322 CALIFORNIA ACADEMY OF SCIENCES. 



which have persisted through all its modifications, this 

 character being lacking in western forms. Sexual selec- 

 tion, aided by further isolation of some sort, next pro- 

 duced one species with a yellow and one with a black 

 throat. The latter by further isolation became subdi- 

 vided into one species with the yellow crown persisting 

 and another in which it is replaced by black, except on 

 the forehead. 



In the west the color of the crown patch became 

 changed from the yellow to a chestnut brown apparently 

 by a process of intensification. The orange-crowned 

 warbler (H. celata) represents the stage intermediate be- 

 tween the yellow and the rufous crown. The general 

 olive green color next became bleached out into an ash, 

 the head being first affected, and at the same time the 

 yellow of the under parts became bleached into white. 

 Sexual selection, in conjunction with isolation, finally 

 produced two species from this last one, one with the 

 rump yellow, the other with the rump chestnut, corre- 

 sponding to the changes of the crown patch. Two 

 eastern species, H. laivrencei and H. pinus, seem to be 

 dichromatic, having the normal olive green and yellow 

 colors, and in the same locality being found with this 

 color replaced by gray and white. The change here is 

 similar to that from H. ruficapilla to H. lucice, only the 

 two forms have not become established as distinct species 

 in the former instance. Mr. Ridgway, in a foot note in 

 the Manual of North American Birds * suggests another 

 explanation, as follows: " In a large series of specimens, 

 every possible intermediate condition of plumage be- 

 tween typical H. pinus and H. leucobronchialis is seen, 

 just as in the case of H. chrysoptera and H. lawrencei. If 

 we assume, therefore, that these four forms represent 



* p. 486. 



