, 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 magnificent green species of
Ornithoptera and the "Aeneas" group of Papilio.

The importance of the two principles--the need of protection and
recognition--in modifying the comparative coloration of the sexes among
butterflies, is beautifully illustrated in the case of the groups which
are protected by their distastefulness, and whose females do not,
therefore, need the protection afforded by sober colours.

In the great families, Heliconidae and Acraeidae, 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 as types--the two sexes are nearly
alike, the male being sometimes more intensely coloured and with fewer
pale markings; but in the American groups--represented by P. aeneas, P.
sesostris, and allies--there is a wonderful diversity, the males having
a rich green or bluish patch on the fore wings, while the females have a
band or spots of pure white, not always corresponding in position to the
green spot of the males. There are, however, transitional forms, by
which a complete series can be traced, from close similarity to great
diversity of colouring between the sexes; and this may perhaps be only
an extreme example of the intenser colour and more concentrated markings
which are a very prevalent characteristic of male butterflies.

There are, in fact, many indications of a regular succession of tints in
which colour development has occurred in the various groups of
butterflies, from an original grayish or brownish neutral tint. Thus in
the "Aeneas" group of Papilios we have the patch on the upper wings
yellowish in P. triopas, olivaceous in P. b