330 University of California Publications in Zoology [Vol. 16 



faunas of other adjacent regions. It must be placed in the Great 

 Basin district of Allen (1892, p. 237). As the Great Basin seems to 

 have had a rather stable climate and a continuous sequence of forms 

 for a long period of geologic time it is allowable to suppose that many 

 of the forms of the Columbia Basin sagebrush fauna originated in the 

 Great Basin and migrated into southeastern Washington at some 

 period later than the Middle Miocene. 



The fauna of the Blue IMountains is ino.st closely I'elated to the 

 fauna of the Rocky Mountains. The Blue Mountain area must be 

 placed in a subdivision of the Canadian subregiou of Holarctica 

 (Lydekker, 1896, p. 360). It seems logical to suppose that the fauna 

 of this area has been derived largely from the North. 



The fauna of the Columbia Basin prairie area is related to the 

 faunas of both the Rocky Mountains and the Great Basin. It has 

 seemingly been produced largely by an admixture of elements from 

 these two places. 



The maintainance of the distinctness of the fauna of the Columbia 

 Basin prairie area must be due to the climatic barriers which separate 

 it from the Columbia Basin sagebrush fauna and from the Blue Moun- 

 tain fauna. Differences in temperature and rainfall and perhaps 

 other factors are effective in separating the fauna of the prairie from 

 that of the Blue Mountains. The difference in temperature between 

 the prairie area and the sagebrush area in southeastern Washington 

 is not marked and the difference in rainfall is probably the chief factor 

 separating the faunas of the two places. 



