1 i H I ill 111 DiHH J 1 1 1 1 1 1 III Ifl|BflB^fl llllflll MARINE BIOLOGICAL LABORATORY. Received F.e.b.r.ua.r.Y.., 19.3.4 Accession No. .^. .?.?..?. .7 ^. . Dr. ; I. '!. Newman Given by liversity of Chicago lace, *** No book or pamphlet is to be removed from the Lab- atory without the permission of the Trustees. ■ ru ' ru i ""^ i o I □ r-q CD m THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS THE BAKER & TAYLOR COMPANY NEW YORK THE CAMBRIDGE UNIVERSITY PRESS LONDON THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI THE COMMERCIAL PRESS, LIMITED SHANGHAI EVOLUTION, GENETICS AND EUGENICS By HORATIO HACKETT NEWMAN Professor of Zoology in the University of Chicago THIRD EDITION THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS COPYRIGHT 1 92I, I925, AND 1 932 BY THE UNIVERSITY OF CHICAGO. ALL RIGHTS RESERVED. PUBLISHED OCTOBER I 92I SECOND EDITION DECEMBER I925. THIRD EDITION APRIL I932 SECOND IMPRESSION SEPTEMBER I 933 COMPOSED AND PRINTED BY THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS, U.S.A. THIS VOLUME IS AFFECTIONATELY DEDICATED TO MY MOTHER PREFACE TO FIRST EDITION This book has been prepared to meet a specific demand, long felt here and elsewhere, for an account of the various phases of evolu- tionary biology condensed within the scope of one volume of moderate size. The present writer has now for sixteen successive years pre- sented in lecture form to large classes of students the subjects of evolution, genetics, and eugenics. Never have we been able to find a single book that would cover the required ground. In fact it has been necessary to require, or at least to recommend, as many as three books. It is believed that the present book will furnish ade- quate reading material for a major or a semester course in evolutionary biology. Some supplementary reading may be necessary in case an instructor wishes to emphasize one or more phases of the subject; but for a first course in the subject we believe that all of the essential reading material will be found within the text itself. An effort has been made to present the subject in the best peda- gogical order. After a general introduction, a rather long chapter appears in which the whole history of the development of evolution- ary science is outlined, together with the names and contributions of the leading evolutionists. Part II is a presentation of the evi- dences of organic evolution, beginning with the bodies of evidence most definite and direct, and ending with the less definite and more controversial. Part III deals with causo-mechanical theories of evolution with Darwinism as the central topic. Part IV concerns itself with genetics or modern experimental evolution, and Part V with eugenics, or genetics as applied to human improvement. The book consists largely of excerpts', some long and some short, from both the older classical evolutionary writers and the modern writers. Our aim has been to select the most significant or character- istic passages from each author. In most cases this ideal has been attained, but it has sometimes happened that we have had to make our selection of material to meet a real need in the book, and accord- ingly have selected from an author a passage he himself might not consider particularly characteristic of his work. We have succeeded, nevertheless, in welding together out of a collection of isolated chapters and passages what seems to us to be a close approach to a coherent unit. Unification has been accomplished by the aid. of editorial connecting passages, introductory statements, criticisms, and sum- maries. In certain cases it became necessary, for a variety of reasons, vii vin PREFACE TO FIRST EDITION for the editor to write short chapters on certain topics that were not presented in the available literature in sufficiently brief compass or in sufficiently non-technical language. The one-man textbook is only too often written to emphasize the author's pet theories and is likely to be unduly biased. The present work is completely non-partisan. It consists of the writ- ings of many authors and presents many diverse theories. The student is left to balance the various views one against another and to form his own judgment. It is very unfortunate, but none the less true, that even in these scientific days, the subject of evolution has a bad name in many communities and in many educational institutions with religious affiliations. The mistake is made of supposing that evolution and religion are diametrically opposed. The present writer has been at some pains to make it clear that evolution and religion are strictly compatible. We teachers of evolution in the colleges have no sinister designs upon the religious faith of our students. While this book is intended primarily for a college textbook, we have also had in mind the general reader. Apart from a few of the more technical details, the text seems to us very readable. The language of the great classic writers— Darwin, Wallace, Romanes, De Vries, Le Conte — is simple and lucid. Among recent biological books few are written so freshly and vividly as those of Professor J. Arthur Thomson. The clearness and scientific accuracy of Conklin, Saleeby, Guyer, Walter, Lull, Osborn, the Coulters, Downing, Shull, Tayler, Popenoe, Johnson, and others, are familiar to American biologists. Scrupulous care has been taken to verify all passages quoted, but it is hardly likely that, in so large a mass of material, all errors shall have been avoided. The author and the publishers would welcome as a favor any suggestions or corrections submitted by interested readers. A list of the books from which material has been quoted is given on pages 612, 613. To the authors and publishers of these books and monographs we wish herewith to tender our grateful acknowledg- ments for their generosity and co-operation. A considerable amount of materia] for which permission to reprint had been granted fails to appear in the present volume. It is hoped to incorporate this material in an appendix to a later edition, or else to use it in the form of a small volume of supplementary readings. H. H. N. August 15, 1921 PREFACE TO SECOND EDITION A book of this sort somewhat resembles a loose-leaf encyclopedia in that it subjects itself very readily to revision and rearrangement and thus may be kept abreast of the times. When certain sections prove through actual class use to need revision or restatement or when new discoveries necessitate a change in conclusions, appropriate cor- rections may be made, new matter added, or whole chapters re- written. When various topics have shown themselves to be either logically or pedagogically in the wrong order, it is easy to rearrange chapters, for the latter are to a large degree independent. When, finally, any new chapter of superior excellence appears in a new pub- lication, it is usually possible through the courtesy of author and pub- lisher to add it to the worthy collection of excerpts already gathered together. The writer has been fortunate in that reviewers and colleagues both in America and in Europe have offered many constructive criticisms of the book and suggestions for its improvement. It is hoped that the present edition will adequately reflect this expert advice. The order of presentation of the evidences of evolution has been changed from one based on the degree of directness of the evidence to one based on the logical succession of topics and their interde- pendence. The chapters on "The Mutation Theory" and "The Inheritance of Acquired Characters" have been placed near the end of the book in order that they may be considered in the full light of our present knowledge of genetics. The chapter on "Linkage and Crossing-over" has been rewritten in a more elementary and cir- cumstantial style in order to overcome, if possible, the difficulty that students have always encountered in understanding the somewhat condensed and technical account of Professor Castle. The discussion of mutations has been modernized and considerably extended through the addition of an article written especially for this book by Professor R. Ruggles Gates and of a paper by Professor H. F. Muller. The section on eugenics has been strengthened by the addition of a lucid iz X PREFACE TO REVISED EDITION chapter from Albert Edward Wiggam's book The Fruit of the Family Tree. The writer has introduced a considerable amount of new matter of his own which consists chiefly of relatively short introductory, con- necting, explanatory, and summarizing statements that serve to cement the otherwise somewhat disconnected excerpts into a coherent whole. Because of the fact that so many of the writer's own contribu- tions are scattered through the book, it is deemed wise to omit his name from all such chapters and passages, with the understanding that all matter not specifically credited to others is his own. Excerpts from books commonly contain undefined technical terms that perplex the beginning student. This difficulty has been overcome through the addition of a full glossary defining nearly all of the bio- logical terms used in the book. Grateful acknowledgments for the use of new materials are here- with given to the following authors and publishers: E. G. Conklin, R. R. Gates, S. J. Holmes, D. F. Jones, T. H. Morgan, H. F. Muller, G. H. Thayer, A. E. Wiggam, E. B. Wilson; The Bobbs-Merrill Com- pany, Henry Holt and Company, The Macmillan Company, The Princeton University Press, The Williams and Wilkins Company. H. H. Newman University of Chicago April 6, 1925 PREFACE TO THIRD EDITION The first edition of this book was avowedly a book of readings, a source book, consisting almost exclusively of extracts from leading writers on evolutionary topics. The character of the book made it somewhat disjointed. It was criticized for lack of unity of plan and was called by one critic a hodge-podge. In spite of these defects many teachers found the book valuable as a reading source to accompany lectures. In the second edition an attempt was made to unify the material by the addition of a considerable amount of editorial comment in the form of short introductory and transitional chapters. This method of unification was, however, not sufficiently radical. The result was nei- ther a source book nor an organized textbook. Believing that what most teachers want is primarily a textbook and only secondarily a source book, the author has attempted to make the book a text by reorganizing the previously less organized material and by taking out and putting into the Appendix chapters that break the continuity of treatment but are useful for collateral reading. Teachers using the book recommended that the historical account of evolution and the section on evidences of evolution be left substantially as they were, but they suggested that the section on genetics be largely rewritten. This has been done. The plan followed is that of dealing with genetics as the study of the mechanism of evolution, the main factors of which are (i) the persistence factor, heredity; (2) the diver- sity factor, sexual reproduction and Mendelian heredity; (3) the change factor, mutation; (4) guiding factors, selection, and possibly the Lamarckian factor and orthogenesis; and (5) the dividing factor, isola- tion in the broad sense. The part on eugenics has also been extensively revised and rewritten, and contains a good deal more factual material. It is hoped that in its present form the book may more nearly meet the needs of college classes in evolutionary biology. It gives us pleasure to acknowledge gratefully the permission given us by The Macmillan Company to use figures 40, 41, 42, 43, 44, 45, and 51. H. H. Newman University of Chicago January 16, 1932 • i \ y« ,: ' TABLE OF CONTENTS PAGE List of Illustrations xxi PART I. INTRODUCTORY AND HISTORICAL Chapter I. Introduction 3 What Organic Evolution Is — Definitions 3 The Modern Attitude as to the Truth of the Evolution Doctrine . 5 What Organic Evolution Is Not 8 Chapter II. Historical Account of the Development op the Evolution Theory ic Evolution among the Greeks n Post-Aristotelians 14 The Early Theologians 14 The Revival of Science 15 The Great Naturalists of the Eighteenth Century 16 Lamarck 18 Cuvier and Geoff roy St. Hilaire 21 Catastrophism and Uniformitarianism 22 The Reawakening of the Evolution Idea 23 Charles Darwin 24 Summary of Darwin's Theories 25 Contemporary Opinion Regarding the Validity of Darwin's Views 2 7 Isolation Theories 32 Orthogenesis Theories 33 The Mutation Theory of De Vries 36 The Rise and Vogue of Biometry 3S Experimental Breeding 39 Mendel's Laws 4 1 Hybridization and the Origin of Species 43 Neo-Mendelian Developments 43 Heredity and Sex 44 The Experimental Induction of Hereditary Variations .... 45 The Recent Attack upon Evolution in the United States ... 45 Concluding Remarks 46 PART II. EVIDENCES OF ORGANIC EVOLUTION Chapter III. Is Organic Evolution an Established Principle? . 49 Chapter IV. The Fundamental Postulate Underlying All Evi- dences of Evolution 53 xiii 42797 XIV TABLE OF CONTENTS PAGE Chapter V. Evidences from Morphology (Comparative Anat- omy). George John Romanes 58 Chapter VI. Evidences from Classification 93 The Principles of Classification. A. F. Skull 93 The Method of Classification. Charles Darwin 96 What Is a Species? 97 Chapter VII. Evidence from Blood Tests. W. B. Scott . . . 100 Chapter VIII. Evidences from Embryology 105 The Facts of Reproduction and Development 105 Outline of Animal Development. D. S. Jordan and V. L. Kellogg. 106 Chapter IX. Critique of the Recapitulation Theory. IV. B. Scott 1 14 Chapter X. Evidences from Palaeontology 124 Strength and Weakness of the Evidence 124 Other Opinions as to the Adequacy of the Evidences from Palaeon- tology 125 What Fossils Are and How They Have Been Preserved . . . 126 Fossils Classified 126 On the Conditions Necessary for Fossilization 127 On the Lapse of Time during Which Evolution Is Believed to Have Taken Place 130 On the Principal General Facts Revealed by a Study of the Fossils 132 Fossil Pedigrees of Some Well-known Vertebrates 133 Pedigree of the Horse 133 Pedigree of the Camels. W. B. Scott 136 Evolution of the Elephants. A. Franklin Skull 139 Chapter XI. The Evolution of Man: Palaeontology. Richard Swann Lull 144 Origin of Primates 144 Origin of Man 145 Fossil Man 147 Evidences of Human Antiquity 157 Future of Humanity 158 Sinanthropus pekinensis 159 Chapter XII. Evidences from Geographic Distribution . . . 160 Principles of Geographic Distribution 160 Some of the More Significant Facts about the Distribution of Ani- mals 164 The Fauna of Oceanic Islands. George John Romanes . . . .164 The Fauna of Continental Islands — Madagascar and New Zealand. A. R. Wallace 173 TABLE OF CONTENTS XV PAGE The Distribution of Marsupials. A. R. Wallace 174 The Distribution of Birds. A. R. Wallace 175 Summary of Mammalian Dispersal. Hans Gadow ...... 177 Summary of the Argument for Evolution as Based on Geographic Distribution 178 PART III. THE MECHANISM OF EVOLUTION (GENETICS) Chapter XIII. Introductory Statement 183 The Nature and Scope of Genetics 183 Prerequisites for the Study of Genetics 184 The Mechanism of Evolution 185 The Main Causal Factors of Evolution 186 Chapter XIV. The Biological Background of Genetics . . 190 Races and Individuals 190 The Cellular Make-up of Individuals 190 A Typical Cell and Its Component Parts 191 Cell Division — Mitosis 194 Differentiation 195 The Origin of New Individuals 196 Modes of Reproduction 196 The Origin of Gametes 200 The Maturing of Gametes 203 The Union of Gametes — Fertilization 207 Chapter XV. Introduction to the Study of Persistence Factors 209 Persistence and Diversity Mechanisms Contrasted 210 A Short Lesson in Biometry 210 Chapter XVI. Heredity in Puke Lines 212 The Nature of Pure Lines 212 The Pure-Line Experiments of Johannsen 212 Other Examples of Pure Lines 215 Exactly What Is Inherited? 217 Chapter XVII. Sex Determination and Sex Differentiation . 219 Sex Determination 219 The Chromosomal Mechanism of Sex Determination . . . 221 Sex Differentiation 227 Chapter XVIII. Mendel's Laws of Heredity 232 Mendel's Life and Character. /. Arthur Thomson 232 Mendel's Discoveries 232 Mendel's Explanations. John M. and Merle C. Coulter .... 23S Illustrations of Simple Mendelian Inheritance in Both Animals and Plants. /. Arthur Thomson 245 xvi TABLE OF CONTENTS PAGE Chapter XIX. The Factor Hypothesis as Applied to Plants. John M. and Merle C. Coulter 253 I. Presence and Absence Hypothesis 253 II. Blends 255 III. The Factor Hypothesis 257 Chapter XX. The Factor Hypothesis as Applied to Animals . 269 Illustrations of the Factor Hypothesis 269 The Factorial Analysis of Color in Mice 269 Different Kinds of Albinos 270 Factorial Analysis of Coat Color in Swine 271 Coat Colors in Guinea Pigs 272 Chapter XXI. Review op Mendelism 273 Chapter XXII. Sex-linked Heredity 277 Chapter XXIII. Linkage, Crossing-over, and the Architecture of the Germ Plasm 285 Linkage 285 Crossing-over 288 Chromosome Maps Indicating the Arrangement of Mendelian Fac- tors, or Genes, in the Chromosomes 291 Linkage in Other Organisms 296 Chapter XXIV. Cross-breeding and Inbreeding 299 The Role of Hybridization in Evolution 299 Some Animal Hybrids 300 Secondary Effects of Cross-breeding and Inbreeding . . . .302 A. Cross-breeding 302 B. Inbreeding 304 Chapter XXV. Change Factors. Introduction 308 Changing Views as to the Origin of New Hereditary Characters .' 308 Lamarck 308 Darwin 308 Weismann 309 Mutations 312 Chapter XXVI. The Mutation Theory 313 New Species (Mutants) of Oenothera. Hugo Dc Vries . . . . 315 Summary of De Vries's Mutation Theory. Thomas Hunt Morgan . 322 Later Investigations of Mutations 327 The Neo-Mutationist Position. R. Ruggles Gates 328 Chromosome Mutations 329 Mutation. H. J. Midler 333 TABLE OF CONTENTS xvii PACE The Causes of Mutations 340 Mutation and Evolution. T. H. Morgan 341 Chapter XXVII. Guiding Factors. Introduction .... 344 Preliminary Discussion 344 Adaptations 344 Orthogenetic Trends 345 Progressive Evolution . 345 Specialization 346 Increase in Size 348 Increase in Intelligence 348 Chapter XXVIII. Adaptation in Nature 349 The Nature of Adaptations 340 Two Categories of Adaptations 353 Adaptations Classified 354 Some Special Adaptations 355 Parasitism and Degeneration 356 Adaptations of Deep-Sea Animals and of Cave Animals . . .35c; Color and Pattern in Animals 360 Osborn's Laws of Adaptation 366 Chapter XXIX. The Web of Life. J. Arthur Thomson . . . 370 Chapter XXX. Natural Selection 383 Summary of Darwin's Natural-Selection Theory. Vernon L. Kellogg 383 Objections to Selection 385 Defense of Selection 390 General Defense of Selection. J . L. Tayler 391 Experimental Support of the Effectiveness of Natural Selection . 394 The Present Status of Natural Selection 396 The Relation of Mendelism and the Mutation Theory to Natural Selection. C. C. Nutting 396 Chapter XXXI. Are Acquired Characters (Modifications) Hereditary? 401 Misunderstandings as to the Question at Issue. /. Arthur Thomson 401 The Inheritance or Non-Inheritance of Acquired Characters. Edwin Grant Conklin 408 The Other Side of the Question 414 A Possible Mechanism for the Transmission of Acquired Characters. Michael F. Guyer 416 Recent Experiments Believed to Favor the Lamarckian Theory . 423 Chapter XXXII. Other Possible Guiding Factors .... 426 Orthogenesis 426 Vitalistic Theories 428 xviil TABLE OF CONTENTS PAGE Chapter XXXIII. Dividing Factors. Isolation 430 Introductory 430 "Isolation" Used in the Broadest Sense 431 Isolation and Inbreeding 43 1 Isolation and Selection 43 2 The Various Categories of Isolation 432 a) Geographic Isolation 43 2 b) Isolation Due to Sheer Distance Apart 435 c) Climatic Isolation 436 d) Biotic Isolation 436 e) Reproductive Isolation 436 /) Psychic Isolation 438 PART IV. EUGENICS Chapter XXXIV. Introduction to Eugenics 441 Definitions of Eugenics 441 The Scope of Eugenics 442 The Aims and Ideals of Eugenics 443 Methods of Research in Eugenics 444 Chapter XXXV. Human Heredity as Revealed by Pedigrees . 446 Weaknesses of the Pedigree Method 446 Some Hereditary Traits in Man 447 Some Pedigrees of Dominant Characters 450 A Typical Pedigree of a Recessive Character 453 Heredity of Mental Traits in Man 456 The Heredity of Feeble-mindedness 458 Insanity and Heredity 461 Pedigrees of Royal Families 462 Chapter XXXVI. The Statistical Study or Heredity in Man . 470 Introduction 470 Galton's Statistical Studies of Heredity in Man 471 Criticism of Galton's Work 474 Galton's Data in the Light of Genetics 475 Chapter XXX VII . Twins and Heredity 476 I. The Biology of Twins 476 Various Kinds of Twins 476 The Origin of Identical Twins 478 II. The Twin Method of Studying Heredity in Man .... 480 III. Crime and Destiny 484 IV. Twins and the Relative Potency of Heredity and Environment 485 TABLE OF CONTENTS xix PAGE Chapter XXXVIII. Does Heredity or Environment Make Men? Albert- Edward Wiggam 491 Chapter XXXIX. Eugenics and Euthenics. Paul Popenoe and Roswell II. Johnson 508 Chapter XL. Human Conservation. Herbert, E. Walter . . . 521 1. How Mankind May Be Improved 521 2. More Facts Needed 521 3. Further Application of What We Know Necessary .... 522 4. The Restriction of Undesirable Germ Plasm 523 5. The Conservation of Desirable Germ Plasm 528 6. Who Shall Sit in Judgment? 530 Chapter XLI. The Promise of Race Culture. Caleb Williams Saleeby 532 PART V. ACCESSORY READING Chapter XLII. The Relation of Evolution to Materialism. Joseph Le Conte 547 Chapter XLIII. The Statistical Study of Variation .... 555 Statistical Methods 555 Bimodal and Multimodal Curves 558 The Coefficient of Correlation 559 Chapter XLIY. The Physical Basis of Mendelism. Ernest B. Babcock and Roy E. Clausen 561 Chapter XLV. Natural Selection. Charles Darwin 573 Foundation Stones of Natural Selection 573 Darwin's Own Estimate as to the Role of Natural Selection in Evolution 573 Effects of Habit and of the Use or Disuse of Parts; Correlated Variation; Inheritance 574 Darwin's Idea of the Causes Responsible for the Origin of Domes- tic Races 575 Darwin's Idea of the Origin of Varieties, Species, and Genera in Nature 575 The Term "Struggle for Existence" Used in a Large Sense . . 576 Geometrical Ratio of Increase 577 Natural Selection; Or the Survival of the Fittest 577 Sexual Selection 584 Illustrations of the Action of Natural Selection, or the Survival of the Fittest 586 Summary of Chapter on Natural Selection 5S7 Difficulties and Objections to Natural Selection as Seen by Darwin 590 XX TABLE OF CONTENTS PAGE Answer to the First Difficulty 591 Answer to the Second Difficulty: Organs of Extreme Perfection and Complication 591 Darwin's Summary of His Answer to the Third Difficulty, That of Accounting for the Acquisition and Modification of In- stincts through Natural Selection 595 Darwin's Summary of His Answer to the Difficulty as to the In- ability of Natural Selection to Account for the Fact That Spe- cies When Crossed Are Sterile or Produce Sterile Offspring, Whereas When Varieties Are Crossed Their Fertility is Unim- paired 596 Bibliography 599 Glossary 602 Index 611 LIST OF ILLUSTRATIONS FIGURE PAGE i. Skeleton of Seal 59 2. Skeleton of Greenland Whale 61 3. Paddle of Whale Compared with Hand of Man .... 62 4. Wing of Reptile, Mammal, and Bird 63 5. Skeleton of Dinar 11 is gravis 66 6. Hermit Crabs Compared with Cocoa-Nut Crab .... 68 7. Rudimentary or Vestigial Hind Limbs of Python ... 70 8. Apteryx austral is 71 9. Illustrations of the Nictitating Membrane in Various Animals 76 10. Rudimentary, or Vestigial and Useless, Muscles of the Human Ear 77 n. Portrait of a Young Gorilla 78 12. Lower Extremities of a Young Child ....*. 79 13. An Infant, Three Weeks Old, Supporting Its Own Weight for Oyer Two Minutes 80 14. Sacrum of Gorilla Compared with That of Man ... 81 15. Diagrammatic Outline of the Human Embryo When about Seven Weeks Old 82 16. Front and Back View of Adult Human Sacrum .... 82 17. Appendix Vermiformts in Orang and in Man 83 18. Appendix Vermiformis in Orang and in Man, Showing Va- riation in the Orang 83 19. Human Ear Modeled and Drawn by Mr. Woolner ... 84 20. Foetus of an Orang 85 21. Vestigial Character of Human Ears 86 22. Hair Tracts on the Arms and Hands of Man, as Compared with Those of the Chimpanzee 88 23. Molar Teeth of Lower Jaw in Gorilla, Orang, and Man . 90 24. Perforations of the Humerus in Three Species of Quadru- mana 91 25. First Stages in the Embryonic Development of the Pond Snail, Lymnaeus 107 26. Stages in the Development of the Prawn, Peneus potimirium 111 xxi xxii LIST OF ILLUSTRATIONS FIGDRE PACK 27. Later Stages in the Development of the Prawn, Pcnais potimiriiim m 28. Metamorphosis of a Barnacle, Lepas 112 29. Embryos in Corresponding Stage of Development of Shark, Fowl, and Man 118 30. Total Geologic Time Scale 131 31. Feet and Teeth in Fossil Pedigree of the Horse . . . 135 32. Four Stages in the Evolution of the Cameline Skull . . 137 33. Four Stages in the Evolution of the Cameline Fore Foot . 138 34. Evolution of Head and Molar Teeth of Mastodons and Elephants 140 35. Skull of Java Ape-Man, Pithecanthropus erectus .... 150 36. Jaws of Man and of the Apes 151 37. Restoration of Prehistoric Men 153 38. Neanderthaloid Skull of La Chapelle-aux-Saints . . . 154 39. Skeleton of Neanderthal Man 155 40. Diagram of a Typical Cell 192 41. Diagram of the Early Phases of Mitosis 194 42. Diagram of the Middle Phases of Mitosis 195 43. Diagram of the End Phases of Mitosis 196 44. The Germ Track in Ascaris 201 45. Diagram of the Germ Track in Ascaris 203 46. The Germ Track in Miastor 204 47. Diagram To Illustrate Spermatogenesis 205 48. Diagram To Illustrate Oogenesis 206 49. Diagram Showing Parallel between Maturation of Sperm-Cell and of Ovum 207 50. Diagram To Illustrate Fertilization 208 51. Diagram To Illustrate the Results of Selection in Pure Lines 216 52. An Armadillo Egg Showing Quadruplet Embryos . . . 220 53. Diagram Showing Chromosome Relations in the Determina- tion of Sex 222 54. A Typical Opposite-sexed Pair of Cattle Twins . . . 230 55. Diagram Illustrating Behavior of Chromosomes in Men- del's Cross of Tall and Dwarf Peas 240 56. Diagram Illustrating Behavior of First Hybrid Genera- tion When Inbred 241 LIST OF ILLUSTRATIONS xxiii FIGUBE PAGE 57. Diagram Illustrating Dihybrid Ratio 244 58. Diagram Showing How the Original Scheme Must Be Modi- fied To Satisfy the Presence and Absence Hypothesis . 254 59. Diagram Showing How Presence and Absence Scheme Is Actually Used 255 60. Diagram Illustrating Blending Inheritance .... 256 61. Diagram Illustrating Behavior of Complementary Factors 258 62. Diagram Illustrating Behavior of Inhibitory Factor . . 261 63. Diagram Showing Some Possible Combinations in F_, When Fj of Figure 62 Is Inbred 262 64. Diagram Showing the Heterozygote Situation .... 262 65. Diagram Illustrating Action of Supplementary Factor . 263 66. Diagram Illustrating Nilsson-Ehle's Explanation of the 15:1 Ratio in F 2 of Hybrid between Red- and White- Grained Wheat 265 67. Another Method of Visualizing Nilsson-Ehle's 15:1 Ratio 266 68. Diagram Illustrating Nilsson-Ehle's 63 : 1 Ratio . . . 267 6q. Sex-Linked Inheritance of White and Red Eyes in Droso- pl/ilii 280 70. Reciprocal Cross to That Shown in Figure 60 ... 281 71. Sex-Linked Inheritance of Barred and Unbarred (Black) Plumage in Poultry 282 72. Reciprocal Cross to That Shown in Figure 71 . . . . 283 73. Diagram Showing the Mechanism of Crossing-over . . 291 74. The Chromosome Map of Drosophila melanogaster .... 295 75. Oenothera lamarckiana 314 76. A Series Showing Oenothera lamarckiana and Several of Its Mutants Growing Shoe by Side 319 77. Diagram Showing in C< >ndensed Form the Genealogy of the Oenothera lamarckiana Family and Its Various Mutants . 324 78. Fierasfer acus, Penetrating the Anal Openings of Holothur- IANS 35S 79. Kallima, the "Dead-Leaf Butterfly" 364 80. Three Aquatic Types of Vertebrate, To Illustrate Con- vergent Adaptation 367 81. Pedigree Showing Heredity of Brachydactyly .... 451 xxiv LIST OF ILLUSTRATIONS FIGURE PAGE 82. Pedigree Showing Heredity of Cataract 452 83. Pedigree Showing Heredity of Albinism 454 84. Pedigree Showing Heredity of Night-blindness . . . .455 85. Pedigree of the Darwin-Galton-Wedgewood Family . . 457 86. Pedigree Showing Heredity of Feeble-mindedness . . . 460 87. Pedigree Showing Heredity of Feeble-mindedness . . . 460 88. Diagram To Illustrate Galton's "Law of Filial Regres- sion" 472 89. Diagram To Illustrate Galton's "Law of Ancestral Shares in Inheritance" 473 go. Typical Fraternal Twins 477 91. Typical Identical Twins 478 92. Polygon of Variation 556 93. Bimodal Polygon of Variation 559 94. Correlation Table of Sixty-Day Oats 560 95. Chromosomes of Drosophila melanogaster 562 96. Diagram of Mitosis 564 97. The Reduction Division 566 98. Diagram of Chromatin Interchange 56S 99. Diagram of Independent Segregation 569 100. Diagram Showing Chromosome Relations in the Inheri- tance of Sex 571 PART I INTRODUCTORY AND HISTORICAL CHAPTER I INTRODUCTION WHAT ORGANIC EVOLUTION IS — DEFINITIONS The following selections are representative both of the older and of the newer attitudes of thinkers on the subject of organic evolution. The earlier writers were greatly impressed with the sublimity of the idea and found it in full accord with their religious faith. The later writers are less awed by the vastness of the process and hence adopt a more completely materialistic attitude. It is not necessary, how- ever, to discard one's religious beliefs in order to adopt a scientific attitude toward the problems of organic evolution. 1 These points of view are well expressed in the following quotations. "The world has been evolved, not created; it has arisen little by little from a small beginning, and has increased through the activity of the elemental forces embodied in itself, and so has rather grown than suddenly come into being at an almighty word. What a sublime idea of the infinite might of the great Architect! the Cause of all causes, the Father of all fathers, the Ens entium! For if we could compare the Infinite it would surely require a greater Infinite to cause the causes of effects than to produce the effects themselves. "All that happens in the world depends on the forces that prevail in it, and results according to law; but where these forces and their substratum, Matter, come from, we know not, and here we have room for faith. " — Erasmus Darwin, 2 as interpreted by Weismann. "When I first came to the notion, .... of a succession of extinc- tion of species, and creation of new ones, going on perpetually now, and through an indefinite period of the past, and to continue for ages to come, all in accommodation to the changes which must continue in the inanimate and habitable earth, the idea struck me as the grandest which I had ever conceived, so far as regards the attributes of the Presiding Mind. " — From a letter of Sir Charles Lyell to Sir John Herschel, 1836. 1 See Joseph Le Conte, Relation of Evolution to Materialism, Appendix. 2 From R. S. Lull, Organic Evolution (The Macmillan Company. Reprinted by permission). 3 4 EVOLUTION, GENETICS, AND EUGENICS "It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so com- plex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduc- tion; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the condition of life, and from use and disuse ; a Ratio of Increase so high as to lead to a struggle for Life, and as a consequence to Natural Selection, entailing Diver- gence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is a grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, while this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. " — Charles Darwin, Origin of Species, conclud- ing paragraph. "Speaking broadly we find as a fact that transmutation of species through the geologic ages has been accompanied by increasing diver- gence of type, by the increased specialization of certain forms, and by the closer and closer adaptation to conditions of life on the part of the forms most highly specialized, the more perfect adaptation and the more elaborate specialization being associated with the greatest variety or variation in the environment. Accepting for this process the name organic evolution, Herbert Spencer has deduced from it the general law, that as life endures generation after generation, its character, as shown in structure and function, undergoes constant differentiation and specialization. In this view, the transmutation of species is not merely an observed process, but a primitive necessity involved in the very organization of life itself." — D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (1908), p. 4. "The Doctrine of Evolution is a body of principles and facts con- cerning the present condition and past history of the living and lifeless things that make up the universe. It teaches that natural processes INTRODUCTION 5 have gone on in the earlier ages of the world as they do to-day, and that natural forces have ordered the production of all things about which we know." — Henry Edward Crampton, The Doctrine of Evolu- tion (1911), p. 1. "Evolution is the gradual development from the simple unorgan- ized condition of primal matter to the complex structure of the physi- cal universe; and in like manner, from the beginning of organic life on the habitable planet, a gradual unfolding and branching out into all the varied forms of beings which constitute the animal and plant kingdoms. The first is called Inorganic, the last Organic Evolution. " — Richard Swann Lull, Organic Evolution (1917), p. 6. THE MODERN ATTITUDE AS TO THE TRUTH OF THE EVOLUTION DOCTRINE "Among that public which, though educated and intelligent, is not yet professionally scientific, there has been, of late, a widespread belief that naturalists have become very doubtful as to the truth of the theory of evolution and are casting about for some more satisfactory substitute, which shall better explain the infinitely varied and mani- fold character of the organic world. This belief is an altogether mis- taken one, for never before have the students of animals and plants been so nearly unanimous in their acceptance of the theory as they are to-day. It is true that there are still some dissentient voices, as there have been ever since the publication of Darwin's 'Origin of Species,' but the whole trend of scientific opinion is strongly in favor of the evolutionary hypothesis." — William Berryman Scott, The Theory oj Evolution, p. 1. "But the biological sciences were still slower [than the physical sciences] to come to their true position as dignified science. Here was the last stronghold of the supernaturalist. Thrust out from the field of 'physical science' it was in the phenomena of life that the last stand was made by those who claim that supernatural agency intervenes in nature in such a way as to modify the natural order of events. When Darwin came to dislodge them from this, their last intrenchment, there was a fight, intense and bitter, but, like all attempts to stay the prog- ress of human knowledge, this final struggle of the supernaturalists was foredoomed to failure. The theory of evolution has taken its place beside the other great conceptions of natural relations, and largely through its establishment biology has become truly a science 6 EVOLUTION, GENETICS, AND EUGENICS with a large group of phenomena consistently arranged and properly classified. The discussion which followed the publication of Darwin's 'Origin of Species ' lasted for nearly a generation, but it is now practi- cally closed, so far as any attempt to discredit evolution as a true scientific generalization is concerned. Scientists are no longer ques- tioning the fact of evolution; they are busied rather with the attempt to further explore and more perfectly understand the operation of the factors that are at work to produce that development of animals and plants which we call organic evolution." — Maynard M. Metcalf, An Outline of the Theory of Organic Evolution (191 1), pp. xxii-xxiii. "Biologists turned aside from general theories of evolution and their deductive application to special problems of descent, in order to take up objective experiments on variation and heredity for their own sake. This was not due to any doubts concerning the reality of evolution or to any lack of interest in its problems. It was a policy of masterly inactivity deliberately adopted; for further discussions concerning the causes of evolution had clearly become futile until a more adequate and critical view of existing genetic phenomena had been attained." — E. B. Wilson (address as president of the American Association for the Advancement of Science, 1914). "The theory of development, as it was revived by Darwin nearly half a century ago, is in its modern form prevailingly unhistorical. True, it has forced beneath its sceptre the methods of investigation of all the sciences which deal with the living world and to-day almost completely controls scientific thought And yet science does not sincerely rejoice in its conquests. Only a few incorrigible and uncritically disposed optimists steadfastly proclaim what glorious progress we have made; otherwise, in scientific as in lay circles, there prevails a widespread feeling of uncertainty and doubt. Not as though the correctness of the principle of descent were seriously questioned; rather does the conviction steadily grow that it is indispensable for the comprehension of living nature, indeed self- evident." — Gustav Steinmann (translated by W. B. Scott from Die Abstammungslehre [1908], pp. 1-2). "The many converging lines of evidence point so clearly to the central fact of the origin of forms of life by an evolutionary process that we are compelled to accept this deduction, but as to almost all the essential features, whether of cause or of mode, by which specific INTRODUCTION 7 diversity has become what we perceive it to be, we have to confess an ignorance nearly total." — William Bateson, Problems of Genetics (1913), p. 248. "The demonstration of evolution as a universal law of living nature is the great intellectual achievement of the nineteenth century. Evolution has outgrown the rank of a theory, for it has won a place in natural law beside Newton's law of gravitation, and in one sense holds a still higher rank, because evolution is the universal master, while gravitation is among its many agents. Nor is the law of evolu- tion any longer to be associated with any single name, not even with that of Darwin, who was its greatest exponent. It is natural that evolution and Darwinism should be closely connected in many minds, but we must keep clear the distinction that evolution is a law, while Darwinism is merely one of the several ways of interpreting the work- ings of this law. "In contrast to the unity of opinion on the law of evolution is the wide diversity of opinion on the causes of evolution. In fact, the causes of the evolution of life are as mysterious as the law of evolution is certain. Some contend that we already know the chief causes of evolution, others contend that we know little or nothing of them. In this open court of conjecture, of hypothesis, of more or less heated controversy the names of Lamarck, of Darwin, of Weismann figure prominently as leaders of different schools of opinion; while there are others, like myself, who for various reasons belong to no school, and are as agnostic about Lamarckism, as they are about Darwinism or Weismannism, or the more recent form of Darwinism, termed Muta- tion by De Vries. "In truth, from the period of the earlier stages of Greek thought man has been eager to discover some natural cause of evolution, and to abandon the idea of supernatural intervention in the order of nature. Between the appearance of The Origin of Species, in 1859, and the present time there have been great waves of faith in one explanation and then in another: each of these waves of confidence has ended in disappointment, until finally we have reached a stage of very general scepticism. Thus the long period of evolution, experi- ment, and reasoning which began with the French natural philosopher, Buffon, one hundred and fifty years ago, ends in 1916 with the general feeling that our search for causes, far from being near completion, ba c only just begun. 8 EVOLUTION, GENETICS, AND EUGENICS "Our present state of opinion is this: we know to some extent how plants and animals and man evolve; we do not know why they evolve. We know, for example, that there has existed a more or less complete chain of beings from monad to man, that the one-toed horse had a four-toed ancestor, that man has descended from an unknown ape-like form somewhere in the Tertiary. We know not only those larger chains of descent, but many of the minute details of these transformations. We do not know their internal causes, for none of the explanations which have in turn been offered during the last hun- dred years satisfies the demands of observation, of experiment, of reason. It is best frankly to acknowledge that the chief causes of the orderly evolution of the germ are still entirely unknown, and that our search must take an entirely fresh start." — H. F. Osborn, The Origin and Evolution of Life (Charles Scribner's Sons), 191 8, pp. viii-x. WHAT ORGANIC EVOLUTION IS NOT i. The evolution doctrine is not a creed to be accepted on faith, as are religious faiths or creeds. It appeals entirely to the logical faculties, not to the spiritual, and is not to be accepted until proved. 2. It does not teach that man is a direct descendant of the apes and monkeys, but that both man and the modern apes and monkeys have been derived from some as yet unknown generalized primate ancestor possessing the common attributes of all three groups and lacking their specializations. 3. It is not synonymous with Darwinism, for the latter is merely one man's attempt to explain how evolution has occurred. 4. Contrary to a very widespread idea, evolution is by no means incompatible with religion. Witness the fact that the early Christian theologians, Augustine and Thomas Aquinas, were evolutionists, and the majority of thoughtful theologians of all creeds are today in accord with the evolution idea, many of them even applying the prin- ciple to their studies of religion; for religious ideas and ideals, like other human characters, have evolved from crude beginnings and are still undergoing processes of refinement. 5. The evolution idea is not degrading. Quite the contrary; it is ennobling as is well brought out by the classic statement of Darwin on page 4 and by that of Lyell, on page 3. 6. The evolution doctrine does not teach that man is the goal of all evolutionary process, but that man is merely the present end product of one particular series of evolutionary changes. The goal INTRODUCTION Q of evolution in general is perfection of adaptation to the conditions of life as they happen to be at any particular time. Many a highly perfected creature has reached the goal of its evolutionary course only to perish because it was too highly perfected for a particular environment and could not withstand the hardships incident to radi- cally changed world-conditions. Many evolutions therefore have been completed, while others are still awaiting the opportunity to speed up toward a new goal. 7. Evolution is therefore not entirely a thing of the past. Obvi- ously some species, including Man perhaps, are nearly at the end of their physical evolution, but there are always certain generalized plastic types awaiting the next great opportunity for adaptive speciali- zation. CHAPTER n HISTORICAL ACCOUNT OF THE DEVELOPMENT OF THE EVOLUTION THEORY The chief sources of material for the present chapters are: Osborn's From the Greeks to Darwin 1 and Judd's The Coming of Evolution.* Professor Osborn studies the evolution of the evolution idea as a biologist would investigate the evolution of a group of species, using all of the available sources of evidence at his disposal. The fragments of ancient writing and the crude imaginings of early natural philoso- phers are the fossils of the evolution idea, many of them ancestors of modern principles; fragments of ancient or discarded ideas that still persist, though irrelevant to modern thought, are the vestigial structures that proclaim kinship between the past and the present; parallelisms between the development of ideas in the minds of inde- pendent thinkers do not prove plagiarism, but indicate common descent from the same ancestral ideas. This whole history is an important chapter in the story of human evolution in general, for it deals with the evolution of a characteristic human faculty — that of appreciating the broad relations that exist between the past and the present. This faculty has evolved as truly as has an organic system such as the nervous system, and is unques- tionably closely bound up with the latter. The evolution theory is a vast fabric of interrelated and inter- dependent facts and principles. The fabric has been gradually woven out of separate threads and now stands strong though flexible, with strands reaching into all sciences and tending to unify all science. It was only after the lesser ideas came to be clearly apprehended that it was possible for the master minds of Lamarck and of Darwin to weave them together into a consistent fabric and to bring the facts together under the one great conception, that of organic evolution. Classification was a science, comparative anatomy had made much progress, the principles of embryology were fairly well understood, 1 II. F. Osborn, From the Greeks to Darwin (The Macmillan Company, 1908). 2 John W. Judd, The Coming of Evolution (Cambridge University Press, 1911J. 10 HISTORICAL ACCOUNT OF EVOLUTION THEORY II much palaeontological discovery had been made, before it was found that the facts from these sources all pointed to one general principle, and only one, that master-principle "organic evolution." We shall now trace the development of the evolution idea from its inception among the Greeks to its present status, and shall first give a brief account of Greek evolution. EVOLUTION AMONG THE GREEKS The early Greek thinkers were sea people. "Along the shores and in the waters of the blue Aegean," says Osborn, "teeming with what we now know to be the earliest and simplest forms of animals and plants, they founded their hypotheses as to the origin and succession of life The spirit of the Greeks was vigorous and hopeful. Not pausing to test their theories by research, they did not suffer the disappointments and delays which come from one's own efforts to wrest truths from Nature. " The Greeks were anticipators of Nature. Their speculations out- stripped the facts; in fact were usually made with "eyes closed to the facts." Their theories were inextricably bound up with current mythology, were naive, vague, and, from our modern point of view, ridiculous; yet they contained many grains of truth and were the germs out of which grew the saner ideas of subsequent thinkers. Thales (624-548 B.C.) was the first of the Greeks to theorize about the origin of life. "He looked upon the great expanse of mother ocean and declared water to be the mother from which all things arose, and out of which they exist. " This idea anticipates the modern idea of the aquatic or marine origin of life, and also the present idea as to the indispensability of water in all vital processes. Anaximander (611-547 B.C.) has been called the prophet of Lamarck and of Darwin. While his theories were highly mythical in character, he conceived the idea of a gradual evolution from a formless or chaotic condition to one of organic coherence. He saw vaguely the idea of transformation of aquatic species into terrestrial, even deriving man from aquatic nshlike men (mythical mermen) who were able to emerge from the water only after they had undergone the necessary changes required for land life. This idea involves that of adaptation, one of the cornerstones of the modern evolutionary structure. Anaximenes (588-524 B.C.), a pupil of Anaximander, "found in air the cause of all things. Air, taking the form of soul, imparts life, motion, and thought to animals. " It is questionable whether this is a 12 EVOLUTION, GENETICS, AND EUGENICS prophecy of the importance of oxygen and oxidation in vital processes. Anaximenes also introduced the idea of abiogenesis (spontaneous generation of living substance), his idea being that animals and plants arose out of a primordial terrestrial slime wakened into life by the sun's heat. This primordial terrestrial slime is perhaps a prophecy of Oken's "Urschleim" or of protoplasm. Xenophanes (576-480 B.C.), probably another pupil of Anaxi- mander, " agreed with his master so far as to trace the origin of man back to the transition period between the fluid or water and solid or land stages of the development of the earth." He was the first to recognize fossils as the remains of animals once alive, and to see in them proof that once the seas covered the entire surface of the earth. Heraclitus (535-475 B.C.), the first of a group of physicists, was the great proponent of the philosophy of change. He was imbued with the idea that all was motion, that nothing was fixed. ''Everything was perpetually transposed into new shapes." Although Heraclitus did not apply his ideas to living creatures and their evolutions, his philosophy was influential in molding the ideas of his successors. Empedocles (495-435 B.C.) " took a great stride beyond his predeces- sors, and may justly be called the father of the Evolution idea He believed in Abiogenesis, or spontaneous generation, as the explana- tion of the origin of life, but that Nature does not produce the lower and higher forms simultaneously or without an effort. Plant life comes first, and animal life developed only after a long series of trials." He thought that all creatures arose through the fortuitous combina- tion of scattered and miscellaneous parts which were attracted or repelled by the forces of love or hate (the two great forces in Nature). Thus arose every sort of combination of parts, some more or less har- monious and complete, others with ill-assorted organization, lacking in some parts, double or triple in others. Some of these combinations could not survive, because of their incompleteness and incongruity, but "other forms arose which were able to support themselves and multiply. " This is a sort of vague prophecy of the survival of the Qttest or of natural selection. Four sparks of truth may be found in Empedocles' philosophy, "first, that the development of life was a gradual process; second, that plants were evolved before animals; third, that imperfect forms were gradually replaced (not succeeded) by perfect forms; fourth, that the natural cause of the production of perfect forms was the extinction of the imperfect. " HISTORICAL ACCOUNT OF EVOLUTION THEORY 13 Democritus (b.450 B.C.), said to have been the first comparative anatomist, contributed to the substructure of evolution the idea of the "adaptation of single structures and organs to certain purposes." Anaxagoras (500-428 B.C.) was the first of the Greeks " to attribute the adaptations of Nature to Intelligent Design, and was thus the founder of Teleology," an idea that has played a retarding function in the history of evolution. "With Aristotle (384-322 B.C.) we enter a new world," says Osborn. "He towered above his predecessors, and by the force of his genius created Natural History." The evolution idea took a great step forward with Aristotle and reached a stage beyond which it did not go for many centuries. He covered nearly the whole field, touching upon most of the foundation stones of the complex problem. His ideas, like those of all the Greeks, were often vague and, in the light of present knowledge, incoherent; but, considering the meager factual background with which he had to work he had a surprising grasp of the whole situation. Some of his principal ideas were: 1. He had a clear idea of laws of Nature ("Necessity"), and attributed all evolutionary changes to natural causes. 2. He opposed the ideas of Empedocles as to the fortuitous origin of adaptive characters, and favored the idea of intelligent design in nature. He was therefore a teleologist. 3. Hence he rejected the hypothesis of the survival of the fittest, because it was based on chance. 4. He "had substantially the modern conception of the Evolution of life, from a primordial soft mass of living matter. " 5. He had an idea of a linear phylogenetic series, beginning with plants, then plant-animals, such as sponges and sea anemones, then animals with sensibility, and thence by graded stages up to Man. 6. "He perceived the unity of type in certain classes of animals, and considered rudimentary organs as tokens whereby Nature sustains this unity. " 7. "He anticipated Harvey's doctrine of Epigenesis in embryonic development." 8. "He fully perceived the forces of hereditary transmission, of the prepotency of one parent or stock, and of Atavism and Reversion. " 9. He is the father of that ancient fallacy called "prenatal influ- ences," and believed in the inheritance of acquired characters, as is shown in the following passage: 14 EVOLUTION, GENETICS. AND EUGENICS " Children resemble their parents not only in congenital characters, but in those acquired later in life. For cases are known where parents have been marked by scars and children have shown traces of these scars at the same points; a case is also reported from Chalcedon in which a father had been branded with a letter, and the same letter somewhat blurred and not sharply defined appeared upon the arm of the child." POST-ARISTOTELIANS With Aristotle the evolution idea reached a high watermark and thereafter the tide steadily declined. Pliny, Epicurus, Lucretius, and others kept the idea alive, but added nothing of importance to Aristotle's contribution. Lucretius (90-55 B.C.) appears to have been chiefly a follower of Empedocles in so far as his ideas as to the origin of animals are con- cerned. He ignored Aristotle and his much more advanced phi- losophy of Nature, finding the earlier, more mythical conceptions better suited to poetic expression. He was not truly an evolutionist, for he believed that all animals and plants arose fully formed from the earth. Lucretius is of importance chiefly as a retarding factor, for his ideas were accepted and admired even up to the eighteenth century; witness Milton's immortal verse: "The Earth obey'd, and straight, Op'ning her fertile womb, teem'd at a birth Innumerous living creatures, perfect forms, Limb'd and full grown." THE EARLY THEOLOGIANS The evolution idea made no progress from the time of Aristotle until the revival of learning in the Middle Ages. The chief inhibiting factor was the church, which favored traditional knowledge and the special-creation idea in its most literal form. Yet the early theo- logians, such as Gregory, Augustine, and Thomas Aquinas, were open- minded about the evolution idea and attempted to reconcile it with the scriptural account of creation. "Gregory of Nyssa (331-396 a.d.) taught," says Osborn, "that Creation was potential. God imparted to matter its fundamental properties and laws. The objects and completed forms of the Universe developed gradually out of chaotic material. " HISTORICAL ACCOUNT OF EVOLUTION THEORY 15 Augustine (353-430 a.d.) conceived the idea, now so generally adopted by theologians, that the biblical account of creation is alle- gorical. "In explaining the passage 'In the beginning God created heaven and the earth,' he says: " In the beginning God made the heaven and the earth, as if this were the seed of the heaven and the earth, although as yet all the matter of heaven and of earth was in confusion, but because it was certain that from this the heaven and the earth would be, therefore the material itself is called by that name. " Thomas Aquinas (1225-74), who wrote much later and was one of the leading church authorities, satisfied himself with merely expound- ing Augustine: "As to the production of plants, Augustine holds a different view, .... for some say that on the third day plants were actually produced, each in its kind — a view favoured by the superficial reading of Scripture. But Augustine says that the earth is then said to have brought forth grass and trees causal iter; that is, it then received the power to produce them. For in those first days .... God made creation primarily or causaliter, and then rested from His work." THE REVIVAL OF SCIENCE During the long centuries until the awakening of science in the Middle Ages the evolution idea smouldered along in the minds of a few thinkers, but it was only when a few daring spirits broke the trammels of scholasticism and began once more to give free rein to observation and speculation that the idea once more burst into flame and began its second great period of advance. A small group of natural philosophers, scarcely more scientific in their methods than the Greeks, were the first to revive interest in the evolution idea. Of these the names of Bacon, Descartes, Leib- nitz, and Kant are the most famous. Francis Bacon (1561-1626) did much to revive the vogue of Aris- totelian ideas. He also added some new ideas: (1) that the muta- bility of species was the result of the accumulation of variations; (2) that variations of an extreme kind, equivalent to "mutations," some- times occur; (3) that new species might arise by a degenerative process from old species. Emmanuel Kant (1724-1804) was purely a philosopher, not an observing naturalist, but he profited by the writings of the contem- porary naturalists, especially those of Buffon and Maupertuis. His 16 EVOLUTION, GENETICS, AND EUGENICS general ideas of evolution were comprehensive and summed up the best features of all preceding writers, but he did not contribute any- thing new to the pressing problem of the causes of evolution. Real progress was not to be made through further speculation. What was most needed was facts, and it was the task of the naturalists to furnish these. The earliest of the eighteenth-century naturalists were still anticipators of Nature in that their theories outran their facts. Of these the names of Bonnet and Oken are the best known. Bonnet (1720-93) was an evolutionist only in the sense that he believed that the adult organism is present in the egg and evolves from it by a process of unfolding or expansion. He was a zoological observer of some note, however, and made some of the most important contributions of his time to the general subject. He believed "that the globe had been the scene of great revolutions, and that the chaos described by Moses was the closing chapter of one of these; thus the Creation described in Genesis may be only a resurrection of animals previously existing." This theory admits of no progress and is scarcely worthy of the name evolution. Oken (1776-1851) is known chiefly for his "Urschleim" doctrine and his ideas of cells as vesicular units of life. According to him, "Every organic thing has arisen out of slime and is nothing but slime in various forms. This primitive slime originated in the sea from inorganic matter." These ideas are purely speculative, but suggest our modern ideas of protoplasm and cells. THE GREAT NATURALISTS OF THE EIGHTEENTH CENTURY Three great names stand out above all the rest during this period: those of Linnaeus, Buffon, and Erasmus Darwin. Linnaeus (1707-78) was the father of taxonomy. He contributed facts rather than theories; he invented our present system of binomial nomenclature of both animals and plants, and a great many of his generic and specific names still persist. Unfortunately he was an ardent advocate of the special-creation idea, holding that all of the true species were created as they are known today, except that new combinations may have arisen through hybridization or through degeneration. His influence was great, but was reactionary and proved a serious hindrance to the progress of the evolution idea. Buffon (1707-88), born the same year as Linnaeus, has been recognized as the father of the modern applied form of the evolution idea. He attempted to explain particular cases on an evolutionary HISTORICAL ACCOUNT OF EVOLUTION THEORY 17 basis. He lived at a time when it was dangerous to express views that might be interpreted as unorthodox, and this may account for the apparent lack of conviction in his own ideas; for he wavered between special creation and evolution. His chief contribution is the idea of the direct influence of the environment in the modification of the structure of animals and plants and the conservation of these modifi- cations through heredity. This seems to imply that he believed in the inheritance of acquired characters. He expressed himself as believing that climate has had a direct effect in the production of various races of man, that new varieties of animals have been formed through human intervention (an idea implying artificial selection), that similar results are produced by geographic migration and through isolation. He expressed the view that there is a great struggle for existence among animals and plants to prevent overcrowding and to maintain the balance of Nature. This appears to be an anticipation of Malthus' ideas on population, which were so influential in shaping the theories of Charles Darwin and of Wallace. While many of his ideas appear to be highly advanced for his time, his special applications are open to serious criticism. He reasons, for example, that the pig as it exists at present could not have been formed on any original complete and perfect plan, but seems to have been formed as a compound from other animals. It has useless parts which could hardly have been a part of a perfect plan as originally conceived. He thought that " the ass is a degenerate horse, and the ape a degenerate man. " On the whole Buffon was not a strong advocate of evolution and his influence was far from being as important as some recent writers appear to believe. Erasmus Darwin (1731-1802), grandfather of Charles Darwin, was a physician, a naturalist, and a minor poet. Undoubtedly he transmitted to his grandson his thoughtful habit and love of science and was influential in shaping his ideas on evolution. The elder Darwin's theories as to the causes of evolution closely paralleled those of Lamarck, his distinguished contemporary in France, but it is now very generally conceded that the ideas of the two men were independently derived from similar materials. Erasmus Darwin laid little emphasis on the direct action of the environment, which had been Buffon's main dependence, and dwelt on the internal origin of adap- tive characters. "All animals," he said, "undergo transformations which are in part produced by their own exertions, in response to r8 EVOLUTION, GENETICS, AND EUGENICS pleasures and pains, and many of these acquired forms or propensities are transmitted to their posterity." One could ask for no clearer statement of the idea that acquired characters are inherited. The fierceness of the struggle for existence was clearly recognized by Dr. Darwin. He considers that this struggle is beneficial to Nature as a whole because it checks the too rapid increase of life. One step farther in the argument, and he would have arrived at the idea of the survival of the fittest, but he never took that step. He agreed with the early Christian fathers in his belief that the powers of development were implanted within the first organisms by the Creator and that subsequent evolution of adaptive characters went on without further divine intervention. The power of improvement rests within the creature's own organizations and is due to his own efforts. The effects of these efforts, he believes, are transmitted to offspring so that there might be a cumulative effect throughout many generations of the results of effort. Erasmus Darwin was perhaps the first to express clearly the ideas that millions of years have been required for the processes of organic evolution and that all life arose from one primordial protoplasmic mass. He writes as follows: " From thus meditating upon the minute portion of time in which many of the above changes have been produced, would it be too bold to imagine, in the great length of time since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, that all warm-blooded animals have arisen from one living filament, which the first great Cause imbued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations, and thus possess- ing the faculty of continuing to improve by its own inherent activity, and of delivering down these improvements by generation to pos- terity, world without end?" LAMARCK Lamarck (1744-1829), the greatest of French evolutionists, is now looked upon as "the founder of the complete modern Theory of Descent. " Osborn considers him " the most prominent figure between Aristotle and Darwin. One cannot compare his Philosophic zoologique with all previous and contemporary contributions to the evolution theory or learn the extraordinary difficulties under which he laboured, and that his work was put forth only a few years after he had turned HISTORICAL ACCOUNT OF EVOLUTION THEORY 10 from Botany to Zoology, without gaining the greatest admiration for his genius. No one has been more misunderstood, or judged with more partiality by over or under praise. The stigma placed upon his writ- ings by Cuvier, who greeted every fresh edition of his words as a 'nouvelle folie,' and the disdainful illusions to him by Charles Darwin (the only writer of whom Darwin ever spoke in this tone) long placed him in the light of a purely extravagant, speculative thinker. Yet, as a fresh instance of the certainty with which men of science finally obtain recognition, it is gratifying to note the admiration which has been accorded to him in Germany by Haeckel and others, by his countrymen, and by a large school of American and English writers of the present day; to note, further, that his theory was finally taken up and defended by Charles Darwin himself, and that it forms the very heart of the system of Herbert Spencer." Lamarck's main theory of evolution was expressed by him in the form of his four "laws": I. Life ; by its proper forces, continually tends to increase the volume of every body which possesses it, and to increase the size of its parts, up to a limit which brings it about. II. The production of a new organ in the animal body results from the supervention of a new want which continues to make itself felt, and a new movement which this want gives rise to and maintains. III. The development of organs and their powers of action are constantly in ratio to the employment of these organs. IV. Everything which has been acquired, impressed upon, or changed in the organization of individuals during the course of their life is preserved by generation and transmitted to new individuals which have descended from those which have undergone these changes. It is about the last "law" that the controversy rages, for it upholds the idea that acquired characters are inherited, now known as the "Lamarckian doctrine." A somewhat more specific statement of Lamarck's theory of evolution may be summed up in the following list of factors which he considered as playing an essential role in evolution. 