4 PECKHAM. [Vol. 1. 



females. The most vigorous and energetic usually being able 

 to rear most offspring, intensity of color, if dependent on or cor- 

 related with vigor, would tend to increase. But as differences 

 of color depend upon minute chemical or structural differences 

 in the organism, increasing vigor acting unequalljr on different 

 portions of the integument, and often producing at the same 

 time abnormal developments of hair, horns, scales, feathers, 

 etc., would almost necessarily lead also to variable distribution 

 of color and thus to the production of new tints and markings. 

 These acquired colors would * * * be transmitted to both 

 sexes, or to one only, according as they first appeared at an 

 early age, or in adults of one sex j * * * but in all cases where 

 an increasing development of color became disadvantageous to 

 the female, it would be checked by natural selection ; and thus 

 produce the numerous instances of protective coloring in the 

 female only, which occur in these two groups, birds and butter- 

 flies."^ 



We have here two theories offered to explain sexual differ- 

 ences in color: the first is, that natural selection modifies 

 color in the female for purposes of protection ; and the second, 

 that color may be produced or intensified where there is a sur- 

 plus of vital energy, as in male animals generally, and some- 

 times in the females, and more especially at the breeding 

 season. We will here consider the second theory, since Mr 

 Wallace regards this as the more important in making intel- 

 ligible cases of more brilliant coloring in the male as compared 

 with the female. 



What is meant by an excess of vital energy is not quite 

 clear. Does this term imply tliat the colored modifications of 

 the integument represent the excess of nutriment over expen- 

 diture? This seems scarcely probable, and yet what other 

 interpretation is to be put upon such a statement ? Supposing 

 this interpretation to be correct, if the color or development of 

 plumage represents the surplus over ordinary expenditure, 

 should not the least active animals, rather than the most vigor- 



1 Tropical Nature, pp. 187 and rJS-1%. 



