ectively coloured on the under side which is exposed to
view when they are at rest. Such are our peacock, tortoise-shell, and
red admiral butterflies; while in the tropics we more commonly find that
the females are less conspicuous on the upper surface even when
protectively coloured beneath.

We may here remark, that the cases already quoted prove clearly that
either male or female may be modified in colour apart from the opposite
sex. In Pieris pyrrha and its allies the male retains the usual type of
coloration of the whole genus, while the female has acquired a distinct
and peculiar style of colouring. In Adolias dirtea, on the other hand,
the female appears to retain something like the primitive colour and
markings of the two sexes, modified perhaps for more perfect protection;
while the male has acquired more and more intense and brilliant colours,
only showing his original markings by the few small yellow spots that
remain near the base of the wings. In the more gaily coloured Pieridae,
of which our orange-tip butterfly may be taken as a type, we see in the
female the plain ancestral colours of the group, while the male has
acquired the brilliant orange tip to its wings, probably as a
recognition mark.

In those species in which the under surface is protectively coloured, we
often find the upper surface alike in both sexes, the tint of colour
being usually more intense in the male. But in some cases this leads to
the female being more conspicuous, as in some of the Lycaenidae, where
the female is bright blue and the male of a blue so much deeper and
soberer in tint as to appear the less brilliantly coloured of the two.

_Probable Causes of these Colours._

In the production of these varied results there have probably 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, preventing the
female from acquiring these same tints, or modifying her colours in
various directions to secure protection by assimilating her to her
surroundings, or by producing 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