evolution op the colors op birds. 329 



Genus Geothlypis. The Kentucky Warbler, etc. 



(7) Adult male similar to female (but more con- 

 spicuously colored); young similar. to female, or (8) with 

 a more simple plumage than the adult. 



Prevailing colors, black, white, ashy, brown, olive 

 green, yellow. 



The characteristic colors of the family, black, yellow 

 and olive green, are conspicuously present in this genus 

 — the result of pigment assortment. This genus is a 

 particularly good example of the combined action of iso- 

 lation, sexual selection, and climatic influences in the 

 production of different species. Geothlypis has more 

 geographical races than is usual in the family, and these 

 are clearly the result of environmental influences. This 

 is conspicuously the case with the different varieties of 

 G. trichas. Such species as G. melanops and G. beldingi 

 are merely climatic races in which the modification has 

 becorae extreme enough to establish them as distinct 

 from G. trichas. The Mexican yellow throats are dis- 

 tinguished from the Guatemalan species by the eyelid 

 being black in the latter and white in the former. This 

 cannot be solely the result of climatic influence, but is 

 due either to isolation alone or to isolation and selec- 

 tion. The specific distinctions between G. formosa, G. 

 agilis, G. Philadelphia and G. macgillivrayi are due to 

 isolation and sexual selection. G. 'macgillivrayi appears 

 to be a retarded form of G. Philadelphia. 



Genus Icteria. The Chats. 



Genus Sylvania. The Hooded Warbler, etc. 



(8) Adult male more conspicuously colored than 

 female; young with a peculiar first plumage. 



Prevailing colors, black, white, gray, olive green, yel- 

 low. 



The colors of these two genera are typical of the 



