CHAPTER X 



COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 



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 

 eff"ect 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 j^urpose, 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 diff'erences of colour or of ornamental 

 appendages in the two sexes. These diff'erences 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 comi)aring 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 



