approximately equal successive divisions. Then select an average segment, expose its pollen fully, cover and count, and multiply to get the whole. Very small anthers, like those of the dandelion, are likely to be quite transparent, and need only to be mounted and covered, and their pollen content may be counted at once. It will be necessary to 404 GENERAL BIOLOGY Split the anther tube of the dandelion, and spread it out flat before covering (fig. 236). When the pollen cavities are so filled that they appear dark, a little pressure on the cover will often burst them and scatter the pollen, so that it may be counted. The gist of this study is in the ratios of the last colurnn. For ready com- parison they should be reduced to the form x:i. With perfect flowers the ratio of pollen grains to ovules produced will be the same for the whole plant as for the single flower, but with moncecious (fig. 235) and dioecious species it will be necessary to count and estimate for equivalent pro- portions of the total of male and female inflorescence. The record. — In conclu- sion, ascertain from the facts of the completed table whether the form of the cluster or the manner of flower aggregation in it have had any effect on the amount of pollen produced. Fig. 236. A single dandelion removed from the flower head. /, stigma; k, the anther ring, split and unrolled at o, the separate filaments shown at p: M, strap-shaped corolla; I, calyx (pap- pus) or bristles; m, ovule case. III. ADJUSTMENT IN FORM AND APPEARANCE. When we have gotten to this division of our subject, it haS •already been illustrated in manifold ways by the organisms we have had before us. Nevertheless, it will still repay a more careful examination. ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 405 There is the adjustment of the individual to external conditions, and there is the adjustment of the race. The Fig. 237. Cross-section of an elm bough with its history written in its wood , rings. former is familiar to our experience. The tanning of the skin with exposure to the sun, the strengthening of the 4o6 GENERAL BIOLOGY muscles through use, acclimation, immunisation; these and many others are every day illustrations of the response of the individual to conditions of environment. Figure 237 shows the record in wood of a series of successive responses on the part of the bough of an elm tree during the 2 5 years of its life. The five dark rings in the center represent the first five years of erect growth (187 8- 1882), while it was still near the top of the tree, and abundantlj^and symmetrically lighted. It started in 1878 from a bud formed on the west side of the top shoot of a three year old sapling. The twelve close set rings following represent the scanty growth of the next twelve years (1883-1894), during which it was struggling for light beneath the higher branches that had overtopped it. The larger growth ring for 1888 represents the result of a windy season, when the tossing about of the upper branches allowed this one to get more light! During this time the bough was leaning slightly to westward, as indicated by the greater thickness of the rings on that side— the lower side in the figure. The ensuing sudden unilateral enlargement of the rings was due to an accident. Some children climbing in the tree bent this bough down, and left it in a somewhat drooping position. Thus, it was brought out from the shadow into the light again, and the rapid growth that followed was, in consequence of its position, on the under side of the bough at the bend where this section was made. It will be observed that for four years (1895— 1898) the addition of woody tissue was bilaterally sjmi- metrical upon the lower side. Then another accident changed the stress upon it and caused it to grow obliquely. It chanced to overhang a walk, and in the spring of 1899 to correct its drooping it was hung up lightly on a wire at- attached to a fork above and a little to one side. The puH w^as to the northward during the ensuing four years (1899- 1902), and this, assisted by prevaiUng south-west windSr ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 407 caused more woody tissue to be formed on that side. In the winter of 1 90 2, the bough was cut, and its autobiography- was interpreted with the aid of competent testimony that was still available. Our practical studies shall be of the modifications of form and appearance that belongs to racial, and not to indi- vidual history. I. The re -adaptation of insects to aquatic life. It seems now quite clear that insects were primitively terrestrial. They are covered with a tough chitinized skin, well adapted to resist evaporation. They are provided with a respiratory apparatus of distinctively aerial type. They breathe through open spiracles, that lead to inter- communicating air tubes (tracheae) within the body. As adults, they all breathe