le 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
difference 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 above-named genera--Papilio, Pieris, and Diadema--or in
any other butterfly, of a male _alone_, mimicking one of the Danaidae or
Heliconidae. 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 greater danger to
which she is exposed, and her greater importance to the existence of the
species; and this she always acquires, in one way or another, through
the action of natural selection.

In his "Origin of Species," fourth edition, p. 241, Mr. Darwin
recognises the necessity for protection as sometimes being a cause of
the obscure colours of female birds; but he does not seem to consider it
so very important an agent in modifying colour as I am disposed to do.
In the same paragraph (p. 240), he alludes to the fact of female birds
and butterflies being sometimes very plain, sometimes as gay as the
males; but, apparently, considers this mainly due to peculiar laws of
inheritance, which sometimes continue acquired colour in the line of one
sex only, sometimes in both. Without denying the action of such a law
(which Mr. Darwin informs me he has facts to support), I impute the
difference, in the great majority of cases, to the greater or less need
of protection in the female sex in these groups of animals.

This need was seen to exist a century ago by the Hon. Daines Barrington,
who, in the article already quoted (see p. 220), after alluding