EVOLUTION OF THE COLORS OF BIRDS. 271 



age of the young is simply less pigmented than that of 

 the adult. 



Genus Falco. The Falcons. 



(2) Adult male generally like female; young with 

 peculiar first plumage. In some species (7) male un- 

 like female; young resembling the latter. 



Prevailing colors, black, white, brown, rusty, bluish,, 

 gray. 



The colors in this widely dispersed genus are partly 

 in accordance with the general laws of growth, in nearly 

 all species enhanced by sexual selection (?), and with 

 recognition marks, sometimes as is F. sparveriiis in the 

 form of characteristic head markings, but more often in 

 variations of the tail bars. The markings of F. 8j)ar- 

 vtriiis are peculiarly pronounced, for, besides the head 

 markings which would alone serve to distinguish it, the 

 flanks have large circular spots of black, the wing feath- 

 ers are barred, and the tail markings are peculiar. The 

 distinction between F . sparverius and F. spciTverioides is 

 a peculiarly interesting one. In the females of both 

 the back is rufous, but in the former the male is 

 similarly colored, while the back of the latter is plum- 

 beous or dark bluish gray. No appeal to the laws of 

 growth would explain this, and it is evidently to be ac- 

 counted for either as a discriminative mark, or as due 

 to sexual selection. Probably both factors were instru- 

 mental in its development for in many instances charac- 

 ters which at first were only useful for sexual recognition, 

 in leading the female to select a mate of her own kind, 

 afterwards became attractive to her and were still 

 further developed by sexual selection. Certain species 

 have been directly influenced by their environment and 

 divided into geographical races, as for instance, F. eol- 

 iimharius suckleyi, the dark form of the Northwest coast 

 region. 



