Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

260.4 THEORY OF BIRDS' NESTS.
above-named genera-Papilio, Pieris, and Diadema-or
in any other butterfly, of a male alone, mimicking
one of the Dainaidcl or Heliconidm. 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'i 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.