No. 567] EFFECT OF DISTRIBUTION ON SPEC1AT10N 1 37 



what conditions in number of genera and species would 

 be expected in these three areas. The San Bernardinos, 

 being almost as small a faunal unit as should be sepa- 

 rately considered, we should expect to approach a mini- 

 mum index of modification, i. e., a minimum number of 

 species per genus, approaching one as a limit. On the 

 San Jacintos, these being smaller than the San Bernardi- 

 nos, we should expect fewer types according to the law 

 suggested by Grinnell and Swarth (1913), that the num- 

 ber of persistent types in a disconnected area varies 

 directly with the size of the area. On the entire Sierra 

 range we should expect, due to the greatly increased 

 territory, a considerable increase in genera, but a very 

 much greater increase in species. Looking now at Table 

 III, we find that with the single exception of the car- 

 nivores on the San Bernardino Mountains, not one dis- 

 crepancy exists. The Ungulates, Insectivores and bats 

 are represented by the same numbers of genera and 

 species on both of the small areas, and all of them show 

 a marked increase in genera and species on the larger 

 area, in every case with an increase in the index of 

 modification. 



The rodents, which show a larger degree of differentia- 

 tion than any of the other groups, show a very interesting 

 advance in the index of modification as the area is ex- 

 tended. The carnivores, as stated above, show a seeming 

 discrepancy, inasmuch as there are six genera and six 

 species existing on the San Jacintos, and only two genera 

 and two species on the San Bernardinos, whereas, if they 

 conformed with our laws of distribution, we should expect 

 at least six, and possibly seven or eight, species to be 

 found there. On page 35 of GrinnelPs ' 1 Biota of the San 

 Bernardino Mountains " (1908) we find reference to a 

 number of carnivores now rare or extinct on the San 

 Bernardinos, which undoubtedly have been exterminated 

 V man within the last fifty years. Counting these forms, 

 ^hich it seems to me we are justified in doing, the table 

 bears out the law without a single exception, not only for 



