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 “ yEneas ” 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, Heliconidae and Acrreidae, 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 Danaidae 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 
