4 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 Heliconide, Danaide and 
Acroeidze, 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 
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