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