CHAPTER X 
COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 
Sex colours in the inollusca 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 
