476 C. L. HUBBS 



Despite the evidence of extreme climatic change at the close of 

 Wisconsin time and during the Postpleistocene millenia, Martin (15) 

 holds to the view that the extinction of the large Pleistocene mam- 

 mals is attributable not to climatic change, but to man. I favor the 

 theory of a combination of factors. 



After the recurrent restoration of humidity in the Pleistocene, 

 the trend toward aridity seems to have continued in the West. The 

 deserts seem to have marched northward and to have spread out 

 like a vast desiccating fan toward the Pacific Coast and onto the 

 Great Plains. This trend is just mentioned in the abstract by 

 Stebbins (7), and it is considered, but I believe probably set too far 

 back in Cenozoic time, by Peabody and J. M. Savage (8). Desicca- 

 tion is plausibly held by R. R. Miller (9) and by Pennak (10) to have 

 been largely responsible for the impoverished freshwater fauna of the 

 West and for the high incidence of local endemism. A. H. Miller 

 (6) attributes the high ratio of endemism among the birds of the 

 Californian fauna to the isolation of this fauna by deserts. 



In some groups, as the Orthoptera (Rehn, 12) and Reptilia, in 

 contrast, the intensification and spread of the Sonoran region seems 

 to have been a potent evolutionary stimulus. 



Redispersals following the vast displacements of the Pleistocene 

 are held to have induced some very interesting speciational situa- 

 tions. Blair (17) postulates the genetic responses, during Recent 

 sympatry, of cognates that had been isolated in the Floridan and 

 Mexican refugia. Hovanitz (14) similarly treats the consequences of 

 cohabitation of butterflies previously segregated by the Wisconsin 

 ice sheet. One pair, he states, has, by hybridization, thus produced a 

 third species. 



Geologically recent topographic changes are held to have condi- 

 tioned other significant speciational events. Hovanitz (14) attributes 

 the high incidence of endemism of butterflies in the Andes to the 

 new environments suddenly furnished by the rapid uplift of the 

 Cordillera, and he regards this type of response as of general signifi- 

 cance. Peabody and Savage (8) explain speciational relations among 

 amphibians and reptiles in California on the basis of the establish- 

 ment of a Coast Range Corridor. They cite evidence that the Sierra 

 Nevada and Coast Ranges were long separated on the south by a 

 marine barrier, so that the forms on the two ranges became sub- 

 specifically difi"erentiated, though intergrading where their ranges 



