No. 567] EFFECT OF DISTRIBUTION ON SPEC L 1 TION 1 4 1 



forms, or forms which are poor in numbers of individ- 

 uals, are poor in genera and species, possibly due to the 

 comparative uniformity of their environment, which is 

 usually coincident. Most marine mammals are of these 

 kinds, and their paucity of types is emphasized by the 

 comparative uniformity of their environment, even in the 

 most widespread groups. From a distributional point of 

 view, i. e., taking into account life zones, faunas and 

 associations, a cosmopolitan, oceanic, surface group of 

 animals does not range through as great a variety of 

 ecologic niches and environmental and climatic condi- 

 tions as does a cosmopolitan terrestrial group. 



In order to determine whether the principles of distri- 

 bution and differentiation here set forth would apply to 

 birds as well as to mammals, a number of series of com- 

 parisons was made as with mammals, and with exactly 

 comparable results. 



TABLE VI 



(191SB), (1908), Willett (1912) 



Table VI gives a comparison of genera and species 

 of resident birds of (a) the San Bernardino Mountain 

 region, (&) Southern California, and (c) California as 

 a whole. Almost without exception, in each individual 

 group of birds there is a reduction in the index of modi- 



