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Show CHAPTER X COLOURS AND ORNAMENTS CHARACTERISTIC OF ~EX Sex colours in the mollusca and crustacea-In insects-In butterflies and moths- Probable causes of these colours- Sexual selection a:; ;t supposed cause-Sexual coloration of bird :-Caus~ of dull colour:; of female birds-Relation of sex colour to nestmg hab1ts- exual colours of other vertebrates-Sexual selection by the struggle of malesSexual characters due to natural selection-Decorative plumage of males and its effect on the females-Display of decorative plumage hy the males-A theory of animal coloration-The origin of accesso ry plumes-Development of acces~ ory plum~s and their display -:-Thc effect of female preference will be neutralised by natural selectwnGenerallaws of animal coloration-Concluding remarks. IN the preceding chapters we have dealt chiefly with the coloration of animals as distinctive of the several ~peci ' :-i ; and we have seen that, in an enormous number of cases, the colours can be shown to have a definite purpose, and t o bo useful either as a means of protection or conce:1lment, of warning to enemies, or of recognition by their own kind. \V e have now to consider a subordinate but very widespread phenomenon-the differences of colour or of ornamental appendages in the two sexes. These differences ~Lre found to have special relations with the three classes of coloration above referred to, in many cases confirming the expl:umtion already given of their purport and use, and furni ·hing us with important aid in formulating a general theory of animal coloration. In comparing the colours •)f the two sexes we find '"" perfrct gradation, from absolute identity of colour up to such extreme difference that it is difficult to believe that the two forms can belong to the same species; and this diver ity in the OIL x 'OLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 269 colours of the sexes does not bear any constant relation to affinity or systematic position. In both insects and birds we find examples of complete identity and extreme diversity of tho sexes ; and these differences occur sometimes in the same tribe or family, and sometimes even in the same genus. It is only among tho higher and more activo animals that sexual differences of colour acquire any prominence. In the mollusca the two sexes, when separated, are always alike in colour, and only very rarely present slight differences in the form of the shell. In the extensive group of crustacea the two sexes n,s a rule are identical in colour, though there are often differences in the form of the prehensile organs ; but in n, very fev1,. cases there are differences of colour also. Thu , in a Brazilian pecies of shore-crab (Gelasimus) the female is grayish-brown, while in the male the posterior part of the cephalo-thorax is pure white, with the anterior part of a rich green. This colour is only acquired by the males when they become mature, and is liable to rapid change in a few minutes to dusky tints.l In some of the fresh-water fleas (Daphnoiclre) the males are ornamented with red and blue spots, while in others similar colours occur in both sexes. In spiders also, though as a rule the two sexes arc alike in colour, there arc a few exceptions, the males being ornamented with brilliant colours on the abdomen, while the female is dull coloured. Sexual Colomtion in Insects. It is only when we come to the winged insects that we find any large amount of peculiarity in sexual coloration, and even here it is only developed in certain orders. Flies (Diptera), field-bugs (Hemiptera), cicadas (Homoptera), and the grasshopper., locusts, and crickets (Orthoptera) present very few and unimportant sexual differences of colour ; but the last two groups have special musical organs very fully developed in the males of some of the species, and these no doubt enable the sexe to discover and recognise each other. In some cases, however, when the female is protectively coloured, as in the well-known leaf-insects already rcfenccl to (p. 207), the male 1 Darwin's Descent of lflan, p. 271. |