free air, and are adapted only by secondary makeshifts to aquatic life. It is only the larvae of scattering groups that have become properly aquatic, and able to breathe the air that is dissolved in the water — all the larvae of a few small groups, and scattering members of most of the larger orders. Among these, therefore, we should be able to see the result of the fitting of diverse forms to the new conditions. When, with the luxuriant development of the insect group, the press of life on land crowded some insects back into the water, the problem of getting air was the chief one to be encountered. Its full solution lay in the development of suitable respiratory apparatus. An impervious chiti- nized skin perforated by open air tubes stood in the way of ready re-adaptation. Adult insects merely adopted various devices for carrying or otherwise obtaining free air when .in the water, without altering their mode of respiring it: many insect larvae, also, get their air supply only at the surface (fig. 238). But the softer and more plastic larvae. 4o8 GENERAL BIOLOGY thin skinned and permeable, are able to get oxygen from the water, and have become strictly aquatic. Among aquatic insect larvae (properly so-called) are found three respiratory types: 1 . Those without gills. —^ These are minute larvae, like those of the biting midges (Ceratopogon, fig. 239) that live in floating masses of filamentous alg£e, where liberated oxygen is abundant, or, if of larger size, as in the case of some stoneflies (Perlidce) they live in rapid and well aerated water. The larger of these although lacking gills have an abundant development of fine air tubes in the thin mem- branes joining the Fig. 238. The larva of a swale fly (Sepedon ^^ . j_ r j 1 fuscipennis). a. Pulling away from the tnOraClC Segments OI the surface film, the guard hairs surrounding ^ j , , , ., the breathing pores convergent at tips; b, DOuy On tlie Ventral end of body as seen when resting on the . j surface, hairs outspread. Side. 2. Those with blood gills. — These most nearly approximate aquatic vertebrate larvae in their mode of respiration. Blood gills are protrusions of the body wall through which the blood flows; the exchange of gases in respiration takes place between the blood inside •'3and- the water without. Blood gills are developed in many dipterous larvae, and oftenest, about the posterior end of the alimentary canal (fig 239^). In dipterous larvae the tracheae are often somewhat reduced. ADJUSTMENT OP ORGANISMS TO ENVIRONMENT 409 3. Those with tracheal gills. — These, comprising the larger larvae of all the more generalized orders of aquatic insects, have adhered more strictly to the tracheate type of respiratory apparatus. Tracheal gills are protrusions of the body- wall with fine tracheal tubes grown out into them, and the exchange of gases in respiration is between the air in the tubes and the water outside the gill. The tracheal system, therefore, instead of being reduced, is increased by the out- growth of the additional parts that penetrate the gills. I I I -Ll a=i=#". Fig. 239. Larvae of dipterous insects, jc, the piitikie (,Ceratopogpn). y .the pliantom larva of Corethra; z, a "blood worm" — the larva of 'a , midge (fihironomus) ;/, floats (expansions of the main air tubes) ; g, g. g, blood gills. Tracheal gills may be external as in the, case of the damsel- fly nymph shown in fig. 225, or internal, as in the case of the larger dragonfly njonph shown in figure 240. Whatever their position, number or arrangement, they conform more or less closely in shape to two types, filiform or cylindric, and lamelliform or flat. Their, diversity in form, position, arrangement and num- ber and size will be seen in the series of larvae selected for study. 4IO GENERAL BIOLOGY Study 5J. A preliminary examination in living specimens of tlie principal gill types of aquatic insects. Materials needed: Living larvae to illustrate : 1. Blood gills (larvae of Culex, the mosquito, Corethra or Chironomus) . 2. Tracheal gills: „ ^ j Filamentous(larvae of a caddis fly, etc.) : I Lamellif orm (nymphs of a damselfly or mayfly) . Internal (nymphs of the dragonfly, Libellula). Mount a larva having blood gills in a copious supply of water, cover and study the gills directly, noting their number, position and relations. Focus carefully upon one gill to see the outline of its internal cavity, and to see the leuco- cytes that drift about in it. To study the ex- ternal tracheal gills, snip off a few gills with fine scissors and mount them in water ; cover and examine at once, to see the tracheoles before the penetration of the water into them has rendered these invisible. While filled with air they appear as sharply defined black lines. They are Fig. 240. Dragonfly nymph (Cdithemis eponina). ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 411 not visible in preserved specimens; hence, living larvse must be had for this. Study especially the division of the large tracheae into fine tracheoles and the disposition of the latter and their intercommunications. The internal giUs of a dragonfly (fig. 241) are arranged in rows upon the inner walls of a gill chamber, made out of the posterior third of the alimentary canal. It is so fine a piece of respiratory apparatus, so unique in plan and it exhibits such delicacy and refinement of structure it is well worth a careful examination. It will be well first to see the external evidences of its operation. Regular respiratory movements of the abdomen can usually be seen in a nymph that lies quietly in a shallow dish of water. They may often be seen intensified if the nymph be turned over on its back. With the expansion of the abdomen water is slowly taken in through the anal aperture to be expelled with its contraction. The currents of the water may be demonstrated by placing some colored fluid in the water close beside the anal opening. This is best done by holding the point of a copying pencil in that position until its color is imparted to the water. The forcible e jec- tion of water from this gill chamber as an aid to propulsion may be seen while the nymph is swimming about. Some idea of the force of the expulsion may be gained by tilting the abdomen of a swimming nymph upward until it touches the surface of the water, when the water in the gill chamber will be shot into the air. To study the structure of the gill chamber and of the gills themselves, the following method will be found to.be expedi- tious and satisfactory. Kill the nymph by snipping off its head. Then snip off the abdomen at its base ; trim off its sharply triangular lateral margins for its whole length ; pin it down to the waxed bottom of a dissecting dish that is small enough for use on the stage of a dissecting microscope, 412 GENERAL BIOLOGY or under a pocket lens ; carefully lift off the roof of the abdomen, (already loosened at the sides by the trim-oflf of the margins,) by seizing it in front with the forceps. This will expose the gill chamber, which occupies the greater part of the abdominal cavity, and terminates the alimentary canal. The severed posterior end of the stomach will be seen in the middle in front, terminated in the rear by a dense cluster of nephridia (Malpighian tubules) and followed by a slender, white, ventrally curved and much concealed intestine, joining it to the gill chamber. On either side of the stomach will be seen a large, silvery white air trunk, which breaks up posteriorly into a great brush of lesser branches that penetrate the walls of the gill chamber. This chamber itself, will be somewhat collapsed ; it may be distended by injecting air or water through the anal aper- ture with a fine-pointed pipette; its longitudinal extent may be seen by lifting the stomach with a forceps and drawing it forward. If turned to one side, a ventral longitudinal tracheal trunk may be seen on either side of the body, breaking up in the rear, like the dorsal trunk, into a multitude of branches, and entering the walls of the gill chamber from below. Through the transparent walls of the gill chamber may be seen lines of the black pigment that occupies the bases of the internal gill plates. Discovering thus the location of the rows of gills, the chamber may be safely opened by inserting the point of a fine scissors and cutting the wall for its entire length between two rows. The circular muscles of the wall will, by their contraction turn the whole organ inside out, and fully expose the rows of beautiful, feathery, purplish tinted gill plates. Then if a row be isolated with scissors and mounted on a slide in water, a few individual gills may readily be isolated with needles under a dissecting lens, covered, and studied with ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 413 a microscope. The accompanying figures (fig. 241) will assist in identifying all the structures present. The record for this study may be in the form of sketches and diagrams of the respiratory apparatus studied. Fig. 241. Diagram of the gill chamber of the nymph of a dragonfly (j4«o« Junius) from drawings by Miss Elizabeth Andrews, a, cross section of the gill chamber; d, d, dorsal tracheal trunks; u, u, ventral trunks; /, tuft of filamentous gills; m, longitudinal muscle; h, a single gill filament, showing traches and tracheoles. Study 54. The comparative development of respiratory apparatus in aquatic insect larvce. Materials needed: Either preserved or fresh specimens of larvae of the following: Ceratopogon, or some other gill- less form (perlid or trichopter will do as well) . I. Two or more dipterous larvae having blood gills of different sort: Chironomus (the larger "blood worms," with ventral abdominal gills) and Simulium will be best, 414 GENERAL BIOLOGY jSJir \ iS ' ■■ i wi?; '^