EVOLUTION OF THE COLORS OF BIRDS. 321 



of specialization of the rest of the family. The fact 

 which looks toward degeneration is that neither the 

 adult nor the young show any tendency toward a streaked 

 plumage (unless the specialized head stripes of Helmi- 

 therus be taken into account) which we would expect to 

 find if the birds still retained a primitive coloration. 

 The general aspect of both these species is that of forms 

 which had been brightly colored and lost all but a vestige 

 of their former splendor. They might be looked upon 

 then as geratologous forms (see ante, p. 78). 



Genus Helminthophila. Golden-winged Warbler, 



ETC. 



(7) Adult male usually more conspicuously colored 

 than female [(2) adults sometimes alike]; young usually 

 similar to female, but sometimes (8) with a peculiar first 

 plumage. 



Prevailing colors, black, white, ash gray, brown, 

 chestnut, olive green, yellow. 



Sexual selection, aided by isolation, has doubtless 

 been the chief factor in the origination of the markings 

 of this genus. The colors may all be explained as sug- 

 gested^ in speaking of the family in harmony with the 

 law of the assortment of pigments. Geographical isol- 

 ation has been an important, though probably not the 

 only form of segregation. The different stages through 

 which the genus has progressed are well shown by the 

 forms of to-day. The young of the Tennessee warbler 

 (H. peregrina) which is plain olive green, paler on the 

 breast, represents the primitive plumage of the genus. 

 Through pigment assortment by sexual selection a yellow 

 crown patch was developed. Then the species was isol- 

 ated in tw^o districts, east and west of the Rocky Moun- 

 tains, by the ice age, apparently. The eastern species 

 developed w^hite recognition markings upon the tail 

 21 



