156 THE MALAYAN PAPILIONIDE AS 
sexual selection. For instance, in most polygamous 
animals the males fight for the possession of the 
females, and the victors, always becoming the pro- 
genitors of the succeeding generation, impress upon 
their male offspring their own superior size, strength, 
or unusually developed offensive weapons. It is thus 
_ that we can account for the spurs and the superior 
strength and size of the males in Gallinaceous birds, 
and also for the large canine tusks in the males of 
fruit-eating Apes. So the superior beauty of plumage . 
and special adornments of the males of so many birds 
can be explained by supposing (what there are many 
facts to prove) that the females prefer the most beau- 
tiful and perfect-plumaged males, and that thus, slight 
accidental variations of form and colour have been 
accumulated, till they have produced the wonderful 
train of the Peacock and the gorgeous plumage of 
the Bird of Paradise. Both these causes have no 
doubt acted partially in insects, so many species 
possessing horns and powerful jaws in the male sex 
only, and still more frequently the males alone re- 
joicing in rich colours or sparkling lustre. But there 
is here another cause which has led to sexual differ- 
ences, viz., a special adaptation of the sexes to diverse 
habits or modes of life. This is well seen in female 
Butterflies (which are generally weaker and of slower 
flight), often having colours better adapted to con- 
cealment; and in certain South American species (Pa- 
pilio torquatus) the females, which inhabit the forests, 
resemble the Auneas group of Papilios which abound 
