260 A THEORY OF BIRDS' NESTS. 



above-named genera Papilio, Pieris, and Diadema or 

 in any other butterfly, of a male alone, mimicking 

 one of the Danaida3 or Heliconidae. 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 

 clanger, 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 imroriance 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. 



