EVOLUTION OF THE COLORS OP BIRDS. 149 



In general, it may be said that the tendency is for the 

 darlc markings, whetlier black or some other shade, to 

 occur in the greatest masses upon the back. As will be 

 seen later, this might be accounted for by supposing 

 that light is inclined to favor the deposit of pigment, 

 and as the back has always received a more considerable 

 share of it than the breast, a rhythm had in the course 

 of ages been established, inclining the larger amount of 

 pigment to those parts of the body. As if to bid defiance 

 to any such laws as we may make upon the subject, a 

 irumber of species are found in which this order is 

 directly reversed. Thus, in the chestnut-colored long- 

 spur fCulcarius ornatus) the top of the head and entire 

 lower parts, except throat are black. The same parts 

 are colored black in the grassquit (Eiietlieia bicolor) and 

 in the bobolink (Dolichonyx oryzivovus). In all these 

 cases the upper parts contain the lighter markings. 



These black markings seem as a rule to spread from 

 certain well marked areas. Those on the lower parts 

 appear to spread from the throat or breast patch, while 

 those of the upper parts usually come first on the head. 

 This is by no means universally the case, as, for in- 

 stance, with the genus Icterus (Plates XVI-XVII). In 

 the females the only patch of black is the one on the 

 throat. It does not extend from there at first, however, 

 but the back darkens, as in the adult of Icterus cucuUaius. 

 In Icterus bullocki the black has extended over the head, 

 while in the next stage, Icterus galbula, the patches on 

 throat and back of head become confluent. The point to 

 be noted is that there is no species in which the top of 

 the head is black while the back remains light colored. 



Sometimes when the entire bird is black or some dark 

 shade, its antecedent stage is black above and light 

 below. A good example of this is the purple martin 

 (Progne subis), the male being entirely dark glossy blue 



