1916] Taylor: Beavers of Western North America 465 



these characters are really not adaptive or not correlated with some 

 adaptation. However this may be, it would seem that the possibility 

 that geographical isolation alone, with no assistance from natural 

 selection, has been a condition in speciation of beavers, is by no means 

 excluded. 



Evidence from Certain Other Families of Mammals 

 Its great geographical extent and wide diversity of topographical 

 and environmental conditions make California probably as favorable a 

 geographic unit as could be found for the study of problems concern- 

 ing the origin and maintenance of vertebrate species ; and since the 

 writer is more familiar with the mammalian fauna of California than 

 with that of any other area, this particular field has furnished most 

 of the material used. But published facts from mammalian distribu- 

 tion in the Great West, and in the continent generally, have been 

 freely drawn upon. 



In cases where the family or genus has recently been monographed, 

 relationships as outlined may be regarded as more dependable than 

 in those instances in which the group has not undergone adequate 

 revision. In the latter the conclusions reached are tentative, and 

 are based upon the writer's familiarity with the mammalian fauna 

 in question. 



Sorex vagrans vagraiis, the range of which within California 

 (Grinnell, 1913a, p. 270) includes the Upper Sonoran, Transition and 

 Boreal zones in the western portion of the state, east to Shasta County 

 and south as far as Monterey, finds in Sorex vagrans amoenus its 

 nearest relative within the state. The range of the latter form takes 

 in the Transition and Boreal zones of the Sierra Nevada Mountains, 

 at least from Mono County north to Mount Shasta (Merriam, C. II., 

 1895, p. 68, and 1899, p. 87). Sorex halicoetes, of the salt marshes 

 bordering the south arm of San Francisco Bay, is more closely allied 

 to Sorex vagrans vagrans than it is to any other species of Sorex 

 (Grinnell, 1913a, p. 184). It will be remembered that Sorex vagrans 

 vagrans is found coastwise in California as far south as Monterey. 



Sorex sinuosus of the brackish marshes of Grizzly Island, Suisun 

 Bay, California, is most closely related to Sorex calif or nicus of the 

 neighboring uplands. 



Note should be made of the distribution of the tenellus group of 



