EVOLUTION OF THE COLORS OF BIRDS. 261 



Prevailing colors, black, white, chestnut, brown. 



The remarkable adaptations of this species to a sum- 

 mer and winter environment, brown at the former and 

 pure white at the latter season, are too universally known 

 to need mention. The winter plumage, however, may 

 not be exclusively intended for protection. White feath- 

 ers are much denser and warmer than colored ones, and, 

 moreover, it is claimed by some, that the intense cold 

 has a direct effect in causing the feathers to turn white. 

 Nevertheless there can be no doubt that protection is 

 one of the chief if not the exclusive object of the white 

 plumage. The different races, and even species, have 

 become modified by geographical isolation. L. rupestris 

 is the stock form inhabiting Arctic America in general; 

 L. rupestris reinhardti, is found only in Greenland, 

 northern Labrador, etc. ; L. rupestris nelsoni is restricted 

 to the island of Unalaska, in the Aleutian chain, and 

 Z. rupestris atkensis to the island of Atkha in the same 

 group, while L. welchi is confined to Newfoundland. 

 L. leucurus is found upon the Alpine summits of the 

 Rocky Mountains, where it also is effectually isolated, 

 for the birds are resident wherever found. Little differ- 

 ences have thus originated which are of no particular 

 utility, and may not be due even to the action of the 

 environment, but simply to geographical isolation. 



GENUS TYMPANUCHUS. PRAIRIE HENS. 



(2) Male like female; young like some ancestral stage 

 of the adult. 



Prevailing colors, brown, dusky, buff, white. 



The colors in this genus are almost entirely protective, 

 the brown stripe below the eye being the only recogni- 

 tion mark. The ruffs and inflatable sacks on sides of 

 neck of male are the only sexual characters, the former 

 having been imperfectly inherited by the female. The 



