EVOLUTION OF THE COLORS OF BIRDS. 191 



operative in a species of the same genus so closely allied. 

 In the rose-breasted grosbeak, the band of white situ- 

 ated apparently in almost the same place as in the 

 wax-wing, is produced not by wdiite terminals on the 

 primary coverts, but by a broad patch of white at the 

 base of the primaries themselves, which is partly cut off 

 by the coverts, leaving only a white band. In the Cal- 

 ifornian woodpecker (MeUinerpes formicivorusbairdi) the 

 white markings on several of the feathers run through 

 the center of the primaries leaving both the base and 

 tip uniformly dark. 



The tail markings also vary in different forms to such 

 an extent as to be inexplicable by any factor but selec- 

 tion. There are, on the other hand, certain points in 

 regard to them which demand an appeal to internal laws 

 for explanation. It is a noticeable fact that the two 

 outer tail feathers have the markings most strongly de- 

 veloped, and that they decrease as we approach the 

 central feather. Of course, it would be said that the 

 outer feathers are the most conspicuous, and the mark- 

 ings would be of more use here than on the inner feath- 

 ers, and this is doubtless the true explanation in such 

 forms as the junco {Junco hyemalis), where only the two 

 outer feathers are thus marked. In the blue jay {Gyano- 

 citta cristata), on the other hand, the tail is marked with 

 a terminal band of white, which diminishes in a regular 

 sequence towards the center. The two central feathers 

 have only a minute tip of white, which would be in- 

 visible even to the closest scrutiny at the distance of a 

 few feet, and hence could not have been produced by 

 selection. This is another example of repetitive marks, 

 it seems to me. Natural selection has developed this 

 white bar on the outer feathers as a recognition mark, 

 and it has been repeated according to the law of corre- 

 lation in a decreasing; series toward the center. In the 