1. "Favorable circumstances attending changes of environment, soil, food, temperature, etc., supposed to act directly in the case of plants, indirectly in the case of animals and man." 2. "Needs, new physical wants or necessities induced by the changed conditions of life. Lamarck believed that change of habits 20 EVOLUTION, GENETICS, AND EUGENICS may lead to the origination or modification of organs; that changes of function also modify or create new organs. By changes of environ- ment animals become subjected to new surroundings, involving new ways and means of living. Thus, certain land birds, driven by neces- sity to obtain their food in the water, gradually assumed characters adapting them for swimming, wading, or for searching for food in the shallow water, as in the case of the long-necked kinds." 3. "Use and disuse. To use an organ is to develop it; not to use it is to eventually lose it. The anterior limbs of birds became capable of sustained flight through use; the hind limbs of whales are lost through disuse, etc." 4. "Competition. Nature takes precautions not to overcrowd the earth. The stronger and larger living things destroy the smaller and weaker. The smaller multiply very rapidly, the larger slowly. A physiological balance is maintained. " 5. "The transmission of acquired characters. The advantages gained by every individual as the result of the structural changes resulting from use or disuse are handed down to its descendants who begin where the parent leaves off, and so are able to continue the pro- gression or retrogression of the character. " 6. "Cross-breeding. 'If when any peculiarity of form or any defects whatsoever are acquired, the individuals in this case, always pairing, they will produce the same peculiarities, and if for successive generations confined to such unions, a special distinct race will then be formed. But perpetual crosses between individuals which have not the same peculiarities of form result in the disappearance of all the peculiarities acquired by the particular circumstances.'" 7. "Isolation. 'Were not man separated by distances of habita- tion, the mixtures resulting from crossing would obliterate the general characters which distinguish different nations.' This thought is expressed in his account of the origin of men from apes, and is not applied to living things in general." In addition to his theories as to the causes of evolution, Lamarck was the first to present the idea of the tree of life, or phylogenetic tren, as a mode of representing animal relationships. All previous classifi- cations had been based on the idea of a single linear phylogenetic series, each lower group being supposedly ancestral to a higher group, and all in a single chain. We may best sum up Lamarck's work and influence in the words of Osborn: HISTORICAL ACCOUNT OF EVOLUTION THEORY 21 "Lamarck, as a naturalist, exhibited exceptional powers of defini- tion and description, while in his philosophical writings upon Evolu- tion, his speculation far outran his observations, and his theory suffered from the absurd illustrations which he brought forward in support of it His critics spread the impression that he believed animals acquired new organs simply by wishing for them. His really sound speculation in Zoology was also injured by his earlier thoroughly worthless speculation in Chemistry and other branches of science. Another marked defect was, that Lamarck was completely carried away with the belief that his theory of the transmission of acquired characters was adequate to explain all the phenomena. He did not, like his contemporaries, Erasmus Darwin and Goethe, perceive and point out, that certain problems in the origin of adaptations were still left wholly untouched and unsolved His arguments are, in most cases, not inductive, but deductive, and are frequently found not to support his law but to postulate it. "It is now a question whether Lamarck's factor is a factor in Evolution at all! If it prove to be no factor, Lamarck will sink gradually into obscurity as one great figure in the history of opinion. If it prove to be a real factor, he will rise into a more eminent position than he now holds, — into a rank not far below Darwin." CUVIER AND GEOFFROY ST. HILAERE Georges Cuvier (1769-1832) deserves especial mention as one of the strongest negative factors in the development of the evolution idea. He was, first of all, an opponent of Lamarck, and, second, of evolution in general. He ranged himself with Linnaeus as a special creationist and advocated the idea of fixity of species. "All the beings, " said he, "belonging to one of these forms (perpetual since the beginning of all things, that is, the Creation) constitute what we call species." So able was Cuvier and so much in favor at the French court that he succeeded in throwing Lamarck's views into disrepute and thus greatly retarded the progress of evolution. He was brilliant as a comparative anatomist and palaeontologist and will long be known for his discoveries in these fields. E. Geojjroy St. Eilaire (1 772-1844) did his best to defeat the retarding influence of Cuvier. The two engaged in a long and bitter controversy over the evolution idea. While not a supporter of Lamarckism proper, he was a thoroughgoing evolutionist, favoring 22 EVOLUTION, GENETICS, AND EUGENICS the doctrine of Buflon, that the direct action of the environment was the sole cause of evolution. He also, in a sense, anticipated De Vries, in that he believed that new species might be formed by transmutation or sudden large variations occurring in one generation. "Hence the underlying causes of transformations," he said, "were profound changes induced in the egg by external influences, accidents as it were, regulated by law. " The controversy between Cuvier and St. Hilaire was a losing one for the latter. The cards were stacked against him and after him the evolution idea was retired to comparative obscurity until revived by Charles Darwin. CATASTROPHISM AND UNIFORMITARIANISM The development of the science of geology had a profound influence upon that of evolution. The prevailing theories as to historical geology during the Middle Ages involved the idea of "catastrophism. " 'According to this view all important changes in the earth's crust represented sudden radical transformations, involving earthquakes, volcanic outbursts, floods, sudden upliftings of submerged areas, or equally sudden submergence of land bodies. From these ideas natu- rally grew the related idea of great, world-wide destructions of animals and plants, followed by re-creation of new faunas and floras. Cuvier, for example, interpreted the more or less distinct fossil strata as being the result of a series of tremendous cataclysms, the last of which had been the great deluge of Scripture, in which Noah figured prominently. He thought that at each cataclysm great floods of water had covered the earth, that the existing animals had been buried in mud and thus preserved as fossils, and that a new creation followed each cataclysm. The great strength of this conception was that it appeared to give scientific support to both special creation and the Mosaic account of the "Flood." As compared with the pure evolutionary conception, this alternative was highly acceptable to the church and was pro- claimed as orthodox. The Scotch philosopher and geologist, Hutton, who lived during the last half of the eighteenth century, combated the idea of catastrophism by advocating the doctrine of "uniformitari- anism," a view involving the idea that past changes on the earth were the result of the same sort of gradual changes as are observed to be taking place today— in brief, that there has been a strict uni- formity of change throughout the entire period of geologic history. There may have been, according to this view, local catastrophes, HISTORICAL ACCOUNT OF EVOLUTION THEORY 23 such as volcanic outbursts, earthquakes, and floods, but the main trend of change has been - slow and constant, due largely to erosion and allied phenomena. This view had practically no influence on the ideas of the time and for a long period the idea of catas- trophism triumphed over the more truly evolutionary view of uni- formitarianism ; thus the evolution idea was destined to he dormant till revived bv Charles Darwin. THE REAWAKENING OF THE EVOLUTION IDEA A number of important influences paved the way for the rehabili- tation of the evolution idea at the hands of the younger Darwin. Which of these was the most important it is difficult to say. Prob- ably Charles Lyell's Principles of Geology and Malthus' On Population were the most suggestive works that Darwin encountered. He was also doubtless influenced by Robert Chambers' Vestiges of Natural History of Creation which appeared in 1844. Charles Lyell (1 797-1875) so successfully rehabilitated the doctrine of unif ormitarianism in geology that it became very generally accepted, thus paving the way for a more favorable consideration of the idea of organic evolution. Charles Darwin as a very young man took Lyell's Principles of Geology with him on his voyage on the " Beagle " and read it with the greatest devotion, as is evidenced by his dedication of the journal of his voyage: "To Charles Lyell, Esq., F.R.S., this second edition is dedicated with grateful pleasure, as an acknowledgment that the chief part of whatever scientific merit this Journal and other works of the author may possess, has been derived from studying the well-known, admirable Principles of Geology." Malthus' influence on Darwin's ideas is well expressed by Judd as follows: "Fifteen months after this 'systematic inquiry' began [referring to Darwin's exhaustive working over of his notes taken during his voyage on the 'Beagle'], Darwin happened to read the celebrated work of Malthus 'On Population' for amusement, and this served as a spark falling on a long prepared train of thought. The idea that as animals and plants multiply in geometrical progression, while the supplies of food and space to be occupied remain nearly constant, and that this must lead to a struggle for existence of the most desperate kind, was by no means new to Darwin, for the elder De Candolle, Lyell, and others had enlarged upon it; yet the facts with regard to 24 EVOLUTION, GENETICS AND EUGENICS the human race, so strikingly presented by Malthus, brought the whole question with such vividness before him that the idea of 'Natural Selection' flashed upon Darwin's mind." CHARLES DARWIN (1809-82) Charles Darwin is without question the foremost figure in the development of the evolution idea and probably in the development of science in general. The publication of his book, The Origin oj Species, in 1859, was the most important event in biological history. As has been already shown, Darwin's chief ideas had been anticipated not by one but by several of his predecessors. Nevertheless, he was the first to furnish a really adequate proof of the fact of evolution and his causo-mechanical theory to explain the method of evolution was supported by a mass of systematically arranged data such as has been paralleled neither before nor since. Darwin was the first evolu- tionist effectively to employ the inductive method, that of everywhere seeking facts first and then devising theories to fit the facts. He never allowed speculation to outstrip observation, as nearly all of his predecessors had done, but made theory await the amassing of facts in its support, until the accumulation of the latter seemed almost to speak out the theory of themselves. Our greatest debt to Darwin is due to his establishment of the factual basis of evolution ; his selection theory was relatively of minor significance in so far as its value in the development of the evolution idea was concerned. Yet this latter theory gained the widest acceptance among the scientifically inclined during the entire post-Darwinian period. It has been viciously assailed on all sides and has tottered repeatedly under the attacks of well-trained adversaries. Some of the weaker elements of the theory have given way under stress, and the whole selection factor as a primary causal factor in evolution has been seriously called into question; but since Darwin's time the fact of evolution has been almost universally accepted. The story of Darwin's life is almost a romance. " Born in 1809, " says Lull, 1 "this emancipator of human minds from the shackles of slavery to tradition saw the light of day upon the very day that ushered in the life of Abraham Lincoln, the emancipator of human bodies from a no more real physical bondage. Darwin studied first at Edinburgh, but finding medicine unsuited to his tastes, entered Christ's College, Cambridge, as a candidate for the church. His love 1 Richard Swann Lull, Organic Evolution (The Macmillan Company, 1917). HISTORICAL ACCOUNT OF EVOLUTION THEORY 25 of Nature, however, dominated all other interests and shortly after graduation an opportunity came to join the ship ' Beagle ' as naturalist in a voyage of exploration around the world. The five years spent upon this memorable journey, the narrative of which is so admirably set forth in the book, A Naturalist's Voyage around theWorld, resulted in the accumulation of the first of Darwin's great series of observations, the final decision to devote his life to zoological research, and the beginning of that illness which made him a life-long invalid. This last factor necessitated a retired life and thus proved of indirect bene- fit, as it enabled him to accomplish the immense amount of work which he did without being impeded by the distractions of a public career." SUMMARY OF DARWIN'S THEORIES Since two subsequent chapters are to be devoted to Darwinism, only an outline of Darwin's theories need be presented in the present historical account. Although Darwin was an all-round biologist and gave attention to practically every phase of evolutionary biology, he is known espe- cially for his selection theories. There are three of these: the theory of artificial selection, the theory of natural selection, and the theory of sexual selection. a) Artificial selection. — According to Darwin the commonest method of producing, under human culture, new races of animals and plants is that of selection. The breeder selects from among the highly variable individuals of a parent-race those which possess the begin- nings of desired modifications, and he breeds them together, expecting that the offspring will show the desired character, some in a more highly perfected condition, others in a less. The ones that vary favorably are again selected for breeding stock, and the same process is carried on until the desired character has been perfected. Although we now know that this is far from being a typical experi- ence among breeders, it appeared to Darwin to be so typical that he transferred the selection idea from the breeder to Nature, making Nature the selecting agency responsible for the production of natural wild species. His argument is as follows: b) Natural selection. — The following factors are involved : 1. All animals and plants tend to multiply in geometrical ratio. 2. There is not food or room for a much larger number of animals and plants than now exist. 26 EVOLUTION, GENETICS, AND EUGENICS 3. All members of a species vary in many if not all directions. 4. Those that vary in the more favorable directions, so as better to fit them to meet the conditions of life, survive in larger numbers than those varying in less favorable directions. This is Spencer's " survival of the fittest. " 5. The survivors of one generation become the parents of the next and, therefore, the more favorable characters are passed on more largely than the less favorable. 6. There is in each generation a slow but definite approach toward complete adaptation to life-conditions. 7. Variations neither useful nor harmful would not be affected by natural selection, and would be left either as fluctuating variations or as polymorphic characters. c) Sexual selection. — This theory was offered to supplement that of natural selection, because Darwin considered the latter as inade- quate to explain the facts of sexual dimorphism, or secondary sexual characters. The theory is as follows: There is always a contest among males for possession of females, in which the inferior males are eliminated either because they are, on the one hand, less courageous or weaker or less well equipped with weapons of combat, or because, on the other hand, the more attractive males, whether on account of colors, odors, phosphorescence, behavior, etc., would succeed in winning mates fiom those less endowed. Thus would be enhanced the sexual dimovphism until it reaches extremes in many cases that are truly remarkable. The name of Alfred Russell Wallace (1822-1913) will always be associated with that of Charles Darwin as co-author of the theory of natural selection. Wallace at the age of twenty-six went on a natural- istic expedition, primarily for collecting specimens from new regions. He covered almost the same ground as did Darwin in his voyage on the "Beagle." Wallace had read Lyell's Principles of Geology, Malthus' On Population, Chambers' Vestiges of Creation. While in Sarawak he tells us: "I was quite alone with one Malay boy as cook, and during the evenings and wet days, I had nothing to do but to look over my books and ponder over the problem which was rarely absent from rny thoughts. " While thus engaged the idea of natural selection came to him as though by a sudden flash of insight. When the idea was still in process of formation he wrote it out on thin paper and mailed it to Darwin, stating that he considered the idea new and asking Darwin to show it to Lyell, who had expressed interest in a HISTORICAL ACCOUNT OF EVOLUTION THEORY 27 former paper of Wallace. The ideas were expressed under the title On the Tendency of Varieties to Depart Indefinitely from the Original Type, and it proved to be an unusually concise and lucid statement of the main points of the natural-selection theory. Darwin at once wrote to Lyell as follows: "I never saw a more striking coincidence; if Wallace had my MS sketch, written in 1842, he could not have made a better short abstract! Even his terms now stand as heads of my chapters. Please return to me the MS which he does not say he wishes me to publish but I shall, of course, at once write and offer to send it to any journal. So all my originality, whatever it may amount to, will be smashed, though my book, if it ever have any value, will not be deteriorated, as all the labour consists in the application of the theory. I hope you will approve of Wallace's sketch, that I may tell him what to say." Lyell insisted that Darwin publish an abstract of his own work simultaneously with that of Wallace, and this course was carried out. Darwin's generosity was equaled by that of Wallace who wrote, in 1870: "I have felt all my life and still feel the most sincere satisfaction that Mr. Darwin had been at work long before me, and that it was not left for me to attempt to write The Origin of Species. I have long since measured my own strength and know well that it would be quite unequal to the task. " Still later he wrote: "I was then (and often since) the 'young man in a hurry,' he [Darwin] the painstaking student, seeking ever the full demonstration of the truth he had discovered, rather than to achieve immediate personal fame." One must perforce admit the nobility of character of both men; but there can be no serious competition between the two for the honor of being called the originator of the natural-selection theory. CONTEMPORARY OPINION REGARDING THE VALIDITY OF DARWIN'S VIEWS At first Darwin was inclined to believe that the selection factor was all-sufficient to account for the origin of species, as well as that of adaptations; but as time passed he modified his earlier more sanguine views and came to the conclusion " that natural selection has been the main but not the exclusive means of modification." Many of his followers went to such extremes in their advocacy of the all-sufficiency of natural selection as would not have met with Darwin's approval. 28 EVOLUTION, GENETICS, AND EUGENICS "The first effect of Darwin's works," says McFarland, 1 "was to carry the world of science by storm, but at the same time to arouse intense hostility on the part of the theologians who found the theory of descent .... incompatible with the doctrines of Creation. In this conflict Darwin took no part, but was championed by Huxley, while Bishop Wilberforce led the opposition. The battle was long and bitter, there was much acrimonious writing on both sides, but the theory of descent — the doctrine of evolution — was found to be invulnerable and at present the theologians themselves have accepted it and even make use of it in their own work. "But as the years flew by the Darwinian doctrines began to meet with assaults from the scientists themselves, who, having endeavored to prove their validity, began to find them inadequate to the require- ments of expanding knowledge. The question was asked, 'What is the origin of the fittest ?' Given the fittest, we easily understand how it is perpetuated, but how does it arise ? In the striking phrase of someone: 'Natural selection might explain the survival of the fittest but fails to account for the arrival of the fittest!'" Darwin's main supporters during the most trying controversial period were Herbert Spencer and Thomas H. Huxley. Herbert Spencer (1820-1903) was an extremely able supporter of the general theory of evolution, but was more definitely an advocate of Lamarckism than of natural selection. His role was that of a champion of the whole philosophy of evolution as opposed to special creation, and it was largely due to his forceful writings that Darwinism won the battle against dogmatism. Spencer tried to explain the structure of protoplasm (living substance) on a physicochemical basis. He thought of the structural units of protoplasm as compa- rable with the molecules of chemical compounds, each local region of the protoplasm in the organism being made up of different kinds of units, which he called "physiological units." This conception of the physical basis of organic structure had a considerable influence in shaping Darwin's ideas and was probably the basis of the latter's provisional theory of "pangenesis." This theory was probably the first consistently worked out theory of the mechanics of heredity. It was thought that every part of the body is continually giving off its particular kind of units ("gemmules") into the blood. These gem- mules are transported by the blood stream to all parts of the body and 1 J. McFarland, Biology, General and Medical (The Macmillan Company, 1918). HISTORICAL ACCOUNT OF EVOLUTION THEORY 29 collect in the germ cells. This was supposed to account for the fact that from the germ cell will develop an organism like the parent in various details. If a part of the body was modified through func- tioning or through changed environment, it would have modified gemmules, which, in turn, would go to the germ cells and carry over the modification to the next generation. This theory was not satis- factory even to Darwin and is now only of historical interest. Spencer is best known in the history of the evolution theory as an ardent neo-Lamarckian. He states his belief as follows: "Change of function produces change of structure; it is a tenable hypothesis that changes of structure so produced are inherited. " This idea prevailed until it was cast down by Weismann. Thomas Henry Huxley (1825-95), one of the keenest, most analyti- cal thinkers of the nineteenth century, not only defended the general doctrine of evolution against Bishop Wilberforce and his aids, but was an able investigator in the fields of comparative anatomy and embry- ology. "At the British Association at Oxford in i85o, " says Judd, "after an American professor had indignantly asked 'Are we a fortuitous concourse of atoms?' as a comment on Darwin's views, Dr. Samuel Wilberforce, the Bishop of Oxford, ended a clever but flippant attack on the Origin by enquiring of Huxley, who was present as Darwin's champion, if it ' was through his grandfather or his grand- mother that he claimed his descent from a monkey ? ' "Huxley made the famous and well-deserved retort: 'I asserted — and I repeat — that a man has no reason to be ashamed of having an ape for his grandfather. If there were an ancestor whom I should feel ashamed of recalling, it would rather be a man — a man of restless and versatile intellect — who not content with success in his own sphere of activity, plunges into scientific questions with which he has no real acquaintance, only to obscure them by aimless rhetoric, and distract the attention of his hearers from the real point at issue by eloquent digressions and skilled appeals to religious prejudice!' "Huxley himself accepted the theory of Natural Selection — but not without some important reservations — these, however, did not prevent him from becoming its most ardent and successful champion. Darwin used to acknowledge Huxley's great service to him in under- taking the defense of the theory- — a defense which his own hatred of controversy and state of health made him unwilling to undertake — by laughingly calling him 'my general agent' while Huxley himself in replying to the critics, declared he was 'Darwin's bulldog.'" 30 EVOLUTION, GENETICS, AND EUGENICS Ernst Haeckel (1834-1919) was one of the earliest and most influential followers of Darwin in Germany. In his Generelle Mor- phologic, published in 1866, seven years after the Origin of Species first appeared, he applied the doctrine of evolution, and especially the theory of natural selection, to the whole field of vertebrate mor- phology. Beyond question Haeckel overapplied the theory and in a sense weakened its influence by his rather uncritical use of materials. His writings have been translated into most languages and "are popularly believed to represent the best scientific thought on the matter." Biologists today, however, are apt to look askance at Haeckel's works and to consider that they did more harm than good to Darwinism. August Weismann (1834-19 14) was the first really original evolutionist after Darwin. Like other thinkers of his time, he realized that further progress in the knowledge of the causal basis of evolution lay in further investigation of the causes of variation and the physical basis of heredity. Weismann has been classed as a neo-Darwinian because he was a strong advocate of some form of selection, but his "selection" was not the selection of Darwin. Realizing that the greatest weakness of the natural-selection theory lay in its inadequacy as an originator of variations, he proposed the "germinal-selection" theory. He contended that all heritable variations have their origin in the germ cell, and therefore that a new type of organism arises only from a changed type of germ cell. The germinal-selection theory stands out in striking contrast with Darwin's "pangenesis" theory. The former is centrifugal, the latter centripetal. "Determiners" of new characters, according to Weismann, arise in the germ plasm and work outward to all parts of the developing body; while the "gem- mules," Darwin's equivalent of determiners, originate in the body tissues and are carried to the germ cells in each generation. Accord- ing to Weismann, there is a struggle among the determiners for the available food and favorable positions in the germ cell, and those that receive the most food and the best positions gain an initial advantage, so that they are able to initiate the development of larger or more perfectly adapted organs. The descendants through cell division of these favored determiners are in a position to compete with other determiners on a more favorable footing in each succeeding generation, so that the character represented by them steadily increases in a linear or definitely directed fashion until it reaches the state of complete adaptation or fitness. Such a character may even continue its direct line of advance beyond the point of maximum fitness and result in HISTORICAL ACCOUNT OF EVOLUTION THEORY 31 what are known as overspecializations. The theory therefore would, if well founded, account not only for the initial stages of new adaptive characters, but also for overspecializations, two phenomena that natural selection was unable to account for. Not only were pro- gressive evolutionary changes explained by germinal selection, but regressive changes seemed to be even more readily accounted for on this basis. In the struggle among determiners in the germ cell some of the less favored units would be handicapped at the outset by insufficient food or unfavorable position and would produce smaller or less effective structures. Progressively, from generation to generation, these weakened determiners would lose ground and become less and less successful in competition until they were weaklings among determiners and would be able to initiate only degenerate or vestigial structures, or else would die out and lose their place altogether, thus accounting for total losses of structures. This theory does not exclude natural selection, but rather increases its importance, for every structure that arises to the threshold of utility or disutility meets the winnowing process of natural selection. The fitter individuals survive in the long run and these perpetuate the germ cells in which the successful determiners reside. A slightly different explanation of degenerating structures in- volves the principle of "panmixia. " According to this idea, changing environmental conditions may render certain adaptive organs of lessened value or of no value, as would be the case in the eyes of cave animals. In different individuals the eye determiners would vary in their success in competition with other determiners, and since natural selection would no longer put a premium on perfect eyes, all grades of eyes would be equally inherited and gradually the poorer or degenerate eyes would become more numerous, till finally there would be no good eyes in the race. Thus it will be seen that the germinal-selection theory was auxiliary to natural selection and tended to support the latter at two of its weakest points. But the supporting theory itself has the fundamental weakness of lacking a factual basis. It is purely hypothetical and cannot be put to an experimental test. Every time an objection to the theory was raised an auxiliary hypothesis was added to explain away the difficulty, till finally it fell to the ground through sheer top-heaviness, unable further to support its intricate structure of interrelated hypotheses. A much more valuable and lasting contribution of Weismann was his theory of "germinal continuity" and of the "apartness of the germ plasm. " The whole theory has come to be known as the " germ-plasm 32 EVOLUTION, GENETICS, AND EUGENICS theory," which forms the framework of nearly all of our modern genetics. According to this view the germ plasm is immortal in that it is perpetuated from generation to generation through the instrumentality of mitotic cell division, each germ cell being the prod- uct of the division of a previous germ cell back to the first germ cell that arose at the dawn of life. Thus a germ cell cannot be a product of the soma, but the soma is the product of germ cells. The soma loses its generalized characters and specializes in various ways. Once specialized, soma cells are believed to have lost their capacity to play a germinal role. Specialization means mortality. Thus the relation- ship between parent and offspring is not that the parent gives rise to the offspring, but that the same germ plasm gives rise to both parent and offspring. The logical conclusion to which this line of reasoning leads is that the changes in the soma, no matter how produced, are helpless to produce any effect upon the germ plasm, since germ cells come only from germ cells and not from soma cells. Consequently Weismann led the assault against Lamarckism and won the day so conclusively that even in these modern times few biologists have the temerity to express aloud any definite belief in the inheritance of acquired charac- ters. Weismann's germ-plasm idea is the cornerstone of modern genetics, though there are some forward-looking biologists who, looking at things with a physiological bias, cannot make themselves believe in the total independence of any tissue — even the sacred germ plasm. Weismann's influence was very great, especially during the last decade of the nineteenth century, and his theories gave rise to an immense amount of research, chiefly of a cytological and embryo- logical character. ISOLATION THEORIES Among the theories subsidiary to natural selection as an aid to species forming are the various isolation theories. One of the weak- nesses inherent in natural selection had to do with the probable swamping out of new types by promiscuous breeding with the more numerous individuals of the older types. "Anything," says Metcalf, "which divides a species into groups, which do not freely interbreed, is said to segregate (isolate) the members of the species into these sub- divisions." Some American writers, especially Jordan and Kellogg, Gulick, and Crampton, have dealt with the isolation factor in evolution and believe HISTORICAL ACCOUNT OF EVOLUTION THEORY 33 that it is a major factor of as great importance in species forming, or nearly so, as natural selection. But the prevailing opinion seems to be that isolation is really a kind of selection, more like artificial selection than anything else, which separates out certain pure lines and prevents promiscuous interbreeding. Various agents are known to produce isolation by erecting barriers to interbreeding between groups of individuals within a species. These segregative factors may be geographical, climatic, reproductive, physiological, or, in plants, the result of soil diversity. Thus a mountain range, on the two sides of which a species migrates, effectively separates the species into two independent groups. Heat, cold, moisture, etc., separate others. Reproductive incompatibility between new and older types is equally effective, as is assortative mating of like with like. Like natural selec- tion, isolation has nothing to do with the origin of new types, but merely aids in the preservation of types when once formed. Were there not spontaneous variations among animals and plants, there would be nothing to isolate. Therefore isolation plays only an auxiliary role, helping to preserve new races once they are formed. ORTHOGENESIS THEORIES "The orthogenetic evolution theories of various authors, based upon the assumed occurrence of variations in determinate lines or directions (a restricted and determinate variation as compared with the nearly infinite, fortuitous, and indeterminate variation assumed in the selection theories), are of several types. The mention of two will reveal pretty well the more important characters of all. Not a few biologists have always believed in the existence of a sort of mystic, special vitalistic force or principle by virtue of which determination and general progress in evolution is chiefly fixed. Such a capacity, inherent in living matter, seems to include at once possibility of pro- gressive or truly evolutionary change. Not all evolution is in a single direct line, to be sure; ascent is not up a single ladder or along a single geological branch, but these branches are few (as indeed we actually know them to be, however the restriction may be brought about) and the evolution is always progressive, that is, toward what we, from an anthropocentric point of view, are constrained to call higher and higher or more ideal life stages and conditions. "Other naturalists also seeming to see this source of determinate or orthogenetic evolution, but not inclined to surrender their dis- belief in vitalism, in forces over and beyond the familiar ones of the 34 EVOLUTION, GENETICS, AND EUGENICS physicochemical world, have tried to adduce a definite causomechani- cal explanation of orthogenesis. The best and most comprehensive types of this explanation are those essentially Lamarckian in principle, in which the direct influence of environmental conditions, the direct reactions of the life stuff to stimuli and influences from the world outside, are the causal factors in such an explanation. But while every naturalist will grant that such factors do change and control in a considerable degree the life of the individual, most see no mechan- ism or means of extending this control directly to the species. " The above-quoted paragraphs from Jordan and Kellogg 1 will serve to place before the reader the general ideas involved in the orthogenesis conception. A brief account of the various special theories of orthogenesis follows: Carl von NageWs ideas of orthogenesis involve a belief in a sort of mystical principle of progressive development, a something, quite intangible, that exists in organic nature, which causes each organism, to strive for or at least make for specialization or perfect adaptation. This idea of an inner driving and directing force reminds one of the "entelechy" of Driesch, or Bergson's "creative evolution." Nageli believed that animals and plants would have developed essentially as they have without any struggle for existence or natural selection. This form of orthogenesis theory, then, is alternative to natural selection. Theodore Eimer's theory of orthogenesis is more scientific and less mystical than Nageli's. He believed that lines of evolution were not miscellaneous and haphazard, but were confined to a few definite directions, determined at their initial stages not by natural selection but by the laws of organic growth, aided by the inheritance of acquired characters. A new character makes a beginning as would the first step in a slow chemical change, or series of such changes, and it must go through to a fixed end, under given conditions, just as surely as does the chemical process. Only when a given character or line of evolu- tion results in the production of a very positive advantage or dis- advantage to the species does natural selection step in to interfere with orthogenesis. The causes of orthogenesis are said "to lie in the effects of external influences, climate, nutrition, or the given constitu- tion of the organism." Actual species-forming, or the breaking-up into specific units of the orthogenetic lines of change, depends, according to Eimer, upon 1 Jordan and Kellogg, Evolution and Animal Life (D. Appleton and Company). HISTORICAL ACCOUNT OF EVOLUTION THEORY 3$ three factors: a standstill or cessation of development on the part of some lines; sudden development by leaps (practically mutations); and hindrance or difficulty of reproduction (the type of thing that Romanes emphasized as physiological isolation ten years later). Eimer illustrated his theories by the evolution of color patterns in lizards and those on the wings of butterflies. In both he believed that longitudinal stripes were primitive, that rows of dots followed these which were in turn followed by crossbands, reticular patterns, and finally by solid coloration. This hypothetical phylogenetic order is more or less closely paralleled by the ontogenetic order, in the lizards at least. It will be noted that Eimer 's theory places natural selection in a subordinate position, but does not dismiss it altogether, as is done by Nageh. It aids natural selection in explaining adaptations in that it furnishes for natural selection various characters of selective value, which may be either perpetuated or eliminated according to their itility. E. D. Cope, a leading American palaeontologist of the past cen- tury, had an orthogenetic theory involving his ideas of "bathmism" (growth force), "kinetogenesis" (direct effect of use and disuse and environmental influence), and "archaesthetism" (influence of primi- tive consciousness). It may be said that his ideas were Lamarckian throughout. In common with the majority of palaeontologists of later date — Osborn, Williston, Hyatt, Smith, and others — Cope felt the need of some factor other than natural selection to explain the apparent steady progress of characters in definitely directed lines as seen in the fossils. It is natural therefore that palaeontologists almost universally lay hold of both Lamarckian and orthogenesis ideas. Charles Otis Whitman, who, until his death over twenty years ago, was considered the leading American zoologist, had strong leanings toward orthogenesis. In one of his few publications he says: "Natural selection, orthogenesis, and mutation appear to present fundamental contradictions; but I believe that each stands for truth, and reconciliation is not far distant. The so-called mutations of Oenothera are indubitable facts; but two leading questions remain to be answered. First, are these mutations now appearing, as is agreed, independently of variation, nevertheless the products of variations that took place at an earlier period in the history of these plants ? Secondly, if species can spring into existence at a single leap, without the assist- ance of cumulative variations, may they not also originate with such 36 EVOLUTION, GENETICS, AND EUGENICS assistance ? That variation does issue a new species, and that natural selection is a factor, though not the only factor, in determining results, is, in my opinion, as certain as that grass grows although we cannot see it grow. Furthermore, I believe I have found indubitable evidence of species-forming variation advancing in a definite direction (ortho- genesis), and likewise of variations in various directions (amphi- genesis). If I am not mistaken in this, the reconciliation for natural selection, and orthogenesis is at hand." In concluding this brief account of orthogenesis, it should be said that definitely directed evolution is now believed to be one of the laws of organic evolution, but that we have no clear ideas as yet as to what are its underlying causes. Therefore orthogenesis is not a causo- mechanical theory of evolution at all. THE MUTATION THEORY OF DE VRIES The theory of "mutations" is associated with the name of Hugo De Vries, the well-known Dutch botanist; that of "heterogenesis," with the name of H. Korchinsky, a Russian. Though Korchinsky anticipated De Vries by several years, his work was not supported by the large amount of experimental data that characterized that of the great Dutch worker. The relative claims for recognition as the founder of the mutation theory are almost on a par with those of Darwin and Wallace for the natural- selection theory. Both Darwin and De Vries held back their theo- ries until they appeared to be adequately supported by personally collected facts. There is a striking parallelism between the ideas and conclusions of De Vries and those of Korchinsky, and since this is true a resume of De Vries's better-known work will serve to give the essentials of the whole conception. De Vries began his genetic experiments by a study of the variations of plants in the field. After learning their normal variability in nature, he transferred them to the experimental garden and there attempted to improve them by selection. He found that the improved living conditions due to better soil and cultivation induced a wider range of variability in size, luxuriance, and fecundity. Such variations were plus or minus in their character, fluctuating about a mean or average. It was exactly this type of variability that Darwin empha- sized as the raw material of evolution; but De Vries found by experi- ment that selection had no permanent hereditary effect when based HISTORICAL ACCOUNT OF EVOLUTION THEORY 37 to fluctuating variations, since the latter were merely somatic responses on variable growth conditions. This negative finding led him to renewed interest in discontinuous or saltatory variations as the only alternative to fluctuating or continuous variations. He looked far and wide among species of wild plants for a species that might exhibit a significant amount of saltatory variation and finally discovered in the evening primrose {Oenothera lamarckiana) what seemed to exhibit exactly the hoped-for characteristics. This large, stately plant with conspicuous yellow blooms had escaped from cultivation and was growing wild in the fields. In addition to a large number of plants that showed only minor differences among them- selves, De Vries found several individuals growing among the typical individuals which differed not merely in degree but in kind. These were as different as distinct varieties, and, when the seeds were planted in the garden they bred true to their kind. The only ques- tion now was whether they had actually arisen from typical parents. To test this possibility, seeds of several typical plants were planted in the garden; the result being not only a repetition of the peculiar types observed in the field, but of about a dozen other true breed- ing types with well-marked differences from the parent-species and among themselves. These new types De Vries considered as new elementary species and he called them "mutants." They came into existence suddenly in one generation and, as a rule, bred true. Whatever factors were responsible for mutations, the seat of origin must have been in the germ cell and not in the soma. Consequently they were inherited fully from the start. The same mutations occurred in considerable numbers and in successive years. In one case a given mutation occurred only once in eight years of observation. Some mutants were robust and successful, others were weak and incapable of living under natural conditions, others were sterile. On the basis of these results, which are reported in detail in chapter xxvi, De Vries came to the conclusion that evolution was based upon the sudden appear- ance of new varieties or elementary species and not upon the natural selection of fluctuating variations. The mutation theory compared and contrasted with the natural selection theory. — It will be recalled that the raw material upon which natural selection works is the minute individual or continuous varia- tion that is universal in all living forms and is known to be largely somatic in character and due to differences in environment. Darwin 38 EVOLUTION, GENETICS, AND EUGENICS did not distinguish between somatic and germinal variations. The essential feature of mutations is that they are germinal in origin and therefore come forth full-fledged in the first generation arising from the changed germ. Darwin recognized "saltatory variations" or "sports," which are mutations, but did not consider them of suffi- ciently frequent occurrence to furnish an adequate material for selection. De Vries, on his side, did not discard the principle of selection, but showed that selection acted as between mutants, serving to elimi- nate those which are unfit and allowing the sufficiently fit to survive alongside the parent-types. According to Darwin's view, the new types arose only at the expense of the old, for only through the elimina- tion of the old (less fit) types could the new types progress toward further fitness. Darwin's view was ill suited to explain the origin of new distinct types, because the process of selection proceeded by imperceptible steps. De Vries's view gives us distinctly different, pure breeding types at once that, if isolated, would be new elementary species from the first. In conclusion it may be said that the mutation theory was at first intended as a substitute for natural selection, but that later the selection idea was adopted as a directive principle, guiding mutations toward adaptiveness. THE RISE AND VOGUE OF BIOMETRY No historical account of the development of the evolution idea would be complete without a statement of the role played by biometry in the study of evolutionary data. Biometry is the statistical study of variation and heredity. During the last decade of the nineteenth century it became obvious to those who had followed the progress of the subject that farther advance toward the solution of the problem of the causes of evolution must come from a better under- standing of variation and heredity, the two fundamental factors involved. Three main modes of attack were developed during these years: the statistical (biometry), the experimental (chiefly breeding work), and the microscopical (cytology or the study of the minute structure of the germ cells). Sir Francis Gallon, a cousin of Charles Darwin, was the founder of biometry. He applied- certain already understood principles that had been developed mainly in the study of the laws of chance to the study of variations, and, by comparing the boiled-down formulas HISTORICAL ACCOUNT OF EVOLUTION THEORY 39 resulting from his computations of parental generations with those of offspring, he arrived at two laws of heredity: the law of filial regres- sion, and that of ancestral shares of inheritance. The essence of the first was that the offspring of exceptional parents tend to regress toward mediocrity in proportion to the degree of parental excep- tionalness. The second law was really explanatory of the first, for it was found that the offspring inherit not only from parents, but from the various grades of ancestors, and it was the pulldown of a miscel- laneous ancestry that made for regression toward mediocrity. It appeared that half of the hereditary influence could be assigned to parents, half of the remainder to grandparents, half of the remaining remainder to great-grandparents, and so on down the line. Karl Pearson, a pupil and follower of Galton, has carried the study of biometry to a more highly refined state. His attempt has been to apply to the study of evolution the precise quantitative methods which are used in physics and in chemistry. While much of Pearson's work is far beyond the range of the average professional biologist today, it is extremely useful as a tool in handling data in which great accuracy is demanded. Frequently, however, the methods are far too refined for the material, and much time is wasted in handling crude data by means of highly refined instruments of measurement and ultra- accurate mathematical methods. On the whole the contributions of biometry to our understanding of the causes of evolution are rather disappointing. About the only clean-cut finding has been the discovery that some variations are continuous and others discontinuous. The former are capable of being expressed in a single curve with a single mode, while the latter are expressed in bimodal or polymodal curves. If material is homo- geneous to start with it is likely to give monomodal curves, but if it is heterogeneous, its heterogeneity will be revealed by the plural modes. In a subsequent connection (chapter xliii) some further account of the details of biometry will be presented. We must for the present be content with having placed biometry in its setting as one step in the advance of the evolution idea. EXPERIMENTAL BREEDING "While De Vries," says Castle, 1 "was engaged in his studies of the evening primrose he hit upon an idea far more important, as most biologists now believe, than the idea of mutation, though De Vries 1 W. E. Castle, Genetics atui Eugenics (Harvard University Press, 1920), p. 82 40 EVOLUTION, GENETICS, AND EUGENICS himself, both before and since, has seemed to regard it as of minor importance. He called this the 'law of splitting of hybrids.' The same law, it is claimed, was independently discovered about the same time by two other botanists, Correns in Germany, and Tschermak in Austria. Further, historical investigations made by De Vries showed that the same law had been discovered and clearly stated many years previously by an obscure naturalist of Briinn, Austria, named Gregor Mendel, and we have now come to call this law by his name, Mendel's Law. Mendel was so little known when his discovery was published that it attracted little attention from scientists and was soon forgotten, only to be unearthed and duly honored years after the death of its author. Had Mendel lived forty years later than he did, he would doubtless have been a devotee of biometry, for he had a mathematical type of mind and his discovery of a law of hybridization was due to the fact that he applied to his biological studies methods of numerical exactness which he had learned from algebra and physics. In biology he was an amateur, being a teacher of the physical and natural sciences in a monastic school at Briinn. Later he became head of the monastery and gave up scientific work, partly because of other duties, partly because of failing eyesight." There had been plant-hybridizers before Mendel, but their lack of exactness in technique had prevented them from discovering the law of segregation or splitting of hybrids. Joseph Gottlieb Kolreuter (1733-1806), who really belonged to the period of Lamarck, barely missed making the discovery that was afterward made by Mendel. The salient features of his work are according to Castle: 1 " 1. Kolreuter established the occurrence of sexual reproduction in plants by showing that hybrid offspring inherit equally from the pollen plant and the seed plant. "2. He showed that hybrids are commonly intermediate between their parents in nearly all characters observed, such for example as size and shape of parts. "3. Many hybrids are partially or wholly sterile, especially when the parents are very dissimilar (belong to widely distinct species). Such hybrids often exceed either parent in size and vigor of growth. "4. Kolreuter did not observe the regular splitting of hybrids which Mendel and De Vries record, but some of bis successors did, particularly Thomas Knight (1799) and John Goss (1822) in England, 1 Op. cit., p. 86. HISTORICAL ACCOUNT OF EVOLUTION THEORY 41 who were engaged in crossing the garden peas with a view to producing more vigorous and productive varieties, and Naudin (1862) in France, who made a comprehensive survey of the facts of hybridization in plants and came very near to expressing the generalization which Mendel reached four years later." mendel's laws "The earliest experimental investigations of heredity," says Locy 1 in a concise summary of Mendel's work, "were conducted with plants, and the first epoch-making results were those of Gregor Mendel (1822-1884), a monk and later abbot, of an Augustinian monastery at Briinn, Austria. In the garden of the monastery, for eight years before publishing his results, he made experiments on the inheritance of individual (or unit) characters in twenty-two varieties of garden peas. Selecting certain constant and obvious characters, as color, and form of seed, length of stem, etc., he proceeded to cross these pure races, thus producing hybrids, and thereafter, to observe the results of self-fertilization among the hybrids. "The hybrids were produced by removing the unripe stamens of certain flowers and later fertilizing them by ripe pollen from another pure breed having a contrasting character. The results showed that only one of a pair of unit characters appeared in the hybrid of the next generation, while the other contrasting character lay dormant. Thus, in crossing a yellow-seeded with a green-seeded pea, the hybrid genera- tion showed only yellow seeds. The character thus impressing itself on the entire progeny was called dominant, while the other that was held in abeyance was designated recessive. "That the recessive color was not blotted out was clearly demon- strated by allowing the hybrid generation to develop by self-fertiliza- tion. Under these circumstances a most interesting result was attained. The filial generation, derived by self-fertilization among the hybrids, produced plants with yellow and green seeds, but in the ratio of three yellow to one green. All green-seeded individuals and one-third of the yellow proved to breed true, while the remaining two thirds of the yellow-seeded plants, when self-fertilized, produced yellow and green seeds in the ratio of three to one. "Subsequent breedings gave an unending series of results similar to those obtained with the first filial generation. 