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