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Show 186 DARWINISM CHAP. VJT are . usually correlated with variations of form or of colour. Hence, as fixed differences of form and colour, slowly gained by natural selection in adaptation to changed conditions, a.re what essentially characterise distinct species, some amount of infertility between species is the usual result. Here the problem was left by Mr. Darwin; but we have shown that its solution may be carried a step further. If we accept the association of some degree of infertility, however slight, as a not unfrequent accompaniment of the external differences which always arise in a state of nature between varieties and incipient species, it has been shown that na.tural selection has power to increase that infertility just as it has power to increase other favourable variations. Such an increase of infertility will be beneficial, whenever new species ari. c in the same area with the parent form; and we thus sec how, out of the fluctuating and very unequal amounts of infertility correlated with physical variations, there may have arisen that larger and more constant amount which appears usually to .characterise well-marked species. The great body of facts of which a condensed account has been given in the present chapter, although from an experimental point of view very insufficient, all point to the general conclusion we have now reached, and afford us a not unsatis factory solution of the great problem of hybridism in relation to the origin of species by means of natural selection. Further experimental research is needed in order to complete the elucidation of the subject; but until these additional facts arc forthcoming no new theory seems required for the cxplanaLiou of the phenomena. CHAPTER VIII THE ORIGIN AND Uf:lES 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 environm ent- Arctic animals white-Exceptious prove the ruleDesert, fore.-t, noctumal, and oceanic animals- Geneml theories of animal colour- Variable protective colou ring-l\1r. Poulton's experiments- Special or local colonr adaptations- Imitation of particular objects- How they have been produced- Special protec tive colouring of butterflies- Protective resemblance among marine animals-Protection by terrifying enemies- Alluring coloration- The coloration of birds' eggs--Colour a· a means of recognition- Summary of the preceding expositiou- Influencc of locality or of climate on colourCoucluding remarks. AMoNG the numerous applications of the Darwinian theory in the interpretation of the complex phenomena presented by the organic world, none have been more successful, or are more interesting, than those which deal with the colours of animals and plants. To the older school of naturalists colour was a trivial character, eminently unstable and untrustworthy in the determination of species; and it appeared to have, in most cases, no use or meaning to the ohjects which displayed it. The bright and often gorgeous coloration of insect, bird, or flower, was either looked upon as having been created for the enjoyment of mankind, or as due to unknown and perhaps undiscoverable laws of nature. But the researches of Mr. Darwin totally changed onr point of view in th!s matter. He showed, clearly, that some of the colours of animals are useful, some hurtful to them ; and he believed that many of the most brilliant colours were clcvclopccl by sexual choice; while his great general principle, that all |