1 Wiliiam A. Locy, The Main Currents of Zoology (Henry Holt & Company, iqi8), pp. 37-30- 42 EVOLUTION, GENETICS, AND EUGENICS "This great principle of alternative inheritance was exhibited throughout the extensive experiments of Mendel, and it is now recog- nized as one of the great biological discoveries of the nineteen 1 h century." The essential feature of Mendel's discovery was not the phenome- non of dominance, for relatively few instances of pure dominance have been discovered; but it was the phenomenon of segregation. By segregation is meant that although determiners for opposed heredi- tary characters derived from diverse parental sources may unite in a common germ plasm for one generation, they segregate out pure, or unmodified by their association together, in the next and subsequent generations. This law of segregation depends on the idea that the germ cell is composed of bundles of separately inheritable unit charac- ters, which may be paired or grouped, shuffled and redealt like cards, so as to give an infinite number of permutations and combinations without affecting the unit determiners themselves. From the evolutionary standpoint it is supposed that new unit characters arise by mutations and are fully hereditary. They cannot be swamped out by interbreeding unless they are recessive, for they will dominate the old characters. Even recessive characters could be perpetuated by segregation, or by the union of two individuals possess- ing the determiner in the recessive condition as well as the dominant. Thus a knowledge of the behavior of unit characters in heredity reveals part of the mechanism for conserving new characters if they are advantageous or even sufficiently fit to survive. New types or species might arise through processes of hybridiza- tion and the survival of individuals possessing the most favorable combinations of characters. "Evolution from this point of view," says Morgan, 1 "has consisted largely in introducing (by mutations) new factors that influence characters already present in the animal or plant. "Such a view gives us a somewhat different picture of evolution from the old idea of a ferocious struggle between the individuals of a species with the survival of the fittest and the annihilation of the less fit. Evolution assumes a more peaceful aspect. New advantageous characters survive by incorporating themselves into the race, improv ing it and opening to it new opportunities. In other words, the emphasis may be placed less on the competition between the indi- 1 T. H. Morgan, A Critique of the Theory of Evolution (Princeton University Press, 1916), pp. 87, 88. HISTORICAL ACCOUNT OF EVOLUTION THEORY 43 viduals of a species (because the destruction of the less fit does not in itself lead to anything that is new) than on the appearance of new characters and modifications of old characters that become incorpo- rated in the species, for on these depends the evolution of the race." HYBRIDIZATION AND THE ORIGIN OF SPECIES As a consequence of the great interest aroused by Mendel's hybridization experiments the question has arisen as to the role of hybridization in organic evolution. Certain it is that a vast number of animal and plant races now existing are mixed or hybrid in nature and are continually splitting up into various Mendelian segregates. How many pure races are there today ? Some authors think that no variable races today are pure. Lotsy goes so far as to claim and attempt to prove that unit characters are fixed and that the only source of variation is hybridization, or amphimixis. Biologists today would not be willing to go thus far with Lotsy, but it seems beyond question that hybridization has played an important role in the pro- duction of very many groups now living. It is of interest to recall that Linnaeus, though a special creationist, admitted the possibility of the origin of new species by hybridization. NEO-MENDELIAN DEVELOPMENTS Since the rediscovery of Mendel's paper by De Vries and its perusal by thousands of biologists the world over, Mendelian breeding experi- ments with all manner of animals and plants has been the ruling passion of geneticists. Among the leading neo-Mendelians are Bate- son, Morgan, Castle, Correns, East, Hurst, Shull, Tschermak, and the pupils of these. Perhaps the first two mentioned, Bateson and Morgan, have con- tributed most largely to an understanding of the intricacies of the Mendelian operations. Bateson has become so imbued with the idea that all mutations are the result of the loss of factors that he proposes the hypothesis that " evolution has taken place through the steady loss of inhibiting factors," as Morgan puts it. "Living matter was stopped down, so to speak, at the beginning of the world. As the stops are lost, new things emerge. Living matter has changed only in that it becomes simpler." It is quite probable that Bateson, in pro- posing so radical a view, intended to be taken only half-seriously. Apart from this, his best-known expression of opinion, Bateson is the 44 EVOLUTION, GENETICS, AND EUGENICS author of a large amount of fine work in genetics and will rank high in the history of the subject. T. H. Morgan, our leading American geneticist, is best known for his researches into the mechanism of Mendelian inheritance. Through the statistical study of ratios and linkages of characters in the fruit fly Drosophila, it has been possible to chart the localities of the deter- miners or genes of at least 400 mutant characters. He has shown that four linked groups of genes exist, corresponding to the four kinds of chromosomes of the germ cells; one of these groups is sex-linked and is therefore to be assigned to the X-chromosome of the mutant male. Two other large groups are to be located in the two large autosomes, and one very small group is assumed to be located in the microsome. Not only have characters, or their determiners, been assigned to given chromosomes, but they have been located in a linear series on a given chromosome. So accurately have these loci been determined that they may be used to predict unknown breeding ratios. It would seem that when a theory serves so well that it may be used to predict the results of experiments, such a theory must be founded on facts. Morgan and his collaborators in genetics are now convinced that they have discovered the actual mechanism of heredity in the behavior of the chromosomes in maturation and fertilization and that it is unex- pectedly simple. Their views have aroused considerable opposition, but they have apparently met successfully all attacks up to the present. If it be true that the actual machinery of variation and heredity has oeen discovered, we are farther along in our understanding of the causo-mechanical basis of evolution than we could have hoped to be at so early a date. HEREDITY AND SEX Since Darwin's theory of sexual selection, sex has been a compli- cating factor in evolutionary theories, and one of the chief advances of the present century has been in connection with the factors con- trolling sex determination and sex differentiation. The evolution of sex has also been a subject for considerable research. It now appears that sex is an inherited Mendelian character, the determiner of which is carried in a definite chromosome or group of chromosomes. Cytological examination of germ cells, under the able leadership of E. B. Wilson, has now made it certain that sex, if not directly the result of the presence or absence of specific chromo- somes, at least is absolutely correlated with such chromosomes. It appears, however, that the sex which is settled by the chromosome HISTORICAL ACCOUNT OF EVOLUTION THEORY 45 mechanism at the time of fertilization may or may not realize its normal somatic differentiation, depending upon the presence or absence of the proper environment. Cases are on record in which an individual germinally determined as a female may be caused to develop the secondary sexual characters of the male, or even to pro- duce sperms instead of eggs. A great deal of extremely interesting work on sex control and sex reversals has been done within the last half-dozen years and new discoveries are being made almost daily. In fact, it might be said that the genetic study of sex marks the high-tide level of modern genetic advance. THE EXPERIMENTAL INDUCTION OF HEREDITARY VARIATIONS With the problem of the mechanism of the heredity of individual differences solved, at least in its more important essentials, attention has gradually shifted to the problem as to how individual differences arise. They seem to arise suddenly and as though of their own accord, and the study of their heredity does not throw much light on the prob- lem of their origin. At the present time a massed attack is being made upon the problem of the mode or modes of origin of new hereditary characters. The most striking success in the artificial induction of mutations has been obtained by H. F. Midler, using as his material the already intensively studied fruit fly, Drosophila melanogaster. By the use of rather heavy doses of X-rays he succeeded in increasing the fre- quency of mutations about 1,500 per cent. Nearly all of the mutations produced by this method were the same as those that occur spontane- ously, but many more occur in a given time. Many other investigators, following Muller's methods, have suc- ceeded in greatly hastening the pace of mutation in various animals and plants. The ability to produce such large numbers of mutants at will furnishes abundant material for genetic investigation and promises greatly to increase our knowledge of the intimate details of the mechan- isms of variation and heredity. The most pressing problem of the present is that of discovering how and when genes act during the course of development in producing the characters of the organism. Some progress has been made in this direction. THE RECENT ATTACK UPON EVOLUTION IN THE UNITED STATES The recent highly advertised attack upon the validity of the principle of evolution by certain individuals and religious bodies is hardly to be considered as forming a part of the history of the science, 46 EVOLUTION, GENETICS, AND EUGENICS but it is significant as an influence that may serve either greatly to accelerate or to retard the progress of our science. The writer's own experience is that the controversy has greatly enhanced popular inter- est in this subject, as evidenced by the growing demand for books on evolution and allied subjects and the marked increase in the numbers of students in the colleges who wish to elect courses along these lines. CONCLUDING REMARKS Now that we have traced the evolution of the science of organic evolution from its crude beginnings among the Greeks up to the present, we are in a position to go back and make a systematic study of some of the more important phases of evolutionary science. Charles Darwin found it necessary to prove the fact of organic evolu- tion before attempting to discover its causes. His method of proof was to marshal a great array of facts which agree with the idea of descent with modification; and we shall follow Darwin's method in the subsequent chapters dealing with the evidences of evolution. Note. — In the first half of the present historical account many short passage? are. presented in quotation marks without mentioning the source of the quotation. In all such cases it will be understood that these passages are from H. F. Osborn's book, From the Creeks to Darwin (The Macmillan Company). part n EVIDENCES OF OllGANIC EVOLUTION CHAPTER HI IS ORGANIC EVOLUTION AN ESTABLISHED PRINCIPLE? i. Is there definite proof of organic evolution? 2. If so, what is the nature of the proof ? 3. What are the evidences of evolution, and in what ways do these bear witness that evolution has occurred and is still occurring ? Before presenting in any detail the several bodies of data that constitute the "evidences of evolution," let us anticipate a little by attempting to answer the three questions just propounded. 1. Reluctant as he may be to admit it, honesty compels the evolutionist to admit that there is no absolute proof of organic evolution. But, for that matter, there is no absolute proof of any- thing that depends on records of past events. We have no absolute proof that Caesar or Napoleon once lived, or fought, or conquered. All we have are the accounts left by the historians which we accept without question because they are the products of human thought and imagination. There is no absolute proof for either of the more or less directly opposed theories of the origin of the material universe: the "nebular hypothesis" of Laplace, and the " planetesimal hypothesis" of Chamberlin and Moulton. Both of these theories rest upon exactly the same types of evidences as does the theory of organic evolu- tion, viz., the amassing of facts which appear to be explicable on the assumption that the one or the other theory is true. If all of the facts are in accord with it, and none are found that are incapable of being reconciled with it, a working hypothesis is said to have been advanced to the rank of a proved theory. As yet it is impossible to say that either of these theories as to the origin of the universe has been proved. Yet there is much less popular opposition to the acceptance of these theories as facts than there is to the general theory of organic evolu- tion. Similarly, there are certain widely accepted theories of the origin of the present conditions of the earth's crust, and its liquid and gaseous envelopes. The accepted theory, as given us by Hutton and especially by Lyell, is essentially an evolutionary theory and depends for its proof on almost exactly the same types of evidence as does that 49 5° EVOLUTION, GENETICS, AND EUGENICS of organic evolution. The basis of the accepted theory of geological evolution is the " uniformitarian doctrine" of Lyell, which assumes that the key to the past lies in the present, that the changes that are going on today are of the same order and kind as those of the past, and, finally, that there is neither beginning nor end to the earth's evolutionary history, but that a slow and orderly development has gone on and will continue indefinitely. The proof of this conception consists of an array of facts derived from a study of the earth's crust, including its stratified structure, of traces of animal and plant life preserved in the rocks, of observed changes in continental contours going on today, of erosion going on in coasts and streams, and of a considerable array of facts derived from a study of other worlds than ours in the making. The theory of geologic evolution meets with scarcely any opposition today, although its foundations are no more securely based than are those of organic evolution. In a sense the proofs of the atomic, ionic, and electron theories are even less absolutely established than is that of organic evolution, because no one has ever seen nor ever can see an atom, an ion, or an electron. Chemical and physical facts are rationalized by assuming the existence of these units with their various properties. The only evidences of the existence of atoms, ions, and electrons appear in the facts that, on the assumption that they exist, the whole array of observed chemical and physical phenomena are rationalized and bound together into a coherent, consistent, and intelligible system. In other words, with the atomic, ionic, and electron theories chemistry and physics are highly rational sciences; without these theories the phenomena of physics and chemistry would be a hopeless hodgepodge. Yet who would say that these fundamental theories are absolutely proved ? The only type of proof of phenomena that cannot be directly observed or that pertain to the remote past is circumstantial proof. By analogy we conclude that certain changes took place thus and so in the past because we observe similar changes going on today. Every past event has left a trace, and it is the task of the historian, anti- quarian, or evolutionist to discover and to interpret these traces. Some- times the traces exist as vestiges in modern life and are meaningless unless related to their origin in the past. The task of the student of organic evolution is to gather all of the traces of past changes both in living creatures today and in the preserved remains of creatures of the remote past. A collection of traces of evolution involves many IS ORGANIC EVOLUTION ESTABLISHED? 51 apparently unrelated bodies of phenomena. There are evidences of evolution in the grouping of animals into phyla, classes, orders, families, genera, species, varieties, and races; in the homologies that exist in general structure and in particular organs between different <:n^ups of animals and plants; in the orderly process of ontogeny or embryonic development of the individual; in actual blood relation- ship, based upon chemical reactions; on the succession of extinct animals and plants found as fossils imbedded in the geologic strata; in the present geographical distribution of the various groups of animals and plants, in the light of data derived from a study of geological changes; and finally, in experimental evolution, which involves the observation under experimental control of changes in organisms and the origin of new varieties or elementary species. 2. The nature of the proof of organic evolution, then, is this: that, using the concept of organic evolution as a working hypothesis it has been possible to rationalize and render intelligible a vast array of observed phenomena, the real facts upon which evolution rests. Thus classification (taxonomy), comparative anatomy, embryology, palaeontology, zoogeography and phytogeography, serology, genetics, become consistent and orderly sciences when based upon evolu- tionary foundations, and when viewed in any other way they are thrown into the utmost confusion. There is no other generalization known to man which is of the least value in giving these bodies of fact any sort of scientific coherence and unity. In other words, the working hypothesis works and is therefore acceptable as truth until overthrown by a more workable hypothesis. Not only does the hypothesis work, but, with the steady accumulation of further facts, the weight of evidence is now so great that it overcomes all intelligent opposition by its sheer mass. There are no rival hypotheses except the outworn and completely refuted idea of special creation, now retained only by the ignorant, the dogmatic, and the prejudiced. 3. In answer to the question, "What are the evidences of evolution and in what ways do these bear witness that evolution has occurred and is still occurring?" we may present an ordered list of subjects that are to be taken up serially in detail. In connection with each of these bodies of evidence the. character of their witness-bearing will be discussed. Some of the evidences are more direct and freer from purely inter- pretative construction than others. Some evidences are primary and foundational; some are in themselves rather inconclusive, but serve 52 EVOLUTION, GENETICS, AND EUGENICS to confirm other facts, and, when reinforced by other evidences, are themselves strongly substantiated. Perhaps the crowning evidence of the truth of evolution is that all of these diverse bodies or phenomena invariably support one another and all point in the same direction and to the same conclusion, viz., that organic evolution is a fact. In the former edition of this book the evidences of evolution were presented in a somewhat arbitrary order, the evidences that seemed to furnish the most direct proof being, for pedagogical reasons, presented first and the more controversial evidences last. Experience, however, has shown that for an appreciation of the data from paleontology and from geographic distribution the student must have a knowledge of the principles of morphology (comparative anatomy) and of classifica- tion. We have, therefore, changed the order of presentation of the evidences to one that has the authority of precedent. The order of treatment will be as follows: I. The fundamental assumption underlying all the evidences. II. Comparative anatomy {homologies and vestigial structures) : the evidence of the fact that structures in unlike organisms have a com- mon plan and mode of origin; that changes have occurred that are in some way related to changes in habit or environment. III. Classification: the evidence that the present groups of animals and plants have arisen by "descent with modification."" IV. Serology {blood-precipitation tests): the evidence that the chemical specificity of the blood parallels taxonomic specificity. V. Embryology {the doctrine of recapitulation) : the evidences that the embryonic development of the individual follows the main out- lines of the evolutionary history of its ancestors. VI. Paleontology: the evidences afforded by a study of the distri- bution in time (vertical distribution in the earth's strata) of the fossil remains of extinct animals and plants. VII. Geographic distribution: the evidences afforded by present (also, to some extent, past) horizontal distribution of contemporaneous animals and plants. VIII. Genetics {experimental evolution): evidences that heritable variations have occurred under observation in large numbers and in many species of animals and plants, and that new varieties of animals and plants have been produced by processes known to man and to a large extent controlled by him. CHAPTER IV THE FUNDAMENTAL POSTULATE UNDERLYING ALL EVIDENCES OF EVOLUTION Every science rests in last analysis upon certain postulates or justi- fiable assumptions, certain verified or verifiable truths that must be admitted before any progress can be made in gaining a further under- standing of the content of that science. Geology, for example, must assume as valid the dynamical laws of Newton and the law of gravita- tion, as well as basic laws of chemistry. Biology assumes the validity of the laws of physics and chemistry, for biology is the fundamental science of the transformations of matter and of energy in living matter; but, in addition, there are also some biological postulates that seem to be so well established that they have come to be thought of as truisms. One of the truisms of biology is the familiar fact that like produces like. How surprised one would be if sparrows had anything but spar- rows for offspring, or if two Caucasic parents were to have a Negro child ! Now, a careful survey of the situation reveals the fact that the only postulate the evolutionist needs is no more nor less than a logical extension of what the layman considers a truism or a self-evi- dent fact, namely, that fundamental structural resemblance signifies genetic relationship; that, generally speaking, the degree of closeness of structural resemblance runs essentially parallel with closeness of kinship. Most biologists would say that this is not merely a postulate, but one of the best-established laws of life. However obvious the validity of this postulate may be, it is the plain duty of one who attempts to justify the evolutionary principle to avoid taking any steps that are open to the least bit of valid criticism. If we cannot rely upon this postulate, which may be called the principle of homology, we can make no sure progress in any attempt to establish the validity of the principle of evolution. The postulate we are now discussing is tantamount to an affirma- tion of the fact of heredity. We rely upon this fact in our everyday life. When we plant a certain kind of seed we expect to get a certain kind of plant; when we breed a certain kind of dog we expect offspring 53 54 EVOLUTION, GENETICS, AND EUGENICS of the same breed. It would be a freak of nature were we to discover any marked exception to the laws of heredity. Furthermore, our ordinary daily contacts with other members of our own species have taught us that, as a rule, the more closely alike people are, the more closely are they related. We recognize that children of the same fam- ily are more alike in their personal characteristics than are members of the same race not so closely related. Whenever we see two people whose resemblance is very great we assume a relatively close kinship. Thus, everyone has had the experience of meeting two people so strikingly alike that it is almost impossible to distinguish them apart, and of immediately assuming that such persons are identical, or dupli- cate, twins. Now the interesting thing about such twins is that they are vastly more closely related than are ordinary brothers and sisters, or even than are fraternal twins, who are only brothers and sisters that happen to have been conceived and born simultaneously as the result of the fertilization of two egg cells. For duplicate twins are the products of the early division into two equivalent parts of a single embryo derived from one fertilized egg. No closer kinship can well be imagined than this, for the two individuals bear the same relationship to each other as do the bilateral halves of one individual. The writer has had an exceptional opportunity of determining the exact degree of resemblance existing between separate offspring de- rived from a single egg. It so happens that a peculiar species of mammal, the nine-banded armadillo of Texas, always gives birth to four young at a time. These quadruplets are invariably all of the same sex in a Utter and are nearly identical even in their finest ana- tomical details, such as the numbers and arrangements of the plates and scales in the armor and the numbers of hairs in a given area of the skin. A detailed study of the embryonic history of this species has proved beyond any question that in every case the four young in a litter result from a very early division of a single embryo derived from a single fertilized egg (see Fig. 52). Large numbers of sets of quadru- plets were studied statistically to determine the exact degree of their resemblance to one another. A comparison of over two hundred sets revealed the somewhat startling fact that on the average they were over 93 per cent identical (more technically, they showed a coefficient of correlation of over .93). The remarkable closeness of this degree of resemblance may be fully appreciated when it is realized that the only structural resemblance belonging to this order of closeness is that existing between the right and left antimeric halves of a single indi- POSTULATE UNDERLYING ALL EVIDENCES OF EVOLUTION 55 vidual, such as the right and left sides of your own face or your two hands, and that the next degree of closeness of resemblance is that between siblings (brothers and sisters), who are only 50 per cent identi- cal (having a coefficient of correlation of only .5); while cousins of various grades have proportionately lower and lower degrees of re- semblance in exact ratio with their grades of kinship. This, then, is a crucial test of the validity of the postulate that closeness of resemblance is in proportion to closeness of kinship, for we have in identical twins and in armadillo quadruplets the closest re- semblance associated with the closest possible genetic relationship, and we also see that there is an exact proportion between all other known grades of kinship and their relative degree of resemblance. Employing the principle of homology in a somewhat broader way, and in a way that is hardly likely to be questioned even by the most captious, we account for the common possession of certain structural peculiarities by all members of a given kind or species of animal or plant by saying that such characters have been derived from a com- mon ancestor. It is only a short step in logic to conclude that two similar kinds or species of animal have been derived one from the other or both from a common ancestral species. Once having taken this step, we are on the road that leads inevitably to an evolutionary in- terpretation of natural groups. If the principle of heredity holds for siblings (offspring of the same parents), for races, for species, where are we to draw the line? It does not seem reasonable to admit that structural resemblances between siblings, between races, between species, are accounted for as the product of heredity, and to deny that equally plain resemblances of essentially the same sort among the species of a genus or among the genera of a family have a similar hereditary basis. It is logically impossible to draw the line at any level of organic classification and say that structural resemblance is the product of heredity up to such and such a level, but that beyond this arbitrarily chosen point heredity ceases to operate. The principle of heredity and its necessary implications constitute the only postulate that is necessary for the evolutionist to make in order to go ahead on a sound basis with a presentation of the evidences of evolution. Give him this one point, and he asks no further con- cessions. And this is not so much of a concession as it might seem at first blush, for the special creationist assumes more potency for heredity than does the evolutionist, since he believes in descent with- out modification, a sort of stereotyped heredity, slavishly duplicating 56 EVOLUTION, GENETICS, AND EUGENICS forever a fixed set of structural patterns without variation or improve- ment. Since, then, both special creationist and evolutionist find it equally necessary to assume the principle of heredity, there should be little argument on this score. But let the reader beware at this point in the discussion, for if he admits the postulates already presented — and how can he help but admit them? — he cannot avoid the inevit- able conclusion that the theory of descent with modification is the only reasonable explanation of organic resemblances and differences. HOMOLOGY VERSUS ANALOGY Much difficulty in connection with the study of resemblances and differences in animals and plants is occasioned by a failure to under- stand the fact that there are two kinds of resemblances and differences. Structures that are similar in anatomical detail and in their mode of embryonic origin, irrespective of whether they perform the same or different functions, are known as homologous. The test of homological equivalence is a study of the anatomical details of the adult structure followed by a study of the developmental history of the part in ques- tion. If the part under examination be a bone, for example, this bone must have a certain relation to the other bones, must occur in a certain part of the body, must be supplied with certain muscle attachments, in order to be considered homologous with another bone that has the same relations. If two structures have the same anatomical relations and arise from equivalent embryonic rudiments they are said to be homologous, in spite of small or great differences in relative size, ap- pearance, or function. If structures are homologous it is believed that they represent the same hereditary units and that these equivalent hereditary units have been derived from the same or similar ancestors. Analogous structures are of an entirely different sort. They may be more or less superficially alike in form or in function, usually in both, though anatomically quite different. As an example of analo- gous structures let us examine the three types of aquatic vertebrates shown in Figure 80. These three kinds of vertebrates, one a fish, one a reptile, and the third a mammal, might be mistaken by the casual observer to be all fishes of different kinds. All have the same fusiform body with lines best adapted for swift locomotion in the water; all have median, paired, and caudal fins; all swim in about the same way. Yet the resemblance is only skin-deep, as it were, for beneath the sur- face the one is all fish, the second all reptile, and the third all mammal. The structures that look alike and function alike are, from the stand- POSTULATE UNDERLYING ALL EVIDENCES OE EVOLUTION 57 point of anatomical relations and embryonic derivation, entirely differ- ent. The resemblances which are so obvious superficially are examples of analogy, not of homology, and are the result of molding unlike materials into a semblance of likeness in adaptation to a common en- vironment. Analogous structures, while not considered as evidences of kinship, are strong evidences of descent with modification, for their very existence implies that they have been changed from a former condition to one in which they are adapted to a new medium. To illustrate this point, call to mind that both the ichythyosaur and the porpoise (Fig. 80, B and C) belong to groups that are fundamentally terrestrial air-breathing vertebrates, and that whatever they have that is fishlike must be interpreted as adaptive modifications for aquatic life. This type of conception and the way in which it bears witness for organic evolution is well brought out in the next chapter by George John Romanes, a chapter that for a generation has been considered a classic. A few of the statements in this chapter would, in all probabil- ity, be somewhat altered if the author were to rewrite it in the light of newer knowledge, but on the whole the statements made would still have the support of the most critical of modern anatomists. CHAPTER V EVIDENCES FROM MORPHOLOGY (COMPARATIVE ANATOMY) 1 GEORGE JOHN ROMANES The theory of evolution supposes that hereditary characters admit of being slowly modified wherever their modification will render an organism better suited to a change in its conditions of life. Let us, then, observe the evidence which we have of such adaptive modifi- cations of structure, in cases where the need of such modification is apparent. We may begin by again taking the case of the whales and porpoises. The theory of evolution infers, from the whole structure of these animals, that their progenitors must have been terrestrial quadrupeds of some kind, which gradually became more and more aquatic in their habits. Now the change in the conditions of their life thus brought about would have rendered desirable great modifica- tions of structure. These changes would have begun by affecting the least typical — that is, the least strongly inherited — structures, such as the skin, claws, and teeth. But, as time went on, the adaptations would have extended to more typical structures, until the shape of the body would have become affected by the bones and muscles required for terrestrial locomotion becoming better adapted for aquatic locomotion, and the whole outline of the animal more fish-like in shape. This is the stage which we actually observe in the seals, where the hind legs, although retaining all their typical bones, have become shortened up almost to rudiments, and directed backwards, so as to be of no use for walking, while serving to complete the fish-like taper of the body (Fig. i). But in the whales the modification has gone further than this so that the hind legs have ceased to be apparent externally, and are only represented internally — and even this only in some species — by remnants so rudimentary that it is difficult to make out with certainty the homologies of the bones; moreover, the head and the whole body have become completely fish-like in shape (Fig. 12). But profound as are these alterations, they affect only 1 From G. J. Romanes, Darwin and after Darwin (copyright 1892). Used by special permission of the publishers, The Open Court Publishing Company. 58 EVIDENCES FROM MORPHOLOGY 59 05 (U O M CO O 60 EVOLUTION, GENETICS, AND EUGENICS those parts of the organism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus the arm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all enclosed in a fin-shaped sack, so as to render them useless for any purpose other than swimming (Fig. 3). Similarly, the head, although it so closely resembles the head of a fish in shape, still retains the bones of the mammalian skull in their proper anatomical relations to one another; but modified in form so as to offer the least possible resistance to the water. In short, it may be said that all the modifi- cations have been effected with the least possible divergence from the typical mammalian type, which is compatible with securing so perfect an adaptation to a purely aquatic mode of life. Now I have chosen the case of the whale and porpoise group, because they offer so extreme an example of profound modification of structure in adaptation to changed conditions of life. But the same thing may be seen in hundreds and hundreds of other cases. For instance, to confine our attention to the arm, not only is the limb modified in the whale for swimming, but in another mammal — the bat — it is modified for flying, by having the fingers enormously elongated and overspread with a membranous web. In birds, again, the arm is modified for flight in a wholly different way — the fingers here being very short and all run together, while the chief expanse of the wing is composed of the shoulder and forearm. In frogs and lizards, again, we find hands more like our own; but in an extinct species of flying reptile the modification was extreme, the wing having been formed by a prodigious elongation of the fifth finger, and a membrane spread over it and the rest of the hand (Fig. 4). Lastly, in serpents the hand and arm have disappeared altogether. Thus, even if we confine our attention to a single organ, how wonderful are the modifications which it is seen to undergo, although never losing its typical character. Everywhere we find the distinction between homology and analogy which was explained in the last chapter — the distinction, that is, between correspondence of structure and correspondence of function. On the one hand, we meet with structures which are perfectly homologous and yet in no way analogous; the structural elements remain, but are profoundly modified so as to perform wholly different functions. On the other hand, we meet with structures which are perfectly analogous, and yet in no way homologous; totally different structures are modified EVIDENCES FROM MORPHOLOGY 61 o c/) 55 M o J ' — I c c o to -*-» a LI 1) s .■; cd co -a M • a; cu a •: 0> 62 EVOLUTION, GENETICS, AND EUGENICS to perform the same functions. How, then, are we to explain these things ? By design manifested in special creation, or by descent with adaptive modification ? If it is said by design manifested in special creation, we must suppose that the Deity formed an archetypal plan of certain structures, and that he determined to adhere to this plan through all the modifications which those structures exhibit. But, if so, why is it that some structures are selected as typical and not others ? Why should the vertebral skeleton, for instance, be tortured Fig. 3. — Paddle of whale compared with hand of man. {From Romanes.) into every conceivable variety of modification in order to subserve as great a variety of functions; while another structure, such as the eye, is made in different sub-kingdoms on fundamentally different plans, notwithstanding that it has throughout to perform the same func- tion ? Will any one have the hardihood to assert that in the case of the skeleton the Deity has endeavored to show his ingenuity, by the manifold functions to which he has made the same structure sub- servient; while in the case of the eye he has endeavored to show his resources, by the manifold structures which he has adapted to serve the same function? If so, it becomes a most unfortunate circum- stance that, throughout both the vegetable and animal kingdoms, all cases which can be pointed to as showing ingenious adaptation of the EVIDENCES FROM MORPHOLOGY Fig. 4. — Wing of reptile, mammal, and bird. {From Romanes) 5 4 EVOLUTION, GENETICS, AND EUGENICS same typical structure to the performance of widely different func- tions — or cases of homology without analogy — are cases which come within the limits of the same natural group of plants and animals, and therefore admit of being equally well explained by descent from a common ancestry; while all cases of widely different structures per- forming the same function — or cases of analogy without homology, are to be found in different groups of plants or animals, and are therefore suggestive of independent variations arising in the different lines of hereditary descent. To take a specific illustration. The octopus, or devil-fish, belongs to a widely different class of animals from a true fish; and yet its eye, in general appearance, looks wonderfully like the eye of a true fish. Now, Mr. Mivart pointed to this fact as a great difficulty in the way of the theory of evolution by natural selection, because it must clearly be a most improbable thing that so complicated a structure as the eye of a fish should happen to be arrived at through each of two totally different lines of descent. And this difficulty would, indeed, be a formidable one to the theory of evolution, if the similarity were not only analogical but homological. Unfortunately for the objection, however, Darwin clearly showed in his reply that in no one anatomical or homologous feature do the two structures resemble one another; so that, in point of fact, the two organs do not resemble one another in any particular further than it is necessary that they should, if both are to be analogous, or to serve the same function as organs of sight. But now, suppose that this had not been the case, and that the two structures, besides presenting the necessary superficial or analogical resemblance, had also presented an anatomical or homologous resem- blance, with what force might it have then been urged, — your hypo- thesis of hereditary descent with progressive modification being here excluded by the fact that the animals compared belong to two widely different branches of the tree of life, how are we to explain the identity of type manifested by these two complicated organs of vision ? The only hypothesis open to us is intelligent adherence to an ideal plan or mechanism. But as this cannot now be urged in any comparable case throughout the whole organic world, we may, on the other hand, present it as a most significant fact, that while within the limits of the same large branch of the tree of life we constantly find the same typical structures modified so as to perform very different functions we never find any of these particular types of structure in other large branches of the tree. That is to say, we never find typical structures EVIDENCES FROM MORPHOLOGY 65 appearing except in cases where their presence may be explained by the hypothesis of hereditary descent ; while in thousands of such cases we find these structures undergoing every conceivable variety of adaptive modification. Consequently, special creationists must fall back upon another position and say, — Well, but it may have pleased the Deity to form a certain number of ideal types, and never to have allowed the structures occurring in one type to appear in any of the others. We answer, — Undoubtedly such may have been the case; but, if so, it is a most unfortunate thing for your theory, because the fact implies that the Deity has planned his types in such a way as to suggest the counter-theory of descent. For instance, it would seem most capri- cious on the part of the Deity to have made the eyes of an innumerable number of fish on exactly the same ideal type, and then to have made the eye of the octopus so exactly like these other eyes in superficial appearance as to deceive so accomplished a naturalist as Mr. Mivart, and yet to have taken scrupulous care that in no one ideal particular, should the one type resemble the other. However, adopting for the sake of argument this great assumption, let us suppose that God did lay down these arbitrary rules for his own guidance in creation, and then let us see to what the assumption leads. If the Deity formed a certain number of ideal types, and determined that on no account should he allow any part of one type to appear in any part of another, surely we should expect that within the limits of the same type the same typical structures should always be present. Thus, remember what efforts, so to speak, have been made to maintain the uniformity of type in the case of the fore-limb as previously explained, and should we not expect that in other and similar cases a similar method should have been followed ? Yet we repeatedly find that this is not the case. Even in the whale, as we have seen, the hind-limbs are either alto- gether absent or dwindled almost to nothing; and it is impossible to see in what respects the hind-limbs are of any less ideal value than the fore-limbs — which are carefully preserved in all vertebrated animals except the snake, and the extinct Dinornis, where again we meet in this particular with a sudden and sublime indifference to the main- tenance of a typical structure (Fig. 5). Now I say that if the theory of ideal types is true, we have in these facts evidence of a most unrea- sonable inconsistency. But the theory of descent with continued adaptive modification fully explains all the known cases; for in every case the degree of divergence from the typical structure which an 66 EVOLUTION, GENETICS AND EUGENICS organism presents corresponds, in a general way, with the length of time during which the divergence has been going on. Thus we Fig. 5. — Skeleton of Dinornis gravis, ^ B nat. size. Drawn from nature (British Museum). As separate cuts on a larger scale are shown, (1) the sternum as this appears in mounted specimens, and (2) the same in profile, with its (hypothetical) scapulo-coracoid attached. (From Romanes.) scarcely ever meet with any great departure from the typical form with respect to one of the organs, without some of the other organs being so far modified as of themselves to indicate, on the supposition EVIDENCES FROM MORPHOLOGY 67 of descent with modification that the animal or plant must have been subject to the modifying influences for an enormously long series of generations. And this combined testimony of a number of organs in the same organism is what the theory of descent would lead us to expect, while the rival theory of design can offer no explanation of the fact, that when one organ shows a conspicuous departure from the supposed ideal type, some of the other organs in the same organism should tend to keep it company by doing likewise. As an illustration both of this and of other points which have been mentioned, I may draw attention to what seems to me a particularly suggestive case. So-called soldier- or hermit-crabs are crabs which have adopted the habit of appropriating the empty shells of mollusks. In association with this peculiar habit, the structure of these animals differs very greatly from that of all other crabs. In particular, the hinder part of the body, which occupies the mollusk-shell, and which therefore has ceased to require any hard covering of its own, has been suffered to lose its calcareous integument, and presents a soft fleshy character, quite unlike that of the most exposed parts of the animal. Moreover, this soft fleshy part of the creature is especially adapted to the particular requirements of the creature by having its lateral appendages — i. e., appendages which in other Crustacea perform the function of legs — modified so as to act as claspers to the inside of the mollusk-shell; while the tail-end of the part in question is twisted into the form of a spiral, which fits into the spiral of the mollusk-shell. Now, in Keeling Island there is a large kind of crab called Birgus latro, which lives upon land and there feeds upon cocoa-nuts. The whole structure of this crab, it seems to me, unmistakably resembles the structure of a hermit-crab (Fig. 6). Yet this crab neither lives in the shell of a mollusk, nor is the hinder part of its body in the soft and fleshy condition just described; on the contrary, it is covered with a hard integument like all the other parts of the animal. Consequently, I think we may infer that the ancestors of Birgus were hermit-crabs living in mollusk-shells; but that their descendants gradually relin- quished this habit as they gradually became more and more terrestrial, while, concurrently with these changes in habit, the originally soft posterior parts acquired a hard protective covering to take the place of that which was formerly supplied by a mollusk-shell. So that, if so, we now have, within the limits of a single organism evidence of a whole series ot morphological changes in the past history of its species. First, there must have been the great change from an 68 EVOLUTION, GENETICS, AND EUGENICS EVIDENCES FROM MORPHOLOGY 69 ordinary crab to a hermit-crab in all the respects previously pointed out. Next, there must have been the change back again from a hermit-crab to an ordinary crab, so far as living without the necessity of a mollusk-shell is concerned. From an evolutionary point of view, therefore, we appear to have in the existing structure of Birgus a morphological record of all these changes, and one which gives us a reasonable explanation of why the animal presents the extraordinary appearance which it does. But, on the theory of special creation, it is inexplicable why this land-crab should have been formed on the pattern of a hermit-crab, when it never has need to enter the shell of a mollusk. In other words, its peculiar structure is not especially in keeping with its present habits, although so curiously allied to the similar structure of certain other crabs of totally different habits, in relation to which the peculiarities are of plain and obvious significance. I will devote the remainder of this chapter to considering another branch of the argument from morphology, to which the case of Birgut serves as a suitable introduction: I mean the argument from rudi- mentary structures. Throughout both the animal and vegetable kingdoms we con- stantly meet with dwarfed and useless representatives of organs, which in other and allied kinds of animals and plants are of large size and functional utility. Thus, for instance, the unborn whale has rudi- mentary teeth, which are never destined to cut the gums; and throughout its life this animal retains, in a similarly rudimentary condition, a number of organs which never could have been of use to any kind of creature save a terrestrial quadruped. The whole anatomy of its internal ear, for example, has reference to hearing in air, as Hunter long ago remarked, "is constructed upon the same principle as in the quadruped"; yet, as Owen says, "the outer open- ing and passage leading therefrom to the tympanum can rarely be affected by sonorous vibrations of the atmosphere, and indeed they are reduced, or have degenerated, to a degree which makes it difficult to conceive how such vibrations can be propagated to the ear-drum during the brief moments in which the opening may be raised above the water." Now, rudimentary organs of this kind are of such frequent occur- rence, that almost every species presents one or more of them — usually, indeed, a considerable number. How, then, are they to be accounted for ? Of course the theory of descent with adaptive modi- fication has a simple answer to supply — namely, that when, from 7o EVOLUTION, GENETICS. AND EUGENICS changed conditions of life, an organ which was previously useful becomes useless, it will be suffered to dwindle away in successive generations, under the influence of certain natural causes which we shall have to consider in future chapters. On the other hand, the theory of special creation can only maintain that these rudiments are formed for the sake of adhering to an ideal type. Now, here again the former theory appears to be triumphant over the latter; for, without waiting to dispute the wisdom of making dwarfed and useless structures merely for the whimsical motive assigned, surely if such a F^/K RtJDiptCflTAp.y HttfD-LlpiBS A. \lEfiT, 8. HOMy TEW/VflTIO/J or JllflD-LipMi. Fig. 7. — Rudimentary or vestigial hind limbs of python, as exhibited in the skeleton and on the external surface of the animal. Drawn from nature, J nat. size. {From Romanes.) method were adopted in so many cases, we should expect that in con- sistency it would be adopted in all cases. This reasonable expectation, however, is far from being realized. We have already seen that in numberless cases, such as that of the fore-limbs of serpents, no vestige of a rudiment is present. But the vacillating policy in the matter of rudiments does not end here; for it is shown in a still more aggravated form where within the limits of the same natural groups of organisms a rudiment is sometimes present and sometimes absent. For instance, although in nearly all the numerous species of snakes there are no vestiges of limbs, in the Python we find very tiny rudiments of the hind-limbs (Fig. 7). Now, is it a worthy conception of Deity that, while neglecting to maintain his unity of ideal in the case of EVIDENCES FROM MORPHOLOGY 71 nearly all the numerous species of snakes, he should have added a tiny rudiment in the case of the Python — and even in that case should have maintained his ideal very inefficiently, inasmuch as only two limbs, instead of four, are represented ? How much more reasonable is the naturalistic interpretation; for here the very irregularity of their appearance in different species, which constitutes rudimentary structures one of the crowning difficulties to the theory of special design, furnishes the best possible evidence in favour of hereditary Fig. S. — Aptcryx austral is. Drawn from life in the Zoological Gardens, g nat. size. The external wing is drawn to a scale in the upper part of the cut. The surroundings are supplied from the most recent descriptions. (From Romanes.) descent; seeing that this irregularity then becomes what may be termed the anticipated expression of progressive dwindling due to inutility. Thus, for example, to return to the case of wings, we have already seen that in an extinct genus of bird, Dinomis, these organs were reduced to such an extent as to leave it still doubtful whether so much as the tiny rudiment hypothetically supplied to Figure 5 was present in all the species. And here is another well-known case of another genus of still existing bird, which, as was the case with Dinomis, occurs only in New Zealand (Fig. 8). Upon this island there are no four-footed enemies — either existing or extinct — to escape from which the wings of birds would be of any service. Conse- 72 EVOLUTION, GENETICS, AND EUGENICS quently we can understand why on this island we should meet with such a remarkable dwindling away of wings. Similarly, the logger-headed duck of South America can only flap along the surface of the water, having its wings considerably reduced though less so than the Apteryx of New Zealand. But here the interesting fact is that the young birds are able to fly perfectly well. Now, in accordance with a general law to be considered in a future chapter, the life-history of an individual organism is a kind of con- densed recapitulation of the life-history of its species. Consequently, we can understand why the little chickens of the logger-headed duck are able to fly like all other ducks, while their parents are only able to flap along the surface of the water. Facts analogous to this reduction of wings in birds which have no further use for them, are to be met with also in insects under similar circumstances. Thus, there are on the island of Madeira somewhere between 500 and 600 species of beetles, which are in large part peculiar to that island, though related to other — and therefore presumably parent — species on the neighboring continent. Now, no less than 200 species — or nearly half the whole number — are so far deficient in wings that they cannot fly. And, if we disregard the species which are not peculiar to the island — that is to say, all the species which likewise occur on the neighboring continent, and therefore, as evolu- tionists conclude, have but recently migrated to the island, — we find this very remarkable proportion. There are altogether 29 peculiar genera, and out of these no less than 23 have all their species in this condition. Similar facts have been recently observed by the Rev. A. E. Eaton with respect to insects inhabiting Kerguelen Island. All the species which he found on the island — viz., a moth, several flies, and numerous beetles — he found to be incapable of flight; and therefore, as Wallace observes, "as these insects could hardly have reached the islands in a wingless state, even if there were any other known land inhabited by them, which there is not, we must assume that, like the Madeiran insects, they were originally winged, and lost their power of flight because its possession was injurious to them" — Kerguelen Island being "one of the stormiest places on the globe, " and therefore a place where insects could rarely afford to fly without incurring the danger of being blown out to sea. Here is another and perhaps an even more suggestive class ot facts. EVIDENCES FROM MORPHOLOGY 73 It is now many years ago since the editors of Silliman's Journal requested the late Professoi Agassiz to give them his opinion on the following question. In a certain dark subterranean cave, called the Mammoth Cave, there are found some peculiar species of blind fishes. Now the editors of Silliman's Journal wished to know whether Profes- sor Agassiz would hold that these fish had been specially created in these caves, and purposely devoided of eyes which could never be of any use to them; or whether he would allow that these fish had prob- ably descended from other species, but, having got into the dark cave, gradually lost their eyes through disuse. Professor Agassiz, who was a believer in special creation, allowed that this ought to constitute a crucial test as between the two theories of special design and heredi- tary descent. "If physical circumstances," he said, "ever modified organised human beings, it should be easily ascertained here." And eventually he gave it as his opinion, that these fish "were created under the circumstances in which they now live, within the limits over which they now range, and with the structural peculiarities which now characterise them." Since then a great deal of attention has been paid to the fauna of this Mammoth cave, and also to the faunas of other dark caverns, not only in the New, but also in the Old World. In the result, the following general facts have been fully established. 1. Not only fish, but many representatives of other classes, have been found in dark caves. 2. Wherever the caves are totally dark, all the animals are blind. 3. If the animals live near enough to the entrance to receive some degree of light, they may have large and lustrous eyes. 4. In all cases the species of blind animals are closely allied to species inhabiting the district where the caves occur; so that the blind species inhabiting the American caves are closely allied to American species, while those inhabiting European caves are closely allied to European species. 5. In nearly all cases structural remnants of eyes admit of being detected, in various degrees of obsolescence. In the case of some of the crustaceans of the Mammoth cave the foot-stalks of the eyes are present, although the eyes themselves are entirely absent. Now, it is evident that all these general facts are in full agreement with the theory of evolution, while they offer serious difficulties to the theory of special creation. As Darwin remarks, it is hard to imagine conditions of b'fe more similar than those furnished by deep 74 EVOLUTION GENETICS, AND EUGENICS limestone caverns under nearly the same climate in the two continents of America and Europe; so that, in accordance with the theory of special creation, very close similarity in the organizations of the two sets of faunas might have been expected. But, instead of this, the affinities of these two sets of faunas are with those of their respective continents — as of course they ought to be on the theory of evolution. Again, what would have been the sense of creating the useless foot- stalks for the imaginary support of absent eyes, not to mention all the other various grades of degeneration in other cases ? So that, upon the whole, if we agree with the late Professor Agassiz in regarding these cave animals as furnishing a crucial test between the rival theories of creation and evolution, we must further conclude that the whole body of evidence which they now furnish is weighing on the side of evolution. So much, then, for a few special instances of what Darwin called rudimentary structures, but what may be more descriptively desig- nated — in accordance with the theory of descent — obsolescent or vestigial structures. It is, however, of great importance to add that these structures are of such general occurrence throughout both the vegetable and animal kingdoms that, as Darwin has observed, it is almost impossible to point to a single species which does not present one or more of them. In other words, it is almost impossible to find a single species which does not in this way bear some record of its own descent from other species; and the more closely the structure of any species is examined anatomically, the more numerous are such records found to be. Thus, for example, of all organisms that of man has been most minutely investigated by anatomists; and therefore I think it will be instructive to conclude this chapter by giving a list of the more noteworthy vestigial structures which are known to occur in the human body. I will take only those which are found in adult man, reserving for the next chapter those which occur in a transitory manner during earlier periods of his life. But, even as thus restricted, the number of obsolescent structures which we all present in our own person is so remarkable, that their combined testimony to our descent from a quadrumanous ancestry appears to me in itself conclusive. I mean, that even if these structures stood alone, or apart from any more general evidences of our family relationships, they would be sufficient to prove our parentage. Nevertheless, it is desirable to remark that of course these special evidences which I am about to detail do not stand alone. Not only is there the general analogy EVIDENCES FROM MORPHOLOGY 75 furnished by the general proof of evolution elsewhere, but there is likewise the more special correspondence between the whole of our anatomy and that of our nearest zoological allies. Now the force of this latter consideration is so enormous that no one who has not studied human anatomy can be in a position to appreciate it. For without special study it is impossible to form any adequate idea of the intricacy of structure which is presented by the human form. Yet it is found that this enormously intricate organisation is repeated in all its details in the bodies of the higher apes. There is no bone, muscle, nerve, or vessel of any importance in the one which is not answered to by the other. Hence there are hundreds of thousands of instances of the most detailed correspondence, without there being any instances to the contrary, if we pay due regard to vestigial characters. The entire corporeal structure of man is an exact anatomical copy of that which we find in the ape. My object, then, here is to limit attention to those features of our corporeal structure which, having become useless on account of our change in attitude and habits, are in the process of becoming obsolete, and therefore occur as mere vestigial records of a former state of things. For example, throughout the vertebrated series, from fish to mammals, there occurs in the inner corner of the eye a semi- transparent eye-lid, which is called the nictitating membrane. The object of this structure is to sweep rapidly, every now and then, over the external surface of the eye, apparently in order to keep the surface clean. But although the membrane occurs in all classes of the sub-kingdom, it is more prevalent in some than in others — e.g., in birds than in mammals. Even, however, where it does not occur of a size and mobility to be of any use, it is usually represented, in animals above fishes, by a functionless rudiment, as here depicted in the case of man (Fig. 9). Now the organisation of man presents so many vestigial structures thus referring to various stages of his long ancestral history, that it would be tedious so much as to enumerate them. Therefore I will yet further limit the list of vestigial structures to be given as examples, by not only restricting these to cases which occur in our own organisa- tion; but of them I shall mention only such as refer us to the very last stage of our ancestral history — viz., structures which have become obsolescent since the time when our distinctively human branch of the family tree diverged from that of our immediate forefathers, the Quadrumana. 