Darwinism | Page 3

deer--Of the scale-ornamentation of reptiles--Instability of non-adaptive characters--Delboeuf's law--No "specific" character proved to be useless--The swamping effects of intercrossing--Isolation as preventing intercrossing--Gulick on the effects of isolation--Cases in which isolation is ineffective

CHAPTER VII
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING
Statement of the problem--Extreme susceptibility of the reproductive functions--Reciprocal crosses--Individual differences in respect to cross-fertilisation--Dimorphism and trimorphism among plants--Cases of the fertility of hybrids and of the infertility of mongrels--The effects of close interbreeding--Mr. Huth's objections--Fertile hybrids among animals--Fertility of hybrids among plants--Cases of sterility of mongrels--Parallelism between crossing and change of conditions--Remarks on the facts of hybridity--Sterility due to changed conditions and usually correlated with other characters--Correlation of colour with constitutional peculiarities--The isolation of varieties by selective association--The influence of natural selection upon sterility and fertility--Physiological selection--Summary and concluding remarks

CHAPTER VIII
THE ORIGIN AND USES OF COLOUR IN ANIMALS
The Darwinian theory threw new light on organic colour--The problem to be solved--The constancy of animal colour indicates utility--Colour and environment--Arctic animals white--Exceptions prove the rule--Desert, forest, nocturnal, and oceanic animals--General theories of animal colour--Variable protective colouring--Mr. Poulton's experiments--Special or local colour adaptations--Imitation of particular objects--How they have been produced--Special protective colouring of butterflies--Protective resemblance among marine animals--Protection by terrifying enemies--Alluring coloration--The coloration of birds' eggs--Colour as a means of recognition--Summary of the preceding exposition--Influence of locality or of climate on colour--Concluding remarks

CHAPTER IX
WARNING COLORATION AND MIMICRY
The skunk as an example of warning coloration--Warning colours among insects--Butterflies--Caterpillars--Mimicry--How mimicry has been produced--Heliconidae--Perfection of the imitation--Other cases of mimicry among Lepidoptera--Mimicry among protected groups--Its explanation--Extension of the principle--Mimicry in other orders of insects--Mimicry among the vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among birds--Objections to the theory of mimicry--Concluding remarks on warning colours and mimicry

CHAPTER X
COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX
Sex colours in the mollusca and crustacea--In insects--In butterflies and moths--Probable causes of these colours--Sexual selection as a supposed cause--Sexual coloration of birds--Cause of dull colours of female birds--Relation of sex colour to nesting habits--Sexual colours of other vertebrates--Sexual selection by the struggles of males--Sexual characters due to natural selection--Decorative plumage of males and its effect on the females--Display of decorative plumage by the males--A theory of animal coloration--The origin of accessory plumes--Development of accessory plumes and their display--The effect of female preference will be neutralised by natural selection--General laws of animal coloration--Concluding remarks

CHAPTER XI
THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
The general colour relations of plants--Colours of fruits--The meaning of nuts--Edible or attractive fruits--The colours of flowers--Modes of securing cross-fertilisation--The interpretation of the facts--Summary of additional facts bearing on insect fertilisation--Fertilisation of flowers by birds--Self-fertilisation of flowers--Difficulties and contradictions--Intercrossing not necessarily advantageous--Supposed evil results of close interbreeding--How the struggle for existence acts among flowers--Flowers the product of insect agency--Concluding remarks on colour in nature

CHAPTER XII
THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
The facts to be explained--The conditions which have determined distribution--The permanence of oceans--Oceanic and continental areas--Madagascar and New Zealand--The teachings of the thousand-fathom line--The distribution of marsupials--The distribution of tapirs--Powers of dispersal as illustrated by insular organisms--Birds and insects at sea--Insects at great altitudes--The dispersal of plants--Dispersal of seeds by the wind--Mineral matter carried by the wind--Objections to the theory of wind-dispersal answered--Explanation of north temperate plants in the southern hemisphere--No proof of glaciation in the tropics--Lower temperature not needed to explain the facts--Concluding remarks

CHAPTER XIII
THE GEOLOGICAL EVIDENCES OF EVOLUTION
What we may expect--The number of known species of extinct animals--Causes of the imperfection of the geological record--Geological evidences of evolution--Shells--Crocodiles--The rhinoceros tribe--The pedigree of the horse tribe--Development of deer's horns--Brain development--Local relations of fossil and living animals--Cause of extinction of large animals--Indications of general progress in plants and animals--The progressive development of plants--Possible cause of sudden late appearance of exogens--Geological distribution of insects--Geological succession of vertebrata--Concluding remarks

CHAPTER XIV
FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
Fundamental difficulties and objections--Mr. Herbert Spencer's factors of organic evolution--Disuse and effects of withdrawal of natural selection--Supposed effects of disuse among wild animals--Difficulty as to co-adaptation of parts by variation and selection--Direct action of the environment--The American school of evolutionists--Origin of the feet of the ungulates--Supposed action of animal intelligence--Semper on the direct influence of the environment--Professor Geddes's theory of variation in plants--Objections to the theory--On the origin of spines--Variation and selection overpower the effects of use and disuse--Supposed action of the environment in imitating variations--Weismann's theory of heredity--The cause of variation--The non-heredity of acquired characters--The theory of instinct--Concluding remarks

CHAPTER XV
DARWINISM APPLIED TO MAN
General identity of human and animal structure--Rudiments and variations showing relation of man to other mammals--The embryonic development of man and other mammalia--Diseases common to man and the lower animals--The animals most nearly allied to man--The brains of man and apes--External differences of man and apes--Summary of the animal characteristics of man--The geological antiquity of man--The probable birthplace of man--The origin of the moral and intellectual nature of man--The argument from continuity--The