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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 poecilomeres—?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