76 EVOLUTION, GENETICS, AND EUGENICS Plica Semilunaris ^%^-x JVl/J/f Wflpf Fig. q. — Illustrations of the nictitating membrane in the various animals named, drawn from nature. The letter N indicates the membrane in each case. In man it is called the plica semilunaris and is represented in the two lower drawings under this name. In the case of the shark (Galeus), the muscular membrane is shown as dissected. {From Romanes.) EVIDENCES FROM MORPHOLOGY 77 I. Muscles of the external ear. — -These, which are of large size and functional use in quadrupeds, we retain in a dwindled and useless condition (Fig. 10). This is likewise the case in anthropoid apes; but in not a few other Quadrumana (e. g., baboons, macacus, magots^ etc.) degeneration has not proceeded so far, and the ears are voluntarily movable. Fig. i o.— Rudimentary, or vestigial and useless, muscles of the human ear. {From Romanes, after Gray.) 2. Panniculus carnosis. — A large number of the mammalia are able to move their skin by means of subcutaneous muscle, as we see, for instance, in a horse, when thus protecting himself against the sucking of flies. We, in common with the Quadrumana, possess an active remnant of such a muscle in the skin of the forehead, whereby we draw up the eyebrows; but we are no longer able to use other considerable remnants of it, in the scalp and elsewhere, — or more correctly it is rarely that we meet with persons who can. But most of the Quadrumana (including the anthropoids) are still able to do so. 78 EVOLUTION, GENETICS, AND EUGENICS There are also many other vestigial muscles, which occur only in a small percentage of human beings, but which, when they do occur, present unmistakable homologies with normal muscles in some of the Quadrumana and still lower animals. 3. Feet. — -It is observable that in the infant the feet have a strong reflection inwards, so that the soles in considerable measure face one another. This peculiarity, which is even more marked in the embryo than in the infant, and which becomes gradually less and Fig. 11. — Portrait of a young gorilla. {From Romanes, after Hartmann.) less conspicuous even before the child begins to walk, appears to me a highly suggestive peculiarity. For it plainly refers to the condition of things in the Quadrumana, seeing that in all these animals the feet are similarly curved inwards, to facilitate the grasping of branches. And even when walking on the ground apes and monkeys employ to a great extent the outside edges of their feet, as does also a child when learning to walk. The feet of a young child are also extraordinarily mobile in all directions, as are those of apes. In order to show these points, I here introduce comparative drawings of a young ape and the EVIDENCES FROM MORPHOLOGY 79 lower extremities of a still younger child. These drawings, moreover, serve at the same time to illustrate two other vestigial characters, which have often been previously noticed with regard to the infant's foot. I allude to the incurved form of the legs and the lateral exten- sion of the great toe, whereby it approaches the thumb-like character of this organ in the Quadrumana. As in the case of the incurved position of the legs and feet, so in this case of the lateral extensibility of the great toe, the peculiarity is even more marked in embryonic Fig. 12. — Lower extremities of a young child. Drawn from life, when the mobile feet were for a short time at rest in a position of extreme inflection. {From Romanes.) than in infant life. For, as Professor Wyman has remarked with regard to the foetus when about an inch in length, "The great toe is shorter than the others; and, instead of being parallel to them, is projected at an angle from the side of the foot, thus corresponding with the permanent condition of this part in the Quadrumana." So that this organ, which, according to Owen, "is perhaps the most characteristic peculiarity of the human structure," when traced back to the early stages of its development, is found to present a notably less degree of peculiarity. So EVOLUTION, GENETICS, AND EUGENICS 4. Hands. — Dr. Louis Robinson has recently observed that the grasping power of the whole human hand is so surprisingly great at birth, and during the first few weeks of infancy, as to be far in excess of present requirements on the part of a young child. Hence he con- cludes that it refers us to our quadrumanous ancestry — the young of anthropoid apes being endowed with similar powers of grasping, in order to hold on to the hair of the mother when she is using her arms for the purposes of locomotion. This inference appears to me justifiable, llPf:''^ 111 '"nllWMIIIM Fig. 13. — An infant, three weeks old, supporting its own weight for over two minutes. The attitude of the lower limbs, feet, toes, is strikingly simian. Repro- duced from an instantaneous photograph, kindly given for the purpose by Dr. L. Robinson. (From Romanes.) inasmuch as no other explanation can be given of the comparatively inordinate muscular force of an infant's grip. For experiments showed that very young babies are able to support their own weight, by holding on to a horizontal bar, for a period varying from one half to more than two minutes. With his kind permission, I here reproduce one of Dr. Robinson's instantaneous, and hitherto unpub- lished, photographs of a very young infant. This photograph was taken after the above paragraph (3) was written, and I introduce it here because it serves to show incidentally — and perhaps even better than the preceding figure — the points there mentioned with regard EVIDENCES FROM MORPHOLOGY 8l to the feet and great toes. Again, as Dr. Robinson observes, the attitude, and the disproportionately large development of the arms as compared with the legs give all the photographs a striking resem- blance to a picture of the chimpanzee "Sally" at the Zoological Gardens. For " invariably the thighs are bent nearly at right angles to the body, and in no case did the lower limbs hang down and take the attitude of the erect position." He adds, "In many cases no sign of distress is evinced, and no cry uttered, until the grasp begins to give way." MAN Gorilla Fig. 14. — Sacrum of gorilla compared with that of man, showing rudimentary tail bones of each. Drawn from nature. (From Romanes.) 5. Tail. — The absence of a tail in man is popularly supposed to constitute a difficulty against the doctrine of his quadrumanous descent. As a matter of fact, however, the absence of an external tail in man is precisely what this doctrine would expect, seeing that the nearest allies of man in the quadrumanous series are likewise destitute of an external tail. Far, then, from this deficiency in man constituting any difficulty to be accounted for, if the case were not so — i.e., if man did possess an external tail, — the difficulty would be to understand how he had managed to retain an organ which had been renounced by his most recent ancestors. Nevertheless, as the anthro- 82 EVOLUTION, GENETICS, AND EUGENICS poid apes continue to present the rudimentary vestiges of a tail in a few caudal vertebrae below the integuments, we might well expect to find a similar state of matters in the case of man. And this is just Fig. 15. — Diagrammatic outline of the human embryo when about seven weeks old, showing the relations of the limbs and tail to the trunk. (After Allen Thompson.) r, the radial, and u, the ulnar, border of the hand and forearm; /, the tibial, and/ the fibular, border of the foot and lower leg; au, ear; u , spinal cord; u , umbilical cord; b, bronchial gill slits; c, tail. (From Romanes.) ^vfPyfl-sh/ods ha. dvky-. r3 u To c EVIDENCES FROM MORPHOLOGY 87 this refined distinction does not hold. On the one hand, the com- paratively hairless chimpanzee which died last year in the Zoological Gardens (T. calvus) was remarkably denuded over the back; and, on the other hand, men who present a considerable development of hair over the rest of their bodies present it also on their backs and shoul- ders. Again, in all men the rudimentary hair on the upper and lower arm is directed towards the elbow — a peculiarity which occurs nowhere else in the animal kingdom, with the exception of the anthropoid apes and a few American monkeys, where it presumably has to do with arboreal habits. For, when sitting in trees, the orang, as observed by Mr. Wallace, places its hands above its head with its elbows pointing downwards; the disposition of hair on the arms and fore-arms then has the effect of thatch in turning the rain. Again, I find that in all species of apes, monkeys, and baboons which I have examined (and they have been numerous), the hair on the backs of the hands and feet is continued as far as the first row of phalanges; but becomes scanty, or disappears altogether, on the second row; while it is invariably absent on the terminal row. I also find that the same peculiarity occurs in man. We all have rudimentary hair on the first row of phalanges, both of hands and feet: when present at all, it is more scanty on the second row; and in no case have I been able to find any on the terminal row. In all cases these peculiarities are congenital, and the total absence or partial presence of hair on the second pha- langes is constant in different species of Quadrumana. For instance, it is entirely absent in all the chimpanzees, which I have examined, while scantily present in all the orangs. As in man, it occurs in a patch midway between the joints. Besides showing these two features with regard to disposition of hair on the human arm and hand, the woodcut on page 88 (Fig. 22) illustrates a third. By looking closely at the arm of the very hairy man from whom the drawing was taken, it could be seen that there was a strong tendency towards a whorled arrangement of the hairs on the backs of the wrists. This is likewise, as a general rule, a marked feature in the arrangement of hair on the same places in the gorilla, orang, and chimpanzee. In the specimen of the latter, however, from which the drawing was taken this characteristic was not well marked. The downward direction of the hair on the backs of the hands is exactly the same in man as it is in all the anthropoid apes. Again, with regard to hair, Darwin notices that occasionally there appears in man a few hairs in the eye- brows much longer than the others; and that they seem to be 88 EVOLUTION, GENETICS, AND EUGENICS ^^\W //Al r dH>M?^ ZE ^' Fig. 22. — Hair tracts on the arms and hands of man, as compared with those of the chimpanzee. Drawn from life. {From Romanes.) EVIDENCES FROM MORPHOLOGY 89 representative of similarly long and scattered hairs which occur in the chimpanzee, macacus, and baboons. Lastly, it may be here more conveniently observed than in the next chapter on Embryology, that at about the sixth month the human foetus is often thickly coated with somewhat long dark hair over the entire body, except the soles of the feet and palms of the hands, which are likewise bare in all quadrumanous animals. This covering, which is called the lanugo, and sometimes extends even to the whole fore- head, ears, and face, is shed before birth. So that it appears to be useless for any purpose other than that of emphatically declaring man a child of the monkey. 9. Teeth. — Darwin writes: "It appears as if the posterior molar or wisdom teeth were tending to become rudimentary in the more civilized races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are usually sound (i.e., not specially liable to decay); they also differ from the other molars in size, less than in the Caucasian races." Now, in addition to these there are other respects in which the dwindling condition of wisdom-teeth is manifested— particularly with regard to the pattern of their crowns. Indeed, in this respect it would seem that even in the anthropoid apes there is the beginning of a tendency to degeneration of the molar teeth from behind forwards. For if we compare the three molars in the lower jaw of the gorilla, orang, and chimpanzee, we find that the gorilla has five well-marked cusps on all three of them ; but that in the orang the cusps are not so pronounced, while in the chimpanzee there are only four of them on the third molar. Now in man it is only the first of these three teeth which normally presents five cusps, both the others presenting only four. So that, comparing all these genera together, it appears that the number of cusps is being reduced from behind forwards; the chimpanzee having lost one of them from the third molar, while man has not only lost this, but also one from the second molar, — and it may be added, likewise partially (or even totally) from the first molar, as a frequent variation among civilized races. But, on the other hand, variations are often met with in the opposite direction, where the 90 EVOLUTION, GENETICS, AND EUGENICS second or the third molar of man presents five cusps — in the one case following the chimpanzee, in the other the gorilla. These latter varia- tions, therefore, may fairly be regarded as reversionary. For these facts I am indebted to the kindness of Mr. C. S. Tomes. 10. Perforations of the humerus. — The peculiarities which we have to notice under this heading are two in number. First, the supra-condyloid foramen is a normal feature in some of the lower Quadrumana (Fig. 24), where it gives passage to the great nerve of N AT. SIZE OK.A.N"G- /<\AjS. Fig. 23. — Molar teeth of lower jaw in gorilla, orang, and man. Drawn from nature, nat. size. (From Romanes.) the forearm, and often also to the great artery. In man, however, it is not a normal feature. Yet it occurs in a small percentage of cases — viz., according to Sir W. Turner, in about one per cent, and therefore is regarded by Darwin as a vestigial character. Secondly, there is inter-condyloid foramen, which is also situated near the lower end of the humerus, but more in the middle of the bone. This occurs, but not constantly, in apes, and also in the human species. From the fact that it does so much more frequently in the bones of ancient — and also of some savage — races of mankind (viz. in 20 to 30 per cent of cases), Darwin is disposed to regard it also as a vestigial feature. EVIDENCES FROM MORPHOLOGY 9* On the other hand, Prof. Flower tells me that in his opinion it is but an expression of impoverished nutrition during the growth of the bone. 11. Flattening of Tibia. — In some very ancient human skeletons there has also been found a lateral flattening of the tibia, which rarely occurs in any existing human beings, but which appears to have been usual among the earliest races of mankind hitherto discovered. x\ccording to Broca, the measurements of these fossil human tibiae resemble those of apes. Moreover, the bone is bent and strongly JAVAI7 L0RJS GAPVCHII/ Fig. 24. — Perforations of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in man, where it does not normally occur. Drawn from nature. (From Romanes.) convex forwards, while its angles are so rounded as to present the nearly oval section seen in apes. It is in association with these ape-like human tibiae that perforated humeri of man are found in greatest abundance. On the other hand, however, there is reason to doubt whether this form of tibia in man is really a survival from his quadrumanous ancestry. For, as Boyd-Dawkins and Hartmann have pointed out, the degree of flattening presented by some of these ancient human bones is greater than that which occurs in any existing species of anthropoid ape. Of course the possibility remains that the unknown species of ape from which man descended may have had its tibia more flattened than is now observable in any of the existing species. Never- 02 EVOLUTION, GENETICS, AND EUGENICS theless, as some doubt attaches to this particular case, I do not press it — and, indeed, only mention it at all in order that the doubt may be expressed. Similarly, I will conclude by remarking that several other instances of the survival of vestigial structures in man have been alleged, which are of a still more doubtful character. Of such, for example, are the supposed absence of the genial tubercle in the case of a very ancient jaw-bone of man, and the disposition of valves in human veins. From the former it was argued that the possessor of this very ancient jaw-bone was probably speechless, inasmuch as the tubercle in existing man gives attachment to muscles of the tongue. From the latter it has been argued that all the valves in the veins of the human body have reference, in their disposition, to the incidence of blood-pressure when the attitude of the body is horizontal, or quadrupedal. Now, the former case has already broken down, and I find that the latter does not hold. But we can well afford to lose such doubtful and spurious cases, in view of all the foregoing unquestionable and genuine cases of vestigial structures which are to be met with even within the limits of our own organization — and even when these limits are still further limited by selecting only those instances which refer to the very latest chapter of our long ancestral history. CHAPTER VI EVIDENCES FROM CLASSIFICATION THE PRINCIPLES OF CLASSIFICATION 1 A. F. SHULL The International Code. — Some of the essential features of the International Code are as follows. The first name proposed for a genus or species prevails on the condition that it was published and accompanied by an adequate description, definition or indication, and that the author has applied the principles of binomial nomenclature. This is the so-called law of priority. The tenth edition of the Sytema Naturae of Linnaeus is the basis of the nomenclature. The author of a genus or species is the person who first publishes the same in connec- tion with a definition, indication or description, and his name in full or abbreviated is given with the name; thus, Bascanlan anthonyi Stejneger. In citations the generic name of an animal is written with a capital letter, the specific and subspecific name without initial capital letter. The name of the author follows the specific name (or subspecific name if there is one) without intervening punctuation. If a species is transferred to a genus other than the one under which it was first described, or if the name of a genus is changed, the author's name is included in parentheses. For example, Bascanion anthonyi Stejneger should now be written Coluber anthonyi (Stejneger), the ge- neric name of this snake having been changed. One species constitutes the type of the genus; that is, it is formally designated as typical of the genus. One genus constitutes the type of the subfamily (when a subfamily exists), and one genus forms the type of the family. The type is indicated by the describer or if not indicated by him is fixed by another author. The name of a subfamily is formed by adding the ending -inae, and the name of a family by adding -idae to the root of the name of the type genus. For example, Colubrinae and Colubri- dae are the subfamily and family of snakes of which Coluber is the type genus. The basis of classification.— Early systematists largely employed superficial characters to differentiate and classify animals, and their 1 From A. F. Shull, Principles of Animal Biology (copyright iq2o). Used by special permission of The McGraw-Hill Book Company 93 94 EVOLUTION, GENETICS, AND EUGENICS classifications were thus largely artificial and served principally as convenient methods of arrangement, description and cataloging. Since the time of the development of the theory of descent with modifications by Lamarck (1809) and Darwin (1859), there has been an attempt to base the classification on relationships. Very nearly related animals are put into the same species. They are related because they descend from a common ancestry, and that common ancestry could not in most cases have been very ancient, otherwise evolution within the group would have occurred and the species would have been split into two or more species. Species that are much alike are included in one genus, being thus marked off from the species of another genus. The similarity of the species of a genus is held to indicate kinship, but since there is greater diversity among the indi- viduals of a genus than among the members of a species, the common stock from which the species of a genus have sprung must have existed at an earlier time, in order that evolution could bring about the degree of divergence now observed. In like manner, a family is made up of genera, and their likeness is again a sign of affinity. But to account for the greater difference between the extreme individuals belonging to a family, evolution must have had more time, that is, the common source of the members of a family must have antedated the common source of the individuals of a genus. Orders, classes, and phyla are similarly regarded as having sprung from successively more remote ancestors, the time differences being necessary to allow for the differ- ences in the amount of evolution. This statement is in general correct. However, since evolution has probably not proceeded at the same rate at all periods, nor in all branches of the animal kingdom at any one time, the time relations of the groups of high or low rank must not be too rigidly assigned. Thus certain genera, in which evolution has been slow, are probably much older than some families in which evolution has been rapid. It is not improbable, also, that some genera are quite as old as the families which include them; but in no case can they be older. Furthermore, different groups are classified by taxonomists of different temperaments, so that groups of a given nominal rank may be much more inclusive (and hence older) in one branch of the animal kingdom than in another. On the whole, nevertheless, the groups of higher rank have sprung from ancestry more remote than that of the groups of lower rank. The means of recognizing the kinship implied in classification permit some differences of opinion. It is recognized that likeness in EVIDENCES FROM CLASSIFICATION 95 structural characters is the chief clue to affinities. However, the evidential value of similarity in one or several structures unaccom- panied by the similarity of all parts is to be distrusted, since animals widely separated and dissimilar in most characters may have certain other features in common. Thus, the coots, phalaropes and grebes among birds have lobate feet but, as indicated by other features, they are not closely related; and there are certain lizards (Amphisbaenidae) which closely resemble certain snakes (Typholopidae) in being blind, limbless, and having a short tail. The early systematists were very liable to bring together in their classification analogous forms, that is, those which are functionally similar; or animals which are super- ficially similar. In contrast with the early practice, the aim of taxonomists at the present time is to group forms according to homol- ogy, which is considered an indication of actual relationship. Since a genetic classification must take into consideration the entire animal, the search for affinities becomes an attempt to evaluate the results of all morphological knowledge, and it is also becoming evident that other things besides structure may throw light upon relationships. The fossil records, geographical distribution, ecology and experi- mental breeding may all assist in establishing affinities. The method of taxonomy. — It is evident that before the relation- ships of animals can be determined the forms must be known, for unknown forms constitute breaks in the pedigrees of the groups to which they belong. Moreover, as pointed out above, the structural characters, variation and distribution must be known before a form can be placed in the proper place in a genetic system. For these reasons an important part of systematic work is the description of forms and an analysis of their differences. After the Linnaean system was adopted zoologists attacked this virgin field and for many years "species making" predominated. Even at the present time when other aspects of zoology have come to receive relatively more attention it is an interesting fact that the analytical method prevails in systematic studies, and taxonomy suffers from, and in part merits, the criticism that it is a mere cataloging of forms and ignores the higher goal of investigation, namely, the discovery of the course of evolution. Many systematists, however, recognize that the ultimate purpose of taxonomic work is to discover the relationships as well as the differences between the described forms in order that the course of evolution may be determined. In other words, it is appreciated that while analytical studies are necessary they are only preliminary, and o6 EVOLUTION, GENETICS, AND EUGENICS that upon their results must be built synthetic studies, if taxonomy is to fulfil its purpose. THE METHOD OF CLASSIFICATION CHARLES DARWTN 1 Naturalists, as we have seen, try to arrange the species, genera, and families in each class, on what is called the Natural System. But what is meant by this system ? Some authors look at it merely as a scheme for arranging together those living objects which are most alike, and for separating those which are most unlike; or as an artificial method of enunciating, as briefly as possible, general propositions, — that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then, by adding a single sentence, a full description is given of each kind of dog. The ingenuity and utility of this system are indisputable. But many naturalists think that something more is meant by the Natural System; they believe that it reveals the plan of the Creator; but unless it be specified whether order in time or space, or both, or what else is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Expressions such as that famous one by Linnaeus, which we often meet with in a more or less concealed form, namely, that the characters do not make the genus, but that the genus gives the charac- ters, seem to imply that some deeper bond is included in our classifica- tions than mere resemblance. I believe that this is the case, and that community of descent — the one known cause of close similarity in organic beings — is the bond which, though observed by various degrees of modification, is partially revealed to us by our classifications. Let us now consider the rules followed in classification, and the difficulties which are encountered on the view that classification either gives some unknown plan of creation, or is simply a scheme for enunciating general propositions and of placing together the forms most like each other. It might have been thought (and was in ancient times thought) that those parts of the structure which determined the habits of life, and the general place of each being in the economy of nature, would be of very high importance in classification. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of any 1 From The Origin of Species EVIDENCES FROM CLASSIFICATION 97 importance. These resemblances, though so intimately connected with the whole life of the being, are ranked as merely "adaptive or analogical characters": but to the consideration of these resemblances we shall recur. It may even be given, as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification. As an instance: Owen, in speaking of the dugong, says, "The generative organs, being those which are most remotely related to the habits and food of an animal, I have always regarded as affording very clear indications of its true affinities. We are least likely in the modifications of these organs to mistake a merely adaptive for an essential character." With plants how remarkable it is that the organs of vegetation, on which their nutrition and life depend, are of little signification; whereas the organs of reproduction, with their product the seed and embryo, are of paramount importance! So again in formerly discussing certain morphological characters which are not functionally important, we have seen that they are often of the highest service in classification. This depends on their constancy throughout many allied groups; and their constancy chiefly depends on any slight deviations not having been preserved and accumulated by natural selection, which acts only on serviceable characters. WHAT IS A SPECIES? "Each kind of animal or plant, that is, each set of forms which in the changes of the ages has diverged tangibly from its neighbors, is called a species. There is no absolute definition for the word species. The word kind represents it exactly in common language, and is just as susceptible to exact definition. The scientific idea of species does not differ materially from the popular notion. A kind of tree or bird or squirrel is a species. Those individuals which agree very closely in structure and function belong to the same species. There is no absolute test, other than the common judgment of men competent to decide. Naturalists recognize certain formal rules as assisting in such a decision. A series of fully intergrading forms, however varied at the extremes, is usually regarded as forming a single species. There are certain recognized effects of climate, of climatic isolation, and of the isolation of domestication. These do not usually make it necessary to regard as distinct species the extreme forms of a series concerned." 1 ' From D. S. Jordan and V. L. Kellocg, Evolution and Animal Life. 9 8 EVOLUTION, GENETICS, AND EUGENICS "The term 'species' was thus defined by the celebrated botanist De Candolle: 'A species is a collection of all the individuals which resemble each other more than they resemble anything else, which can by mutual fecundation produce fertile individuals, and which repro- duce themselves by generation, in such a manner that we may from analogy suppose them all to have sprung from one single individual. ' And the zoologist Swainson gives a somewhat similar definition: 'A species, in the usual acceptation of the term, is an animal which, in a state of nature, is distinguished by certain peculiarities of form, size, colour, or other circumstances, from another animal. It propagates, after its kind, individuals perfectly resembling the parent; its pecu- liarities, therefore, are permanent.' " ' As will have become apparent, the significant assumption underlying classification is that the closest fundamental similarities between animals (or plants) are found in the forms most closely related and that the greatest differences are found in those forms which are unrelated or at best very distantly related. The assumption implies the idea of descent with modification, which is no more nor less than evolution. Using this evolutionary basis, we can arrive at an extremely satisfactory classification both of living and of extinct forms ; and there is no other basis of classification that works. The question might well be asked whether it is possible to test the validity of the assumption that degrees of resemblance vary directly with closeness of blood relationship ? Two direct tests of this may be and have been made. The closest of blood relatives possible are individuals that have been derived by the dividing of a single egg. Armadillo 2 quadruplets have been shown to be thus derived, and detailed studies of the closeness of resemblance existing between members of a given set indicate that they are vastly more alike than are the simultaneously born offspring of animals which give birth to several young, but in which each young is derived from a separate egg. If we use the index of correlation to indicate the degree of similarity between individuals we find that ordinary brothers or sisters are only about 50 per cent alike, while armadillo quadruplets are over 90 per cent alike. Identical or duplicate twins in human beings are believed to have an origin from one egg, after the fashion of the armadillo, 1 From A. R. Wallace, Darwinism. 'See H. H. Newman, The Biology of Twins (191 7), University of Chicago Press. EVIDENCES FROM CLASSIFICATION 99 though the proof has not been forthcoming. Everyone is familiar with the remarkable similarity, amounting almost to identity, between such twins. Thus we are able to show that the closest blood relation- ship known is associated with the closest resemblance. The next degree of resemblance is between members of the same family, brothers, sisters, cousins, etc., and we do not hesitate to explain this resemblance as due to blood relationship. In this we merely accept the known principles of heredity. The second direct test of the validity of the assumption that degrees of resemblance run parallel with degrees of blood relationship is found in connection with "blood-precipitation tests." This evi- dence, as presented by Professor Scott, forms the substance of the next chapter. CHAPTER VII EVIDENCE FROM BLOOD TESTS 1 W. B. Scott Here may be conveniently considered the very interesting and significant blood tests which have been made in the last fifteen years by various physiologists and especially by Dr. George H. F. Nuttall, of the University of Cambridge. Though there are several methods of making these tests, the "precipitation method" employed by Dr. Nuttall will be quite sufficient for the ends sought in these lec- tures. The method and significance of the tests can best be explained by taking as an example human blood, which, of course, has been most extensively and minutely studied, because of its legal importance as well as its scientific interest. Ordinary chemical analysis is unable to determine the differences in blood-composition between various animals, but that there were important differences had long been understood. This was shown by the fact that, in performing the operation for the transfusion of blood, it was not practicable to substitute animal for human blood, since the former might cause serious injury to the patient. The precipitation method of making blood tests is as follows: Freshly drawn human blood is allowed to coagulate or clot, which it will do in a few minutes, if left standing in a dish, and then the serum is drained away from the clot. Blood-serum is the watery, almost colourless part of the blood, which remains after coagulation. Small quantities of this serum are injected, at intervals of one or two days, into the veins of a rabbit and cause the formation in the rabbit's blood of an anti-body, analogous to the anti-toxin which is produced in the blood of a horse by the injection of diphtheria virus. After the last injection the rabbit is allowed to live for several days and is then killed and bled, the blood is left until it clots and the serum drained off and preserved. The serum obtained thus from a rabbit is called "anti-human" serum and is an exceedingly delicate test for human blood, not only when the latter is fresh, but also when it is in the form of old and dried blood-stains, or even when the blood is 1 From W. B. Scott, The Theory of Evolution (copyright 1917). Used by special permission of the publishers, The Macmilhin Company. TOO EVIDENCE FROM BLOOD TESTS ioi putrid. Stains, for example, are soaked in a very weak solution of common salt and, if necessary, the blood solution is filtered until it is quite limpid and clear. Into the blood solution a few drops of the anti-human serum are conveyed and, if the stains are of human blood, a white precipitate is formed and thrown down, but if the stains are of the blood of some domestic animal, such as a pig, sheep, or fowl, no such reaction follows. In the same manner as above described, we may prepare anti-pig, anti-horse, anti-fowl, etc., etc., sera by injecting the fresh-drawn serum of a pig, horse, fowl, or any other animal into the rabbit, instead of human blood-serum. In some countries, notably in Germany and Austria, this test has already been adopted by the courts of justice and has been found extremely useful in the detection of crime. Further investigation showed that these blood tests might be employed to determine the degrees of relationship between different animals, for, although a prompt and strong reaction is usually obtained only from the blood of the same species as that from which the original injection into the rabbit was taken, the blood of nearly allied species, such as the horse and donkey, for example, gives a weaker and slower precipitation. By using stronger solutions and allowing more time, quite distant relationships may be brought out. Nuttall and his collaborator, Graham-Smith, made many thousands of such experi- ments bearing upon the problems of relationship and classification and it is of great significance to note that their highly interesting and important results contain few surprises, but, in almost all cases, merely serve to confirm the conclusions previously reached by other methods, such as comparative anatomy and palaeontology. It will be instructive to quote some of these results, the quotations being taken from "Blood Immunity and Blood Relationship, by G. H. F. Nuttall, including Original Researches by G. L. Graham-Smith and T. S. P. Strangeways, " Cambridge, 1904. "In the absence of palaeontological evidence the question of the interrelationship amongst animals is based upon similarities of struc- ture in existing forms. In judging of these similarities, the subjective element may largely enter." "The very interesting observations upon the eye made by Johnson also demonstrate the close relationships between the Old World forms and man, the macula lutea tending to disappear as we descend in the scale of New World Monkeys and being absent in the Lemurs. The results which I published upon my tests with precipitins directly supported this evidence, for the reactions 102 EVOLUTION, GENETICS, AND EUGENICS obtained with the bloods of Simiidae (i.e., Man-like Apes) closely resemble those obtained with human blood, the bloods of Cercopithe- cidae (Old World Monkeys) came next, followed by those of Cebidae and Hapalidae (New World Monkeys and Marmosets) which gave but slight reactions with anti-human serum, whilst the blood of Lemuroidea gave no indication of blood-relationship." "A perusal of the pages relating to the tests made upon the many bloods I have examined by means of precipitating anti-sera, will very clearly show that this method of investigation permits of our drawing certain definite conclusions. It is a remarkable fact .... that a common property has persisted in the bloods of certain groups of animals throughout the ages which have elapsed during their evolution from a common ancestor, and this in spite of differences of food and habits of life. The persistence of the chemical blood-relationship between the various groups of animals serves to carry us back into geological times, and I believe we have but begun the work along these lines, and that it will lead to valuable results in the study of various problems of evolution." The general conclusions on interrelationships, so far as they are of particular interest for our purpose, reached by Nuttall and Graham- Smith as the result of many thousands of blood tests, may be summa- rized as follows: i. If sufficiently strong solutions be used and time enough be allowed, a relationship between the bloods of all mammals is made evident. 2. The degrees of relationship between man, apes and monkeys have already been noted. 3. Anti-carnivore sera show "a preponderance of large reactions amongst the bloods of Carnivora, as distinguished from other Mam- malia; the maximum reactions usually take place amongst the more closely related forms in the sense of descriptive zoology." 4. Anti-pig serum gives maximum reactions only with the bloods of other species of the same family, moderate reactions those of rumi- nants and camels, and moderate or slight reactions with those of whales. Anti-llama serum gives a moderate reaction with the blood of the camel, and the close relationship between the deer family and the great host of antelopes, sheep, goats and oxen is clearly demonstrated. 5. An ti- whale serum gives maximum reactions only with the bloods of other whales and slight reactions with those of pigs and ruminants. EVIDENCE FROM BLOOD TESTS I03 6. A close relationship is shown to exist between all marsupials, with the exception of the Thylacine, or so-called Tasmanian Wolf. 7. Strong anti-turtle serum gives maximum reactions only with the bloods of turtles and crocodiles; with those of lizards and snakes the results are almost negative. With the egg-albumins of reptiles and birds a moderate reaction is given. 8. Anti-lizard serum produces maximum results with the bloods of lizards and reacts well with those of snakes. 9. These experiments indicate that there is a close relationship between lizards and snakes, on the one hand, turtles and crocodiles on the other. They further indicate that birds are more nearly allied with the turtle-crocodile series than with the lizard-snake series, results for which palaeontological studies had already prepared us. 10. "Tests were made by means of anti-sera for the fowl and ostrich upon 792 and 649 bloods respectively. They demonstrate a similarity in blood constitution of all birds, which was in sharp con- trast to what had been observed with mammalian bloods, when acted upon by anti-mammalian sera. Differences in the degree of reaction were observed, but did not permit of drawing any conclusions." 11. I have already called attention to the fact that the prob- lematical Horseshoe-crab is indicated by its embryology to be related to the air-breathing spiders and scorpions rather than to the marine Crustacea. It is of exceptional interest to learn that embryology is supported by the results of the blood tests. It must not be supposed that there is any exact mathematical ratio between the degrees of relationship indicated by the blood tests and those which are shown by anatomical and palaeontological evidence. Any supposition of the kind would be immediately nega- tived by the contrast between the blood of mammals and that of birds. It could hardly be maintained that an ostrich and a parrot are more nearly allied than a wolf and a hyena and yet that would be the inference from the blood tests. Like all other anatomical and physiological characters, the chemical composition of the blood is subject to change in the course of evolution and these developmental changes do not keep equal pace in all parts of the organism. It is the rule rather than the exception to find that one part of the structure advances much more rapidly than other parts, such as the teeth, the skull, or the feet. The human body is, fortunately for us, of rather a primitive kind, while the development of the brain is far superior to that of any other mammal and this great brain development has 104 EVOLUTION, GENETICS, AND EUGENICS necessitated a remodeling of the skull. On the other hand, the skeleton, limbs, hands and feet are but slightly specialized. In the elephant tribe, so far as we can trace them back in time, there has been little change, save in size, in the structure of the body or limbs, while the teeth and skull have passed through a series of remarkable changes. It is for this reason that it is unsafe to found a scheme of classification, which is meant to be a brief expression of relationship, upon a single character, for the result is almost invariably misleading. The results of blood tests must be critically examined and checked by a comparison with the results obtained by other methods of investiga- tion, but after every allowance has been made, these tests are very remarkable. The blood tests have brought very strong confirmation to the theory of evolution and from an entirely unexpected quarter; they come as near to giving a definite demonstration of the theory as we are likely to find, until experimental zoology and botany shall have been improved and perfected far beyond their present state. CHAPTER VIII EVIDENCES FROM EMBRYOLOGY THE FACTS OF REPRODUCTION AND DEVELOPMENT It is now definitely known that all living creatures are mortal, at least as individuals, but they all have the capacity of continuing their life by the reproduction of offspring. This physical immortality is based upon an actual transmission from parent to offspring of some material substance which is so organized chemically as to be fully representative of the race or stock to which the parent belongs. Reproduction may be asexual or sexual. In asexual development a new individual may be produced by a process of fission (dividing the parent into two or more parts, each of which has the capacity to develop into a whole new individual); by budding (the production of new individuals by means of outgrowths of the parent-body) ; or by giving off spores or eggs capable of development without fertiliza- tion (parthenogenesis). In sexual reproduction two kinds of parent- individuals exist: one a female which is capable of giving off relatively large single cells, called eggs (ova) ; and the other a male, which is capable of producing minute, usually motile cells, called spermatozoa. A union of ovum and spermatozoon is usually necessary before the ovum can begin its development. It is the sexual method of repro- duction that will chiefly concern us here, and, for present purposes, we may omit any further mention of the various asexual methods. An ovum may be conceived of as an individual of some definite species or race reduced to the very lowest terms. It exhibits the characteristic cell structure, consisting of cytoplasm and nucleus, cell membrane, nuclear membrane, usually a centrosome (Fig. 40). Further details as to the minute structure of the nucleus are given in chapter xliv, where the mechanism of Mendelian heredity is dealt with. "The reproductive cells from the two sexes," says Wright, 1 "have very different appearances. In mammals, the ovum is a relatively large, spherical cell, just visible to the naked eye. ' From Sewall Wright, Principles of Livestock Breeding, United States Depart ment of Agriculture, Bulletin No. 905. 1D 6 EVOLUTION, GENETICS, AND EUGENICS "In birds, the yolk of an egg is really a single ovum, distended to an enormous size by food material. The sperm cell is very much smaller and can be seen well only with a high-power microscope. It is something like a tadpole in shape, having a small cell body, containing a little nucleus, and attached to this a long, whiplike process which beats rapidly while the cell is alive, enabling it to seek out and unite with the large passive egg in the act of fertilization. Enormous num- bers of sperm cells are produced by the male, but only one takes part in fertilization. After the first has penetrated the membrane of an egg cell, a change takes place in the latter which prevents the entrance of others. "The sperm activates certain formerly inert substances in the egg and the new combination cell (the zygote) starts almost at once to produce a new individual." OUTLINE OF ANIMAL DEVELOPMENT 1 D. S. JORDAN AND V. L. KELLOGG The embryonic development is from the beginning up to a certain point practically alike, looked at in its larger aspect, for all the many- celled animals. That is, there are certain principal or constant characteristics of the beginning development which are present in the development of all many-celled animals. The first stage or phenome- non of development is the simple fission of the germ cell into halves (Fig. 25, b). These two daughter cells next divide so that there are four cells (c) ; each of these divides, and this division is repeated until a greater or lesser number (varying with the various species or groups of animals) of cells is produced. These cells may not all be of the same size, but in many cases they are, no structural differentiation whatever being apparent among them. The phenomenon of repeated division of the germ cell is called cleavage, and this cleavage is the first stage of development in the case of all many-celled animals. The germ or embryo in some animals consists now of a mass of few or many undifferentiated primitive cells lying together and usually forming a sphere (Fig. 25, e), or perhaps separated and scattered through the food yolk of the egg. The next stage of development is this: the cleavage cells arrange themselves so as to form a usually hollow sphere or ball, the cells lying side by side to * From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copyright 1907). Used by special permission of the publishers, D. Appleton & Company. EVIDENCES FROM EMBRYOLOGY 107 form the outer circumferential wall of this hollow sphere (/). This is called the blastida or blastoderm stage of development, and the embryo itself is called the blastula or blastoderm. This stage also is common to all the many-celled animals. The next stage in embryonic develop- ment is formed by the bending inward of a part of the blastoderm cell layer, as shown in (g) (or the splitting off inwardly of cells from a special part of the blastula cell layer) . This bending in may produce a small depression or groove; but whatever the shape or extent of the sunken-in part of the blastoderm, it results in distinguishing the blastoderm layer into two parts, a sunken-in or inner portion called Fig. 25. — First stages in the embryonic development of the pond snail, Lymnaeus. a, egg cell; b, first cleavage; c, second cleavage; d, third cleavage; e, after numerous cleavages; /, blastula — in section; g, gastrula just forming — ■ in section; h, gastrula completed — in section. (From Jordan and Kellogg, after Rabl.) the endoblast and the other unmodified portion called the ectoblast. Endo- means within, and the cells of the endoblast often push so far into the original blastoderm cavity as to come into contact with the cells of the ectoblast and thus obliterate this cavity (h). This third well-marked stage in the embryonic development is called the gastrula stage, and it also occurs in the development of all or nearly all many- celled animals. In the case of a few of the simple many-celled animals the embryo hatches — that is, issues from the egg at the time of or very soon after reaching the gastrula stage. In the higher animals, however, develop- ment goes on within the egg or within the body of the mother until the embryo becomes a complex body, composed of many various 108 EVOLUTION, GENETICS, AND EUGENICS tissues and organs. Almost all the development may take place within the egg, so that when the young animal hatches there is necessary little more than a rapid growth and increase of size to make it a fully developed mature animal. This is the case with the birds; a chicken just hatched has most of the tissues and organs of a full-grown fowl, and is simply a little hen. But in the case of other animals the young hatches from the egg before it has reached such an advanced stage of development; a young starfish or young crab or young honeybee just hatched looks very different from its parent. It has yet a great deal of development to undergo before it reaches the structural condition of a fully developed and fully grown starfish or crab or bee. Thus the development of some animals is almost wholly embryonic develop- ment — that is, development within the egg or in the body of the mother — while the development of other animals is largely post- embryonic, or larval development, as it is often called. There is no important difference between embryonic and postembryonic develop- ment. The development is continuous from egg cell to mature animal, and whether inside or outside of an egg it goes on regularly and uninter- ruptedly. The cells which compose the embryo in the cleavage stage and blastoderm stage, and even in the gastrula stage, are apparently all similar; there is little or no differentiation shown among them. But from the gastrula stage on, development includes three important things; the gradual differentiation of cells into various kinds to form the various kinds of animal tissues; the arrangement and grouping of these cells into organs and body parts ; and finally the developing of these organs and body parts into the special condition characteristic of the species of animal to which the developing individual belongs. From the primitive undifferentiated cells of the blastoderm, develop- ment leads to the special cell types of muscle tissue, of bone tissue, of nerve tissue; and from the generalized condition of the embryo in its early stages, development leads to the specialized condition of the body of the adult animal. Development is from the general to the special, as was said years ago by von Baer, the first great student of development. A starfish, a beetle, a dove, and a horse are all alike in their beginnings — that is, the body of each is composed of a single cell, a single structural unit. And they are all alike, or very much alike through several stages of development; the body of each is first a single cell, then a number of similar undifferentiated cells, and then a EVIDENCES FROM EMBRYOLOGY io 9 blastoderm consisting of a single layer of similar undifferentiated cells. But soon in the course of development the embryos begin to differ, and as the young animals get further and further along in the course of their development, they become more and more different until each finally reaches its fully developed mature form, showing all the great structural differences between the starfish and the dove, the beetle and the horse. That is, all animals begin development apparently alike, but gradually diverge from each other during the course of develop- ment. There are some extremely interesting and significant things about this divergence to which attention should be given. While all animals are apparently alike structurally at the beginning of development, so far as we can see, they do not all differ noticeably at the time of the first divergence in development. The first divergence in development is to be noted between two kinds of animals which belong to different great groups or classes. But two animals of different kinds, both belonging to some one great group, do not show differences until later in their development. This can best be understood by an example. All the butterflies and beetles and grasshoppers and flies belong to the great group or class of animals called Insecta, or insects. There are many different kinds of insects, and these kinds can be arranged in subor- dinate groups (orders), such as the Diptera, or flies, the Lepidoptera, or butterflies and moths, and so on. But all have certain structural characteristics in common, so that they are comprised in one great class — the Insecta. Another great group of animals is known as the Vertebrata, or backboned animals. The class Vertebrata includes the fishes, the batrachians, the reptiles, the birds and the mammals, each composing a subordinate group, but all characterized by the possession of a backbone or, more accurately speaking, of a notochord, a back- bonelike structure. Now, an insect and a vertebrate diverge very soon in their development from each other; but two insects, such as a beetle and a honeybee, or any two vertebrates, such as a frog and a pigeon, do not diverge from each other so soon, 'fhat is, all vertebrate animals diverge in one direction from the other great groups, but all the members of the great group keep together for some time longer. Then the subordinate groups of the Vertebrata, such as the fishes, the birds, and the others, diverge, and still later the different kinds of animals in each of these groups diverge from each other. That the course of development of any animal from its beginning to fully developed adult form is — in all its essentials — fixed and certain no EVOLUTION, GENETICS, AND EUGENICS is readily seen. All rabbits develop in the same way; every grass- hopper goes through the same developmental changes from single egg cell to the full-grown, active hopper as every other grasshopper of the same kind — that is, development takes place according to certain natural laws; the laws of animal development. These laws may be roughly stated as follows: All many-celled animals begin life as a single cell, the fertilized egg cell; each animal goes through a certain orderly series of developmental changes which, accompanied by growth leads the animal to change from a single cell to the many-celled, com- plex form characteristic of the species to which the animal belongs; this development is from simple to complex structural condition; the development is the same for all individuals of one species. While all animals begin development similarly, the course of development in the different groups soon diverges, the divergence being of the nature of a branching, like that shown in the growth of a tree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed condition, all standing more or less clearly apart from each other. But in tracing back the development of any kind of animal we soon come to a point where it very much resembles or becomes apparently identical with the development of some other kind of animal, and, in addition, the stages passed through in the developmental course may very much resemble the fully devel- oped, mature stages of lower animals. To be sure, any animal at any stage in its existence differs absolutely from any other kind of animal, in that it can develop into only its own kind of animal. There is something inherent in each developing animal that gives it an identity of its own. Although in its young stages it may be hardly distin- guishable from some other kind of animal in similar stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. A very young fish and a very young sala- mander are almost indistinguishably alike, but one is sure to develop into a fish and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law of heredity. Viewed in the light of development, there must be as great a difference between one egg and another as between one animal and another, for the greater difference is included in the less. The significance of the developmental phenomena is a matter about which naturalists have yet very much to learn. It is believed, how- ever, by practically all naturalists that many of the various stages in 'he development of an animal correspond to or repeat, in many EVIDENCES FROM EMBRYOLOGY Hi fundamental features at least, the structural condition of the animal's ancestors. Naturalists believe that all backboned or vertebrate Fig. 26 —Stages in the development of the prawn, Pcncus potimirium. A Nauplius larva; B, first zoea stage; C, second zoea stage. {From Jordan and Kellogg, after Fritz Midler.) D ^#1P Fig. 27. — Later stages in the development of the prawn, Pcncus potimirium. D, Mysis stage; E, adult stage. (From Jordan and Kellogg.) IT2 EVOLUTION, GENETICS, AND EUGENICS animals are related to each other through being descended from a common ancestor, the first or oldest backboned animal. In fact, it is because all these backboned animals — the fishes, the batrachians, the reptiles, the birds, and the mammals — have descended from a common ancestor that they all have a backbone. It is believed that the descendants of the first backboned animal have in the course of many generations branched off little by little from the original type until there came to exist very real and obvious differences among the back- boned animals — differences which among the living backboned animals are familiar to all of us. The course of development of an individual animal is believed to be a very rapid and evidently much condensed and changed recapitulation of the history which the species or kind of animal to which the developing individual belongs has passed through in the course of its descent through a long series of gradually changing ancestors. If this is true, then we can readily understand why a fish and a salamander, a tortoise, a bird, and a rabbit, are all much alike, as they really are, in their earlier stages of development, and gradually come to differ more and more as they pass through later and later developmental stages. A crab has a tail in one of its developmental stages, so that at that time it looks like and really is like the mature stage of some tailed crustacean like a crayfish. A barnacle, which looks little like a crayfish or crab in its ma- ture stage, is hardly to be distinguished in its immature life from a young crab or lobster. Sacculina, which is a still more degenerate crustacean, is only a sort of feeding sac with rootlet-like processes projecting into the body of the host crab on which it lives as a parasite, but the young free-swimming Sacculina is essentially like a barnacle, crayfish, or crab in its young stage. However, it is obvious that this recapitulation or repetition of ancestral stages is never perfect, and it is often so obscured and modi- fied by interpolated adaptive stages and characters that but little of an animal's ancestry can be learned from a scrutiny of its development. Fig. 28.