D. D.
F. F.
CONTENTS
PAGE
[CHAPTER I] 1 Rise of the Theory of Natural Selection and its Subsequent Development Pre-Darwinian Evolutionists—Causes which led to the speedy triumph of the theory of Natural Selection—Nature of the opposition which Darwin had to overcome—Post-Darwinian biology—Usually accepted classification of present-day biologists as Neo-Lamarckians and Neo-Darwinians is faulty—Biologists fall into three classes rather than two—Neo-Lamarckism: its defects—Wallaceism: its defects—Neo-Darwinism distinguished from Neo-Lamarckism and Wallaceism—Neo-Darwinism realises the strength and weakness of the theory of Natural Selection, recognises the complexity of the problems which biologists are endeavouring to solve. [CHAPTER II] 30 Some of the more Important Objections to the Theory of Natural Selection Brief statement of Theory—Objections to the Theory fall into two classes—Those which strike at the root of the Theory—Those which deny the all-sufficiency of Natural Selection—Objections which strike at root of Theory are based on misconception—Objections to Wallaceism—The Theory fails to explain the origin of Variations—Natural Selection called on to explain too much—Unable to explain beginnings of new organs—The Theory of change of function—The co-ordination of variations—The fertility of races of domesticated animals—Missing links—Swamping effects of intercrossing—Small variations cannot have a survival value—Races inhabiting same area—Excessive specialisation—Chance and Natural Selection—Struggle for existence most severe among young animals—Natural Selection fails to explain mimicry and other phenomena of colour—Conclusion, that scarcely an organism exists which does not possess some feature inexplicable on the theory of Natural Selection as held by Wallace and his followers. [CHAPTER III] 52 Variation The assumption of Darwin and Wallace that variations are haphazard in origin and indefinite in direction—If these assumptions be not correct Natural Selection ceases to be the fundamental factor in evolution—Darwin’s views regarding variation underwent modification—He eventually recognised the distinction between definite and indefinite variations, and between continuous and discontinuous variations—Darwin attached but little importance to either definite or discontinuous variations—Darwin’s views on the causes of variations—Criticism of Darwin’s views—Variations appear to occur along certain definite lines—There seems to be a limit to the extent to which fluctuating variations can be accumulated—De Vries’ experiments—Bateson on “discontinuous variation”—Views held by De Vries—Distinction between continuous and discontinuous variations—The work of De Vries—Advantages enjoyed by the botanist in experimenting on the making of species—Difficulties encountered by the animal breeder—Mutations among animals—The distinction between germinal and somatic variations—The latter, though not transmitted to offspring, are often of considerable value to their possessor in the struggle for existence. [CHAPTER IV] 111 Hybridism The alleged sterility of hybrids a stumbling-block to evolutionists—Huxley’s views—Wallace on the sterility of hybrids—Darwin on the same—Wallace’s theory that the infertility of hybrids has been caused by Natural Selection so as to prevent the evils of intercrossing—Crosses between distinct species not necessarily infertile—Fertile crosses between species of plants—Sterile plant hybrids—Fertile mammalian hybrids—Fertile bird hybrids—Fertile hybrids among amphibia—Limits of hybridisation—Multiple hybrids—Characters of hybrids—Hybridism does not appear to have exercised much effect on the origin of new species. [CHAPTER V] 133 Inheritance Phenomena which a complete theory of inheritance must explain—In the present state of our knowledge it is not possible to formulate a complete theory of inheritance—Different kinds of inheritance—Mendel’s experiments and theory—The value and importance of Mendelism has been exaggerated—Dominance sometimes imperfect—Behaviour of the nucleus of the sexual cell—Chromosomes—Experiments of Delage and Loeb—Those of Cuénot on mice and Castle on guinea pigs—Suggested modification of the generally-accepted Mendelian formulæ—Unit characters—Biological isomerism—Biological molecules—Interpretation of the phenomena of variation and heredity on the conception of biological molecules—Correlation—Summary of the conception of biological molecules. [CHAPTER VI] 170 The Colouration of Organisms The theory of protective colouration has been carried to absurd lengths—It will not bear close scrutiny—Cryptic colouring—Sematic colours—Pseudo-sematic colours—Batesian and Müllerian mimicry—Conditions necessary for mimicry—Examples—Recognition markings—The theory of obliterative colouration—Criticism of the theory—Objections to the theory of cryptic colouring—Whiteness of the Arctic fauna is exaggerated—Illustrative tables—Pelagic organisms—Objectors to the Neo-Darwinian theories of colouration are to be found among field naturalists—G. A. B. Dewar, Gadow, Robinson, F. C. Selous quoted—Colours of birds’ eggs—Warning colouration—Objections to the theory—Eisig’s theory—So-called intimidating attitudes of animals—Mimicry—The case for the theory—The case against the theory—“False mimicry”—Theory of recognition colours—The theory refuted—Colours of flowers and fruits—Neo-Darwinian explanations—Objections—Kay Robinson’s theory—Conclusion that Neo-Darwinian theories are untenable—Some suggestions regarding the colouration of animals—Through the diversity of colouring of organisms something like order runs—The connection between biological molecules and colour—Tylor on colour