260 4A THEORY OF BIRDS’ NESTS, 
above-named genera—Papilio, Pieris, and Diadema—or 
in any other butterfly, of a male alone, mimicking 
one of the Danaidz or Heliconide. Yet the necessary 
colour is far more abundant in the males, and varia- 
tions 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 
tanger, 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 fe- 
male 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. 