— Metamorphosis of a barnacle, Lepas. a-, larva; b, adult. (From Jordan and Kellogg.) EVIDENCES FROM EMBRYOLOGY 113 The fascinating biogenetic law of Mliller and Haeckel summed up in the phrase, "ontogeny is a recapitulation of phytogeny," must not be too heavily leaned on as a support for any speculations as to the phyletic affinities of any species or group of species of organisms. "Embryology is an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand." CHAPTER IX CRITIQUE OF THE RECAPITULATION THEORY 1 W. B. SCOTT Embryology is the study of the development of the individual organism from its beginning in the egg to the attainment of the adult condition. This individual development is called ontogeny and the question of the relation of ontogeny to the ancestral history of the species, or phytogeny, constitutes one of the main problems of embry- ology. Around this problem many controversies have raged, contro- versies which have by no means arrived at a definite solution, even to-day. Thirty years ago the "recapitulation theory" was well-nigh universally accepted, according to which the individual development, or ontogeny, was regarded as an abbreviated repetition of the ances- tral history of the species, or phylogeny. Haeckel called this theory the "fundamental biogenetic law" and upon it he established his whole "History of Creation." Nowadays, that "fundamental law" is very seriously questioned and by some high authorities is altogether denied. However, even those who take this extreme position con- cerning the recapitulation theory see in the facts of embryology one of the strongest supports of the doctrine of evolution. It was very early recognized that the recapitulation theory could not be applied with literal exactness, but was subject to certain important exceptions and qualifications. i. That the history must have been enormously abbreviated. After three weeks of incubation the tiny speck of protoplasm, which forms a circular mark on the yolk of a hen's egg, is developed into a fully formed chick, ready for hatching and able in large degree to take care of itself. On the other hand, the evolution of birds from their invertebrate ancestors, through the fishes, amphibians, and reptiles, the separation of the gallinaceous stock from other birds and the differentiation of this particular species were extremely slow processes, extending through unnumbered millions of years. Admitting reca- pitulation to the fullest extent, it is evidently a physical impossibility 1 From W. B. Scott, The Theory of Evolution (copyright 191 7). Used by special permission of the publishers. The Macmillan Company. 114 THE RECAPITULATION THEORY 115 that it should be a perfect repetition of phylogeny; very much of the long story must of necessity be omitted. 2. Through all the stages of development the embryo must be rendered able to live and grow and thrive through adaptation to its surroundings and changes in its environment. In some animals development takes place within the body of the mother; in others the embryo is protected by the hard egg-shell, as in birds, while the eggs of certain fishes and many invertebrates float freely in the sea and are almost without protection. Such differences in environment necessi- tate differences in the mode of development, while the presence or absence of a large amount of inert food-material, or yolk, exerts a great influence in determining the steps of ontogeny. 3. Many animals pass through a larval stage of development, in which the immature young leads an independent and self-sustaining existence, during which it is very different in appearance and structure from its adult parents. Familiar instances of this mode of develop- ment are to be found in the tadpole, which is the larva of the frog, and the caterpillar, the larva of a butterfly. Larvae are fully subject to the struggle for existence and must adapt themselves to their environ- ment and to changes in that environment, exactly as do adults, if they are to survive. In this way many changes are introduced into the ontogeny which can have no phylogenetic significance. It is found in several known instances, that nearly allied species, living under different conditions, have quite different modes of ontogeny, though their ancestral history must have been substantially identical. In one and the same species of marine worms, for example, which inhabits both the warm Mediterranean and the cold waters of the North Sea, the larva of the northern form is quite distinct from that of the southern. In attempting to interpret the meaning of embryological facts, it is thus necessary to distinguish sharply between those features which are derived from a long inheritance, and are therefore called palingenetic, from those which have been secondarily introduced in response to the changing needs of embryonic or larval life. These secondary features are termed cenogenetic. "If we are compelled to admit that cenogenetic characters are intermingled with palingenetic, then we cannot regard ontogeny as a pure source of evidence regarding phyletic relationships. Ontogeny accordingly becomes a field in which an active imagination has full scope for its dangerous play, but in which positive results are by no means everywhere to be obtained. To attain such results, the palm- Ii6 EVOLUTION, GENETICS, AND EUGENICS genetic and cenogenetic phenomena must be sifted apart, an operation which required more than one critical grain of salt. On what grounds shall this critique be based ? Assuredly not by way of a vicious circle on the ontogeny again; for if cenogenetic characters are present in one case, who will guarantee that a second case, used for a comparison with the first, does not likewise appear in cenogenetic disguise ? If it once be admitted that not everything in development is palingenetic, that not every ontogenetic fact can be accepted at its face value, so to speak, it follows that nothing in ontogeny is immediately available for the critique of embryonic development. The necessary critique must be drawn from another source." These remarks of Gegenbaur's were called forth by the state of wild speculation into which embryological work had fallen. As there were no generally accepted canons of interpretation for the facts of embryological development, different writers interpreted these facts in the most divergent and contradictory manner, resulting in a chaotic confusion, which led to a strong reaction against the whole method, though there can be little doubt that this reaction has gone too far. "It must be evident to any candid observer, not only that the embryological method is open to criticism, but that the whole fabric of morphology, so far as it rests upon embryological evidence, stands in urgent need of reconstruction. For twenty years embryological research has been largely dominated by the recapitulation theory; and unquestionably this theory has illuminated many dark places and has solved many a perplexing problem that without its aid might have remained a standing riddle to the pure anatomist. But while fully recognizing the real and substantial fruits of that theory, we should not close our eyes to the undeniable fact that it, like many another fruit- ful theory, has been pushed beyond its legitimate limits. It is largely to an overweening confidence in the validity of the embryological evidence that we owe the vast number of the elaborate hypothetical phylogenies which confront the modern student in such bewildering confusion. The inquiries of such a student regarding the origin of any of the great principal types of animals involve him in a labyrinth of speculation and hypothesis in which he seeks in vain for conclusions of even an approximate certainty." Many other equally vigorous and well-deserved criticisms of the embryological method might be cited, but it should be emphasized that these criticisms are all directed against the application of the method to the solution of definite and concrete problems of descent and THE RECAPITULATION THEORY II7 relationship. None of them denies and many strongly affirm that embryology affords some of the strongest and most convincing evi- dence in favor of the evolutionary theory. Let us examine some of this evidence. To begin with, it should be noted that, in following out the ontogeny or individual develop- ment, the observer witnesses the formation of something new, not merely the enlargement and unfolding of a pre-existing organism, though the theory of preformation, which was widely accepted in the eighteenth century, looked upon ontogeny precisely in that way, as the growth of a germ which was the miniature of the parent. Such a theory was possible only before the development of microscopic technique had enabled the observer to detect the actual successive steps of change. The egg is a single cell, with the nucleus and all the parts of other undifferentiated cells, though it may be enormously enlarged by the presence of food-yolk. In the hen's egg this food-yolk is quite inert and the activity of development is confined to the minute disc of protoplasm on the outside of the yolk, while in the frog's egg the yolk is disseminated, though not uniformly, throughout the egg and in the mammalian egg, which is microscopic in size, there is no yolk. It is a very remarkable fact that all of the vertebrated animals, lishes, amphibians, reptiles, birds and mammals, however different their habits and modes of life, have a mode of ontogeny which is of even more characteristically and unmistakably the same plan than is the type of their adult structure, which was described in the last chapter. The egg, or the active portion of it, divides in a definite and regular manner into a very large number of cells, which arrange them- selves in definite layers, an outer and an inner, and within these layers cell-aggregates form incipient organs, which, step by step, take on the adult condition. Not only is the plan and type of development essentially similar throughout the whole phylum of the vertebrates, but, in accordance with the recapitulation theory, many structural features which are permanent in lower forms appear in the embryos of higher and more advanced types. In the latter, however, these features aro transitory and, in the course of development, they either disappear, or are so modified as to be very different, sometimes unrecog- nizable, in the adults. At a certain stage of the ontogeny the embryo of a mammal has gill-pouches like a fish, the skeletal supports of the gill-pouches, the arteries and veins which supply them with blood, the structure of the heart, in short, the entire plan of the circulatory system is fish-like. n8 EVOLUTION, GENETICS, AND EUGENICS At a later stage most of the gill-pouches have been obliterated, but one is retained and converted into the Eustachian canal, which connects the throat with the middle ear, inside of the ear-drum. Similarly, the embryological evidence shows that the lungs of air-breathers have been derived from the swim-bladder of fishes, a conclusion which had already been reached by comparative anatomy, for in a remarkable Fig. 29. — Embryos in corresponding stage of development of shark (A), fowl (B), and man (C); g, gill slits. {From Scott.) group, known as the Dipnoi or lung-fishes, the air-bladder is utilized for purposes of respiration. It has been objected that, while embryology may prove relation- ship within a single type, it fails to demonstrate any connection between different types, but this is not altogether true. The Tuni- cata, a curious group of marine animals once referred to the Mollusca, are shown by their ontogeny to be related to the vertebrates and the same is true of certain marine worms (Balanoglossus). Indeed, most modern zoologists have adopted a scheme of classification, in which THE RECAPITULATION THEORY 1 19 the type Chordata includes not only the true vertebrates, but also the Lancelet (Amphioxtis), the tunicates, and Balauoglossus; this scheme is founded upon the embryological evidence. Among the inverte- brates even more remarkable examples have been observed. Such radically different types as the segmented worms and the shell- fish (Mollusca) are brought into relationship by their ontogeny and their closely similar types of larvae, as are also, though less distinctly, the brachiopods or lamp-shells, and the Bryozoa. The Horseshoe- crab, or King-crab, so abundant along our Atlantic coast, was long of uncertain affinities; originally referred to the Crustacea, largely because of its marine habits of life, embryology makes much more probable its relationship to the air-breathing scorpions and spiders, a result which has been examined previously from another point of view in connection with blood-tests. Even before the publication of Darwin's Origin of Species one of the great stumbling blocks in the way of the theory of special crea- tion was the existence in a great many animals of rudimentary organs, or such as are so far reduced and atrophied as to be of no service to their possessors. An analogy employed by my lamented friend, Mr. Richard Lydekker, may be advantageously repeated here. Let us suppose that a screw-steamer, with longitudinal shaft leading aft from the engine-room to the stern, where it carries the propeller, should, on close examination, reveal many signs that it has originally been a "side-wheeler," or paddle-boat. Recognizable remnants of paddle- boxes, of bearings for a transverse shaft, and the like, are found; what would be the inevitable conclusion ? No one would maintain that a naval architect, in possession of his senses, in constructing a screw- steamer would deliberately introduce features which are useful and appropriate only in a paddle-boat. The only reasonable explanation would be that the vessel had originally been built as a paddle-boat and had subsequently been converted into a screw-steamer and in the conversion it had not been found necessary completely to eradicate all traces of the original construction. Obviously, the same reasoning applies to rudimentary organs. The only satisfactory explanation of such useless remnants is that their possessors are descendants of ancestors in which those organs were fully functional. It seems quite absurd to assume that, in a separately and specially created animal, useless structures, reminiscent of other animals in which the same structures are useful and valuable, should be included, merely to indicate ideal relationships and community of plan. 120 EVOLUTION, GENETICS, AND EUGENICS It was sought to break the force of this very serious objection to the theory of special creation by saying that apparently useless organs may nevertheless have functions which are still unknown to us and may be revealed by future discovery. In certain cases, like that of the thyroid gland in the neck, this contention has been justified, but there are many others to which it does not apply. For example, in the great and varied whale-tribe (order Cetacea) which includes the right, or whalebone, whales, the sperm-whales, the porpoises, dolphins, etc., the forelimbs have been converted into swimming paddles, but the hind limbs appear to have vanished completely, leaving no externally visible trace. Internally, however, recognizable remnants of the hind limb-bones may be found in various stages of reduction, which differ in the different members of the order. In the Greenland Right Whale the hip-bone, thigh-bone and shin-bone are indicated; in the Finwhale only the hip-bones and a minute rudiment of the thigh-bone are to be found; in the toothed whales only an almost unrecognizable remnant of the hip-bone is left and in one of the dolphins even that has dis- appeared. Similarly, the snakes have lost their limbs completely, so far as external appearance is concerned, and in most members of the group no trace of limbs is to be found on dissection, but in certain snakes the rudiments of limbs are to be detected. Leaving aside all preconceptions, which is the more probable explanation of such phenomena, the theory of special creation or the theory of evolution ? Even if it were admitted that all rudimentary organs and struc- tures found in the adult have a certain unknown use and value, no one could maintain this with regard to the countless instances of structures which are developed in the embryo, but disappear entirely before birth. It is possible to mention but a very few of such instances out of the great number that have already been observed and recorded, but these few will suffice to illustrate the principle involved. "Examples of this may be cited from the most widely different groups: in the embryo of insects, especially of beetles, pairs of legs are formed within the egg, not only on the head and thorax, but also on the abdomen, but while those on the head are transformed into mouth-parts, those on the thorax are farther developed in their joint- ing and musculature to be locomotive legs, those on the abdomen are again resorbed. In many fresh- water worms, the eggs of which are laid in a cocoon, from which they are hatched as a finished, minute, crawling worm, larval organs are nevertheless formed, which recall those of the Trochophore,the larva of the original worms, which swims THE RECAPITULATION THEORY 121 freely in the sea. However, these larval organs .... are never properly functional, since no actually free-swimming larva is developed but the embryo merely floats in the albuminous fluid of the cocoon. "A particularly beautiful example is offered by the whales in their embryological development, which has been thoroughly studied by Kukenthal. In the adult condition they show only the anterior extremities, but in the embryo the posterior pair, with their skeletal parts, are formed,but are afterwards completely atrophied. Although they are mammals, in the adult condition they have absolutely no covering of hair, since in their aquatic life another and more effective protection against loss of heat is given by means of a thick layer of blubber; only a few coarse bristles, partly with particular functions, have persisted on a few parts of the body. But in the embryo a dense covering of hair is formed, which is later transformed in a peculiar manner and atrophied. Further, a series of whales have no teeth in the adult condition, but only the well-known, eel-trap-like, horny plates, from which whale-bone is produced. Nevertheless, in the embryo there is a dentition of numerous teeth, which are, however, resorbed, without ever piercing the gum." 1 Throughout the great group of the ruminants, which includes the oxen, buffaloes, bison, sheep, goats, antelopes, deer and giraffes, the collar-bone is invariably lacking, since it is superfluous on account of the exclusively locomotive manner in which the fore legs are employed. In the embryo sheep the collar-bone is established and even, to some extent ossified, but is subsequently resorbed and disappears entirely. No doubt, the collar-bone will be found in many other embryo rumi- nants, when the proper examination shall have been made, but its demonstrated presence in the foetal sheep is sufficiently striking. In the higher mammals the number of teeth was originally 44, or 11 on each side of both upper and lower jaws, but in most of the modern or existing groups of these higher mammals this number has been very considerably reduced through the suppression of certain teeth. We have every reason to believe that the ancestors of the forms with reduced dentition possessed teeth in full numbers and that there has actually been a loss of teeth in the course of descent. This conclusion is abundantly confirmed by the facts of embryology. Take, for example, the great group of the gnawing mammals or Rodentia, in which the front teeth or incisors, above and below, are reduced to one on each side, except in the rabbits. The incisors are chisel-shaped and 1 Otto Maas, Die Abslammungslehre, pp. 273-74. 122 EVOLUTION, GENETICS, AND EUGENICS are faced with hard enamel, so that the action of the upper teeth upon the lower keeps the cutting edges extremely sharp; these teeth do not form roots, but continue to grow throughout the lifetime of the animal. Between the chisel-like incisors and the grinding teeth, there is a long toothless gap, which, we assume, was, in the ancestors of the rodents, occupied by the second and third incisors, the canine and two or more grinders. This conclusion is justified by the facts of embryology; for instance, in the embryo of the squirrel several of the missing teeth are begun as distinct tooth-germs, but fail to develop, never cut the gum and are resorbed before birth. All available evidence points to the conclusion that birds are descended from reptiles, a conclusion which is especially strengthened by the facts of palaeontology and will be examined more at length in the following lecture. Such a descent explains many otherwise puzzling features in the ontogeny of birds, in which reptilian charac- teristics appear in transitory fashion and are either modified so as to take on typically bird-like character, or are suppressed altogether. A remarkable example of this is the formation of rudimentary teeth in certain embryonic birds, followed by their resorption and disappear- ance before hatching. It can hardly be contended that these rudimentary structures, which are confined to the embryonic stages of development and of which no trace remains in the adult, are so indispensable to the processes of ontogeny, that they were specially created to serve this temporary purpose. For such a contention there is not a particle of evidence and the theory of evolution, which regards these structures as useless remnants, due to inheritance from ancestors in which the structures are functional, offers much the most satisfactory solution of the problem that has yet been suggested. Embryology further shows that evolution is not invariably an advance from lower and simpler to higher and more complex types, but may be by way of degeneration and degradation. The adoption of a parasitic mode of life is very apt to cause such degradation, and some very remarkable instances of the degeneration of parasites have been observed. An instructive example that may be cited is that of Sacculina, a nondescript creature that is parasitic on certain species of crabs. The parasite is attached to the body of its victim, under- neath the tail, by means of root-like fibres which penetrate and ramify throughout the interior of the crab. The root-like fibres absorb nutri- ment and convey it to the body of the parasite, which is reduced to a THE RECAPITULATION THEORY 12 $ mere bag, without appendages, muscles, nervous system, sensory apparatus, digestive tract, or any determinable organs save those of reproduction. The creature has the power of assimilating the nutri- tive juices which are conveyed to it by the root-like filaments from the body of its host, and the power of reproduction, and it must have some respiratory and excretory capacity, though there are neither gills nor glands. From an examination of the adult parasite alone, it would be quite impossible to classify it and determine the type and class to which it should be referred, but embryology solves the problem. From the egg is hatched a free-swimming larva, which has jointed append- ages, nervous, muscular and digestive systems and, in short, clearly belongs to that group of the Crustacea which includes the barnacles. This is degeneration carried nearly to the utmost possible extreme and yet the individual development shows the derivation of this otherwise problematical parasite and the steps through which it passed in its deterioration. It was stated above that several distinguished naturalists alto- gether reject the recapitulation theory as a means of interpreting the facts of embryology. They do this on the ground that, inasmuch as changes and innovations in form or structure must arise in the germ- plasm, at the very beginning of ontogeny, there is no reason why such changes might not involve the whole course of embryological develop- ment. To my mind this a priori objection to the recapitulation theory is quite without force in view of the great body of observed facts, but there is no time to enter upon a discussion of such an abstract and difficult problem. For our present purpose, however, it is important to note that these objectors are staunch evolutionists and find in the community of mode in ontogeny between different classes of organ- isms one of the strongest arguments in support of the evolutionary doctrine. CHAPTER X EVIDENCES FROM PALAEONTOLOGY STRENGTH AND WEAKNESS OF THE EVIDENCE The word palaeontology means literally the science of ancient life. Practically, it is the study of the fossil remains of extinct animals and plants, including any traces of their existence, such as footprints, impressions in slate, clay, or coal. The evidence from the fossils has definite elements of strength in that it deals with actual organisms that formerly inhabited the earth's surface. Many of these species must have left descendants, some of which are doubtless living in a modified condition today. Palaeontology should be able either strongly to support or to contradict the idea of evolution. If its data accord with the evolution idea and are opposed to the special creation idea, the fossils may be said to be evidences of evolution. The weakness of the study of fossils lies in the fact that extremely few samples of the living forms that have existed in the past have been preserved, and of those that have been preserved only a very small percentage have been dug up and studied by capable scientists. Many types of animals and plants, moreover, are soft and capable of preservation only under such exceptional conditions that but a rare specimen here and there over the world, scattered through various widely separated strata, has been found. Only very common or abundant types are likely to have been preserved and discovered, for the chances of an uncommon form being preserved would be small and the further chances of these infrequently preserved specimens being found would be infinitely smaller. The great majority of fossil remains are fragmentary or preserved very incompletely, so that only the hard parts have come down to us. There are, of course, many important exceptions to this rule, and these are our chief reliance in interpreting ancient life. That Darwin fully realized the vulnerable points in the palaeonto- logical record is shown by the following quotation from the Origin oj Species: "I look at the geological record as a history of the world imper- fectly kept and written in a changing dialect; of this history we possess 124 EVIDENCES FROM PALAEONTOLOGY 125 the last volume alone, relating only to two or three countries. Of this volume only here and there a short chapter has been preserved; and of each page only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, maj' represent the forms of life which are entombed in our successive formations and which falsely appear to us to have been abruptly introduced." OTHER OPINIONS AS TO THE ADEQUACY OF THE EVIDENCES FROM PALAEONTOLOGY "The primary and direct evidence in favour of evolution can be furnished only by palaeontology. The geological record, so soon as it approaches completeness, must, when properly questioned, yield either an affirmative or a negative answer: if Evolution has taken place there will its mark be left; if it has not taken place there will lie its refutation." — T. H. Huxley. "The geological record is not so hopelessly incomplete as Darwin believed it to be. Since The Origin of Species was written our knowl- edge of that record has been enormously extended, and we now possess no complete volumes, it is true, but some remarkably full and illumi- nating chapters. The main significance of the whole lies in the fact that, just in proportion to the completeness of the record is the unequivocal character of its testimony to the truth of the evolutionary theory" — W. B. Scott. " On the other hand, matters have greatly improved since Darwin wrote his oft-cited Chapter X; many lands then geologically unknown have been explored and many of the missing chapters and paragraphs in the history of life have been brought to light. The most ancient biologically intelligible period of the earth's history is called the Cambrian and, compared with the succeeding periods, the Cambrian has always been poor in fossils, great areas and thicknesses of rocks being entirely barren. No one could doubt that our knowledge of Cambrian life was most incomplete and inadequate. A few years ago Dr. C. D. Walcott, Secretary of the Smithsonian Institution, dis- covered in the Canadian Rockies a most marvelous series of Cambrian fossils of an incredible delicacy and beauty of preservation, which have thrown a flood of new and unexpected light into very dark places. It is clear that the Cambrian seas swarmed with a great variety and profusion of life, but thai in only a few places, so far known to us, 126 EVOLUTION, GENETICS, AND EUGENICS were conditions such that these delicate creatures could be preserved. It is not possible to say how far the difficulty caused by the imperfec- tion of the geological record will be removed by the progress of dis- covery. Even as matters stand to-day, the astonishing fact is that so much has been preserved, rather than that the story is so incom- plete. Notwithstanding all the difficulties, the palaeontological method remains one of the most valuable means of testing the theory of evolution, because certain chapters in the history of life have been recorded with a minuteness that is really very surprising." — W. B. Scott, Theory of Evolution. (The Macmillan Company. Re- printed by permission). WHAT FOSSILS ARE AND HOW THEY HAVE BEEN PRESERVED " Fossils are only animals and plants which have been dead rather longer than those which died yesterday." — T. H. Huxley. "Fossils are either actual remains of bones or other parts preserved intact in soil or rocks, or else, and more commonly, parts of animals which have been turned into stone, or of which stony casts have been made. All such remains buried by natural causes are called fossils." — Jordan and Kellogg. FOSSILS CLASSIFIED Class i. The actual remains of recently extinct animals and plants which have been buried or surrounded by some sort of preserv- ing material constitute the first type under consideration. Such remains have undergone little or no change of the original organic matter into inorganic. Thus we find the complete bodies of great hairy mammoths frozen in the arctic ice. These are so well preserved that dogs have fed upon their flesh. Nearly a thousand species of extinct insects, including many ants, have been obtained practically intact from amber, a form of petrified resin. Innumerable mollusk shells, teeth of sharks, pieces of buried logs, bones of animals buried in asphalt lakes and bogs, have been found in a well-preserved condition. Class 2. Petrified fossils. — The process of petrification involves the replacement, particle for particle, of the organic matter of a dead animal or plant by mineral matter. So completely is the finer structure preserved that microscopic sections of preserved tissues, especially of plants, have practically the same appearance as sections made from living organisms. Various mineral materials have been employed in petrification, such as quartz, limestone, or iron pyrites. EVIDENCES FROM PALAEONTOLOGY 1 27 Class 3. Casts and impressions. — Very frequently the animal or plant has been buried in mud or has lain on a soft mud fiat only long enough to have left its impress in the plastic material. Sub- sequently the entire organism has decayed and been dissolved away, and its place has been taken by a mineral deposit. Thus only the external appearance has been preserved, as would be the case in making plaster-of-paris casts. Sometimes traceries of soft-bodied animals have been left upon forming slate or coal that are almost as accurate in detail as a lithograph. Perhaps the most remarkable fossils known are those found by Professor Charles D. Walcott in the marine oily shales of British Columbia. A large number of soft-bodied invertebrates of Cambrian age have been found so wonderfully preserved that not only are the external features revealed, but sometimes even the details of the internal organs may be seen through the transparent integu- ment. Some authorities include among fossils such traces of extinct life as footprints, utensils and tools of extinct man, and even the vestiges of archaic sea beaches. Perhaps this is stretching the definition of the term "fossil" too far. ON THE CONDITIONS NECESSARY FOR FOSSILIZATION "Examination and study of the rocks of the earth reveal the fact that fossils or the remains of animals and plants are found in certain kinds of rocks only. They are not found in lava, because lava comes from volcanoes and rifts in the earth's crust, as a red-hot, viscous liquid, which cools to form a hard rock. No animal or plant caught in a lava stream will leave any trace. Furthermore, fossils are not found in granite, nor in ores of metals, nor in certain other of the common rocks. Many rocks are, like lava, of igneous origin; others, like granite, although not originally in the melted condition, have been so heated subsequent to their formation, that any traces of animal or plant remains in them have been obliterated. Fossils are found almost exclusively in rocks which have been formed by the slow deposition in water of sand, clay, mud, or lime. The sediment which is carried into a lake or ocean by the streams opening into it sinks slowly to the bottom of the lake or ocean and forms there a layer which gradually hardens under pressure to become rock. This is called sedimentary rock, or stratified rock, because it is composed of sedi- 128 EVOLUTION, GENETICS, AND EUGENICS ment, and sediment always arranges itself in layers or strata. In sedimentary or stratified rocks fossils are found. The commonest rocks of this sort are limestone, sandstone, and shales. Limestone is formed chiefly of carbonate of lime; sandstone is cemented sand, and shales, or slaty rocks, are formed chiefly of clay. "The formation of sedimentary rocks has been going on since land first rose from the level of the sea; for water has always been wearing away rock and carrying it as sediment into rivers, and rivers have always been carrying the worn-off lime and sand and clay downward to lakes and oceans, at the bottoms of which the particles have been piled up in layers and have formed new rock strata. But geologists have shown that in the course of the earth's history there have been great changes in the position and extent of land and sea. Sea bottoms have been folded or upheaved to form dry land, while regions once land have sunk and been covered by lakes and seas. Again, through great foldings in the cooling crust of the earth, which resulted in depression at one point and elevation at another, land has become ocean and ocean land. And in the almost unimaginable period of time which has passed since the earth first shrank from its hypo- thetical condition of nebulous vapor to be a ball of land covered with water, such changes have occurred over and over again. They have, however, mostly taken place slowly and gradually. The principal seat of great change is in the regions of mountain chains, which, in most cases, are simply the remains of old folds or wrinkles in the crust of the earth. '•'When an aquatic animal dies, it sinks to the bottom of the lake or ocean, unless, of course, its flesh is eaten by some other animal. Even then its hard parts will probably find their way to the bottom. There the remains will soon be covered by the always dropping sedi- ment. They are on the way to become fossils. Some land animals also might, after death, get carried by a river to the lake or ocean, and find their way to the bottom, where they, too, will become fossils, or they may die on the banks of the lake or ocean and their bodies may get buried in the soft mud of the shores. Or, again, they are often trodden in the mire about salt springs or submerged in quick- sand. It is obvious that aquatic animals are far more likely to be preserved as fossils than land animals. This inference is strikingly proved by fossil remains. Of all the thousands and thousands of kinds of extinct insects, mostly land animals, comparatively few speci- mens are known as fossils. On the other hand, the shell-bearing EVIDENCES FROM PALAEONTOLOGY 129 mollusks and crustaceans are represented in almost all rock deposits which contain any kind of fossil remains." — Jordan and Kellogg. 1 The study of geology teaches us that the earth's outer zones have undergone within the period of vertebrate history numerous profound changes which in general we may term climatic changes. There have been periods of continental subsidence, accompanied by ocean-floor elevations, during which great continental plains have been covered with comparatively shallow seas. The marine faunas of the seas have migrated into these shallows and representatives of them have been buried in sediment. When the reverse change has occurred and the continental plain has been again elevated, the sedimentation of the shallow-sea period forms a great rocky stratum laden with marine fossils. Between periods of subsidence millions of years elapsed, and therefore a break in the continuity of the entombed fossils is to be expected. Discontinuity between the fossil faunas in adjacent strata is the invariable rule. Were it not for this periodicity of subsidence and elevation there would be no boundaries between consecutive geologic strata. In addition to the methods of fossilization mentioned, a few others deserve notice. Many animals of the arid plains have been fossilized by becoming imbedded in dust or sand drifts which have piled up against rocky outcrops or have filled in dried-up arroyos. Some very valuable fossils have been recovered from asphaltic deposits as the result of animals falling into liquid or semiliquid lakes or pools of asphalt. Not only are external organs preserved with precision, but even delicate internal structures, such as the brains or the viscera of verte- brates, have been found in such a perfectly natural shape that the comparative anatomy could be worked out with confidence. On the whole, then, we must conclude that the earlier pessimism regarding the inadequacy and insufficiency of fossil data is giving way before a steadily increasing optimism, due to the very rapid advance in technique and the surprisingly abundant discoveries of the modern palaeontologist. The more enthusiastic of the new school of fossil- hunters do not despair of ultimately bringing to light all of the really essential links in the chain of evidence necessary to place the evolution theory beyond the reach of controversy. 1 From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copy- right 1907). Used by special permission of the publishers, D. Appleton & Company. 130 EVOLUTION. GENETICS, AND EUGENICS ON THE LAPSE OF TIME DURING WHICH EVOLUTION IS BELIEVED TO HAVE TAKEN PLACE "Independently of our not finding fossil remains of such infinitely numerous connecting links [referring to the objection that all steps in the evolution of modern types should be revealed in the fossils], it may be objected that time cannot have sufficed for so great an amount of organic change, all changes having been effected slowly. It is hardly possible for me to recall to the reader who is not a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. He who has read Sir Charles Lyell's grand work on the Principles of Geology, which the future historian will recognize as having produced a revolution in natural science, and yet does not admit how vast have been the past periods of time, may at once close this volume. Not that it suffices to study the Principles of Geology, or to read special treatises by different observers on separate forma- tions, and to mark how each author attempts to give an inadequate idea of the duration of each formation, or even of each stratum. We can best gain some idea of past time by knowing the agencies at work, and learning how deeply the surface of the land has been denuded, and how much sediment has been deposited. As Lyell has well remarked, the extent and thickness of our sedimentary formations are the result and the measure of the denudation which the earth's crust has elsewhere undergone. Therefore a man should examine for him- self the great piles of superimposed strata, and watch the rivulets bringing down the mud, and the waves wearing away the sea-cliffs, in order to comprehend something about the duration of past time, the monuments of which we see all around us." — Charles Darwin, Origin of Species. "In 1862," says Schuchert, 1 "the physicist, Lord Kelvin .... held that as our planet was continually losing energy in the form of heat, the globe was a molten mass somewhere between 20,000,000 and 400,000,000 years ago, with a probability of this state occurring about 98,000,000 years ago. Finally in 1897 he concurred in Clarence King's conclusion that the globe was a molten mass about 24,000,000 years ago. Both of these conclusions, however, were wrought out under the Lap- lacian hypothesis, and now many geologists hold that the earth never was molten. While geologists have not been able to fit their evidence into so short a time, they have ever since been trying to keep their 1 C. Schuchert, Text-Book of Geology, Part II, Historical Geology (t9is). EVIDENCES FROM PALAEONTOLOGY 131 1 L> w cc AGE OF MAN O OUARTFRNARY MILLIONS < AGE O OF YEARS - u OF z Ul TERTIARY 60 ID MAMMALS 1- z Ifi d: < UPPER CRETACEOUS in > N z 3 LOWER O AGE CRETACEOUS O O OF (COMANCHEAN) LI CC CC Ul REPTILES (/) 1 2 d q 111 JURASSIC > c 2 a < u. in 10 TRIASSIC z "- 225 — 2 > Ul -III < z AGE •si O PERMIAN hr O S "• Z u U u PENNSYLVANIAN OF •* (UPPER 1 in a AMPHIBIANS = CARBONIFEROUS) £ MISSISSIPPIAN cc - uiO 2 z u, < -1 w a. O 1 LOWER CARBONIFEROUS) < O r! in O >- AGE OF N O O M DEVONIAN £ Z u. 3 _ q c FISHES Ul < 3 O > u 2! | H 1 1- z SILURIAN < 0. ■ 0" Ul <* in * 1 AGE OF INVERTEBRATES *M O ORDOVICIAN E CAMBRIAN 675 KEWEENAWAN 3§ ANIMIKIAN N O £2 .. Id Z 8 EVOLUTION OF 111 H HURONIAN z°S ^ ^ Ul W INVERTEBRATES H O □ w =! K ce Ui - m < 111 EL OC DC ALGOMIAN cc in a. - mo 3k" >- u h- O O s a. SUDBURIAN o-t Ujui- 1 1,050 O O S z cc 2o ui O q 1- 2 o in cc in »5n. (M J u. CO z~ LAURENTIAN I .0 a- >■ . =. < cc cc U < u 2 LI zoz < I - - EC CL N u z z li z us: O LI < I 11 — LI U O 1- 0) Ul = * in J < 2 c U cc < GRENVILLE (KEEWATIN) (COUTCHICHINC) 1,500 CO >, o o o o o o o 10 -a cu CO CD B bO _o "o o H o CO 132 EVOLUTION, GENETICS, AND EUGENICS estimates within the bounds of Lord Kelvin's older calculations. Wal- cott, in 1893, on the basis of the stratigraphic record and the known discharge of sediment by rivers, concluded that 70,000,000 years had elapsed since sedimentation began in the Archeozoic. Sir Archibald Giekie places the time at 100,000,000 years, and most geologists have tried, although with difficulty, to fit the record within these estimates. "Since the discovery of radium, all of the calculations previously made have been set aside by the new school of physicists, and now the geologists are told they can have 1,000,000,000 or more years as the time since the earth attained its present diameter Even if finally it shall turn out that the physicists have to reduce their estimates as to the age of certain minerals and rocks, geologists nevertheless appear to be on safer ground in accepting their estimates than those based either on sedimentation, chemical denudation, or loss of heat by the earth." The last decade has seen the demise of the outworn objection to evolution based on the idea that there has not been time enough for the great changes that are believed by evolutionists to have occurred. Given 100,000,000 or 1,000,000,000 years since life began, we can then allow 1,000,000 years for each important change to arise and establish itself. We can also understand why it is that so little change can be noted in the majority of wild animals and plants within the historic period. A thousand years in the development of the race is like a second in the development of an individual and, though no one can notice any change in a growing creature in a second or a minute, very radical changes can be noted in an hour or a day or a year. We cannot see any movement in an hour hand of a clock, but it moves with certainty around the dial in a relatively short time. There is there- fore no shortage of time. Evolution may have been infinitely slow, but time has been infinitely long. The accompanying time scale shows the lapse of time and the distribution in time of the main groups of animals (Fig. 3c). ON THE PRINCIPAL GENERAL FACTS REVEALED BY A STUDY OF THE FOSSILS i. None of the animals or plants of the past are identical with those of the present. The nearest relationship is between a few species of the past and some living species which have been placed in the same genera. EVIDENCES FROM PALAEONTOLOGY 133 2. The animals and plants of each geologic stratum are at least generically different from those of any other stratum, though belonging in some cases to the same families or orders. 3. The animals and plants of the oldest (lowest) geologic strata represent all of the existing phyla, except the Chordata, but the representatives of the various phyla are relatively generalized as compared with the existing types. 4. The animals and plants of the newest (highest) geologic strata are most like those of the present and help to link the present with the past. 5. There is, in general, a gradual progression toward higher types as one proceeds from the lower to the higher strata. 6. Many groups of animals and plants reached the climax of specialization at relatively early geologic periods and became extinct. 7. Only the less specialized relatives of the most highly specialized types survived to become the progenitors of the modern representa- tives of their group. 8. It is very common to find a new group arising near the end of some geologic period during which vast climatic changes were taking place. Such an incipient group almost regularly becomes the domi- nant group of the next period, because it developed under the changed conditions which ushered in the new period and was therefore especially favored by the new environment. 9. The evolution of the vertebrate classes is more satisfactorily shown than that of any other group, probably because they represent the latest phylum to evolve, and most of their history coincides with the period within which fossils are known. 10. Most of the invertebrate phyla had already undergone more than half of their evolution at the time when the earliest fossil remains were deposited. FOSSIL PEDIGREES OF SOME WELL-KNOWN VERTEBRATES PEDIGREE OF THE HORSE Of all fossil pedigrees that of the horse is most often mentioned in evolutionary literature. The main facts have been known for about forty years, and there is a rather general consensus of opinion as to the history as a whole. It appears practically certain that the horse family (Equidae) arose from a group of primitive five-toed ungulates or hoofed mammals called Condylarthra that lived in Eocene times. 134 EVOLUTION, GENETICS, AND EUGENICS No particular member of this extinct group has been found that fulfils all the requirements of a primitive horse ancestor, so the chances are that the real ancestral condylarthran has not been discovered. "The course of their [Equidae] evolution," says Dendy, 1 "has evidently been determined by the development of extensive, dry, grass-covered, open plains on the American continent. In adap- tation to life on such areas structural modification has proceeded chiefly in two directions. The limbs have become greatly elongated and the foot uplifted from the ground, and thus adapted for rapid flight from pursuing enemies, while the middle digit has become more and more important and the others, together with the ulna and the fibula, have gradually disappeared or become reduced to mere vestiges. At the same time the grazing mechanism has been gradually perfected. The neck and head have become elongated so that the animal is able to reach the ground without bending its legs, and the cheek teeth have acquired complex grinding surfaces and have greatly increased in length to compensate for the increased rate of wear. As in so many other groups, the evolution of these special characters has been accompanied by gradual increase in size. Thus Eohippus, of Lower Eocene times, appears to have been not more than eleven inches high at the shoulder, while existing horses measure about sixty-four inches, and the numerous intermediate genera for the most part show a regular progress in this respect. "All these changes have taken place gradually, and a beautiful series of intermediate forms indicating the different stages from Eohip- pus to the modern horse [Equus] have been discovered. The sequence of these stages in geological time exactly fits in with the theory that each one has been derived from the one next below it by more perfect adaptation to the conditions of life. Numerous genera have been described, but it is not necessary to mention more than a few." The first indisputably horselike animal appears to have been Eyracotherium, of the Lower Eocene of Europe. Another Lower Eocene form is Eohippus, which lived in North America, probably having migrated across from Asia by the Alaskan land connection which was in existence at that time. In Eohippus the fore foot had four completely developed hoofed digits and a "thumb" reduced to a splint bone; in the hind foot the great toe had entirely disappeared and the little toe is represented by a vestigial structure or splint bone. 1 Arthur Dendy, Outlines of Evolutionary Biology (D. Appleton & Company, IQ i6). EVIDENCES FROM PALAEONTOLOGY 135 Then came in succession Orohippus, of the Upper Eocene, Mesohippus of the Lower Miocene, Pliohippus of the Upper Pliocene, and finally Equus : Qua- ternary and Recent. Pliohippus : Pliocene. Protohippus ; Lower Plio- cene. Miohippus : Miocene. Mesohippus : Lower Mio- cene. Crohippus : Eocene. Fig. 31. — Feet and teeth in fossil pedigree of the horse. {After Marsh.) a, Bones of the fore foot; b, bones of the hind foot; c, radius and ulna; J, fibula and tibia; e, roots of a tooth; / and g, crowns of upper and lower teeth. Equus of the Quaternary and Recent. Other genera might be men- tioned, but the history of this series has been pictured in a classic 136 EVOLUTION, GENETICS, AND EUGENICS diagram by Marsh, and in this (Fig. 31) the reader may trace upward from Orohippus to Equus the steady changes in fore and hind feet, bones of the forearm, bones of the lower leg, and the grinding teeth of upper and lower jaws. So definitely and clearly has the horse pedigree been worked out that, according to Dendy, "the palaeontological evidence amounts to a clear demonstration of the evolution of the horse from a five-toed ancestor along the lines indicated above." For a long time the palaeontological series of the horse was un- rivaled by other vertebrate types, but now we have almost equally complete series for several other modern types, notably the camels and the elephants. We shall present herewith accounts of the pedi- gree of the camels by Professor Scott, and that of the elephants by Professor Shull. And, to conclude the vertebrate pedigrees, we shall present in the next chapter that of man as given by Professor Lull. In extenuation of the use of vertebrate material to the exclusion of invertebrate, the present writer has only this to offer, that verte- brate material is more intelligible to the non-biological reader and is more in his own field of knowledge and interest. PEDIGREE OF THE CAMELS 1 W. B. SCOTT There remains one family of mammals with which it is necessary to deal and that is the camel tribe. This family has two well-defined subdivisions, the camels of the Old World and the llamas, guanacos, etc., of South America. For a very long time, the family was entirely confined to North America and did not reach its present homes until the Pliocene epoch of the Tertiary period. The skeleton of a Patago- nian guanaco may be taken as the starting point of our inquiry. In this animal the third incisor and the canine are retained in the upper jaw, all the incisors and the canine in the lower. The anterior two grinding teeth have been lost and the others are moderately high- crowned. The skull is broad and capacious behind, narrow and tapering in front. The neck is long and its vertebrae very curiously modified. The limbs are long and slender and have undergone nearly the same modifications as in the horses; the ulna is greatly reduced, interrupted in the middle and its separated portions are fused with the radius. In the hind leg the shaft of the fibula has been completely 'From W. B. Scott, The Theory of Evolution (copyright 1917). Used b> special permission of the publishers, The Macmillan Company. EVIDENCES EROM PALAEONTOLOGY 1 37 suppressed; the upper end fuses with the tibia, while the lower remains as a small separate bone, wedged in between the tibia and the heel- bone. The feet are very long and slender, with two toes in each; the Fig. 32. — Four stages in the evolution of the cameline skull. A, Protylopus, Upper Eocene; B, Po'ebr other ium, Lower Oligocene; C, Procamchis, Upper Miocene; D, guanaco, Recent. (From Scott.) long bones of the foot are co-ossified to form a "cannon-bone," the very young skeleton showing that this co-ossification does actually take place. The toes proper are free, giving the "cloven hoof," but the hoofs are very small and the weight is carried upon a soft, thick pad. i 3 8 EVOLUTION, GENETICS, AND EUGENICS && JET JK JD? M Fig. 33. — Four stages in the evolution of the cameline fore foot. A , Protylopus, Upper Eocene; B, Po'ebrotherium, Lower Oligocene; C, Procamelus, Upper Miocene; D. guanaco, Recent. (From Scott.) EVIDENCES FROM PALAEONTOLOGY 139 Were there time enough to do so, we might trace the development of this family backward, step by step, through all the many stages between the Pleistocene and the Upper Eocene in quite as unbroken sequence and in as full detail as*can be done for the horses. We must, however, pass over all the intermediate steps and consider the ances- tral camels of the Upper Eocene. These were very little animals, hardly larger than a jack rabbit, which had the full complement of teeth, 44 in total number, and all with very low crowns. The limbs, and especially the feet, are relatively short, the ulna is complete and separate, as is also the fibula; there are four toes in each foot, though the lateral pair of the hind foot are extremely slender, and there is no co-ossification to form cannon-bones. The hoofs are well developed, in form like those of an antelope, so that there can have been no pad. For the present, the line cannot be carried back of the Upper Eocene, the probable ancestors from the middle and Lower Eocene being, as vet, represented only by fragmentary specimens. In addition to this main stem of cameline descent which resulted in the modern species, there were two short-lived side branches which should be mentioned. One, ending in the Lower Miocene, was the series descriptively called "gazelle-camels," small animals with very long and slender legs, evidently swift runners. The other series, the so-called "giraffe-camels," terminated in the Upper Miocene; these were browsers and display an increasing stature, especially in the length of the neck and fore limbs. They adapted themselves to the growing aridity of the western plains. EVOLUTION OF THE ELEPHANTS 1 A. FRANKLIN SHULL The mastodon-elephant series shows a larger number of obvious changes than most of the other series named, all of these changes except that of the body having to do with features of the head. From the numerous specimens of elephant-like forms available, the following are selected (following Lull) as probably representing a direct line of evolution: Moeritherium from the Upper Eocene of Egypt; Palaeomastodon from the Lower Oliogocene of Egypt, also from India; Trilophodon from the Miocene of Europe, Africa, and North America; Mastodon from the Pliocene and Pleistocene of 1 From A. F. Shull, Principles of Animal Biology (copyright 1920). Used b\ special permission of the publishers. The McGraw-Hill Book Company. 