EVOLUTION OF THE COLORS OF BIRDS. 83 



further developed by the combats of the males for the 

 possession of the females. The most vigorous and ener- 

 getic usually being able to rear the most offspring inten- 

 sity of colour, if dependent on, or correlated with vigor, 

 would tend to increase. But as differences of colour 

 depend npon minute chemical or structual differences in 

 the organism, increasing vigor acting unequally on 

 different portions of the integument, and often produc- 

 ing at the same time abnormal developments of hair, 

 horns, scales, feathers, etc., would almost necessarily 

 lead also to variable distribution of colour and thus to 

 the production of new tints and markings." In follow- 

 ing out this line of argument Mr. Wallace suggests that 

 color is proportionate to integumentary development, 

 reaching its maximum among butterflies and birds 

 where there is the greatest expanse and variation of 

 wings, but it has been asked why, if this hypothesis 

 be a true one, bats are generally so dull colored, or 

 beetles so brightly. " The endless processes of growth 

 and change during the development of feathers, and 

 the enormous extent of this delicately organized sur- 

 face,"- he says,* " must have been highly favorable to 

 the production of varied colour effects, which, when not 

 injurious, have been merely fixed for purposes of specific 

 identification, but have often been modified or sup- 

 pressed whenever different tints were needed for purposes 

 of protection." This appeal to the extent and structure 

 of the feather can have but little weight, for, in the 

 majority of cases, it is the exposed edge of the feather 

 alone which is colored; the basal half being as a rule 

 white or some dull shade of buff or gray. 



Whether the appeal of Mr. Wallace to the general 

 laws of growth is or is not justifiable as an explanation 



*i. c, p. 369. 



