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.
- public@vhost@g@gutenberg@html@files@44063@[email protected]#fig5"