140 EVOLUTION, GENETICS, AND EUGENICS North America, Europe and Asia; Stegodon from the Pliocene of southern Asia; and Elephas from the Pleistocene of the Americas, Europe, and Asia, as well as the living elephants of Asia and Africa. Fig. 34. — Evolution of head and molar teeth of mastodons and elephants. A, A', Elephas, Pleistocene; B, Stegodon, Pliocene; C, C, Mastodon, Pleistocene; D, D', Trilophodon, Miocene; E, E' , Palaeomastodon, Oligocene; F, F', Moe- rithcrium, Eocene. {From Lull.) EVIDENCES FROM PALAEONTOLOGY 141 A study of Figure 34 in connection with the following account will dis- close the more striking steps of evolution. These forms differed from one another in a number of features, but the differences between any member of the series and the one that precedes or that which follows were so small that the series is obviously a continuous one. Moerithe- rium was very different from the modern elephant, but the inter- mediate forms completely bridged the gap. The series exhibits an enormous increase in size of body, changes in the form and size of the teeth, a reduction in the number of teeth, an alteration in the method of tooth succession, the enlargement of certain teeth to become tusks, the elongation and subsequent shortening of the lower jaw, the development of the upper lip and nose into a proboscis, and an increase in the height of the skull through the development of large cavities in the substance of the bone. These features are described in the several forms seriatim. Moeritherium. — The earliest animal recognized as belonging to the elephant series, Moeritherium by name, was recovered from the late Eocene and early Oligocene deposits of northern Egypt. It was slightly over three feet in height. The features suggesting elephantine affinities are the high posterior portion of the skull (Fig. 34, F); composed of somewhat cancellate bone, that is, bone containing open spaces ; the elongation of the second pair of incisors in each jaw to form short tusks; the indication of transverse ridges on the molar teeth (Fig.34,F) ; and the position of the nasal openings some distance back of the tip of the upper jaw, indicating probably a prehensile upper lip. There were 24 teeth, and the neck was long enough to enable the animal to put its head to the ground. It probably fed upon tender shoots and swamp vegetation. Palaeomastodon. — This form also lived in Egypt, but has recently been found in India. It dates irom early Oligocene time. Palaeo- mastodon was of somewhat larger size than the preceding form, the posterior part of the skull was distinctly higher (Fig. 34,22') — with a greater development of cancellate bone, and the neck was somewhat shortened. The upper incisors of the second pair were more elongated as tusks and bore a band of enamel on their front surfaces. The lower second incisors were present, but not enlarged. All other incisors and the canines had disappeared. The molar teeth (E) resembled those of Moeritherium but were larger. The lower jaw was considerably elongated, and the total number of teeth was still high (26). The nasal openings had receded until they were just in front of the eyes, 142 EVOLUTION, GENETICS, AND EUGENICS which is believed to indicate the existence of a short proboscis extending at least to the tips of the tusks. Trilophodon. — Trilophodon, a great migrant and consequently wide-spread over several continents as stated above, exhibited in several respects a striking advance over Palaeomastodon; but this advance was in the main in the same direction as was indicated by the change from Moeritherium to Palaeomastodon. Trilophodon was a huge animal, nearly as large as modern Indian elephants. The tusks were considerably longer (Fig.34,D') and still bore a band of enamel. The molar teeth were large and greatly reduced in number, so that only two were present at any one time on each side of each jaw. The surface of these teeth bore a somewhat larger number of transverse crests (Fig. 34, D) than were present in the earlier forms. The lower jaw was enormously elongated, so that it projected as far forward as the tusks. The great weight of the lower jaw and tusks was associated with a considerable development of cancellate bone in the skull, to which the supporting muscles of the neck were attached. Presumably there was a proboscis which extended to or beyond the tips of the tusks and lower jaw. Mastodon. — The mastodons on the whole represent a line of development which became extinct; but in their incipient stages they appear to have given rise to the succeeding forms leading to the elephants. The body was somewhat larger than that of Trilophodon, being about the size of the Indian elephant. The tusks (C") were much elongated (9 feet or more), but the lower jaw was greatly short- ened and the lower incisor teeth were reduced or wanting. The molar teeth (Fig.34 > C) were scarcely more complex than earlier forms, and numbered two on each side of each jaw. They were still crushing teeth, and the food must have been tender twigs and succulent plants; indeed, remains of such objects have been found in the region of the stomach of the fossil mastodons. Stegodon. — This animal is of interest chiefly because the molar teeth bore five or six well-defined transverse ridges (Fig.34,5). These ridges were due to plates of enamel extending up through the tooth, and inclosing a substance known as dentine. Over the enamel in an unworn tooth was a thin coat of a third substance called cement, but there was not much of this substance between the ridges. In the latter respect Stegodon differed, as is pointed out below, from the elephants and mammoths. On the whole, Stegodon was intermediate between the mastodons and elephants. EVIDENCES FROM PALAEONTOLOGY 143 Elephas. — In this genus are included a number of extinct forms (the mammoths) from three or four continents, and the living ele- phants. The extinct forms, though called mammoths, were not large animals, being no larger than the Indian elephant of today, and not so large as the living African species. Some of the features of the elephants, their size, the short neck, the long proboscis, and the heavy tusks are matters of common observation. The skull is very high and short (Fig. 34, A'). The height is due chiefly to the development of cancellate bone, not to the enlargement of the brain, which is still quite small. As stated above, the high skull affords the necessary leverage for the muscles that support the weight of the tusks. The molar teeth are distinctly grinding teeth (Fig. 34, A). Each tooth bears a number of transverse ridges, about ten in the African elephant and two dozen or more in the Indian species. These ridges are worn down by the chewing of harsh food, so that the upper surface displays a number of flattened tubular plates of enamel inclosing dentine and bound together by cement. A tooth is completely worn out by use, and is replaced by another. The method of replacement, however, is peculiar. While the tusks (incisors) are of two sets, one following the other like milk and permanent teeth of other mammals, the grinders succeed one another in continuous fashion. There are never more than two visible grinders on each side of each jaw. As they wear out they move forward in the jaw, and are replaced by new teeth appearing behind. New molars thus enter at intervals of two to four years in young elephants, and at intervals of 15 to 30 years in later life. If an elephant lives long enough (60 years or more) it develops a total of 28 teeth, including tusks, but has not more than ten (often less) at any one time. Correlated with the nature of the teeth of the elephants are their food and chewing habits. Whereas the ancestral forms whose molars bore prominent elevations lived on twigs and tender herbage which they crushed in mastication, the mammoths with their flattened tooth surfaces devoured grasses, sedges, and other harsh vegetation which they ground with lateral motion of the teeth upon one another. In this respect modern elephants are like the mammoths. In the changes described above is found one of the most beautiful and best established evolutionary series with which the palaeontolo- gist is acquainted. Only a few others equal or approach it in clearness and completeness. CHAPTER XI THE EVOLUTION OF MAN: PALAEONTOLOGY 1 Richard Swann Lull ORIGIN OF PRIMATES Stock. — There is but little doubt that two important orders of modern mammals, the Carnivora and the Primates, had a common origin, diverging mainly along lines determined by a dietary contrast, as the former have become more strictly flesh-eating or predaceous, the latter largely fruit-eating and as a consequence more completely arboreal. Back of each group lie as annectant forms the Insectivora, not perhaps such as are alive to-day, as all these are highly specialized along diverse lines, but generalized insectivores possessing, because of their primitiveness, a wider range of potential adaptation. Mat- thew is " disposed to think of these, our distant ancestors, at the dawn of the Tertiary, as a sort of hybrid between a lemur and a mongoose, rather catholic in their tastes, living among and partly in the trees, with sharp nose, bright eyes, and a shrewd little brain behind them, looking out, if you will, from a perch among the branches, upon a world that was to be singularly kind to them and their descendants." Thus we can define the stock as a relatively large-brained arboreal insectivore, of primitive but adaptable dentition, and especially of progressive mentality. Time. — -The time of primate origin must have been not later than basal Eocene, as primates, clearly definable as such, are found in the Lower Eocene rocks of both Europe and North America. Place. — The simultaneous appearance of the primate in the Old World and the New gives rise to the same conclusions as to their place of origin and their migrations thence as with other modernized mammals. It suffices now to say that their ancestral home was boreal Holarctica, probably within the limits of the present continent of Asia, whence they migrated southward along the three great continental radii. The impelling cause of this migration was the increasing northern cold, before which the boreal limitations of the tropical forests retreated, carrying with them the primates which, in 1 From R. S. Lull, Organic Evolution (copyright 1017). Used by special permission of the publishers, The Macmillan Company. 144 % THE EVOLUTION OF MAN 145 general, are utterly dependent upon such an environment for their sustenance. Geologic record. — Primates are found in the North American sediments from Lower to Upper Eocene time, when they became extinct. Thus, while their remains constitute a relatively large per- centage of the total fauna of the Eocene, primates are utterly unknown on this continent from that time until the coming of man. In Europe the record is similar except that the extinction occurred at a somewhat later date, the Oligocene. Furthermore, they reappear in Europe in the Lower Miocene, at the time of the proboscidean migration out of Africa, whence these primates may also have come. Their second European extinction was in the Upper Pliocene shortly before the first appearance of mankind. But in southern Asia, Africa, and South America the evolution of primates seems to have been continuous since the first great southward migration. The evidence, however, is not so much the historical documents as the presence of primates in those places at the present time, the fossil record is not entirely lacking although highly incom- plete. The South American monkeys may have had their origin in the ancient North American primates, or more doubtfully, the stock may have come by way of Africa. Scott inclines toward the latter view although he says the evidence is by no means conclusive. ORIGIN OF MAN Stock. — According to W. K. Gregory, the stock from which man arose was some big-brained anthropoid related most nearly to the chimpanzee-gorilla group, an assumption based upon anatomical evidences, in spite of wide differences in habitus and consequent adaptation. Place. — Evidences point to central Asia as the place of descent from the trees of the human precursor, the reasons for this belief being several. First, it was central for migrations elsewhere; Europe, on the other hand, where the most conclusive, in fact almost the exclusive evidence for fossil man is found, is too small an area for the divergent evolution of the several human species. Second, Asia is contiguous to the oldest known human remains, which, as we shall see, were found in Java. Third, it was the seat of the oldest civilizations, not only of the existing nations which, like the Chinese, trace their recorded history back to a hoary antiquity, but of nations which preceded them by thousands of years, and whose records have not yet come to light. 146 EVOLUTION. GENETICS, AND EUGENICS • This antiquity vastly exceeds that of the nations of Europe or of the Americans or of Africa. Fourth, central Asia is the source of almost all of our domestic animals, many of which have been subjected to human will and control for thousands of years, and this is equally true of many of our domestic plants. This is not due to the fact that man first reached civilization in Asia, but rather that he chose for his com- panions the highest and best of their several evolutionary lines, and Asia was the place of all others upon earth where the evolution in general of organic life reached its highest development in late Cenozoic time (Williston). Fifth, climatic conditions in Asia in the Miocene or early Pliocene were such as to compel the descent of the prehuman ancestor from the trees, a step which was absolutely essential to further human development. Impelling cause. — We look for a geologic cause back of this most momentous crisis in the evolution of humanity and we find it in conti- nental elevation and consequent increasing aridity of climate, espe- cially to the northward of the Himalayas. With this increased aridity and tempering of tropical heat came the dwindling of the forested areas suitable to primate occupancy. Barrell has suggested that this diminution left residual forests comparable to the diminishing lakes and ponds of the Devonian, which upon final desiccation compelled their denizens to become terrestrial or perish. The dwindling of the residual forests would have an effect upon the tree-dwellers which may be expressed in precisely the same words. Once upon the ground the effect upon even a conservative type — and the primates in general, where constant conditions prevail, are slow of change — would be the rapid acquisition of such adaptations as were necessary to insure sur- vival under the new conditions. The other man-like apes had, unfortunately for their further evolution, reached a region where tropical forests continued to be available and hence have retained their arboreal life and with it a stagnation of progress. The result has been, at any rate on the part of the three larger forms, a degeneracy from the estate of their common ancestry with mankind; the gibbons seem to have deteriorated less, while terrestrial man has risen to the summit of primate evolution. Time. — The time of the descent is not later than early Pliocene nor earlier than Miocene time; when the terrestrial ape-man became what we would call human was perhaps later, but certainly during the Pliocene, which makes the age of man as such measurable in terms of hundreds of thousands of years! THE EVOLUTION OF MAN I47 Significance of the descent from trees. — As a result of the descent from the trees, certain definite factors were called into play, each of which had its effect on the further evolution. Briefly enumerated, these are: (i) Assumption of the erect posture; (2) liberation of the hands from their ancient locomotor function to become organs of the mind; (3) loss of the easily obtainable food of the tropical forests, necessitating the search for sustenance, both plant and animal, and man became a hunter; (4) need of clothing with increasing inclemency of the weather, especially during the long winters; (5) freedom from climatic restrictions — when an omnivorous diet and clothing were acquired man was no longer limited to one definite habitat and the result was dispersal; (6) the development of communal life, rendered possible by the terrestrial habitat. Primates are at best gregarious, submitting, as in the gorilla, to the leadership of the strongest male, but it is only by communal life with its attendant division of labor that man can rise above the level of utter savagery. Evolutionary changes. — Human evolutionary changes which are recorded are: more erect posture, shorter arms, perfection of thumb opposability, reduction of muzzle and of size of teeth, loss of jaw power, development of chin prominence, increase in skull capacity, diminution of brow-ridges, diminution in strength of zygo- matic or temporal arch, increase in size and complexity of brain, especially frontal lobes, development of articulate speech. FOSSIL MAN Fossil remains of man are found under two conditions, in river valley deposits and in limestone caverns which served first as a dwelling-place and later as a sepulture. Of these the caverns have been by far the most productive, but they contain only the remains of the later races, as the caverns according to Penck did not become available for human occupancy before middle Pleistocene time. The rarity of human fossils may be explained, first, by the various burial customs which seldom are sufficiently perfect to preclude the possibility of alternate wetting and drying or of rapid oxidation, both of which are prohibitive of fossilization. If man lived and died in the forests the chances for his fossilization, in common with other forest creatures, was very remote, for the remains of such are almost invari- ably destroyed by other animals, by dampness, or by fungi, and rarely attain a natural burial in sediment. Tf. on the other hand, he dwelt 148 EVOLUTION, GENETICS, AND EUGENICS in the open, the chances of so shrewd a creature being caught in the flood waters and thus buried in sediment were not very great. However we account for it, the fact remains that relics of ancient man are rare and are valued accordingly. In North America. — Repeated instances of seemingly ancient man have been brought to light in North America, such as the "Cale- veras skull" of the California gold-bearing gravels, which was satirized by Bret Harte; the Nebraska "Loess man," and those of the Trenton gravels; none of which, with the possible exception of the last-men- tioned, has proved to be really old in the geologic sense. Indirect evidence of human antiquity, that is, the association of North Ameri- can man with animals which are now extinct, while very rare, has been reported in at least two highly authentic instances. The first of these was at Attica, New York, and is attested by Doctor John M. Clarke, the New York state geologist. Four feet below the surface of the ground, in a black muck, he found the bones of the mastodon (Masto- don americanus), and 12 inches below this, in undisturbed clay, pieces of pottery and thirty fragments of charcoal. The charcoal may have been of natural origin, but the presence of the pottery seems conclu- sive. The other instance was that of the remains of a herd of extinct bison (Bison antiqims) found near Smoky Hill River, Logan County, Kansas, and thus described by Professor Williston: An "arrow-head was found underneath the right scapula of the largest skeleton, embedded in the matrix, but touching the bone itself. The skeleton was lying upon the right side The bone bed when cleared off .... contained the skeletons of five or six adult animals, and two or three younger ones, together with a foetal skeleton within the pelvis of one of the adult skeletons. The animals had evidently all perished together, during the winter. There was no possibility of the accidental intrusion of the arrow-head in the place where found It must have been within the body of the animal at the time of death, or have been lying on the surface beneath its body." What at this writing is claimed to be another genuine case of such an association, this time of the actual human bones, has just been announced from Florida. This find, which has been reported by State Geologist Sellards, was made at Vero, eastern Florida, in 1913. The fossil human bones are from two incomplete skeletons and are found in strata which also contain remains of the following extinct species: Elephas colwmbi, Equus leidyi, a fox, a deer, the ground-sloth, Megalonyx jeffersoni, and the American mastodon. THE EVOLUTION OF MAN 149 In South America. — A number of finds have been recorded from South America, notably by the late Florentino Amegbino of Buenos Aires, who contributed so largely to our knowledge of South American prehistoric life. An expert from Washington, Doctor Ales Hrdlicka, has studied with the utmost care the locality and character of each of these finds in the Western World, and has expressed the opinion that none is of an antiquity greater than that of the pre-Columbian Indians. Further evidence lies in the uniformity of type, except for minor distinctions, of all native American peoples. There is no such racial differentiation as that seen in the Old World, and the argument is that there has not been time for such a deployment. The area and condi- tions as an adaptive radiation center are surely ample. In Africa. — The only African relics thus far reported are those of prehistoric cultures, comparable to those of Southern Europe, in certain caverns of the Barbary States. There has also been reported from Oldoway ravine, German East Africa, a human skeleton of undoubted antiquity. It is described, however, as being neither a very early nor a primitive type. In Asia. — Asia has given us in Pithecanthropus the oldest known relic of the Hominidae, found at Trinil in the island of Java. Osborn says: "It is possible that within the next decade one or more of the Tertiary ancestors of man may be discovered in northern India among the foothills known as the Siwaliks. Such discoveries have been heralded, but none have thus far been actually made. Yet Asia will probably prove to be the center of the human race. We have now discovered in southern Asia primitive representatives or relatives of the four existing types of anthropoid apes, namely, the gibbon, the orang, the chimpanzee, and the gorilla, and since the extinct Indian apes are related to those of Africa and of Europe, it appears probable that southern Asia is near the center of the evolution of the higher primates and that we may look there for the ancestors not only of prehuman stages like the Trinil race but of the higher and truly human types." In Europe. — It is in Europe, however, that the tale of human prehistory is the most complete, not only through the happy accident of preserval, but because it has been much more thoroughly explored than has the Asiatic evolutionary center. The latter, however, holds the greatest hopes for future exploration since, as we have emphasized. Europe is too small to be an adaptive radiation center and European i5° EVOLUTION GENETICS, AND EUGENICS prehistoric man represents waves of migration from the greater continent. Nevertheless the European record has enabled us to name and define a number of distinct human species, and here the record of the cultural evolution of man is also unusually complete. Hence Euro- pean chronology is taken as a standard in describing discoveries from any portion of the world. CHRONOLOGICAL TABLE (Adapted from Osborn, 1915) Postglacial Time 25,000 years Upper Palaeolithic culture Cro-Magnon man Fourth Glacial Stage (Wiirm, Wisconsin) 50,000 years Close of Lower Palaeolithic culture Neanderthal man Third Interglacial Stage 150,000 years Beginning of Lower Palaeolithic culture Piltdown and pre-Neanderthaloid men Third Glacial Stage (Riss, Illinoian) 175,000 years Second Interglacial Stage 375,000 years Heidelberg man Second Glacial Stage (Mindel, Kansas) 400,000 years First Interglacial Stage 475,000 years Pithecanthropus, ape-man First Glacial Stage (Giinz, Nebraskan) 500,000 years Pithecanthropus. — The Java ape-man, Pithecanthropus erectus (Fig. 35) was discovered in Trinil, on the Solo or Bengawan River in central Java, in 1894. The type consists of a calvarium or skull cap, a left thigh bone, and two upper molar teeth. The skull is characterized by its limited capacity, about two- s V — 7^-^^= \f \ thirds that of man; and by the low flat forehead and beetling brows. Hence not only was the brain limited in its total size, but this FlG - 35-— Skull of Java ape-man, Pithccan- . n , c , i thro pas credits. (From Lull, after Dubois.) was especially true of the frontal lobes, which, as we have seen, are the seat of the higher intel- lectual faculties. Thus, as Osborn says, although touch, taste, and THE EVOLUTiON OE MAN ISI vision were well developed there was a limited faculty for profiting by experience and accumulated tradition. The femur associated with (he skull is remarkable for its length and slight curvature as compared with the primitive Neanderthal race of Europe and indicates a creature fully as erect and nearly as tall as the average European of today, the height being estimated at 5 feet 7 inches as compared with 5 feet 3 inches for the Nean- derthals and 5 feet 8 inches, the average height of modern males. The erect posture of course implies the liberation of the hands from any part in the locomotor function. The teeth are somewhat apedike, but are more human than are those of the gibbon, and the human mode of mastication has been acquired. Certain authorities have tried to prove that Pithecanthropus is nothing but a large gibbon, but the weight of authority considers it prehuman, though not in the line of direct development into humanity. It is neverthe- less a highly important transi- tional form. Associated with the Pithe- canthropus remains are those of a number of the contem- porary animals which fix the Fig. 36. — Jaws, left outer aspect, of A, date as either of the Upper Plio- chimpanzee,P(z;f,sp.; B, fossil chimpanzee, cene or lowermost Pleistocene Pan vctus, found in association with Tilt- [od which being rendered down man; C, Heidelberg man, Homo . . , . , ,, • n 1 ti m terms ot vears gives an esti- heidelbergensis; D, modern man, II. sapiens. ■ a {From Lull, after Woodward.) mated age of a.bout 500,000! 1 52 EVOLUTION, GENETICS, AND EUGENICS Heidelberg man. — Homo heidelbergensis, the Heidelberg man, represents the oldest recorded European race, geologically speaking. The type was discovered in 1907 in river sands, 79 feet below the surface, at Mauer, near Heidelberg, South Germany. The relic consists of a perfect lower jaw with the dentition (Fig. 36, C). The description by the discoverer, Doctor Schoetensack, follows (from Osborn) : "The mandible shows a combination of features never before found in any fossil or recent man. The protrusion of the lower jaw just below the front teeth (the chin prominence) which gives shape to the human chin is entirely lacking. Had the teeth been absent it would have been impossible to diagnose it as human. From a fragment of the symphysis of the jaw it might well have been classed as some gorilla-like anthropoid, while the ascending ramus resembles that of some large variety of gibbon. The absolute certainty that these remains are human is based on the form of the teeth — molars, pre- molars, canines, and incisors are all essentially human and although somewhat primitive in form, show no trace of being intermediate between man and the anthropoid apes but rather of being derived from some older common ancestor. The teeth, however, are small for the jaw; the size of the border would allow for the development of much larger teeth. We can only conclude that no great strain was put on the teeth, and therefore the powerful development of the bones of the jaw was not designed for their benefit. The conclusion is that the jaw, regarded as unquestionably human from the nature of the teeth, ranks not far from the point of separation between man and the anthropoid apes. In comparison with the jaws of the Neanderthal races .... we may consider the Heidelberg jaw as pre-Neander- thaloid; it is, in fact, a generalized type." Associated with the Heidelberg jaw is an extensive warm-climate fauna: straight-tusked elephant (E. antiqims), Etruscan rhinoceros, primitive horse, bison, wild cattle {urus), bear, lion, and so on, all of which aid in establishing the date of the jaw as Second Interglacial and its age, conservatively estimated, at from 300,000 to 375,000 years. The cultural evolution of Heidelberg man is indicated by the presence of eoliths, flint implements of the crudest workmanship, if indeed their apparent fashioning is not merely the result of use. Neanderthal man. — The original specimen of the Neanderthal man, Homo neanderthalensis or primigenius (Figs.37, 38,39) was dis- covered in 1856 not far from Dusseldorf in Rhenish Prussia. Here the valley of the Dtissel forms the deep Neanderthal ravine, whose THE EVOLUTION OF MAN 153 limestone walls are penetrated by caverns, in one of which the remains were found. What was doubtless a perfect skeleton at the time of its '^. ■£. "5 £4 V. '?» "« > ~ "=; _i s V. *o t/5 c c-i '-; tf so — O *tr, O £ B ^ M <" "*-> U ■^ © t+H "T ?s rt -l '-= ^> _^ g *^ a O &5 S c cq" O cS Z3 c/3 l—i w **H tf O O l # C rt <-* t'- s c 6 ft) 1 c c to discovery was so injured by its finders that only a portion of it, which is now preserved in the Provincial Museum at Bonn, was saved. : ' This prophet of an unknown race was for a time utterly without honor 154 EVOLUTION, GENETICS, AND EUGENICS though of course the subject of a most heated controversy, being con- sidered as non-human, or, as Virchow believed, owing its distinctive ■characters to disease. The sagacity of Huxley threw true light upon "he problem, though it was not until the mute testimony of other representatives of the race (the men of Spy) was offered that even Huxley's masterful conception of the Neanderthal characters was taken as an accepted fact. Professor Huxley's descrip- tion of the Neanderthal type is classic. He says : "The anatomical char- acters of the skeletons bear out conclusions which are not flattering to the appear- ance of the owners. They were short of stature but powerfully built, with strong, curiously curved thigh bones, the lower ends of which are so fashioned that they must have walked with a bend at the knees. Their long depressed skulls had very strong brow-ridges; their lower jaws, of brutal depth and solidity, sloped away from the teeth down- wards and backwards in consequence of the absence of that especially characteristic feature of the higher type of man, the chin prominence." Subsequently several more specimens have come to light, at Spy in Belgium, at Krapina in Croatia, at Le Moustier, La Chapelle-aux- Saints and La Ferrassie in France, and at Gibraltar, which, while differing in various details, effectually serve to establish the race, whose main characteristics are : Heavy, overhanging brows, retreating fore- head, long upper lip; jaw less powerful than that of the Heidelberg man but very thick and massive; chin generally strongly receding but in process of forming; dentition extraordinarily massive in the La Chapelle specimen, whereas in those of Spy the teeth are small. The skull in many characteristics is nearer to the anthropoids than to modern man. The brain is large and its volume is surely human, but the pro- portions are again less like those of recent man than like the anthro- poids. The chest is large and robust, the shoulders broad, and Fig. 38. — Neanderthaloid skull of La Chapelle-aux-Saints {Homo neandcrtlialaisis). {From Lull, after Boule.) THE EVOLUTION OF MAN •55 the hand large, but the fingers are relatively short, the thumb lacking the range of movement seen in modern man. The knee was some- what bent, the leg powerful, with a short shin and clumsy foot, clearly not of cursorial adaptation. The curve of the bent leg was correlated with a similar curvature of the spine, so that the man could not stand fully erect, as he lacked the fourth or cervical curvature of Homo sapiens. The average stature was 5 feet 3 inches, with a range from 4 feet 10.3 inches to 5 feet 5.2 inches, partly sex differences. Neanderthal man lived in Eu- rope from the Third Interglacial stage through the Fourth Glacial, a duration of thousands of years, and then became extinct, from twenty to twenty-five millenniums ago. He seems to have been an actual lineal successor of the man of Heidelberg, but was throughout his long career an unprogressive static race. One of the most remarkable features in connection with this race, however, w^as the very reverent way in which the dead were buried, with an abun- dance of ornaments and finely Fig. 39. — Skeleton of Neanderthal man. A , Homo neanderthalensis, com- pared with that of a living native Australian; B,Homo sapiens, the latter the lowest existing race. (From Lull, after Woodward.) worked flints. This can have but one interpretation, the awakening within this ancient type of the instinctive belief in immortality! Piltdown man. — In 191 2 was announced the discovery of a very ancient man from the Thames gravels at Piltdown, Sussex, England. Here again the skull was injured and partly lost, so that the question of its proper restoration has been the subject of considerable contro- versy. The material consists of portions of the cranial walls, nasal bones, a canine tooth, and part of a lower jaw. The brain-case in this instance is typically human, except for the remarkably thick cranial wails. The forehead is high and lacks the superorbital ridges of Neanderthal man and Pithecanthropus. While the skull is of com- 156 EVOLUTION, GENETICS, AND EUGENICS paratively high human type, the associated jaw and canine tooth clearly are not, and some difficulty was met in explaining their evolu- tionary discrepancy. That has apparently been answered, however, by the conclusion that the association of the material is purely acci- dental and that the jaw not only does not belong with the skull, but that it is not even human but is that of a fossil chimpanzee. That being the case, there seems to be no reason for the exclusion of the Piltdown man, who has been named Eoanthropus dawsoni, from the direct line of human ancestry. The specimen is not, perhaps, so surely dated as are those of the other European races, but it is associated with a warm-climate fauna and is generally considered to belong to the Third Interglacial stage — from 100,000 to 150,000 years old, and hence vastly more ancient than the more primitive Homo neander- thalcnsis. (See Fig. 36,5.) Cro-Magnon man. — The original finds of the men of the Cro- Magnon race, Homo sapiens, were made at Gower, Wales, and at Aurignac, France. In the latter place seventeen skeletons came to light in 1852, but were buried in the village cemetery and thus lost to science, and not until 1868, when five more skeletons were discovered at Cro-Magnon, France, was the race established. These individuals, an old man, two young men, a woman and a child, are thus the types of the race. This magnificent race is thus characterized : Skull large but narrow, with a broad face, hence disharmonic. Facial angle equalling the highest type of Homo sapiens. Jaw thick and strong, with a narrow but very prominent chin. Forehead high and orbital ridges reduced. Brain not only of high type but very large, that of the women exceeding the average male of to-day. The stature of the old man was 6 feet 4.5 inches; the average for males being 6 feet 1.5 inches, for women 5 feet 5 inches, a great dis- parity. The lower segments of the limbs were long, in contrast with the Neanderthal type, hence the men of Crd-Magnon were swift- footed, while those of Neanderthal were slow. Osborn says: "The wide, short face, the extremely prominent cheekbones, the spread of the palate and a tendency of the upper cutting teeth and incisors to project forward, and the narrow, pointed chin recall a facial type which is best seen to-day in tribes living in Asia to the north and to the south of the Himalayas. As regards their stature the Cro-Magnon race recall the Sikhs living to the south of the Himalayas. In the disharmonic proportions of the face, that is, the combination of broad cheekbones and narrow skull, they resemble the Eskimo. The THE EVOLUTION OF MAN 1 57 sum of the Cro-Magnon characters is certainly Asiatic rather than African, whereas in the Grimaldis (of which specimens have been found in association with Cro-Magnons at the Grotte des Enfants, Mentone) the sum of the characters is decidedly negroid or African." The Cr6-Magnons again show by their elaborate burial customs how old and well founded is the belief in life after death. They are supposed to be the people who left on the walls of the caverns of France and Spain the marvelous examples of Upper Palaeolithic art of which Professor Osborn's book gives so adequate a description. They lived for a while contemporaneously with the men of Neanderthal and may have contributed somewhat to the final extinction of the latter. In the course of time, however, they too declined, although to this day survivors of the race may be seen in Dordogne, at Landes, near the Garonne in Southern France, and at Lannion in Brittany. Osborn says: The decline of the Cr6-Magnons, with their artistic culture, "may have been partly due to environmental causes and the abandon- ment of their vigorous nomadic mode of life, or it may be that they had reached the end of a long cycle of psychic development We know as a parallel that in the history of many civilized races a period of great artistic and industrial development may be followed by a period of stagnation and decline without any apparent environmental cause." Europe was repopulated after Cr6-Magnon decline by later invaders from the Asiatic realm, the so-called Mediterranean narrow- headed and the Alpine broad-headed types, etc., probably differen- tiated in Asia in early Palaeolithic times. The repopulation took place in the Upper Palaeolithic. EVIDENCES OF HUMAN ANTIQUITY Great variation. — These, briefly summarized^ are, first, great variation. If man is monophyletic, that is, derived from a single prehuman species, and there is no reason to believe otherwise, he must be old, for while the adaptations to ground-dwelling after the descent from the trees were doubtless relatively rapidly acquired, the differen- tiation into the various races, due perhaps largely to climatic influ- ences rather than to any notable environmental change, must have been slowly attained. As corroborative evidence we have but to point to the mural paintings on Egyptian monuments, dating back 158 EVOLUTION, GENETICS, AND EUGENICS several thousand years, in which are depicted the Ethiopian, Caucasian, and the like, which are in some instances striking likenesses of the present-day Egyptians. Universal distribution is, in animals, another mark of antiquity: in man, it is probably less so because of his greater intelligence. And yet before transportation had become a science man's spread over land and sea was extremely slow. High intelligence as compared with that of the anthropoids is also a mark of antiquity, for the brain, especially the type of brain found in the higher human races, must have been very slow of development. Our study of fossil man shows this. Communal life, division of labor and all of the complicated interactions which it brings about, and the development of law and religions all have taken time. When we realize that Babylonian texts, twice as remote as the patriarch Abraham, give evidence of highly perfect laws and of a civilization which must have antedated their production by centuries, we gain another yet more emphatic im- pression of human antiquity. Add to all this the palaeontological evidence of man's association with various genera and numerous successive species of prehistoric animals of which he alone survives, and the evidence is complete. FUTURE OF HUMANITY Because of his intelligence and communal co-operation man is no longer subject to the laws which govern the adaptation of animals to their environment. Osborn's law of adaptive radiation, which, as we have seen, applies equally well to the insects, reptiles, and mam- mals, fails in its application to mankind; and yet man has become as thoroughly adapted to speed, flight, to the fossorial and aquatic as they; but his adaptation is artificial and to a very small extent only affects his physical frame, while theirs is natural and the stamp of environment is deeply impressed upon the organism. Man's physical evolution has virtually ceased, but in so far as any change is being effected, it is largely retrogressive. Such changes are: Reduction of hair and teeth, and of hand skill; and dulling of the senses of sight, smell, and hearing upon which active creatures depend so largely for safety. That sort of charity which fosters the physi- cally, mentally, and morally feeble, and is thus contrary to the law of natural selection, must also in the long run have an adverse effect upon the race. THE EVOLUTION OF MAN *59 Man is hardly as yet subject to Malthus' law, for while he is increasing more rapidly than any other animal, owing largely to the care of the young which makes the expectation of life of the new-born relatively very high, his migratory ability, but above all his intelli- gence, save him from the application of the law. A single new dis- covery such as that of electricity may increase his food supply and other life necessities several fold. His future evolution, in so far as it is progressive, will be mental and spiritual rather than physical, and as such will be the logical conclusion of the marvelous results of organic evolution. Sinanlhropus pekinensis A remarkable new discovery has come to light during the last few years and deserves to be added to the foregoing account by Professor Lull: the discovery of Pekin man, Sinanlhropus pekinensis. Next to Pithecanthropus in the fossil series of primitive man, and only a stage more advanced, is this newly discovered genus. A few years ago, Pro- fessor Davidson Black found some fossil teeth near Pekin, China, which, though distinctively human, were different enough from any others known to be assigned to a new genus of man. Search for further remains in the same neighborhood brought to light two lower jaws containing the kind of teeth discovered by Black. The investigation was climaxed a little later when the Chinese anthropologist, W. C. Pei, unearthed in the same neighborhood a complete cranium. At last reports, collectors had brought in parts of skeletons of at least ten indi- viduals. While Sinanlhropus is more like Pithecanthropus than any other extinct genus of man, it is a more advanced type and helps ma- terially to bridge the gap between Pithecanthropus and the higher human genera. At this writing further details about this new genus would be inappropriate, since investigations upon it are still in progress. CHAPTER XII EVIDENCES FROM GEOGRAPHIC DISTRIBUTION PRINCIPLES OF GEOGRAPHIC DISTRIBUTION Just as palaeontology may be said to be a study of the vertical distribution (distribution in time) of organisms, so geographic distribu- tion may be called a study of the horizontal distribution of organisms, on the earth's surface at any given time (spatial distribution). We are chiefly to be concerned with the present spatial distribution of animal and plant species, but equally interesting studies have been and still may be made of the horizontal or contemporaneous existence of extinct forms. Much new knowledge has been gained by combining the data of palaeontology with those of geographic distribution. In fact, neither field can be studied profitably without recourse to the other. This fact was clearly perceived by J. A. Thomson in his little manual on Evolution when he combined the two types of evidence in one chapter under the title "Evidences of Evolution from Explorer and Palaeontologist." It was a consideration of the present and of the past distribution of Edentates that led Charles Darwin to his first clear concept of descent with modification. In his voyage on the "Beagle" he found that present-day Edentates (armadillos, sloths, anteaters), a very peculiar group of archaic mammals, are practically confined to South America. When he also found that the only fossil Edentates, resem- bling but also differing from the existing types, are also confined to South America, he easily arrived at the only inference permitted by the facts: that the present Edentates are the modified descendants of the Edentates of the past. The following quotations from both an older and a recent writer will give the reader a clear idea of the ways in which the general facts of geographic distribution bear witness to the truth of the evolutionary principle. "The theory," says Wallace, 1 "which we may now take as estab- lished — that all the existing forms of life have been derived from other forms by a natural process of descent with modification, and that this same process has been in action during past geological time — should 1 From A. R. Wallace, Darwinism (1889). Used by special permission of the publishers, The Macmillan Company. r6o EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 161 enable us to give a rational account not only of the peculiarities of form and structure presented by animals and plants, but also of their grouping together in certain areas, and their general distribution over the earth's surface. "In the absence of any exact knowledge of the facts of distribution, a student of the theory of evolution might naturally anticipate that all groups of allied organisms would be found in the same region, and that, as he travelled farther and farther from any given centre, the forms of life would differ more and more from those which prevailed at the starting-point, till, in the remotest regions to which he could penetrate, he would find an entirely new assemblage of animals and plants, altogether unlike those with which he was familiar. He would also anticipate that diversities of climate would always be associated with a corresponding diversity in the forms of life. "Now these anticipations are to a considerable extent justified. Remoteness on the earth's surface is usually an indication of diversity in the fauna and flora, while strongly contrasted climates are always accompanied by a considerable contrast in the forms of life. But this correspondence is by no means exact or proportionate, and the converse propositions are often quite untrue. Countries which are near to each other often differ radically in their animal and vegetable productions; while similarity of climate, together with moderate geographical proximity, are often accompanied by marked diversi- ties in the prevailing forms of life. Again, while many groups of animals — genera, families, and sometimes even orders — are confined to limited regions, most of the families, many genera, and even some species are found in every part of the earth. An enumeration of a few of these anomalies will better illustrate the nature of the problem we have to solve. "As examples of extreme diversity, notwithstanding geographical proximity, we may adduce Madagascar and Africa, whose animal and vegetable productions are far less alike than are those of Great Britain and Japan at the remotest extremities of the great northern continent; while an equal, or perhaps even a still greater, diversity exists between Australia and New Zealand. On the other hand, Northern Africa and South Europe, though separated by the Mediterranean Sea, have faunas and floras which do not differ from each other more than do the various countries of Europe. As a proof that similarity of climate and general adaptability have had but a small part in determining the forms of life in each country, we have the fact of the enormous increase 1 62 EVOLUTION, GENETICS, AND EUGENICS of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the common sparrow in North America, though in none of these cases are the animals natives of the countries in which they thrive so well. And lastly, in illustration of the fact that allied forms are not always found in adjacent regions, we have the tapirs, which are found only on opposite sides of the globe, in tropical America and the Malayan Islands; the camels of the Asiatic deserts, whose nearest allies are the llamas and alpacas of the Andes; and the marsupials, only found in Australia and on the opposite side of the globe in America. Yet, again, although mammalia may be said to be universally distributed over the globe, being found abundantly on all the continents and on a great many of the larger islands, yet they are entirely wanting in New Zealand, and in a considerable number of other islands which are, nevertheless, per- fectly able to support them when introduced. "Now most of these difficulties can be solved by means of well- known geographical and geological facts. When the productions of remote countries resemble each other, there is almost always conti- nuity of land with similarity of climate between them. When adjacent countries differ greatly in their productions, we find them separated by a sea or strait whose great depth is an indication of its antiquity or permanence. When a group of animals inhabits two countries or regions separated by wide oceans, it is found that in past geological times the same group was much more widely distributed, and may have reached the countries it inhabits from an intermediate region in which it is now extinct. We know, also, that countries now united by land were divided by arms of the sea at a not very remote epoch, while there is good reason to believe that others now entirely isolated by a broad expanse of sea were formerly united and formed a single land area. There is also another important factor to be taken account of in considering how animals and plants have acquired their present peculiarities of distribution, — changes of climate. We know that quite recently a glacial epoch extended over much of what are now the temperate regions of the northern hemisphere, and that consequently the organisms which inhabit those parts must be, comparatively speaking, recent immigrants from more southern lands. But it is a yet more important fact that, down to middle Tertiary times at all events, an equable temperate climate, with a luxuriant vegetation, extended to far within the Arctic circle, over what are now barren wastes, covered for ten months of the year with snow and ice. The EVIDENCES FROM GEOGRAPHIC DISTRIBUTION l63 Arctic zone has, therefore, been in past times capable of supporting almost all of the forms of life of our temperate regions; and we must take account of this condition of things whenever we have to specu- late on the possible migration of organisms between the old and new continents." "Many of the facts of distribution," says Shull, 1 "are capable of interpretation by the assumption that evolution has operated with the other factors. If each kind of animal has arisen from a pre-existing kind, then each group of related animals must have had an ancestral form, and if the component parts of the groups are widespread the range of the ancestral form may be considered to be the center of dispersal of the group. The facts of distribution can apparently be interpreted only on this basis. "Accepting evolution, along with the other factors which can be recognized, the method of distribution is generally conceived to be as follows. The ancestral form tends to spread in all directions. In some directions it is limited by unfavourable conditions either through- out its life or for some time. In other directions it extends its range. Anywhere within its range new types of individuals may arise through the process of evolution. These new types may be fitted to occupy new regions, and if they are formed near the limits of the range they may find opportunity to spread into areas which are inaccessible to the unaltered members of the species. Thus may arise recognizably distinct forms coincident in range with certain environmental condi- tions. If particular forms, or the individuals of a single form, are accidentally (or possibly by sporadic migration) transferred across barriers the distribution of the group becomes discontinuous. If these processes have been going on for a long time, that is, if the common ancestors of a group of forms existed long ago, the range may have had time to become very extensive, or its discontinuity very marked. If, contrariwise, the ancestors were comparatively recent, the range is likely to be much smaller. For this reason, groups that have diverged far enough to have attained the rank of families are on the whole more widespread than those so nearly allied as to be con- sidered genera. Should the environment become altered within a given range, the occupying form might be driven from it or destroyed. •From A. F. Shull, Principles of Animal Biology (copyright 1920). Used b\ special permission of the publishers, The McGraw-Hill Book Company. 1 64 EVOLUTION, GENETICS, AND EUGENICS If the environment in a region adjoining a range should change in a favourable manner, the range might be extended at that point without any alteration on the part of the animals. "The distribution of animals is inferred to be in harmony with this method, which involves, it will be noted, the factors of migration, evolution, physiological and morphological dependence upon the environment, the diversity and changeableness of the earth's surface, and extinction; and in this manner are explained the differences in geographical position, differences in size of range, differences in the continuity of range and the fact that ranges are at first continuous, differences in physical and biological conditions which characterize the ranges of different forms, and the geographical proximity of apparently related forms." SOME OF THE MORE SIGNIFICANT FACTS ABOUT THE DISTRIBUTION OF ANIMALS THE FAUNA 01 OCEANIC ISLANDS 1 GEORGE JOHN ROMANES Turning now from aquatic organisms to terrestrial, the body of facts from which to draw is so large, that I think the space at my dis- posal may be best utilized by confining attention to a single division of them — that, namely, which is furnished by the zoological study of oceanic islands. In the comparatively limited — but in itself extensive — class of facts thus presented, we have a particularly fair and cogent test as between the alternative theories of evolution and creation. For where we meet with a volcanic island, hundreds of miles from any other land, and rising abruptly from an ocean of enormous depth, we may be quite sure that such an island can never have formed part of a now submerged continent. In other words, we may be quite sure that it always has been what it now is — an oceanic peak, separated from all other land by hundreds of miles of sea, and therefore an area supplied by nature for the purpose, as it were, of testing the rival theories of creation and evolution. For, let us ask, upon these tiny insular specks of land what kind of life should we expect to find ? To this question the theories of special creation and of gradual evolution would agree in giving the same answer up to a certain point. For both theories would agree in supposing that these islands would, at all 1 From G. J. Romanes, Darwin and after Darwin (copyright 1892). Used by special permission of The Open Court Publishing Company. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 165 events in large part, derive their inhabitants from accidental or occa- sional arrivals of wind-blown or water-floated organisms from other countries — especially, of course, from the countries least remote. But, after agreeing upon this point, the two theories must part company in their anticipations. The special-creation theory can have no reason to suppose that a small volcanic island in the midst of a great ocean should be chosen as the theatre of any extraordinary creative activity, or for any particularly rich manufacture of peculiar species to be found nowhere else in the world. On the other hand, the evolution theory would expect to find that such habitats are stocked with more or less peculiar species. For it would expect that when any organisms chanced to reach a wholly isolated refuge of this kind, their descendants should forthwith have started upon an independent course of evolu- tionary history. Protected from intercrossing with any members of their parent species elsewhere, and exposed to considerable changes in their conditions of life, it would indeed be fatal to the general theory of evolution if these descendants, during the course of many genera- tions, were not to undergo appreciable change. It has happened on two or three occasions that European rats have been accidentally imported by ships upon some of these islands, and even already it is observed that their descendants have undergone a slight change of appearance, so as to constitute them what naturalists call local varieties. The change, of course, is but slight, because the time allowed for it has been so short. But the longer the time that a colony of a species is thus completely isolated under changed condi- tions of life the greater, according to the evolution theory, should we expect the change to become. Therefore, in all cases where we happen to know, from independent evidence of a geological kind, that an oceanic island is of very ancient formation, the evolution theory would expect to encounter a great wealth of peculiar species. On the other hand, as I have just observed, the special-creation theory can have no reason to suppose that there should be any correlation between the age of an oceanic island and the number of peculiar species which it may be found to contain. Therefore, having considered the principles of geographical distri- bution from the widest or most general point of view, we shall pass to the opposite extreme, and consider exhaustively, or in the utmost possible detail, the facts of such distribution where the conditions are best suited to this purpose — that is, as I have already said, upon oceanic islands, which may be metaphorir^Tlv regarded as having been 1 66 EVOLUTION, GENETICS, AND EUGENICS formed by nature for the particular purpose of supplying naturalists with a crucial test between the theories of creation and evolution. The material upon which my analysis is to be based will be derived from the most recent works upon geographical distribution — espe- cially from the magnificent contributions to this department of science which we owe to the labours of Mr. Wallace. Indeed, all that follows may be regarded as a condensed filtrate of the facts which he has collected. Even as thus restricted, however, our subject matter would be too extensive to be dealt with on the present occasion, were we to attempt an exhaustive analysis of the floras and faunas of all oceanic islands upon the face of the globe. Therefore, what I propose to do is to select for such exhaustive analysis a few of what may be termed the most oceanic of oceanic islands — that is to say, those oceanic islands which are most widely separated from main- lands, and which, therefore, furnish the most unquestionable of test cases as between the theories of special creation and genetic descent. Azores. — A group of volcanic islands, nine in number, about 900 miles from the coast of Portugal, and surrounded by ocean depths of 1,800 to 2,500 fathoms. There is geological evidence that the origin of the group dates back at least as far as Miocene times. There is a total absence of all terrestrial Vertebrata, other than those which are known to have been introduced by man. Flying animals, on the other hand, are abundant : namely, 53 species of birds, one species of bat, a few species of butterflies, moths and hymenoptera, with 74 species of indigenous beetles. All these animals are unmodified European species, with the exception of one bird and many of the beetles. Of the 74 indigenous species of the latter, 36 are not found in Europe; but 19 are natives of Madeira or the Canaries, and 3 are American, doubtless transplanted by drift-wood. The remaining 14 species occur nowhere else in the world, though for the most part they are allied to other European species. There are 69 knowD species of land-shells, of which 37 are European, and 32 peculiar, though all allied to European forms. Lastly, there are 480 known species of plants of which 40 are peculiar, though allied to European species. Bermudas. — A small volcanic group of islands, 700 miles from North Carolina. Athough there are about 100 islands in the group, their total area does not exceed 50 square miles. The group is sur- rounded by water varying in depth from 2,500 to 3,800 fathoms. The EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 167 only terrestrial Vertebrate (unless the rats and mice are indigenous) is a lizard allied to an American form, but specifically distinct from it, and therefore a solitary species which does not occur anywhere else in the world. None of the birds or bats are peculiar, any more than in the case of the Azores; but, as in that case, a large percentage of the land-shells are so — namely, at least one quarter of the whole. Neither the botany nor the entomology of this group has been worked out; but I have said enough to show how remarkably parallel are the cases of these two volcanic groups of islands situated in different hemispheres but at about the same distance from large continents. In both there is an extraordinary paucity of terrestrial Vertebrata, and of any peculiar species of bird or beast. On the other 'hand, there is in both a marvellous wealth of peculiar species of insects and land-shells. Now these correlations are all abundantly intelligible. It is a difficult matter for any terrestrial animal to cross 900, or even 700 miles of ocean : therefore only one lizard has succeeded in doing so in one of the two parallel cases; and living cut off from intercrossing with its parent form, the descendants of that lizard have become modified so as to constitute a peculiar species. But it is more easy for large flying animals to cross those distances of ocean : consequently, there is only one instance of a peculiar species of bird or bat — namely, a bull-finch in the Azores, which, being a small land-bird, is not likely ever to have had any other visitors from its original parent species coming over from Europe to keep up the original breed. Lastly, it is very much more easy for insects and land-mollusca to be conveyed to such islands by wind and floating timber than it is for terrestrial mammals, or even than it is for small birds and bats; but yet such means of transit are not sufficiently sure to admit of much recruiting from the mainland for the purpose of keeping up the specific types. Consequently, the insects and the land-shells present a much greater proportion of peculiar species — namely, one half and one fourth of the land-shells in the one case, and one eighth of the beetles in the other. All these cor- relations, I say, are abundantly intelligible on the theory of evolution; but who shall explain, on the opposite theory, why orders of beetles and land-mollusca should have been chosen from among all other animals for such superabundant creation on oceanic islands, so that in the Azores alone we find no less than 32 of the one and 14 of the other? And, in this connection, I may again allude to the peculiar species of beetles in the island of Madeira. Here there are an enor- mous number of peculiar species, though they are nearly all related to, 1 68 EVOLUTION, GENETICS, AND EUGENICS or included under the same genera, as beetles on the neighboring conti- nent. Now, as we have previously seen, no less than 200 of these species have lost the use of their wings. Evolutionists explain this remarkable fact by their general laws of degeneration under disuse, and the operation of natural selection, as will be shown later on; but it is not so easy for special creationists to explain why this enormous number of peculiar species of beetles should have been deposited on Madeira, all allied to beetles on the nearest continent, and nearly all deprived of the use of their wings. And similarly, of course, with all the peculiar species of the Bermudas and the Azores. For who will explain, on the theory of independent creation, why all the peculiar species, both of animals and plants, which occur on the Bermudas should so unmistakably present American affinities, while those which occur on the Azores no less unmistakably present European affinities ? But to proceed to other, and still more remarkable, cases. The Galapagos Islands. — This archipelago is of volcanic origin, situated under the equator between 500 and 600 miles from the West Coast of South America. The depth of the ocean around them varies from 2,000 to 3,000 fathoms or more. This group is of peculiar interest, from the fact that it was the study of its fauna which first suggested to Darwin's mind the theory of evolution. I will, therefore, begin by quoting a short passage from his writings upon the zoological relations of this particular fauna. "Here almost every product of the land and of the water bears the unmistakable stamp of the American continent. There are twenty-six land birds; of these, twenty-one, or perhaps twenty-three, are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with the other animals, and with a large pro- portion of the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, feels that he is standing on American land. Why should this be so? Why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which closely resembles the EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 169 conditions of the South American coast; in fact, there is a considerable dissimilarity in all these respects. On the other hand, there is a con- siderable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagoes; but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts such as these admit of no sort of explanation on the ordi- nary view of independent creation; whereas in the view here main- tained it is obvious that the Galapagos Islands would be likely to receive colonists from America, and the Cape de Verde Islands from Africa; such colonists would be liable to modification — the principle of inheritance still betraying their original birthplace. " The following is a synopsis of the fauna and flora of this archi- pelago, so far as at present known. The only terrestrial vertebrates are two peculiar species of land-tortoise, and one extinct species; five species of lizards, all peculiar — two of them so much so as to constitute a peculiar genus; — and two species of snakes, both closely allied to South American forms. Of birds there are 57 species, of which no less than 38 are peculiar; and all the non-peculiar species, except one, belong to aquatic tribes. The true land-birds are represented by 31 species, of which all, except one, are peculiar; while more than half of them go to constitute peculiar genera. Moreover, while they are all unquestionably allied to South American forms, they present a beautiful series of gradations, "from perfect identity with the conti- nental species, to genera so distinct that it is difficult to determine with what forms they are most nearly allied; and it is interesting to note that this diversity bears a distinct relation to the probabilities of, and facilities for, migration to the islands. The excessively abun- dant rice-bird, which breeds in Canada, and swarms over the whole United States, migrating to the West Indies and South America, visiting the distant Bermudas almost every year, and extending its range as far as Paraguay, is the only species of land-bird which remains completely unchanged in the Galapagos; and we may therefore con- clude that some stragglers of the migrating host reach the islands sufficiently often to keep up the purity of the breed" [Wallace]. Again, of the thirty peculiar land-birds, it is observable that the more they differ from any other species or genera on the South American continent, the more certainly are they found to have their nearest relations among those South American forms which have the 170 EVOLUTION, GENETICS, AND EUGENICS more restricted range, and therefore the least likely to have found their way to the islands with any frequency. The insect fauna of the Galapagos Islands is scanty, and chiefly composed of beetles. These number 35 species, which are nearly all peculiar, and in some cases go to constitute peculiar genera. The same remarks apply to the twenty species of land-shells. Lastly, of the total number of flowering plants (332 species) more than one half (174 species) are peculiar. It is observable in the case of these peculiar species of plants — as also of the peculiar species of birds — that many of them are restricted to single islands. It is also observable that with regard both to the fauna and flora, the Galapagos Islands as a whole are very much richer in peculiar species than either the Azores or Bermudas, notwithstanding that both the latter are considerably more remote from the nearest continents. This differ- ence, which at first sight appears to make against the evolutionary interpretation, really tends to confirm it. For the Galapagos Islands are situated in a calm region of the globe, unvisited by those periodic storms and hurricanes which sweep over the North Atlantic, and which every year convey some straggling birds, insects, seeds, etc., to the Azores and Bermudas. Notwithstanding their somewhat greater isolation geographically, therefore, the Azores and Bermudas are really less isolated biologically than are the Galapagos Islands; and hence the less degree of peculiarity on the part of their endemic species. But, on the theory of special creation, it is impossible to understand why there should be any such correlation between the prevalence of gales and a comparative inertness of creative activity. And, as we have seen, it is equally impossible on this theory to under- stand why there should be a further correlation between the degree of peculiarity on the part of the isolated species, and the degree in which their nearest allies on the mainland are there confined to narrow ranges, and therefore less likely to keep up any biological communi- cation with the islands. St. Helena. — A small volcanic island, ten miles long by eight wide, situated in mid-ocean, 1,100 miles from Africa, and 1,800 from South America. It is very mountainous and rugged, bounded for the most part by precipices, rising from ocean depths of 17,000 feet, to a height above the sea-level of nearly 3,000. When first discovered it was richly clothed with forests; but these were all destroyed by human agency during the i6th, 17th, and 18th centuries. The records of civili- zation present no more lamentable instance of this kind of destruction. