x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 273 



Probable Causes of these Colours. 



In the production of these varied results there have prob- 

 ably been several causes at work. There seems to be 

 a constant tendency in the male of most animals — but 

 especially of birds and insects — to develop more and more 

 intensity of colour, often culminating in brilliant metallic blues 

 or greens or the most splendid iridescent hues; while, at 

 the same time, natural selection is constantly at work, pre- 

 venting the female from acquiring these same tints, or 

 modifying her colours in various directions to secure pro- 

 tection by assimilating her to her surroundings, or by pro- 

 ducing mimicry of some protected form. At the same 

 time, the need for recognition must be satisfied ; and this 

 seems to have led to diversities of colour in allied species, 

 sometimes the female, sometimes the male undergoing 

 the greatest change according as one or other could be 

 modified with the greatest ease, and so as to interfere least 

 with the welfare of the race. Hence it is that sometimes 

 the males of allied species vary most, as in the different 

 species of Epicalia; sometimes the females, as in the magnifi- 

 cent green species of Ornithoptera and the " uEneas " group 

 of Papilio. 



The importance of the two principles — the need of pro- 

 tection and recognition — in modifying the comparative colora- 

 tion of the sexes among butterflies, is beautifully illustrated 

 in the case of the groups which are protected by their dis- 

 tastefulness, and whose females do not, therefore, need the 

 protection afforded by sober colours. 



In the great families, Heliconidse and Acrseidae, we find 

 that the two sexes are almost always alike ; and, in the very 

 few exceptions, that the female, though differently, is not less 

 gaily or less conspicuously coloured. In the Danaidse the same 

 general rule prevails, but the cases in which the male exhibits 

 greater intensity of colour than the female are perhaps more 

 numerous than in the other two families. There is, however, a 

 curious difference in this respect between the Oriental and 

 the American groups of distasteful Papilios with warning 

 colours, both of which are the subjects of mimicry. In the 

 Eastern groups — of which P. hector and P, coon may be taken 



T 



