EVOLUTION OF THE COLORS OF BIRDS. 183 



large masses of muscle lying beneath the surface or by 

 nerve terminals, which produce great local activity and 

 excess of nourishment in particular parts of the body. 

 Inasmuch as the large masses of muscle and important 

 nerve terminals are more or less constant, especially 

 among species of the same genus, whereas the areas of 

 color may be profoundly different, it does not seem 

 possible that this factor can be a very strong one, if 

 operative at all among birds. The frequent occurrence 

 of a patch of color on the breast where the largest 

 muscles of the body are situated, might be, to some ex- 

 tent, due to this influence however. With regard to the 

 great frequency of a strongly marked throat patch Dr. 

 Stejneger suggested to me the possibility that the con- 

 stant vibration of the throat in singing might be a 

 factor in the specialization of its color. There could 

 not be any direct connection between the motion of the 

 throat and singing, however, because sometimes the patch 

 is distinguished by the presence of pigment as in Harris's 

 sparrow {Zonotrichiu querula) which has a black throat, 

 while in the same genus even, a species occurs in which 

 the throat is marked off by the absence of pigment — the 

 white-throated sparrow {Zonotrichiu alhicollis). More- 

 over, the canary, which shakes its throat as if it would 

 burst, has not produced any perceptible difference in 

 this region, nor has the mocking bird {Mimus polyglottos) 

 in which the throat and breast are uniform white. Dr. 

 Stejneger's theory has suggested to me another view 

 which is not open to these objections. The frequent 

 vibration of the throat, especially during the courting 

 season, would make it almost the most conspicuous part 

 of the body, and it seems highlj^ probable that when- 

 ever any suggestion of pigment occurred there it would 

 be retained and encouraged by sexual selection. It 

 might also prove a useful recognition mark in many 

 instances. 