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 171 From a merely pecuniary point of view the abolition of these pri- meval forests has proved an irreparable loss; but from a scientific point of view the loss is incalculable. These forests served to harbour countless forms of life, which extended at least from the Miocene age, and which, having found there an ocean refuge, survived as the last remnants of a remote geological epoch. In those days, as Mr. Wallace observes, St. Helena must have formed a kind of natural museum or vivarium of archaic species of all classes, the interest of which we can now only surmise from the few remnants of those remnants, which are still left among the more inaccessible portions of the mountain peaks and crater edges. These remnants of remnants are as follows: There is a total absence of all indigenous mammals, reptiles, fresh-water fish, and true land-birds. There is, however, a species of plover, allied to one in South Africa; but it is specifically distinct, and therefore peculiar to the island. The insect fife, on the other hand, is abundant. Of beetles, no less than 129 species are believed to be aboriginal, and, with one single exception, the whole number are peculiar to the island. "But in addition to this large amount of specific peculiarity (perhaps unequalled anywhere else in the world) the beetles of this island are remarkable for their generic isolation, and for the altogether exceptional proportion in which the great divisions of the order are represented. The species belong to 39 genera, of which no less than 25 are peculiar to the island; and many of these are such isolated forms that it is impossible to find their allies in any particular country" [Wallace]. More than two-thirds of all the species belong to one group of weevils — a circumstance which serves to explain the great wealth of beetle-population, the weevils being beetles which live in wood, and St. Helena having been originally a densely wooded island. This circumstance is also in accordance with the view that the peculiar insect fauna has been in large part evolved from ancestors which reached the island by means of floating timber; for, of course, no explanation can be suggested why special creation of this highly peculiar insect fauna should have run so disproportionately into the production of weevils. About two-thirds of the whole number of beetles, or over 80 species, show no close affinity with any existing insects, while the remaining third have some relations, though often very remote, with European and African forms. That this high degree of peculiarity is due to high antiquity is further indicated, according to our theory, by the large number of species which some of the types comprise. Thus, the 54 species of Cossonidae may be 172 EVOLUTION, GENETICS, AND EUGENICS referred to three types ; the 1 1 species of Bembidium form a group by themselves; and the Heteromera form two groups. "Now, each of these types may well be descended from a single species, which origi- nally reached the island from some other land; and the great variety of generic and specific forms into which some of them have diverged is an indication, and to some extent a measure, of the remoteness of their origin" [Wallace]. But, on the counter-supposition that all these 128 peculiar species were separately created to occupy this particular island, it is surely unaccountable that they should thus present such an arborescence of natural affinities amongst themselves. Passing over the rest of the insect fauna, which has not yet been sufficiently worked out, we next find that there are only 20 species of indigenous land-shells — which is not surprising when we remember by what enormous reaches of ocean the land is surrounded. Of these 20 species no less than 13 have become extinct, three are allied to Euro- pean species, while the rest are so highly peculiar as to have no near allies in any other part of the globe. So that the land-shells tell exactly the same story as the insects. Lastly, the plants likewise tell the same story. The truly indige- nous flowering plants are about 50 in number, besides 26 ferns. Forty of the former and ten of the latter are peculiar to the island, and, as Sir Joseph Kooker tells us, "cannot be regarded as very close specific allies of any other plants at all." Seventeen of them belong to peculiar genera, and the others all differ so markedly as species from their congeners, that not one comes under the category of being an insular form of a continental species. So that with respect to its plants, no less than with respect to its animals, we find that the island of St. Helena constitutes a little world of unique species, allied among themselves, but diverging so much from all other known forms that in many cases they constitute unique genera. Sandwich Islands. — These are an extensive group of islands, larger than any we have hitherto considered — the largest of the group being about the size of Devonshire. The entire archipelago is vol- canic, with mountains rising to a height of nearly 14,000 feet. The group is situated in the middle of the North Pacific, at a distance of considerably over 2,000 miles from any other land, and surrounded by enormous ocean depths. The only terrestrial vertebrates are two lizards, one of which constitutes a peculiar genus. There are 24 aquatic birds, five of which are peculiar; four birds of prey, two of which are peculiar; and 16 land-birds, all of which are peculiar. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 173 Moreover, these 16 land-birds constitute no less than 10 peculiar genera, and even one peculiar family of five genera. This is an amount of peculiarity far exceeding that of any other islands, and, of course, corresponds with the great isolation of this archipelago. The only other animals which have here been carefully studied are the land- shells, and these tell the same story as the birds. For there are no less than 400 species which are all, without any exception, peculiar; while about three-quarters of them go to constitute peculiar genera. Again, of the plants, 620 species are believed to be endemic; and of these 377 are peculiar, yielding no less than 39 peculiar genera. THE FAUNA OF CONTINENTAL ISLANDS — MADAGASCAR AND NEW ZEALAND 1 A. R. WALLACE The two exceptions just referred to are Madagascar and New Zealand, and all the evidence goes to show that in these cases the land connection with the nearest continental area was very remote in time. The extraordinary isolation of the productions of Madagascar — almost all the most characteristic forms of mammalia, birds, and reptiles of Africa being absent from it — renders it certain that it must have been separated from that continent very early in the Tertiary, if not as far back as the latter part of the Secondary period; and this extreme antiquity is indicated by a depth of considerably more than a thousand fathoms in the Mozambique Channel, though this deep portion is less than a hundred miles wide between the Comoro Islands and the main- land. Madagascar is' the only island on the globe with a fairly rich mammalian fauna which is separated from a continent by a depth greater than a thousand fathoms; and no other island presents so many peculiarities in these animals, or has preserved so many lowly organised and archaic forms. The exceptional character of its pro- ductions agrees exactly with its exceptional isolation by means of a very deep arm of the sea. New Zealand possesses no known mammals and only a single species of batrachian; but its geological structure is perfectly conti- nental. There is also much evidence that it does possess one mammal, although no specimens have been yet obtained. Its reptiles and birds are highly peculiar and more numerous than in any truly oceanic island. Now the sea which directly separates New Zealand from Australia is more than 2,000 fathoms deep, but in a north-west direc- 1 From A. R. Wallace, Darwinism (copyright 1889). Used by special permis- sion of the publishers, The Macmillan Company. 174 EVOLUTION, GENETICS, AND EUGENICS tion there is an extensive bank under 1,000 fathoms, extending to and including Lord Howe's Island, while north of this are other banks of the same depth, approaching towards a submarine extension of Queensland on the one hand, and New Caledonia on the other, and altogether suggestive of a land union with Australia at some very remote period. Now the peculiar relations of the New Zealand fauna and flora with those of Australia and of the tropical Pacific Islands to the northward indicate such a connection, probably during the Cre- taceous period; and here, again, we have the exceptional depth of the dividing sea and the form of the ocean bottom according well with the altogether exceptional isolation of New Zealand, an isolation which has been held by some naturalists to be great enough to justify its claim to be one of the primary Zoological Regions. THE DISTRIBUTION OF MARSUPIALS' A. R. WALLACE This singular and lowly organised type of mammals constitutes almost the sole representative of the class in Australia and New Guinea, while it is entirely unknown in Asia, Africa, or Europe. It reappears in America, where several species of opossums are found; and it was long thought necessary to postulate a direct southern con- nection of these distant countries, in order to account for this curious fact of distribution. When, however, we look to what is known of the geological history of the marsupials the difficulty vanishes. In the Upper Eocene deposits of Western Europe the remains of several animals closely allied to the American opossums have been found; and as, at this period, a very mild climate prevailed far up into the arctic regions, there is no difficulty in supposing that the ancestors of the group entered America from Europe or Northern Asia during early Tertiary times. But we must go much further back for the origin of the Australian marsupials. All the chief types of the higher mammalia were in existence in the Eocene, if not in the preceding Cretaceous period, and as we find none of these in Australia, that country must have been finally separated from the Asiatic continent during the Secondary or Mesozoic period. Now during that period, in the Upper and the Lower Oolite and in the still older Trias, the jaw-bones of numerous small mammalia have been found, forming eight distinct genera, which 1 From A. R. Wallace, Darwinism (copyright 1889). Used by special per- mission of the publishers. The Macmillan Company. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 175 are believed to have been either marsupials or some allied lowly forms. In North America also, in beds of the Jurassic and Triassic formations, the remains of an equally great variety of these small mammalia have been discovered; and from the examination of more than sixty speci- mens, belonging to at least six distinct genera, Professor Marsh is of the opinion that they represent a generalised type, from which the more specialised marsupials and insectivora were developed. From the fact that very similar mammals occur both in Europe and America at corresponding periods, and in beds which represent a long succession of geological time, and that during the whole of this time no fragments of any higher forms have been discovered, it seems probable that both the northern continents (or the larger portion of their area) were then inhabited by no other mammalia than these, with perhaps other equally low types. It was, probably, not later than the Jurassic age when some of these primitive marsupials were able to enter Australia, where they have since remained almost com- pletely isolated; and, being free from the competition of higher forms, they have developed into the great variety of types we now behold there. These occupy the place, and have to some extent acquired the form and structure of distinct orders of the higher mammals — the rodents, the insectivora, and the carnivora — while still preserving the essential characteristics and lowly organisation of the marsupials. At a much later period — probably in late Tertiary times — the ances- tors of the various species of rats and mice which now abound in Australia, and which, with the aerial bats, constitute its only forms of placental mammals, entered the country from some of the adjacent islands. For this purpose a land connection was not necessary, as these small creatures might easily be conveyed among the branches or in the crevices of trees uprooted by floods and carried down to the sea, and then floated to a shore many miles distant. That no actual land connection with, or very close approximation to, an Asiatic island had occurred in recent times, is sufficiently proved by the fact that no squirrel, pig, civet, or other widespread mammal of the Eastern hemisphere has been able to reach the Australian continent. THE DISTRIBUTION OF BIRDS* A. R. WALLACE These vary much in their powers of flight, and their capability of traversing wide seas and oceans. Many swimming and wading birds 1 From A. R. Wallace, Darwinism (copyright 1891). Used by special per- mission of the publishers, The Macmillan Company. 176 EVOLUTION, GENETICS, AND EUGENICS can continue long on the wing, fly swiftly, and have, besides, the power of resting safely on the surface of the water. These would hardly be limited by any width of ocean, except for the need of food; and many of them, as the gulls, petrels, and divers, find abundance of food on the surface of the sea itself. These groups have a wide distri- bution across the oceans; while waders — especially plovers, sandpipers, snipes, and herons — are equally cosmopolitan, travelling along the coasts of all the continents, and across the narrow seas which separate them. Many of these birds seem unaffected by climate, and as the organisms on which they feed are especially abundant on arctic, tem- perate, and tropical shores, there is hardly any limit to the range even of some of the species. Land-birds are much more restricted in their range, owing to their usually limited powers of flight, their inability to rest on the surface of the sea or to obtain food from it, and their greater specialisation, which renders them less able to maintain themselves in the new coun- tries they may occasionally reach. Many of them are adapted to live only in woods, or in marshes, or in deserts; they need particular kinds of food or a limited range of temperature; and they are adapted to cope only with the special enemies or the particular group of competi- tors among which they have been developed. Such birds as these may pass again and again to a new country, but are never able to establish themselves in it; and it is this organic barrier, as it is termed, rather than any physical barrier, which, in many cases, determines the presence of a species in one area and its absence from another. We must always remember, therefore, that, although the presence of a species in a remote oceanic island clearly proves that its ancestors must at one time have found their way there, the absence of a species does not prove the contrary, since it also may have reached the island, but have been unable to maintain itself, owing to the inorganic or organic conditions not being suitable to it. This general principle applies to all classes of organisms, and there are many striking illus- trations of it. In the Azores there are eighteen species of land-birds which are permanent residents, but there are also several others which reach the islands almost every year after great storms, but have never been able to establish themselves. In Bermuda the facts are still more striking, since there are only ten species of resident birds, while no less than twenty other species of land-birds, and more than a hundred species of waders and aquatics are frequent visitors, often in great numbers, but are never able to establish themselves. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 177 SUMMARY OF MAMMALIAN DISPERSAL 1 HANS GADOW Australia as the earliest great mass of land permanently severed from the rest is in almost undisturbed possession of the lowest mam- mals. It is the sole refuge of the monotremes, and the marsupials have narrowly escaped a similar fate. They take us to the next independent continent, South America. This had three chances, or epochs, of being stocked with mammals. Within the Cretaceous period it seems to have received its marsupial stock from the north, the pro- genitors of all modern marsupials. A second influx during the early Tertiary brought edentates and rodents as its first Placentals from Africa, and those queer Ungulates, the Toxodonts and Pyrotheria, unless we prefer to look upon these Eocene extinct orders as truly aboriginal to South America, when this was still continuous with the ancient Brazil- Afro-Indian Gondwanaland. The third and last inroad came once more from the north, when with the close of the Miocene permanent connection with North America was re-established. This brought the modern odd-toed and pair-toed Ungulates, with dogs, cats and bears in their wake, and lastly man. There remains the huge North World. Eurasia and North America have always formed a wide circumpolar ring, which repeatedly broke and joined again. Whatever group of terrestrial creatures was developed in the eastern, Asiatic, half, was sure to turn up in the western, and vice versa. Lastly, the mysterious African continent. It began originally as the centre of the ancient equatorial South World; it has lost these con- nections and has become joined to the northland, after many vicissi- tudes. It is therefore most difficult to apportion its fauna rightly; moreover for fossils it is almost a blank, except Egypt. It must have had some share in the evolution of mammals, like edentates, rodents, insectivores, hyrax, elephants, sirenians and lemurs, all groups with an ancient stamp. But what share it had, against Eurasia, in the development of say ungulates, carnivores, monkeys, we do not know. Not much is likely to have originated in Europe; the elephants, rhinos, hippos, lions and hyaenas were migrants rather from than to Africa, rarely across some Mediterranean bridge, usually by Asia Minor. The more dominant forms of our present fauna have originated, to use an expression of Darwin's, "in the larger areas and more efficient 1 From Hans Gadow, Wanderings of Animals (1913), Cambridge University Press. 178 EVOLUTION, GENETICS, AND EUGENICS workshops of the north," and the balance is in favour of Asia as the cradle of modern mammals. Is it an idle dream to think of the future ? A survey of the past reveals the vanishing of whole faunas from extensive countries, which were then repeopled by other forms from elsewhere. What has happened before, may happen in times to come. Countless groups, once flourishing, are no more ; many others have had their day and are now on the decline, whilst others are flourishing now, are even in the increase and seem to have a future before them. Such favoured assemblies are the toads and frogs, lizards and snakes, Passerine birds and rodents, mostly the small-sized members of their tribes; the days of giants are past. All this has happened in the natural course of events, without the influence of man, who only within most recent times has become the most potent and destructive factor to the ancient faunas of the world. SUMMARY OF THE ARGUMENT FOR EVOLUTION AS BASED ON GEOGRAPHIC DISTRIBUTION On the hypothesis of special creation or on any other hypothesis except evolution that has even been suggested, the extremely intricate patchwork of animal and plant distribution remains an unsolvable picture puzzle, without rhyme or reason. When this puzzle is attacked with the aid of the evolutionary idea, the key to the whole maze is furnished and the difficulties clear up with remarkable ease. The whole hodgepodge makes sense and we can understand many pre- viously irreconcilable facts. In no field does the working hypothesis of evolution work to such advantage as in this field. On the basis that a species arises at one place, spreads out over large areas, becoming modified as it goes, that new species are formed from old through modification after isolation from the parent-stock, how do the facts of distribution look when examined in detail ? i. Cosmopolitan groups, those with the widest distribution, are those to whom no barriers are sufficient to check migration, e.g., strong fliers, Man, earthworms carried by Man. 2. Restricted groups are usually those to which barriers are readily set up and are frequently the last remnants of a formerly successful fauna or flora, which continue to survive only in some restricted area where the conditions are rather more favorable than elsewhere. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 179 3. The study of the distribution of species belonging to a single genus reveals that the more primitive or generalized species occupy a central position and the most specialized species are at the outer boundaries of the distributional area. 4. The faunas and floras of continental islands are just what we should expect on the basis that there was at one time a land connection with the nearest continent; that at this time the faunas and floras were the same on both island and continent; that, later, the continent and island were separated by an impassable barrier of ocean ; and that the inhabitants of the two bodies evolved separately. 5. The faunas and floras of oceanic islands are like those of the nearest mainland and are of those types, for the most part, that might most readily have been blown there by the wind or carried on floating debris. 6. The conclusions arrived at by students of geographic distribu- tion, past and present, as to the existence of former land connections, now broken, are borne out by the independent findings of geologists and geographers. PART III THE MECHANISM OF EVOLUTION (GENETICS) . CHAPTER XIII INTRODUCTORY STATEMENT THE NATURE AND SCOPE OF GENETICS The validity of the general principle of evolution rests on the kind of evidence presented in previous chapters. Volumes could be written giving further evidence of the same sort. Very few thoughtful persons, once confronted with these evidences, fail to be convinced as to the reality of evolution. It is one thing to know that evolution has occurred and has fol- lowed certain courses, but quite another thing to understand what forces underly the process. A going process must have causes, and it is our purpose in this section of the course to present an account of what we know, or what we think we know, about the causes of organic evolution. Our method of study is one that depends on the validity of the doctrine of uniformitarianism. Exactly as in the science of geology the method of investigation is that of studying in detail the changes goiig on toda} r , of assuming that present changes are of the same na- ture as changes in the past, and that the past may be interpreted in terms of what we discover about the present. Thus, long series of generations of rapidly breeding animals and plants are studied in- tensively over periods of years, some species having been bred for over twenty-five years, or for at least seven hundred generations, a period equivalent in number of generations to 25,000 years of human life. Hundreds of millions of individuals have been passed in review before the keenly trained eyes of an army of competent investigators, all on the lookout for the slightest change from the normal. Any observed change is then followed through subsequent generations to determine whether it is hereditary and how it affects the success of individuals possessing it. Studies of this sort, together with examinations of the germ cells to see whether or not correlated changes have occurred in their materials, and, mathematical calculations of the relative fre- quencies with which different characters occur in combination with one another, have led to an understanding of the mechanism of evolu- tion far more complete and detailed than anyone a decade or so ago could possibly have hoped to attain. The experimental and analytical study of the processes and mech- 183 1 84 EVOLUTION, GENETICS, AND EUGENICS anisms of evolution is the province of that branch of evolutionary biology known as Genetics. Three principal methods of attack upon the problems of genetics are as follows: a) Experimental breeding. — This method, first carried out system- atically by Mendel, consists of breeding together two individuals dif- fering in certain well-defined characters and of determining the ratios in which the contrasting parental characters reappear in successive generations of descendants. Mendel's method has been extremely fruitful and in connection with the second method, that of cytology, has thrown a flood of fight on the actual workings of the mechanisms of evolution. b) Cytology. — This method involves the microscopic study of the germ cells, especially during the period of reproduction. The ob- served changes that go on in connection with sexual reproduction have been found to constitute a mechanism adequate to explain the peculiar hereditary regularities constituting Mendel's laws of heredity. Also it has been found that most aberrations in heredity are associated with correlated aberrations in the mechanism of reproduction. Experi- mental breeding and cytology have been most intimate and successful collaborators. c) Statistical analysis. — -While experimental breeding and cytology lay stress upon the type of change or the mode of heredity in in- dividuals, statistical methods deal chiefly with group changes and with the characteristics of populations. Often evolutionary trends or group changes, quite undetectable in separate individuals, can be detected by statistical methods. Also, the statistical consequences of a cer- tain type of change in a few or many individuals can be predicted for any given number of generations. While the methods of statistical analysis are as a rule too difficult for any but the expert, they are be- coming more and more essential as a part of the technique of genetics. d) Observations of changes going on in nature. — Attempts are made to discover evidences of changes in nature equivalent to those observed in the laboratory, and there is now considerable evidence favoring the conclusion that laboratory observations are reliable cri- teria of what takes place in nature. PREREQUISITES FOR THE STUDY OF GENETICS In order profitably to pursue the study of genetics, one must first understand the fundamentals of biology in general, for the mechanism INTRODUCTORY STATEMENT 185 of evolution is largely within the living organism itself. One must be familiar with the units of life: cells, organisms, generations. One must understand the methods of reproduction and the mechanisms of heredity and variation resident in the cellular components of the organism. Also, one must never forget that organisms live and grow and reproduce only if in an appropriate environment and that the environment has much to do with the expression of hereditary char- acters. In addition, the environment acts as a guiding factor, direct- ing the course of evolution along lines of fitness or adaption. The highly varied character of the environment, moreover, tends to favor a high degree of diversity in organisms and thus to give rise to a vast multiplicity of different types. In view of all this, it will be necessary for the general student who has had no previous training in biology to learn the fundamentals of this subject before proceeding with the more specific materials of genetics. THE MECHANISM OF EVOLUTION At the beginning of the present century very little was known about the actual mechanism of evolution. We had Darwin's theory of nat- ural selection, Weismann's theory of germinal continuity, and a statistical knowledge of certain aspects of variation and heredity. Something was known about the role of isolation in species-forming, and the general fact of orthogenesis was appreciated. With the re- discovery in 1900 of Mendel's work and the announcement by De Vries of the mutation theory, genetics really began. Thirty odd years of intensive research by hundreds of specialists have contributed so much to our understanding of the workings of evolution that we now con- sider that we know something about the main causes of evolution. Evolution is now looked upon as an extremely complicated process. There is no one cause of evolution, as the extreme proponents of natural selection once held ; rather, there are many causes, each acting upon and in co-ordination with all the others. The mechanism of evolution is like an intricate piece of machinery manufacturing a complex product. Each part is geared up with other parts. Some parts are concerned with feeding in the raw materials, others with separating and dis- tributing the raw materials, others with assembling and shaping up the various parts into something useful and with discarding defective and useless parts, and still others with sorting out the different kinds of products and keeping them in separate lots. 1 86 EVOLUTION, GENETICS, AND EUGENICS No man-made mechanism is so intricate as is the mechanism of evolution, for ail man-made machines are designed to turn out uniform products, while the evolution machine is especially adapted to turn out highly diverse products. Although we recognize that none of the causal factors of evolution are independent, it makes for clearness of exposition to subdivide the complex machine into five separate factors, each in itself complex enough for further subdivision. THE MAIN CAUSAL FACTORS OF EVOLUTION I. Persistence factors. — Under this head are included all agents that make for persistence of type in organisms, resulting in relative constancy of form and function over long periods of time. When characters persist unchanged from generation to generation, they are said to be hereditary. It is part of our plan to find out what parts of the mechanism promote constancy of type. Some of the principal reasons for this constancy are the following : a) The relative stability of the unit materials of heredity, the genes. b) The relative constancy of bundles of genes, the chromosomes. c) The relative constancy and perfection of operation of the mech- anism of mitotic cell division, a mechanism that aids in maintain- ing the constancy of (a) and (b). d) The relative constancy of the principal factors of the environ- ment over long periods of time. II. Diversity factors. — " Diversity" may be defined as variety with- out the introduction of anything definitely new. It involves, first the kaleidoscopic recombination of all the various hereditary unit charac- ters without any change in the unit characters themselves, and, sec- ond, the almost equally varied expression of characters under the in- fluence of a highly variable and diversified environment. The chief agent in promoting diversity of combinations is sexual, or gametic, reproduction. And the specific mechanisms involved are the mechanisms of mciosis and fertilization, which are later to be de- scribed in detail. The recombinations of unit characters, while apparently random in individual cases, give constant statistical ratios that are known as "Mendelian ratios." Mendel's Laws of heredity in general are, in fact, the laws of the random assortment and recombination of unit characters through the instrumentality of meiosis and fertilization. [NTRODUCTORY STATEMENT 187 Much of the endless dive] >ity of living beings is, however, not hereditary at all, but is due to variations in the environment. The differences in the sizes of beans on a single bean plant, for example, are purely environmental in origin, as experiments have shown, and not in the least hereditary. III. Change factors. — Change, in contrast with diversity, involves the introduction of new unit characters or new gene arrangements. It is as though a new piece of colored glass were added to those already present in the kaleidoscope, or a piece of a different color substituted for an old piece, thus changing all future patterns. Such changes re- sult from the rare modification of individual genes, from equally rare changes in the number of chromosomes, and from the shifting of groups of genes within a chromosome or from one chromosome to another. The mechanisms involved are: a) Gene mutations. b) Slips in the regularity of mitosis involving irregularities in the relatively constant operations of heredity. The result is a change in the number of chromosomes. Several kinds of such irregularities are known and all are called chromosomal aberra- tions. c) Breaks in chromosomes, followed by the reunion of the broken pieces in various new arrangements. Such changes are known as translocations. d) While most of the changes under (a), (b), and (c) seem to occur spontaneously, there is some evidence that they may have en- vironmental causes. Hence the environment may be the ulti- mate change factor. IV. Guiding factors. — Evolution has been for the most part order- ly, especially in two respects, (a) The best-known fossil pedigrees indicate that changes from age to age tend to follow certain definite trends. We have previously spoken of this phenomenon as ortho- genesis, (b) Evolutionary changes have also been, at least to a large extent, adaptive. Most changes have been appropriate to changes in the environment. The chief guiding factor commonly held responsible for both ortho- genesis and adaptation is natural selection, a causal factor of great im- portance that will be discussed critically in its proper place. Various mystical guiding factors, such as Driesch's "enteleche" and Bergson's "elan vital," have been posited to account for the apparent 1 88 EVOLUTION, GENETICS, AND EUGENICS purposiveness of evolution, but there is little evidence for the existence of such factors. Lamarck's theory of the inheritance of acquired characters was also designed to explain the adaptive character of evolutionary change, but there are many reasons for doubting the validity of this theory. V. Dividing factors. — One of the most striking features of living organisms is their multiplicity of different kinds: phyla, classes, orders, families, genera, species, and varieties. Evolution has involved the subdividing of old types into numbers of new ones. The whole process has been one of the branching out into numerous diverging lines of descent. All those agencies that promote the splitting of all old species into two or more new ones are classed as isolation factors. There are many means of isolating portions of a species from other portions of the same species. These may for convenience be called: a) Geographic isolation. b) Reproductive isolation. c) Psychic isolation. d) Environmental isolation. In general, any agent that prevents a particular variant of a species from breeding with the more typical members of that species tends to help to establish the variant type and aids in the production of a dis- tinct new variety, species, or genus, depending on the degree of com- pleteness of isolation and the length of time involved. All of these factors are necessary agents in bringing about the kind of evolution that we know has occurred. With perfect heredity and no diversity or change factors, the organic world would be at a stand- still. Successive generations would be exact duplicates of one another. Offspring would be exactly like their parents, and no evolution would be possible. The diversity and change factors, without the conserving factor, heredity, would produce nothing but chaos. Two successive generations would be utterly unrelated, offspring would be nothing like their parents, and change would merely run riot. With the hered- ity, diversity, and change mechanisms operating as they are known to do, but without any guiding factor, living things would be little more than a chaotic collection of monstrosities, unfit and ill assorted. With all the other factors operating as they are known to do, but with no dividing factor, evolution would proceed along but one front line of advance. There would be no multiplicity of groups, no system of INTRODUCTORY STATEMENT 189 branching relationships. Thus it is obvious that the factors of evolu- tion are to be conceived of, not as independent mechanisms, but as interdependent parts of one grand mechanism. It is our purpose to discuss in detail the various component factors of the mechanism of evolution approximately in the order in which they have been listed in this outline. CHAPTER XIV THE BIOLOGICAL BACKGROUND OF GENETICS RACES AND INDIVIDUALS Evolution is racial change. In order that a race may change, the individuals comprising the race must in large numbers exhibit changed conditions. One or a few individuals changing in some peculiar way may not at all affect the status of the species as a whole. It is well, then, to remember that true evolutionary changes are mass changes involving whole populations through successive generations. A race or a species is to be looked upon as a vast unit continuous in time and space. The members of a race are all descended from common an- cestors and are interbreeding in all sorts of ways. If one should work out a diagram of the genetic relationships of the members of a large species, such a diagram of connecting lines of relationship would con- stitute an almost solid mass of interlacing lines. An intricately inter- connected group of individuals constituting a race is then a true evolu- tionary unit. Just as cells are the structural units of life, so races are the evolutionary units of life. Let us now briefly examine the consti- tution of a race. At any given period of time a race consists of a large group of in- dividuals. Some of these are related as parents and offspring, others as brothers and sisters, others as cousins of varying grades; and all doubtless trace back to some common ancestors a score of generations ago. The individuals constituting the race at one period are not at all the same individuals that constituted that race a few years pre- viously, and none of them will be represented in that race a few years hence. Individuals, then, are temporary components of the race; but the race itself is permanent and relatively constant, though slowly changing as a whole. It is our problem to determine the relation of the individual to the race and to account for both the relative constancy and the slow change of the race made up as it is of such mortal units as individuals. More specifically, we must study the make-up of the individual and learn how replacements are made when individuals wear out. THE CELLULAR MAKE-UP OF INDIVIDUALS All living things, except possibly the filterable viruses and bac- teriophages, are made up of vital units known as cells. Many of the ipo THE BIOLOGICAL BACKGROUND OF GENETICS 191 lower organisms are unicellular, but all of the higher organisms are multicellular. A complex organism such as a man or an insect consists of millions of cells. Each cell bears a life of its own, but is dependent for its supplies of food and oxygen upon other cells, and in turn performs some function that is of value to the organism as a whole. The divi- sion of labor and interdependence of the innumerable cells of the cell- republic, the organism, is reflected in the fact that groups of cells of a particular sort constitute tissues, that various combinations of tissues form organs, and that various combinations of organs form systems, such as the nervous system, the digestive system, the circulatory sys- tem, and others. No matter how elaborate the specialization of or- gans and systems may be, every living part is composed of cells. Cells are perhaps the most fundamental units of life in about the same sense that atoms are the fundamental units of matter itself. Also, cells possess certain mechanisms which are the principal agents both in preserving constancy of form and function (heredity) and in promoting diversity and change (variation). Hence, if we desire to understand how evolution takes place, it behooves us to make a careful examina- tion of the constitution of cells. A TYPICAL CELL AND ITS COMPONENT PARTS Cells vary in size and in form according to the special functions they subserve; but in spite of their numerous specializations they have many features in common. The diagram of a cell shown in Figure 40 is not meant to represent any particular kind of cell, but is a composite of many kinds of cells. Of course, no one kind of cell contains all of the cell organs shown in this diagram. The two most important subdivisions of the cell are the nucleus and the cytosome, or cytoplasm. The nucleus is a more or less centrally situated body, commonly spherical in form, and separated most of the time from the cytosome by a nuclear membrane. Within the nucleus is a characteristic material known as chromatin, which at times condenses into definite bodies known as chromosomes, each chromosome consisting of a bundle of genes, or hereditary units. The cytosome consists of all the rest of the cell except the nucleus. All the vast hordes of cells of different sorts constituting a given com- plex organism are identical in their nuclei, but in any cell the cyto- plasm differs in its form and structure according to its location in the 192 EVOLUTION, GENETICS, AND EUGENICS organism and its special function. Thus a nerve cell and a muscle cell of a given organism may be utterly different in general form and function, but the nuclei of both are the same. Since the specific heredi- tary materials of the individual are possessed equally by all of the cells of that individual, it follows that the differences in cells and tissues must be solely cytoplasmic. How these differences arise is a problem Central bodies Golgi bodies Plasraosome or the true nucleolus Basichromatin Oxychromatin or linin Karyosome or chro- matin-nucleolus ■hue wall or membrane Plasma-membrane Cortical layer Plastids Chondriosomes Vacuole Passive metaplasmic or paraplastic bodies Fig. 40. — Diagram of a typical cell. Its cytoplasmic basis is shown as a granu- lar mesh work or framework in which are suspended various differentiated granules, fabrillae, and other formed components. (From E. B. Wilson.) of individual development rather than one of racial evolution. Many of the slow and steady trends in evolution, however, are believed by some authorities to be the result of general hereditary changes in the cytoplasm, but there is a great deal of evidence favoring the conclusion that the nucleus is the chief organ of heredity and of variation in the cell. Details of the nucleus. — The nucleus, except during the process of mitotic cell division, is surrounded by a definite membrane, the nuclear membrane, which separates the clear nuclear sap from the cytoplasm outside the nucleus. During the resting stage, between two cell divi- sions, there is a network of linin fibers running throughout the nucleus. THE BIOLOGICAL BACKGROUND OF GENETICS 193 Upon this linin network are strung numerous granules of a deeply staining substance, or substances, called chromatin granules. Other nuclear bodies, known as nucleoli, plasmosomes, and karyosomes, are found in some nuclei, but they may be ignored so far as our knowledge of their role in evolution is concerned. Chromosomes. — Far more important for our purposes than any other cellular components are the chromosomes. Preparatory to the process of mitosis the diffusely scattered granules of chromatin con- dense into semi-solid masses, sometimes spherical or ovoid, sometimes rodlike, sometimes V-shaped, etc. These are the chromosomes. Each species of organism possesses in all its cells a certain definite number and kind of chromosomes. Man has 48, Drosophila melanogaster has 8, Oenothera lamarckiana has 14, Ascaris megalocephala has 4 in one varie- ty and 2 in another, some crustaceans and some roses have over a hundred chromosomes. Careful studies of the chromosomes of nu- merous species of both animals and plants have revealed the significant fact that, except in the males of some species, the numbers of chromo- somes are even. Furthermore, it is possible to match them up in pairs according to their sizes and shapes. The significance of this becomes clear when we remember the fact that an individual starts out from a zygote (fertilized egg) which is a composite cell to which each parent has contributed one full set of chromosomes. Genes. — At this time it seems well to anticipate the later results of genetic research to the extent of stating that each kind of chromosome contains a unique series of hereditary units (genes). Certain genes lie in certain chromosomes, and all the genes in a particular chromosome are probably arranged in linear order like beads on a string. Since the orderly arrangement of genes in bundles (chromosomes) and their specific arrangement within a given bundle are both important in heredity, it is essential that this specific organization be maintained during development and reproduction. It is also essential for per- sistence of type from generation to generation that the complete or- ganization, not only of nucleus but of the cytosome, be maintained. This is accomplished by that heredity mechanism par excellence, the mechanism of mitosis. The central body. — Imbedded in the cytoplasm, usually in close proximity to the nuclear membrane, lies a structure known as the central body (centrosome). In most animal cells this structure is well defined, but it is absent in most plant cells. The central body is pri- marily associated with cell division, as will be seen later. 194 EVOLUTION, GENETICS, AND EUGENICS CELL DIVISION — MITOSIS Each cell during periods of growth and development grows to a definite size and then divides into two daughter cells. Repeated cell divisions multiply vastly the numbers of cells in a developing embryo, until in higher organisms millions of cells are produced. To make what might easily be a long story as short as pos- sible, let it be sufficient to state that the chief features of mitotic cell divi- sion (mitosis) are as follows: The central body divides, and the two cen- ters migrate apart, spinning between them a spindle of fibers, the mitotic spindle (Fig. 41). The nuclear mem- brane disappears; and, as the two central bodies migrate farther and far- ther apart, the chromosomes, which have in the meantime gradually be- come more and more compact, move toward the equator of the spindle, each attached by two fibers probably com- posed of linin, one fiber leading to one pole of the spindle, the other fiber to the other pole (Fig. 42) . Usually even before migrating into the spindle each chromosome has become a double body, a pair of joined twins, each one the exact duplicate from end to end of the other. Each gene in the gene-chain has previously twinned, so that each gene in each chromosome is a double or twin gene. The chromosomes now arrange themselves in a plane at right angles to the long axis of the spindle, each chromosome having one of its twin halves directed to one pole and the other to the opposite pole. All of the stages up to this point of equilibrium are known as prophases of mitosis, the equilibrium phase being the meta phase (Fig. 42, E). Fig. 41. — Diagram of the early phases of mitosis in Ascaris. A, vegetative nucleus; B, fine spireme; C, coarse spireme; D, late prophase with chromosomes, spindles form- ing. {From E. B. Wilson.) THE BIOLOGICAL I'.ACKGROUND OF GENETICS 195 Now the real separation phase of division begins. The twin com- ponents of each chromosome seem to be pulled apart, possibly by the fibers attached to them, and one twin migrates to one pole, the other twin to the other (Fig. 43, 77). Slowly the various chromosome twins approach opposite poles and finally clus- ter closely about the latter. At this point the cell wall constricts at the equa- tor and soon divides the single cell into twin daughter cells (Fig. 43, 7"). Not only are the chromosomes divided me- ticulously into equal lots, but all the cytoplasmic parts of the cell are so divided that each daughter cell comes to have exactly the same detailed organiza- tion as had the mother cell from which they were derived. The period during which chromosome halves migrate to the poles of the spindle is called the anaphase, while that during which the chromosomes lose their density and reform the chroma- tin network with which we started is called the telophase. By means of this elaborate routine of division the exact specific organization of the cell is maintained, at least in the germ track. In this process of mitosis, we have a view of the mechanics of hered- ity, which, if uninterrupted, would main- tain perfect constancy of type for all time unless a new order of environment were to alter the expression of a fixed hered- -Diagram of the . ii- middle phases of mitosis. E, met- itary complex, or some other mechanism aphase . F> G> ear]ier and ]ater promoting diversity were to intervene. anaphases. (From Wilson.) DIFFERENTIATION Not merely do cells multiply in the process of embryonic develop- ment, but at every step differentiation of cells for the performance of different specialized functions is going on. The mechanism of differen- tiation is at present very incompletely understood, and what we do know is of a technical nature and difficult to present without a back- 196 EVOLUTION, GENETICS, AND EUGENICS ground of embryological lore. In very brief, it must suffice for present purposes to state that differentiation is accomplished partially by un- equal cell division, in which the two daughter cells come to possess different cytoplasmic contents; partially by the differences in position of cells with respect to the environment; partially by differences in the relations of certain cells to other cells that have already as- sumed special characters owing to their original position or their spe- cial relation to the environment; and partially by the fact that cer- tain genes are effective only at a given time in development or when cells are in a certain physiological state. All cells in an organism are believed to possess the same genes, but particular genes become effec- tive only under special conditions of time and place. Some such complex of intricate interrelations, at present very obscure even to the most advanced students of embry- ology, seems to underly cellular differentiation. While the geneti- cist cannot avoid all responsibility for the solution of these insistent problems, he may with some ap- propriateness delegate them, for the present at least, to the experi- mental embryologist. At any rate, it seems best at the present juncture to sidestep the problems of dif- ferentiation and to get back on surer ground, more familiar to geneti- cists and more applicable to their problems. Fig. 43. — Diagram of the end phases of mitosis. //, the beginning of constriction of the cell membrane; I, the completed division. {From Wilson.) THE ORIGIN OF NEW INDIVIDUALS MODES OF REPRODUCTION Since individuals, the temporary components of the race or species, are continually running their courses and dying off, replacements are necessary if the race is to persist. The replacement of old individuals by new is commonly called reproduction. There are numerous modes THE BIOLOGICAL BACKGROUND OF GENETICS 197 of reproduction, but they may all be classified into two main categories: somatogenic and cytogenic. Somatogenic reproduction is accomplished without the instrumentality of sex and involves the subdivision of the parent body into two or many fragments, each of which has the power to reconstitute a whole new individual like the parent. In unicellular organisms somatogenic reproduction involves the division of the one- celled body into two or more cells each capable of growing into a full- sized individual; in multicellular organisms the products of division are multicellular fragments. Cytogenic reproduction is accomplished by means of unicellular germ cells which must pass through processes of growth and division in order to reconstitute the multicellular body typical of the species to which they belong. The essential feature of all reproductive processes is that some por- tion of the parent is isolated, or physically cut off, from the parent's body, an integral part of which it formerly was. On ceasing to be a part of an organized system, it exercises its prerogative of developing a system of its own in the image of the one from which it was derived. This is true whether the isolated part be a single cell or a multicellular fragment. This is perhaps to be expected in view of the fact that each cell possesses the hereditary organization characteristic of the race or species. Varieties of somatogenic reproduction. — In unicellular organisms the principal modes of sexless reproduction are transverse fission, in which the division is across the long axis of the cell; longitudinal fis- sion, in which the division is parallel with the long axis; and multiple fission, in which the nucleus divides several times before the cytoplasm partitions the several nuclei off to form separate cells. In multicellular organisms the principle modes of somatogenic reproduction corre- spond rather closely to those among unicellulars, but several special modes are added. Transverse fission is common among worms of vari- ous kinds and among the larvae of certain jelly fishes. Longitudinal fission takes place in the early embryos of a number of mammals, as in armadillos and in man, resulting in true quadruplets and true twins derived from an original single embryo. Multiple fission occurs in a number of parasitic insects, in which the multicellular embryo sub- divides into hundreds of cell-masses each of which becomes a complete individual. In addition to these types of fission, multicellular or- ganisms reproduce by budding, which may be simple or multiple. Uudding involves the outgrowth of a minor portion of the parent body, the latter body being left intact — this being in contrast with r ;8 EVOLUTION, GENETICS, AND EUGENICS lission processes in which the entire parent body is distributed among the progeny. Gemmidation is a peculiar form of somatogenic repro- duction found among sponges, involving the gathering together of samples of all the various kinds of cells into compact little balls called gemmitles, which survive the dead parent and escape to form new sponges when the parent body disintegrates. Artificial propagation of both plants and animals is made possible through the ability of many organisms to regenerate a whole new in- dividual from a small part of a parent organism. Thus sponges may be cut up into minute fragments and distributed like seeds upon a slab of concrete, each fragment growing up into a sponge of the parental type. Everyone is familiar with the fact that gardeners propagate many plants by planting cuttings, and that potato tubers (enlarged parts of the underground stem) may be cut into as many fragments as there are "eyes" and that each will produce a plant with tubers like the parental tuber. All the cells involved in somatogenic reproduction are the product of ordinary mitotic division, which maintains so painstakingly the hereditary organization of the species. No wonder, then, that this mode of reproduction makes for a high degree of constancy of type. Apart from the effects of differences in the environment, which appear not to be inherited, there is nothing about somatogenic reproduction to favor diversity or change. If one desires to study pure heredity uncomplicated by sex and its diversifying effects, one should study successive generations produced by somatogenic reproduction. Varieties of cytogenic reproduction. — All those forms of reproduc- tion in which single cells become separated from parent bodies to give rise to new individuals fall into this category. The first and simplest form of cytogenic reproduction is spore formation. Among plants, reproduction by spores is well-nigh universal, though many also make use of gametes. A spore is nothing more than a small cell produced by mitosis from previous cells. Spores are commonly motile, some being furnished with flagella by means of which they travel through the water, others being so light as to be wind-borne. To reproduce a new organism, a spore, after a period of rest, merely divides and redivides and thus forms a new multicellular body. In some of the lower plants motile spores, all visibly similar, swim about actively in swarms, and then pair off two by two and fuse to form zygotes from which new plants develop. These mating cells are called gametes. This is the most primitive expression of sex in plants. THE BIOLOGICAL BACKGROUND OF GENETICS 199 Further specialization of sex cells involves, first, the differentiation of gametes into large passive gametes (eggs) and small motile gametes (sperms) and, second, the differentiation of sex individuals, males and females. At this point, it is important to emphasize the fact that sex, al- though it appears to be so intimately associated with reproduction, is in no way essential to reproduction. In fact, it is a hindrance rather than a help to the mere multiplication of individuals. Sex is a mechan- ism that has been superimposed upon reproduction and has a function only remotely associated with the latter. In a word, sex is the diversity mechanism, with which we shall deal fully at the proper time. The female gamete, or egg, is always relatively large. Even the tiniest of eggs, such as those of placental mammals, are relatively large as compared with tissue cells. The human egg (ovum), for example, has a cubic content of about a thousand times that of the average tissue cell, while the eggs of birds and sharks are thousands of times bulkier than mammalian eggs. This great increase in the size of eggs is dut largely to the accumulation of food material (yolk) that serves to sus- tain the embryo and give it a good start in life. Most eggs are, when full grown, spherical or ovoid in form; and most of them are protected by envelopes of one kind or another. Male gametes, or sperms, are highly variable in shape and are rela- tively extremely small and active. Most of the cytoplasm of the sperm is specialized for locomotor purposes. Some sperms have one long tail like a snake (Fig. 47,/), others have two or several tails, and still others have numerous radiating locomotor processes resembling the thorns of a sand burr. In general, a sperm gives the impression of being a highly specialized cell one of whose main functions is that of finding and penetrating the egg. Sperms are thousands, sometimes millions, of times as numerous as eggs, thus making ft more probable that at least one sperm will reach each mature egg. When the sperm approaches the egg, it seems to be guided, at least in some cases, by a specific chemical substance given off by the egg. When egg and sperm meet, the latter penetrates the former more or less completely (Fig. 50, a). Sometimes the whole sperm enters the egg, sometimes only the head of the sperm containing the nucleus and a minute amount of cytoplasm. On entering the egg cytoplasm, the sperm nucleus grows at the expense of the latter and becomes nearly or quite as large as the egg nucleus (Fig. 50, c, d, e). The union of egg and sperm is called fertilization, and the product of the union is a zygote. 