VI 
A THEORY OF BIRDS’ NESTS 
137 
of colour that this protection can be most readily obtained, 
since no other character is subject to such numerous and 
rapid variations. The case I have now endeavoured to illus- 
trate is exactly analogous to what occurs among butterflies. 
As a general rule, the female butterfly is of dull and incon- 
spicuous colours, even when the male is most gorgeously 
arrayed ; but when the species is protected from attack by a 
disagreeable odour and taste, as in the Heliconidse, Danaidae 
and Acreeidje, both sexes display the same or equally brilliant 
hues. Among the species which gain a protection by imitat- 
ing these, the very weak and slow-flying Leptalides resemble 
them in both sexes, because both sexes alike require pro- 
tection, while in the more active and strong-winged genera — 
Papilio, Pieris, and Diadema — 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 characters 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 colour 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. If may, I 
think, be looked upon as a confirmation of this view, that no 
single case is known either in the above-named genera — 
Papilio, Pieris, and Diadema — or in any other butterfly, of a 
male alone mimicking one of the Danaidse or Heliconidse. 
Yet the necessary colour is far more abundant in the males, 
and variations always seem ready for any useful purpose. 
This seems to depend on the general law that each species 
and each sex can only be modified just as far as is absolutely 
necessary for it to maintain itself in the struggle for existence, 
not a step further. A male insect by its structure and habits 
is less exposed to danger, and also requires less protection, 
than the female. It cannot, therefore, alone acquire any 
further protection through the agency of natural selection. 
But the female requires some extra protection, to balance the 
