336 CALIFORNIA ACADEMY OP SCIENCES. 



sified, the back of Sialia. Deepened still further it 

 would become the black of Merula and other species. 

 In the same manner variations of the brown would ac- 

 count for the color of the under parts of the body. 



Sexual selection with isolation has doubtless been an 

 important, though by no means the only agent in the 

 production of generic and specific characters. Recog- 

 nition markings, both directive and discriminative have 

 been developed by natural selection, and direct environ- 

 mental influences have done their share in modifying 

 species, as with Merula continis. 



