EVOLUTION OF THE COLORS OF BIRDS. 319 



Prevailing colors, black, white, olive, brown, gray, 

 yellow. 



The colors of this genus have been developed in ac- 

 cordance with the law of the assortment of pigments, 

 but whether sexual selection or need for recognition has 

 been the more important agent is not easily established. 

 Both factors have apparently been operative. 



FAMILY MNIOTILTIDJE. THE WOOD WARBLERS. 



This extensive family is particularly interesting from 

 the almost uniform degree of specialization of color 

 marks which it presents. They are to be explained 

 principally by the influence of sexual selection supple- 

 mented by the advantage of recognition marks. The 

 different genera, with some few exceptions, are closely 

 related so far as their colors are concerned, making this 

 family one of the best illustrations of the law of the 

 assortment of pigments. The prevailing colors of the 

 entire family are olive green, black and yellow. The 

 gray which so often appears may be due to the bleach- 

 ing of the olive green; the red, as in Setophaga, is ob- 

 viously the result of intensification of the yellow, and 

 the chestnut may be also a modification of the yellow, 

 analagous to the change in Icterus spurius. This leaves 

 the blue still unaccounted for, and it must be confessed 

 that this is the most difficult color to explain in harmony 

 with the law of pigment assortment. It is probably a 

 modification of the black pigment, however. In the 

 casrulean warbler (Dendroica ccerulea), the young is olive 

 green, and the color of the adult is apparently produced 

 by the loss of the yellow pigment. 



The different species of a genus are as a rule sharply 

 separate from one another, and do not blend through 

 local races. Most species are not very widely spread 

 over the country, and accordingly comparatively few are 

 divisible into geographical varieties. 



