CHAPTER VI. 



SEXUAL COLOKATION. 







Sexual Dimorphism in Colour. 



IN many animals where the sexes are separated, the mules 

 and females are to be distinguished by certain " secondary 

 sexual characters," as they have been termed. Familiar in- 

 stances of such are the antlers of the stag, the spurs of the 

 cock and other gallinaceous birds, and the gorgeous plumes 

 found iu the males of the birds of paradise. 



The secondary sexual characters are sometimes confined to 

 differences of colour alone, as is the case with the chaffinch 

 and many other birds. The reasons for these differences in 

 the sexes have been much discussed, and the fact that at 

 least five theories have been put forward at different times 

 is an index of the difficulty of the questions involved. 



An attempt will be made here to give some notion of the 

 arguments for and against the several theories which profess 

 to explain sexual dimorphism. Some notion must, however, 

 be first given of the facts which have to be dealt with. 

 Among the invertebrates, insects furnish the most abundant 

 examples, chiefly because they are the largest group of 

 animals in which the two sexes are distinct. 



In many species of butterflies and moths, the sex cannot 

 Vie distinguished by the colour ; the Vanessidre in general 