200 EVOLUTION, GENETICS, AND EUGENICS The term "zygote" is also sometimes used to designate the organism that develops from the united gametes. Gametic reproduction is very frequently evaded in both animals and plants by the use of a process known as parthenogenesis, or "virgin birth," in which eggs develop without any aid from sperms. Some instances of this peculiar aberration of gametic reproduction will be discussed in a special chapter on the biology of sex (chap. xvii). Here we have the anomalous situation of gametic (or marrying) reproduction without the presence of both sexes. Long-continued parthogenesis involves the reduction of diversity and results in constancy as marked as that which is associated with various forms of sexless reproduction. While the great majority of animals and plants are sexually di- morphic, consisting of male individuals that produce sperms and fe- male individuals that produce eggs, large numbers of both animals and plants are monoecious, having both sexes present in one individual. Such forms among animals are known as hermaphrodites. In some her- maphrodites eggs are fertilized by own sperms, in others mating occurs in which a mutual exchange of sperms takes place. In the latter in- stances the advantages of sex in enhancing diversity are retained, and mating is facilitated because any two adult individuals may mate; but in the former instances where self-fertilization occurs, diversity is re- duced to the level found among organisms reproducing by asexual methods. It is possible, therefore, to produce from such self-fertiliz- ing monoecious species pure lines in which all progeny of a single in- dividual are genetically identical. In a later connection we shall make use of such forms as these to study heredity in its simplest expression (chap. xv). THE ORIGIN OF GAMETES Two important questions arise in connection with gametes: (a) From what cells in the parent body do gametes arise? (b) What changes occur in germ cells that make it possible for them to unite in pairs to form zygotes? The germ track. — The question as to whether germ cells are derived from parental tissues that have been more or less specialized for other functions, or whether they are derived from an unbroken series of germ cells set apart from bodily functioning at all times, is one of great importance for theories of heredity. The prevailing view of biologists is that, at least in the higher animals, and possibly in all animals and plants, the germ cells are produced from cells that have never in any THE BIOLOGICAL BACKGROUND OF GENETICS 20 1 generation gone through a period of specialization for any particular bodily function. In animals these cells are believed to be set apart early in ontogeny (the development of the individual) and localized in sex glands, or gonads. In plants the germ track is believed to be maintained in the meristematic cells of growing points, but germ cells are not so definitely localized as in most animals. Fig. 44. — The germ track in Ascaris. Stages in early cleavage showing the chromatin diminution process in all cells except the stem cell (S). (From Boveri, lSQ2.) In some animals the germ track is very clear and unmistakable from the beginning of one generation to that of the next. The classic instances of germinal continuity are those of the roundworm, Ascaris megalocephala, and the fly, Miastor americana. In the variety of Ascaris known as univalcns, which possesses only two chromosomes to each cell (incidentally the lowest number known), the developmental stages are as follows: at the first division of the zygote two cells are formed in the usual way, each cell with two long loop-like chromosomes (Fig. 44, A). One of these two cells, however, undergoes a striking nuclear change involving the breaking-off of the 202 EVOLUTION, GENETICS, AND EUGENICS ends of the chromosomes and the disintegration into small chromatin granules of the middle portions. The large ends of the chromosomes are discarded into the cytoplasm and absorbed, while the smaller granules are all that the descendants of this cell have for chromosomes. It is significant that the progeny of such cells form only body cells, in this case skin and nerve cells (Fig. 44, A). The other cell remains just like the original zygote and is a stem cell or germ cell. This cell then divides, and one of the daughter cells retains the full germinal charac- ter, while the other breaks down its chromosomes as before and gives rise to tissues that line the digestive tract (Fig. 44, B) Once more the germ cell divides and one of its daughter cells undergoes chromosome breakdown, its cell progeny forming chiefly such tissues as muscles, blood vessels, and connective tissues (Fig. 44, D). From this point on, the germ cells are definitely set apart and contribute no further to the soma. They give rise to nothing but germ cells and ultimately to gametes. The unbroken series of cells with intact chromosomes from the zygote to gametes is the germ track, and the chromosomes of these cells carry the so-called germ plasm. The rest of the embryo consti- tutes the soma or somatoplasm. The relation of body to germ plasm is well shown in the accompanying diagram (Fig. 45). A second example of a well-defined germ track is found in Miastor (Fig. 46, A), in which a single definite germ cell {p.g.c.) is set aside at the very first division of the zygote. The other cell divides repeatedly, casting out parts of the chromosomes, as in Ascaris, and ultimately giving rise to all the soma. Even in a fairly advanced embryo (Fig. 46, B) the large germ cells are clearly seen in a small group (oog 3 ). These give rise ultimately to the gametes. While in vertebrates the germ track is ill defined and difficult to follow, there seems to be no doubt that it exists. Some investigators, however, claim that the original primordial germ cells disintegrate and come to naught and that the gametes arise from a new lot of germ cells that are derived afresh from generalized epithelium. If that be the case, it seems fair to consider this epithelium as part of the germ track and not as a differentiated part of the soma. At least it may be said with confidence that in none of the higher mammals are germ cells ever derived from specialized body cells such as muscle cells, nerve cells, or gland cells. Our question as to the origin of the germ cells seems to be answered. They are derived in an unbroken series from previous germ cells by the process of mitosis. This is the basis of Weismann's concept of the continuity of the germ plasm. THE BIOLOGICAL BACKGROUND OF GENETICS 203 THE MATURING OF GAMETES The primordial germ cells, previously described as having been set apart from the soma at a relatively early period of development and located in germ glands (ovaries and testes), pass through a period of comparative inactivity until sexual maturity arrives. At the dawn » Fig. 45. — Diagram of the germ track in Ascaris. E; egg; Pi, P 2 , Pi, stem (germinal) cells; P A , primordial germ cell. Circles represent somatic cells, while the four black dots outside of the circles represent the masses of chromatin that are eliminated. (From Boveri, igio.) of sexual maturity the germ cells wake up and enter upon a period of great activity, during which mature gametes are produced. The his- tory of the production of gametes differs somewhat in the two sexes. That of the male, called spermatogenesis, is a little simpler than that of the female, called oogenesis, and will be described first. Spermatogenesis. — The first sign of renewed activity of the male germ cells is evidenced by a succession of rapid cell divisions, the 204 EVOLUTION, GENETICS, AND EUGENICS mechanism of mitosis being used. After many thousands of germ cells have been produced, multiplication is stopped and growth sets in, the growth in the male cells being very slight as compared with that of female cells. During the period of growth the chromosomes not only split to form twin chromosomes, as though in preparation for mito- sis, but whole homologous chromosomes unite in pairs, a process known o'6 g Fig. 46. — The germ track in Miastor americana. A, germ-cell (p-g.c.) set apart in the eight-celled stage of cleavage. (After Hegner.) The walls of the remaining seven somatic cells have not yet formed, though the resting or the dividing (M p) nuclei may be seen; C R, chromatin fragments cast off from the somatic cells; B, section lengthwise of a later embryo of Miastor; the primordial egg-cells (obg 3 ) are conspicuous. (From Guyer, after Hegner.) as synapsis. The result is that, instead of the set of single chromosomes characteristic of the zygote, there are groups of four (tetrads), each tetrad composed of two pairs of twin chromosomes bound together. There are just half as many tetrads as there are chromosomes in the zygote. Now follow two special cell divisions, known as meiosis, with- out any further change in the chromosomes. The result is that one chromosome of each tetrad goes to each of the four matured sperm cells. In one of these divisions, the so-called reduction division, both twins of an originally single chromosome pass together to one cell, THE BIOLOGICAL BACKGROUND OF GENETICS 205 which results in a reduction of the number of chromosomes charac- teristic of the zygote to one-half (Fig. 47). The other division simply separates twin chromosomes as in ordinary mitosis. The result is that each gamete has only one of each kind of chromosome and there- fore only one-half the number possessed by the zygote and the pri- mordial germ cells. It is customary to speak of the somatic or zygotic Fig. 47. — Diagram to illustrate spermatogenesis, a, showing the diploid num- ber of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinal} cells and spermatogonia; b, the pairing (synapsis) of corresponding mates in the primary spermatocyte preparatory to reduction; c, each secondary spermatocyte receives three, the haploid number of chromosomes; d, division of the secondary spermatocytes to form e, spermatids, which transform into/,, spermatozoa. {From Guyer.) number as diploid, or 211, and the gametic number as haploid, or n. The final phase of spermatogenesis consists of an elaborate specializa- tion of the male gamete, consisting of the development of locomotor organs and adaptations for penetrating the egg. Oogenesis. — The period of multiplication is essentially the same as in spermatogenesis except that fewer and larger cells are produced. The period of growth is very marked, for it is during this period that the egg accumulates the yolk, which is usually massed at the vegetal pole. The nucleus and central body, confined to a small region of 2o6 EVOLUTION, GENETICS, AND EUGENICS clear protoplasm at the animal pole, are not in a position to bring about an equal cell division. Hence the maturation divisions are very un- equal (Fig. 48). The first maturation division results in a large cell, not appreciably smaller than the full-grown egg, and a very tiny cell, called the first polar body. The second maturation division results in a second polar body and the mature female gamete, or unfertilized egg. Sometimes the first polar body divides again, sometimes not. Typi- cally, however, four female gametes are produced, corresponding in Fig. 48. — Diagram to illustrate oogenesis, a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and in oogonia; b, the pairing of corresponding mates preparatory to reduction; c, d, the reduction division, giving off the first polar body; e, egg preparing to give off the second polar body, first polar body ready for division; /, second polar body ready for division; g, second polar body given off, division of first polar body completed. The egg nucleus, now known as the female pronucleus, and each polar body contain the reduced or haploid number of chromosomes. (From Guyer.) number to male gametes similarly produced; but three out of every four eggs, namely, the polar bodies, are abortive and die because of deficiency of cytoplasm, leaving only the one large well-nourished egg as the progeny of each primordial germ cell that completed the period of growth. Synapsis and the reduction division are the same in oogenesis as in spermatogenesis (Fig. 49). Now, during both spermatogenesis and oogenesis the united pairs of homologous chromosomes, one derived from the father and one from the mother of the previous generation, arrange themselves during the reduction division quite independently of one another, so that some maternal and some paternal chromosomes enter each gamete. Where THE BIOLOGICAL BACKGROUND OF GENETICS 207 there are large numbers of chromosomes, very many different assort- ments of maternal and paternal chromosomes are produced. This shuffling and dealing of parental heredity materials constitutes one of the principal means of increasing diversity in organisms, and therefore one of the most important mechanisms of evolution. A relatively oosrmatogenesis 1 Spermato- gonia {Multiplication Period . Growth Period Goaonia 4^j J Pairing of Chromosomes Spermatocyte l\?J^ / \ } Reducing div'n ision Secondary I &/ Spermoco- CyUS IKT@ © Q © Sperm- atozoa Primary dStylt Secondary oocyte {/lyunj and first fjo'or body) I tf~\ nature ovum J and polar bodies /yf \ nature ofum fo/1 number of C/iromosomes restored Fig. 49. — Diagram showing the parallel between maturation of the sperm- cell and maturation of the ovum. {From Guyer.) simple case of the assortment of different chromosomes to gametes is shown in Figure 99. THE UNION OF GAMETES — FERTILIZATION Once the gametes are mature they are read}' for the fertilization process (Fig. 50). It seems to be a matter of pure chance that any particular sperm finds and enters any particular egg. Since there are usually hundreds of different kinds of eggs and equally large numbers of different kinds of sperms, the number of possible kinds of zygotes produced is extremely great. If, for example, a species has only 208 EVOLUTION, GENETICS, AND EUGENICS twenty chromosomes — less than half that possessed by man — there would be (2) 10 , or 1,024 different gametes in each sex. And since each kind of egg is as likely to be fertilized by one kind of sperm as another, there might be produced (1024) 2 , or 1,048,576 different zygotes. Hence, meiosis and fertilization together constitute an extremely effec- Fig. 50. — Diagram to illustrate fertilization, d% male pronucleus; 9 , female pronucleus; observe that the chromosomes of maternal and paternal origin re- spectively do not fuse. {From Guyer.) tive mechanism for increasing diversity. This is the chief role played by sex in evolution. This abbreviated account of biological processes is all that is needed for our purpose and should, we believe, enable the student to follow intelligently the accounts of heredity, variation, selection, and isola- tion that are to follow. A more detailed account of the processes of mitosis, meiosis, and fertilization may be found in chapter xliv, "The Mechanism of Mendelian Heredity." CHAPTER XV INTRODUCTION TO THE STUDY OF PERSISTENCE FACTORS As was pointed out in chapter xiii, the principal agents that make for constancy or persistence of type from generation to generation are: (a), the relative stability of the units of heredity, genes; (b), the rela- tive stability of the bundles of genes, chromosomes; (c), the relative precision and perfection in operation of mitosis; (d) the relative con- stancy of the environment over long periods. Laborious studies of gene changes, especially in the fruit fly, DrosopJiila, have shown that genes are exceedingly stable. Muller and Altenberg have discovered that a large proportion of the genes of DrosophUa must have a stability comparable with that of the atoms of the element radium, which have an average unchanged life of about two thousand years. Of course, where there are hundreds, possibly thousands, of genes in each cell, the chances of some gene change being observed during any given period will depend upon the numbers of genes present and the number of individuals examined. Chromosomes are probably almost equally stable except for cross- ing-over, a phenomenon that cannot be discussed here. Changes in numbers and composition of chromosomes do occur as rare accidents of mitosis or meiosis, and these changes break up the constancy of cellular organization; but the relative constancy of chromosomal num- bers and of their genie content constitutes one of the most important agents in maintaining persistence of types. Various kinds of irregularities in the characteristically constant and nearly perfect mechanism of mitosis are responsible for rare changes in chromosome numbers and for redistribution of genes in chromosomes, but these events are so rare that they only very slightly : t the constancy of the species of animals and plants in which they ur. Another factor influencing persistence that is likely to be neglected is that of the relative constancy of the environment in any given geo- graphic region. It is well known that, apart from seasonal fluctua- tions, the general climatic conditions in a given region remain un- changed over long periods of time. The mean annual temperature, 209 210 EVOLUTION, GENETICS, AND EUGENICS the mean annual rainfall, and the general character and direction of air movements remain essentially the same for very long periods. Such constancy of environment could hardly fail to exercise a standardizing effect upon animal and plant communities, and thus aid in maintaining constancy in the expression of racial characters. Persistence and diversity mechanisms contrasted. — It is custom- ary in courses in genetics to introduce the study of heredity by present- ing an outline of Mendelian heredity. This seems to us a mistake, for Mendelian heredity is really not essentially a persistence factor, but rather a most effective diversity factor. It breaks up constancy of combinations of unit characters and promotes multiplicity of recom- binations of such characters. For this reason we shall begin our dis- cussion of the persistence factors in evolution with the study of heredity in pure lines, in which the persistence mechanism is free to operate without being complicated by the diversity mechanism. Since bio- metric methods are necessary for the study of variation and heredity in pure lines, it is necessary to introduce a brief statement about these methods. A short lesson in biometry. — When character differences are either qualitative or are sharply defined, they are easily handled by Mendeli- an methods. If for example, all individuals are either black or white, tall or short, heavy or light, and no gradations occur between the two alternatives, it is easy to follow the distinct types through successive generations. If, on the other hand, there occur all gradations between black and white, all gradations between tall and short, and all grada- tions between heavy and light, it is no longer possible to classify each individual in some particular category. When this is the case the only possible method of finding out how much is inherited, and how much is not, is to study whole generations of progeny as units, and to deter- mine what is the characteristic of one whole generation as compared with the next whole generation. Such a comparison requires statistical methods. Suppose, for example, we want to find out whether the size of bean seeds is hereditary or merely environmental, we shall have to measure the seeds of the parent and compare them with those of the offspring. There would be nothing gained by comparing a selected seed of the parent with one of the offspring. We must compare the total of one with the total of the other. It is necessary, then, to find a method or methods of comparing the parent condition as a whole with the off- spring condition as a whole. Some simple method must be devised INTRODUCTION TO THE STUDY OF PERSISTENCE FACTORS 211 that gives due credit to each of the variants in the two generations. One method commonly used is the graphic method. Thus each seed of the parent generation is measured and plotted on a graph in which the horizontal line (the abscissa) represents a series of size classes vary- ing from the smallest ones at one end and the largest at the other, and the perpendicular line (the ordinate) represents the frequencies of indi- viduals in the varying size classes. A line connecting high points in each of the size classes will form a variation curve which will be charac- teristic of the group. Such a curve has a high point near the middle (called the mode), and the curve slopes gradually toward each end. This curve not only represents the distribution of the different sizes in the group but shows the most commonly occurring size class, the modal class. A similar curve is made for the offspring generation, and the parent curve is compared with the offspring curve. The two may be compared with respect to mode, mean, average, and standard devia- tion, and a great deal may be learned, that can be learned in no other way, about the variation and heredity of such graduated or fluctuating characters as weight. For further information about statistical methods in genetics, the reader is referred to a short chapter in the Appendix (chap, xliii). It will hardly be necessary here to do more than indicate that pure-line work, such as that of Johannsen, Jennings, Tower, and Wright, dis- cussed in the next chapter, could not have been done without the use of biometrical methods. CHAPTER XVI HEREDITY IN PURE LINES THE NATURE OF PURE LINES When all the individuals in a family line — offspring, parents, grand- parents, and so forth — have just the same hereditary materials (genes and chromosomes), the whole family connection is known as a "pure line." Pure lines may originate in at least three ways : (a) by asexual modes of reproduction, such as fission and budding; (b) by sexual re- production of hermaphrodite or monoecious parents, such a parent being a double-sexed, male-female, individual, in which both eggs and sperms are provided by the same parent; (c) by prolonged close in- breeding, brother-and-sister mating, for many generations. Members of pure lines, if they are genuinely pure, will all be exactly alike in their bodily characters if they develop under identical environ- mental conditions, unless some change, such as a mutation, occurs. When differences occur among members of a pure line, we can be sure that such differences are not innate, but are due to external, or en- vironmental, causes. Much may be learned about heredity from a study of pedigrees of individuals in pure lines. In pure lines heredity presents itself in its simplest form, uncomplicated by the mechanism of diversity. Here we can study the operations of heredity modified by only one other factor, the environment. Some of the facts that are brought to light by our studies of pure-line heredity will serve to simplify and elucidate the more complex aspects of Mendelian heredity. THE PURE-LINE EXPERIMENTS OE JOHANNSEN One familiar with the field of genetics never thinks of pure-line breeding without thinking of Johannsen and his famous nineteen beans. He was interested in pure -line breeding primarily as a means of im- proving certain kinds of beans for economic reasons. He thought he might be able to produce uniformly bigger beans by always planting only the biggest seeds. In order to avoid the complexities of mixed races, he worked with pure lines, keeping each pure line quite separate from the others. Although he experimented with nineteen pure lines, we shall not attempt to follow up the results of more than two of them. 212 HERED] IT IN PURE LINES 21 The first thing he did was to collect all the seeds (we would call them beans) from a single bean plant, which is a male-female individu- al. These beans were of many different sizes, some quite large, others quite small, and many intermediate. In order to follow the heredity of bean size with accuracy, he weighed each bean on a delicate balance. What he determined was the weight of each bean, but we shall speak of large and small size instead of heavy and light weight. Since some few of the beans were very large and some few very small, what more natural than to select the largest beans to be the seeds for the next generation of plants, and to expect that the beans of the next and the next and the next generation would be larger and larger and larger? If Johannsen was sanguine enough to expect to get such a rapid improvement in beans, he was doomed to disappointment. He had constructed a curve of variation, in the manner described in the last chapter, for beans of the parent plant. This curve had a definite shape and a definite mode. When he constructed a similar variation curve for the progeny derived from the largest beans of the first gener- ation this curve was practically the same as that of the parent genera- tion. The beans were not all large, as he may have hoped, but some Were large, some small, and many intermediate, just as in the parent oration. Continuing to select, for planting, the largest beans for six generations, he found that there had been no change in the average size of beans, and the number of largest beans had not increased. Selection then had been powerless when operating upon individuals with identical heredity. Another series of breeding experiments was carried out, select seed the smallest beans of the original parent plant of Pure Line I. The bean progeny of the second generation differed not at all from those derived from the largest bean of this pure line. They gave the same variation curve and the same average as the latter, and this was maintained for six generations. It made no difference whether the largest, the smallest, or the average beans were selected for planting; so long as they belonged to the same pure line, the same variation curve and the same average w-ere maintained. A second pure line, vrhich we may call Pure Line II, was started from another parent plant, chosen because the average size of beans was distinctly less than that of the first pure line. The statistical study of the beans of this second parent plant revealed another new and significant fact, namely, that the beans, when arranged accord- to sizes, gave a variation curve differing markedly from that of 214 EVOLUTION, GENETICS, AND EUGENICS Pure Line I. The snape of the curve, the mode, and average size were different. Again selection for six generations of, first, the largest beans for seed, and second, the smallest beans for seed, did not affect the variation curve, mode, or average. Once again, selection within a pure line was quite ineffective. The same results were obtained in all the rest of the nineteen orig- inal pure lines with which Johannsen experimented. A somewhat more concrete idea of the results obtained may be secured through the perusal of the following table which gives in terms RESULTS OF SELECTION IN PURE LINE I Harvest Year 1902 I903 1904 I905 1906 1907 Mean Weight of Selected Parent Seed Minus 60 55 50 43 46 56 Plus 70 80 87 73 84 Si Mean Weight of Offspring From Minus Parent 75 54 63 74 69 15 19 59 55 38 07 From Plus Parent 64.85 70.88 56.68 63.64 73.OO 67.66 of average bean weight the results of selecting plus and minus parents for six generations in Pure Line I, the plus parents being largest beans and minus parents being smallest beans. It will be seen that in the last year of this selection experiment (1907) the smallest beans, averaging 56 eg. in weight, produced a prog- eny weighing, on the average, 69.07 eg., while the largest beans of the same year, averaging 81 eg., produced progeny of practically the same average weight as did the smallest beans, namely, 67.66 eg. The dif- ference is not statistically significant. The influence of the environ- ment in bean weight is clearly shown in the data for the year 1904, which was a bad year for growth. In this year, the average weight of progeny from both large and small beans was greatly reduced, being respectively 56.68 eg. and 54.59 eg. That this loss in weight was not inherited is shown by the results in subsequent years in which the average returned to that seen in the first year of selection. It is also interesting to note that in the years 1903, 1906, and 1907 the lighter parents produced a heavier progeny than did the heavier parents. From these experiments Johannsen came to the conclusion that plus and minus fluctuations about the mode were due to differences in HEREDITY IN PURE LINES 215 the environment and were not inherited, and that what was really in- herited was merely a potentiality to produce beans of a certain average weight which may be modified by the environment. It was also ob- vious that these environmental modifications had no effect on subse- quent generations, that is to say, they are not inherited. Selection, therefore, has no effect unless there are hereditary differences among the individuals selected. All the members of a given pure line are identical genetically, that is, there are no differences in their genes. Johannsen called a group of individuals that are alike in their genes a genotype, and spoke of all such individuals, whether alike in seed size or not, as genotypically identical. Hence individuals that breed alike, whether they look alike or not, are members of the same genotype. On the other hand, each of the nineteen pure lines is a different genotype though some beans in each pure line may be exactly of the same size as some beans in others. Johannsen designated individuals that look alike and have the same somatic characteristics, whether they are genotypically alike or not, members of the same phenotype; in other words, they may be phenotypically identical though genotypically different. Phenotypic differences, unless also associated with genotypic differences, are not hereditary. This conclusion is a highly important one for all our fur- ther study of heredity. Another significant conclusion may be reached from these experi- ments, namely, that environmentally produced differences, if they are merely phenotypic in character, play no important part in evolution, except in so far as they may favor the survival of certain individuals possessing genotypic differences. Hence, environmentally produced differences in the soma have only an indirect influence on the course of evolution. OTHER EXAMPLES OF PURE LINES W. L. Tower, in a long series of experiments on the potato beetle, Leptinotarsa decemlineata, came to similar conclusions. He produced a pure line, not by making use of a self-fertilizing species, but by closely inbreeding a bisexual family for a long time. This stock was tested and found to be genotypically pure, yet there was a considerable amount of variability in the shade of color in the color patterns, and in other ways. For twelve generations he selected and bred from the darkest specimens, and from the lightest specimens, keeping the two series separate (Fig. 51). The result was that, instead of getting 2l6 INVOLUTION, GENETICS, AND EUGENICS darker specimens predominating in one series and lighter specimens in the other, he obtained a final generation of individuals showing practi- ce I iy the same range of variability in both series, and in neither series was there any consistent change from the condition present at the beginning of the experiment. More recently Sewall Wright, working in connec- tion with the Bureau of Animal Industry, has pro- duced a considerable num- ber of pure lines by long continued brother-and-sis- ter mating in guinea pigs. Sixteen families have been established by over thirty generations of the closest possible inbreeding. No selection was practiced. The result was that most of the pure lines showed marked reduction in fer- tility and in vigor, as compared with the control cross-bred stock. Each line, however, differed from all the others in these Fig. 51. — Diagram to il- lustrate the results of selection in pure lines. Ineffectiveness of selection through twelve generations within a homozy- gous strain in the case of the Colorado potato-beetle (Lepti- notarsa). In each generation extreme dark specimens were selected as the parents of the succeeding generation but the progeny always swung back to the type. (After Tower.) HEREDITY IN PURE LINE 217 respects, as well as in Mem hers of any pure lines were remarkably uniform but there was also a fair amount of fluctuation with respect to quantitative characters. Even such characters as the shape and extent of color markings, weight, fertility, etc., showed con- siderable individual variability, but none of these differences proved to be hereditary. Jennings tried still another type of pure-line material, using organ- isms that reproduce by binary fission. He worked with a number of differing clones (pure lines produced asexually) of Paramecium, a com- mon ciliate protozoan. Each selected Paramecium was isolated in a separate culture vessel, where it was allowed to multiply for several generations. Size differences and other measurable differences were noted in the original isolated parent individuals, and those of all the progeny were measured, plotted, and averaged. It was found that a different mean, mode, and average size was characteristic for each pure line. If the largest individual is then isolated from each culture and bred for a number of generations, its progeny will be of the same aver- age proportions as that of the stock from which the largest individual had been chosen, for they are genotypically unchanged, though differ- ing phenotypically. Exactly what is inherited? — All that is passed on from one genera- tion to another is an organized mass of protoplasm, or in gametic re- production two such organized masses — an egg and a sperm that unite to form a zygote. Characters, as such, are not transmitted or passed on. A zygote, the biologic heritage, has no eyes, no feet, no brain, no instincts. All it possesses is a very definite nuclear and cytoplasmic organization, which, under an appropriate environment, has the po- tentiality of producing a new individual with characters whose expres- sion may be more or less modified by the differences in the environ- ment within the organism or outside of it. Specifically, let us state exactly what was inherited in Johannsen's pure lines. Pure Line I differs in its hereditary potentialities from Pure Line II in that, under the given conditions of the environment, it produces on the average heavier beans than Pure Line II is able to produce. If a particular bean chosen from Pure Line I and one from Pure Line II were allowed to develop under exactly the same environ- ment, the offspring of Pure Line I would be heavier than that of Pure Line II. But if the environment of Pure Line II were better than that of Pure Line I, it might readily produce larger beans that the latter. But this environmentallv induced difference would not be inherited. 218 EVOLUTION, GENETICS, AND EUGENICS Exactly what, then, is inherited? A certain complex of genes and a certain cytoplasmic organization are all that can be said to be in- herited. These heritages are to be conceived of as potentialities cap- able of working with the environment to produce individuals with certain particular characters. Individuals will differ from one another partly because the materials constituting the zygote are different and partly because their environments are different. To what extent heredity and environment determine individual differences we must not attempt to decide at present. We shall be in a much better posi- tion to deal with this problem after we have discussed the other factors of the evolution mechanism. CHAPTER XVII SEX DETERMINATION AND SEX DIFFERENTIATION Introductory statement. — In chapter xiv we have spoken of sexual reproduction as one of the modes of cytogenic reproduction. We have also referred several times to the ways in which sexual reproduction constitutes the main mechanism of diversity. Two questions with regard to sex, however, have not yet been broached: (a) What de- termines the sex of the individual? (b) How do the secondary sexual characters of individuals develop? These two questions will be an- swered in the present chapter. SEX DETERMINATION The question as to what determines whether an animal shall be a male or a female is a very ancient one, and it is only during the present century that we have solved the puzzle. A great many theories of sex determination have been proposed, some of which are as follows : a) Hippocrates and some subsequent theorists believed that the sex of the offspring depended on the relative vigor of the parents, the more vigorous parent giving his or her sex to the offspring. b) Thury thought that the sex of the offspring depended on the degree of ripeness of the ovum at the time of fertilization. c) Various writers claim that statistics show that germ cells from the right ovary produce males and those from the' left ovary females. d) The nutrition theory. — The egg is a much more highly nourished cell than the spermatozoon, and the idea seems natural that high degrees of nourishment of the mother produce female offspring and lower degrees of nourishment male offspring. Professor Schenk of Vienna gained a huge reputation by controlling the diet of certain royal prospective mothers and predicting the sex of the offspring accordingly. He was correct in his predictions several times, but his success was short-lived. His early predictions were merely lucky, just as one might be who could guess heads or tails correctly several times in succession. Some color is lent to the nutrition hypothesis by the fact, if it is a fact, that after war or famine, when the nutrition of mothers has been 2TQ >20 EVOLUTION, GENETICS, AND EUGENICS low, more males than females are born. This is probably a case of differential prenatal mortality. By that we mean that more females die unborn than males, because the latter are hardier and stand pre- natal malnutrition better. e) Sex is determined at the time of fertilization. — Perhaps the best evidence that sex is determined at the very beginning of development Fig. 52. — An armadillo egg about six weeks after fertilization, showing the quadruplet fetuses derived from the single egg and all destined to be of the same sex. (From Newman.) is derived from one-egg twins and quadruplets. In the nine-banded armadillo practically every female gives birth to quadruplets, four essentially identical young being produced in each litter. All in a given set of quadruplets are invariably of the same sex, either four males or four females. Newman and Patterson have shown that each set of quadruplets comes from a single egg which at a very early stage SEX DETERMINATION AND SEX DIFFERENTIATION 221 divides into four parts to form four fetuses (Fig. 52). The conclusion is that sex was determined before the separation took place. Human identical twins, also always of the same sex in a pair, furnish further evidence in favor of very early sex determination. These and nu- merous other similar facts justify the conclusion that sex is deter- mined at the time of fertilization. THE CHROMOSOMAL MECHANISM OF SEX DETERMINATION In two previous chapters (chaps, xv and xvi) descriptions of the typical modes of chromosomal sex determination have been given. In order to facilitate a clear understanding of this important matter, it seems well to recapitulate one typical instance. Perhaps the best- known instance of sex determination is that of Drosopkila melanogaster , already described and figured (Fig. 53) by Babcock and Clausen. In this insect the female body cells and the unmaturated germ cells are characterized by the presence of two sex chromosomes (X-chromo- somes), which are shown in black at the top of the left-hand column of the accompanying figure. The chromosomes are readily dis- tinguishable by being of medium size and straight. The male body cells and unmaturated germ cells (top of right column) are just like those of the female except that there is substituted for one of the X-chromosomes a hook-like chromosome, known as a Y-chromosome. Now in the process of maturation of the germ cells, which results in the formation of gametes with the haploid or half-somatic number of chromosomes, each of the eggs (female gametes) receives an X-chromo- some. All eggs are therefore alike in their chromosome content, in- cluding the sex chromosome. The case is different on the male side; for two kinds of gametes are formed, one kind with an X-chromosome and the other with a Y-chromosome. These are formed in exactly equal numbers, as one of each is produced at every reduction division. Each egg must be fertilized by one or the other of these two kinds of sperms, and in the long run as many eggs will receive an X-chromosome as will receive a Y-chromosome. Those that receive an X-chromosome will be characterized by having two X-chromosomes, which is the typical female condition, and thus a new female individual is started in life; while those that receive no X-chromosome, but a Y-chromo- some, will have the XY composition characteristic of the male sex, and will give rise to males. The female sex may thus be designated as XX and the male sex as XY. We have shown for Drosopkila the exact mechanism that operates in determining whether an individual 222 EVOLUTION, GENETICS, AND EUGENICS shall be a male or a female, and in addition we have explained why equal numbers of both sexes are continuously produced. How general is the chromosomal mechanism of sex-determina- Fig. 53. — Diagram showing chromosome relations in the determination of sex in Drosopliila ampelopliila. {From Babcock and Clausen.) tion? — "To what extent," says E. B. Wilson, "sex may be determined by an automatically operating nuclear mechanism such as has been here described is unknown; but a mechanism that exists in the same SEX DETERMINATION AND SEX DIFFERENTIATION 223 general form in organisms as diverse as bryophytes, nematodes, echi- noderms, arthropods and vertebrates is beyond a doubt of far-reaching significance, and may be as widely distributed as Mendelian heredity generally." While the same general scheme holds for all forms that have been investigated, there exist many interesting differences in the details of operation of the sex-determining machine. Some of the simpler variations of the process are as follows: a) Variations of the Y -chromosome. — Beginning with a condition such as that described for Drosophila, in which the Y-chromosome is larger than the X-chromosome, there is a long series of species in which the Y-chromosome becomes smaller and smaller until it dwindles away to nothing and the male chromosome condition becomes XO in- stead of XY. In the females of such species the condition remains XX. b) Variations of the X-chromosome. — In a number of species of animals the X-chromosome may be represented by from two to nine components, each of which at times has the appearance of a separate chromosome. In a species of roundworms, Ascaris canis, for example, the diploid chromosome number of the female is thirty-six and that of the male is thirty, the difference being due to the fact that there are two sets of six X-components in the female and only one set in the male. In the reduction division of the male germ cell, the six X-components all go in a group to one gamete and none to the other, so that two kinds of gametes are produced, one with eighteen chromosomes and the other with twelve chromosomes. All the female gametes have eighteen chromosomes. Apart from the fact that the X-chromosome is in six pieces instead of but one, the mechanism of sex determination is the same as it is in a group that has but one X-chromosome. c) Linkage of sex chromosome with autosome. — In a great many species of insects the X-chromosome has been found to be united to one end of one of the autosomes, never losing this relation during the entire chromosome cycle. Apart from this apparently secondary union with an autosome, the behavior of the X-chromosome is the same as in the XO cases described above. Hence the mode of sex deter- mination is in line with the types already discussed. d) Female digamety. — In this mode of sex determination two differ- ent kinds of eggs are produced, while the sperms are all alike. In other words, there is simply an exchange between the sexes of the nuclear differences characterizing males and females. Thus in the Lepidoptera (butterflies and moths) the females have either the XY or the XO type of chromosome complex, while the males always have the XX condi- 224 EVOLUTION, GENETICS, AND EUGENICS tion. Though the cytological evidence is still incomplete, it is prac- tically certain that birds have the same peculiar method of chromo- somal sex determination as the Lepidoptera, for they have the same type of sex-linked heredity as the latter and the opposite of that seen in mammals and most insects. Apart from the change of the digametic condition from one sex to the other, the mechanism remains the same. Sex chromosomes in parthenogenesis. — When it became known that parthenogenetic species (those in which eggs are capable of de- veloping without fertilization) in some cases produce males and in other cases produce females from parthenogenetic eggs, this seemed to be out of accord with the theory of the chromosome mechanism of sex determination. It is interesting to know, however, that, now that we know the histories of the chromosome cycles in these species, the facts are not only fully in accord with the chromosome theory, but greatly strengthen it and enlarge its range of applicability. Two kinds of parthenogenesis are known, which may be designated diploid and haploid. In the former, the developing egg and embryo has the full somatic number of chromosomes; in the latter, only half the somatic number characteristic of the species is present. a) Diploid parthenogenesis. — In these species only one maturation division occurs, and this division is not the reduction division; hence each egg retains the diploid number of chromosomes, including two X-chromosomes (XX). The result is that all eggs that behave in this way develop into females. Thus in aphids and phyloxerans many suc- cessive generations . of all females are produced. After a series of female generations, a mixed generation appears in which males are produced parthenogenetically along with females, but from smaller eggs. Examination reveals the fact that male-producing eggs have, after maturation, two less chromosomes than the female-producing eggs. This was explained by the observation that when the first maturation takes place, two chromosomes (obviously consisting of a double X-element) are cast out into the polar body, while all the auto- somes and two of the X-chromosomes remain in the egg nucleus. In this way the male produced from this egg comes to have only two X-chromosomes, while the female has four. This is really the equiv- alent of XX for the female and XO for the male. In gamete forma- tion the males produce two kinds of gametes, one with the double X-element and the other with no X-element. Only the former of these is viable; and this accounts for the fact that all fertilized eggs produce females, for both gametes supply double X-elements. This SEX DETERMINATION AND SEX DIFFERENTIATION 225 whole rather intricate story is thus seen to be merely a variant upon the typical scheme of chromosomal sex determination. b) Haploid parthenogenesis. — This kind of parthenogenesis is now known to occur in rotifers, in several orders of insects, and in arach- nids. It is practically universal among the Hymenoptera (bees, wasps, ants, etc.), and we may use the case of the honey bee as an illustration. In haploid parthenogenesis the egg develops after having undergone the reduction division ; it therefore has only half the somatic number of chro- mosomes, including but one X-chromosome. Invariably the progeny from haploid parthenogenesis are males, which we might expect from the fact that they have but one X-chromosome. In the bees the queen seems to be able to determine whether an egg gets fertilized or not. An egg descends the oviduct, passes the seminal receptacle containing a supply of sperms acquired during the mating act, and if sperms are given off, fertilization occurs and a female is produced; but if an egg slips past the seminal receptacle without being fertilized, the result is a male (drone). Now these drones are the mates of the future queens, and must supply the spermatozoa for the next generation of eggs. They already possess the reduced number of chromosomes, so they cannot well undergo the reduction division in forming gametes. It is inter- esting to note, however, that a sort of vestigial reduction division takes place resulting in the formation of a tiny cell without any nucleus and a larger cell with all the chromosomes (including one X-chromosome) characteristic of males of the species. A second maturat ion division di- vides chromosomes lengthwise. Since all gametes, both male and fe- male, contain an X-chromosome, fertilization always results in a female. Thus once more the general sex-determination formula is confirmed. Sex-chromosomes in hermaphrodites and gynandromorphs. — Hermaphrodites are individuals which are functionally both male and female, that produce both eggs and sperms in the same body. Her- maphroditism is common in snails, flatworms, earthworms, nematodes, tunicates, and in several other phyla of animals. We have unfortu- nately very little information about the chromosome situations in these forms. In one species of nematode {Angiostomum nigrovenosum) , however, it is known that there is an alternation of generations between a parasitic hermaphroditic generation and a free-living dioecious gen- eration (with separate males and females). In the dioecious genera- tion males and females are about equally numerous. All fertilized eggs of this generation produce parasitic hermaphrodites. These pro- duce from their gonads first oogonia and later spermatogonia, the form- 226 EVOLUTION, GENETICS, AND EUGENICS er producing eggs and the latter spermatozoa. It is known that all eggs of the hermaphrodite generation have six chromosomes, while the sperms have either five or six. Self-fertilization takes place, and half of the fertilized eggs produce males with (eleven chromosomes) and half produce females (with twelve chromosomes) of the free-living gen- eration. The males of the dioecious generation produce two kinds of gametes with respectively five and six chromosomes, and one would expect males and females to be produced from fertilization; but this is not what happens, for only hermaphrodite individuals with twelve chromosomes are produced. It seems certain that only one of the two kinds of spermatozoa (that with six chromosomes) is viable, and that the hermaphrodite generation is chromosomally female. How can a female produce spermatozoa of two kinds, one with six and the other with five chromosomes? This is explained by the fact that in the second maturation division one of the X-chromosomes remains near the equator of the spindle, and does not become included within the daughter-nucleus. Thus one of the daughter-cells is without an X-chromosome and is male-producing when fertilization takes place. Further investigation of the chromosomes of hermaphrodites will doubtless be in agreement with what we already know. Gynandromorphs are individuals made up of some female body regions and some male body regions. Thus, an insect may have male secondary sexual characters on one half of the body and female char- acters on the other; or the anterior end may be male and the posterior, female. The chromosomal basis for these conditions is not entirely clear, but Morgan and Bridges have shown that all of the peculiarities of the hereditary behavior can be explained on the assumption that in the first or second cleavage division one of the X-chromosomes lags behind and is excluded from one of the daughter-cells. Thus one daughter-cell gets XX and the other X, which accounts for the fact that all the cell descendants of one cell have the female characters and all those of the other cell, male characters. Intersexes and their bearing on sex determination. — Bridges, dur- ing his experiments with Drosophila, encountered in certain strains anomalous individuals that were neither male nor female, but inter- sexes. On cytological examination these were found to have a changed chromosome complex. One type, for example, was found to have three of one kind of autosomes (instead of the usual two) but only two X-chromosomes. The interesexual condition in this case might be explained by the assumption that the autosomes have a male-produc- SEX DETERMINATION AND SEX DIFFERENTIATION 227 ing tendency and that one set of extra autosomes is sufficient partially to overcome the female tendency of two X-chromosomes, thus produc- ing intersexes. Again, individuals with three X-chromosomes but only the usual supply of autosomes were super-femaleb somatically, but unbalanced in their physiology and non-viable. These results show that, in the words of E. B. Wilson, "the actual performance of the zygote, therefore, is the common effect of the whole group, and is turned this way or that as the result of a quantitative balance between X-chromo- somes and autosomes." SEX DIFFERENTIATION It now becomes necessary to distinguish clearly between sex determination and sex differentiation. When we say that by means of a chromosomal mechanism sex Is determined, exactly what do we mean ? We answer that the sex of an individual arising from a fertil- ized egg (in the case of parthenogenesis, an unfertilized egg) has been settled. Now as a matter of fact only one thing has been settled irrevo- cably, and that is that one individual will have the chromosome composition characteristic of a male and another individual that of a female. A male is usually an individual that produces spermatozoa and a female one that produces ova. Is it irrevocably settled beyond possibility of reversal that a zygote with the XX chromosome com- position must produce eggs and one with the X composition, sper- matozoa ? This question has apparently been answered by Geoffrey Smith in his work on parasitically castrated crabs and by Richard Goldschmidt on Gypsy moths. In the first case, individual crabs whose testes had been infested by the parasitic cirripede, Sacculina, were gradually changed over in their whole metabolism to such an extent that cells destined to produce spermatozoa produced ova. In the second case, when certain varieties of moth were crossed, all of the germ cells produced females with ova, whereas half of the eggs had the XX and half the X chromosome content. This evidently means that some individuals with the male chromosome character produced eggs. From these results we may be justified in conclud- ing that not even this most fundamental difference of sexes, that of the female producing ova and the male spermatozoa, is irrevocably predetermined at fertilization. Lest the reader be confused, however, we hasten to add that under natural conditions of life an individual with the male chromosomal content produces spermatozoa and one with the female chromosomal content produces eggs, and that only rare accidental or unnatural 228 EVOLUTION, GENETICS, AND EUGENICS conditions disturb the normal course of events. For purposes of practical genetics we may then define a female as an individual that produces ova and a male as one that produces spermatozoa. Secondary sexual characters. — Usually males and females differ from each other in many other characters besides the production of eggs or sperm. Often one sex is larger, stronger, more elaborately ornamented and colored than the other and possesses characteristic accessory sex organs whose function it is to facilitate the bringing together of the eggs and the sperm. All of the differences between the sexes other than the primary difference of egg or sperm production are called secondary sexual characters. Usually very young animals show only slight differences in secondary sexual characters and the differ- ences increase markedly at sexual maturity. We speak of the gradual divergent development of the two sex types as sex differentiation. The question arises as to whether or not the chromosomal differences are the causes of the differentiation of secondary sexual characters. These secondary sexual characters are all somatic, and, since the soma is the product of cell division of the zygote, the soma cells must have either the male or the female chromosomal character. That the chromosomal mechanism in the somatic cells is not sufficient of itself to bring about, unaided, the differentiation of secondary sexual charac- ters can be shown readily in at least many animals. In the mammals, for example, it is known that the early removal of the testes or ovaries results in a retention of the juvenile or undif- ferentiated condition of secondary sexual characters. Evidently some influence is exerted by the tissues of the gonad that is necessary for the full differentiation of sex characters. The current theory is that certain glandular cells that form part of the body of ovary or testes excrete materials into the blood that stimulate various tissues in different ways and produce dimorphic results. The specific sub- stances produced by these glands are called "hormones," for want of a better name. To test the efficiency of these hormones the crucial experiment of taking out the gonads of a young rat or guinea pig and implanting the gonad of an individual of the opposite sex has been many times performed. For example, Steinach castrated young male rats and then successfully grafted into them ovaries from young female rats. The result was that these young rats which started to be males became much altered in a female direction, the mammary glands becoming greatly enlarged, their instincts more feminine than masculine, and in a number of other particulars they showed more or less pronounced evidences of feminization. Conversely, spayed SEX DETERMINATION AND SEX DIFFERENTIATION 229 females with engrafted testes showed a tendency toward male differ- entiation, especially in instincts. These experiments have been largely confirmed by C. R. Moore. In birds it is of interest to note that practically complete reversal of secondary sexual characters may be induced if young females are entirely deprived of the ovary. The condition is described by L. V. Domm as follows: "The larger percentage of our birds have assumed additional male characters following removal of the ovaries, until they are practically complete replicas of the male, and, to those not familiar with their history, they are regarded as unmistakable males. Thus we find that they assume the complete male plumage, spurs grow as they do in the normal cock, head furnishings increase in size until they can not be distinguished from those of the normal male. "Other birds in the pen regard them as males and when a strange cock is introduced they fight as would other cocks, very frequently assuming the initiative, some of them having been observed to come off victorious in such a combat. Many of these birds crow regularly. When aroused by a disturbance, it was found that their reaction is very similar to that of the male; the sounds they make, together with their reaction on such occasions, reminds one very much of the young male just prior to maturity. "One set of experiments may be mentioned as an example: Out of the one lot of fourteen females of the same hatch, one was kept as control and thirteen were operated upon between the ages of six weeks and six months; twelve of these have developed all the characteristics of the male mentioned above, some being completely cock-feathered, while the others are fast becoming so. The other one of the thirteen is very capon-like in appearance except perhaps for size and can not be readily distinguished from her capon brothers by those not know- ing her history. This bird has assumed complete male plumage, is developing spurs; but the comb, wattles and earlobes are pale and small, resembling those of the capon. "In some of our cases individuals which have assumed more or less complete male characters as concerns head furnishings, plumage and spurs, are reverting toward the female type as shown by the female type of plumage reappearing. "Our results indicate that the female in the brown leghorn fowl has many potentialities of the male, which are normally inhibited by the presence of the ovary, and that these potentialities can assert 230 EVOLUTION, GENETICS, AND EUGENICS CO c en en - 43 o TJ CO tn G 4-> to 5 £« co 4>J +j 8 o 4-1 o o to o .s to § fe; M rt 13 g CD u 43 en g a CO to O Oh O ^ £ TJ M-l c8 43 fl £ 03 TJ "a d G 43 i- co be Q co •f4 a CO .S to 'Jn 4= S3 43 to rt +J