A THEORY OF BIRDS' NESTS. 259 



most readily obtained, since no other character is sub- 

 ject to such numerous and rapid variations. The case I 

 have now endeavoured to illustrate is exactly analogous 

 to what occurs among butterflies. As a general rule, 

 the female butterfly is of dull and inconspicuous colours, 

 even when the male is most gorgeously arrayed ; but 

 when the species is protected from attack by a disa- 

 greeable odour, as in the Heliconidaa, DanaidaB and 

 Acrceida?, both sexes display the same or equally bril- 

 liant hues. Among the species which gain a protec- 

 tion by imitating these, the very weak and slow-flying 

 Leptalides resemble them in both sexes, because both 

 sexes alike require protection, while in the more active 

 and strong-winged genera Papilio, Pieris, and Dia- 

 dema it is generally the females only that mimic the 

 protected groups, and in doing so often become actually 

 more gay and more conspicuous than the males, thus 

 reversing the usual and in fact almost universal char- 

 acters of the sexes. So, in the wonderful Eastern leaf- 

 insects of the genus Phyllium, it is the female only 

 that so marvellously imitates a green leaf; and in all 

 these cases the difference can be traced to the greater 

 need of protection for the female, on whose continued 

 existence, while depositing her eggs, the safety of the 

 race depends. In Mammalia and in reptiles, however 

 brilliant the colours may be, there is rarely any differ- 

 ence between that of the sexes, because the female is 

 not necessarily more exposed to attack than the male. 

 It may, I think, be looked upon as a confirmation of 

 this view, that no single case is known either in the 



s 2 



