ADAPTIVE ACCELERATION IN EMBRYONIC GROWTH-RATE. 345 



than adaptive. Shortness of incubation period could very easily 

 have been of enough selective value for natural selection to operate 

 on in a very decided fashion. 



Lest it be thought that the differences in incubation period of 

 the various species of cowbirds are due to differences in the extent 

 of embryonic development, it should be noted that all of these 

 birds hatch out in exactly the same stage of development. The 

 size of the bird incubating the eggs seems to have no noticeable 

 effect on the period of incubation. Birds as large as mocking- 

 birds and as small as vireos hatch out young cowbirds in the same 

 number of days. The size of the eggs of the various species of 

 cowbirds is practically the same in all cases. 