patterns in animals—Bonhote’s theory of pœcilomeres—Summary of conclusions arrived at. [CHAPTER VII] 297 Sexual Dimorphism Meaning of the term—Fatal to Wallaceism—Sexual Selection—The law of battle—Female preference—Mutual Selection—Finn’s experiments—Objections to the theory of Sexual Selection—Wallace’s explanation of sexual dimorphism stated and shown to be unsatisfactory—The explanation of Thomson and Geddes shown to be inadequate—Stolzmann’s theory stated and criticised—Neo-Lamarckian explanation of sexual dimorphism stated and criticised—Some features of sexual dimorphism—Dissimilarity of the sexes probably arises as a sudden mutation—The four kinds of mutations—Sexual dimorphism having shown itself, Natural Selection determines whether or not the organisms which display it shall survive. [CHAPTER VIII] 345 The Factors of Evolution Variation along definite lines and Natural Selection are undoubtedly important factors of evolution—Whether or not sexual selection is a factor we are not yet in a position to decide—Modus operandi of Natural Selection—Correlation an important factor—Examples of correlation—Correlation is a subject that requires close study—Isolation a factor in evolution—Discriminate isolation—Indiscriminate isolation—Is the latter a factor?—Romanes’ views—Criticism of these—Indiscriminate isolation shown to be a factor—Summary of the methods in which new species arise—Natural Selection does not make species—It merely decides which of certain ready-made forms shall survive—Natural Selection compared to a competitive examination and to a medical board—We are yet in darkness as to the fundamental causes of the Origin of Species—In experiment and observation rather than speculation lies the hope of discovering the nature of these causes. [Footnotes] 389 [Index] 389
LIST OF ILLUSTRATIONS
FACING PAGE
[Heck’s Curassow feeding Young Bird, which has the Plumage of the Hens of the Globose Curassow, its Father’s Species] Frontispiece By permission of the Avicultural Society. [A Turbit belonging to Mr H. P. Scatliff] 92 From “The Modern Turbit,” published by “The Feathered World,” London. [Yellow-Rumped and Chestnut-Breasted Finches, with Specimens in Transitional State] 98 On the left, the yellow-rumped finch; on the right, the chestnut-breasted; birds in state of change in the middle. By permission of the Avicultural Society. [Male Amherst Pheasant] 122 The chief colours of this species (Chrysolophus amherstiæ) are white and metallic green, so that it is very different in appearance from its near ally the gold pheasant. [Harlequin Quail (Coturnix delegorguei)] 124 By permission of the Avicultural Society. [Rain Quail (Coturnix coromandelica)] 124 The markings on the throats of these quails are of the type usually put down as “recognition marks,” but as the Harlequin Quail is African and the Rain Quail Indian, the two species cannot possibly interbreed. The pattern, then, can have no “recognition” significance. By permission of the Avicultural Society. [Bouru Friar-Bird] 222 Like most of the group to which it belongs, this honey-eater (Tropidorhynchus bouruensis) is a soberly coloured bird, but is noisy, active, and aggressive. By permission of Messrs Hutchinson & Co. [Bouru Oriole] 222 This “mimicking” oriole (Oriolus bouruensis) is of the same tone of colour as its supposed model the Friar-bird of the same island. By permission of Messrs Hutchinson & Co. [King-Crow or Drongo] 232 This very conspicuous black bird (Dicrurus ater), ranging from Africa to China, is a striking feature of the landscape wherever it occurs. By permission of Messrs Hutchinson & Co. [Drongo-Cuckoo] 232 The fork of the tail in this bird is unique among cuckoos, but is nevertheless much less developed than in the supposed model, and may be an adaptation for evolutions in flight, as such tails usually appear to be. By permission of Messrs Hutchinson & Co. [Shikra Hawk] 236 The upper surface of the tail, not shown in this drawing, exactly corresponds with that of the cuckoo “mimic.” By permission of Messrs Hutchinson & Co. [Hawk-Cuckoo] 236 This species (Hierococcyx varius) is commonly known in India as the “Brain-fever bird.” By permission of Messrs Hutchinson & Co. [Brazilian Troupial] 284 This species (Icterus vulgaris) is that most frequently seen in captivity; the pattern of colour is found in several other allied forms. By permission of Messrs Hutchinson & Co. [Indian Black-Headed Oriole] 284 Several other orioles besides this (O. melanocephalus) have the black head. By permission of Messrs Hutchinson & Co. [Queen Whydah] 298 This species (Tetraenura regia) is a typical example of seasonal sexual dimorphism, the male being long-tailed and conspicuously coloured only during the breeding season, and at other times resembling the sparrow-like female. By permission of the Foreign Bird Club. [Courtship of Skylark] 314 Illustrating display by a species with no decorative colouring or sex difference.
THE MAKING OF SPECIES
CHAPTER I
RISE OF THE THEORY OF NATURAL SELECTION AND ITS SUBSEQUENT DEVELOPMENT
Pre-Darwinian Evolutionists—Causes which led to the speedy triumph of the theory of Natural Selection—Nature of the opposition which Darwin had to overcome—Post-Darwinian biology—Usually accepted classification of present-day biologists as Neo-Lamarckians and Neo-Darwinians is faulty—Biologists fall into three classes rather than two—Neo-Lamarckism: its defects—Wallaceism: its defects—Neo-Darwinism distinguished from Neo-Lamarckism and Wallaceism—Neo-Darwinism realises the strength and weakness of the theory of Natural Selection, recognises the complexity of the problems which biologists are endeavouring to solve.