326 CALIFORNIA ACADEMY OF SCIENCES. 



yellow, and those in which the upper tail coverts are 

 bluish gray. The white and yellow on the right hand 

 side is considerably interrupted by streaks, but in gen- 

 eral is fairly constant except in D. cestiva and its allies. 

 From this it seems undoubted that the ancestor of the 

 genus was an olive green bird, probably streaked both 

 above and below. The olive green is a combination of 

 black and yellow, which were apparently the original 

 pigments of the genus. They were thrown off indis- 

 criminately producing the olive green effect, and the 

 bulk of the pigment following the shaft of the feather 

 would give a streaked appearance to the bird. Excess of 

 sunlight upon the back would cause the greater part of 

 the pigment to lodge there, and the result would be a 

 bird not unlike a composite of the females here repre- 

 sented. 



Then sexual selection would step in as a factor in pro- 

 ducing change. Occasionally a single feather or a par- 

 ticular spot on the bird would receive a trifle more of 

 either the yellow or the black pigment than usual, from 

 a cause which, for lack of a better name, we may call 

 fortuitous, and this bird would be conspicuous among 

 his fellows, and more easily gain a mate and leave off- 

 spring. In the course of time this character would be- 

 come exaggerated, from the continual selection, into a 

 specific character. Olive green, black and yellow would 

 then be the three colors of the genus, the back being 

 darker than the breast, and the black and yellow occur- 

 ring upon the most conspicuous parts of the body. It 

 will be noticed from the diagram that these colors occur 

 most frequently upon the top of the head, rump, upper 

 tail coverts, ear coverts, throat and breast, where they 

 would be most noticeable. An intensification of the 

 yellow would produce orange, as in D. blackburnice, while 

 a darkening of this, perhaps by reintroducing black, 



