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. In the preceding chapters we have dealt chiefly with the coloration of animals as distinctive of the several species; and we have seen that, in an enormous number of cases, the colours can be shown to have a definite purpose, and to be useful either as a means of protection or concealment, of warning to enemies, or of recognition by their own kind. We have now to consider a subordinate but very widespread phenomenon---the differences of colour or of ornamental appendages in the two sexes. These differences are found to have special relations with the three classes of coloration above referred to, in many cases confirming the explanation already given of their purport and use, and furnishing us with important aid in formulating a general theory of animal coloration. In comparing the colours of the two sexes we find a perfect 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 diversity in the 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 the sexes; and these differences occur sometimes in the same tribe or family, and sometimes even in the same genus.