1913] Grinnell-Swarth : Birds and Mammals of San Jacinto 389 



degree of difference is so great that violent contrast is afforded 

 in meteorological conditions and in their resultant effects on the 

 plant and animal life. 



The relatively semi-humid province occupying the Pacific 

 slope of southern California has been termed the San Diegan 

 District. The opposite slope leads down to the Colorado Desert, 

 and this latter geographic name may be used for the faunal 

 division which includes it. 



The transition from semi-humid to extreme arid does not 

 occur at the very summit of the divide at any point. Because 

 of the prevailingly westerly winds the San Diegan condition and 

 biota are carried over upon the desert slopes to varying distances 

 beyond the crest according to steepness and trend of the ridges 

 which deflect the moist air-currents. 



An analysis of the Sonoran birds and mammals of the San 

 Jacinto area by species leads to the following groupings accord- 

 ing to nature of distribution: (1) Species which occur regularly 

 in both the San Diegan District and on the Colorado Desert, and 

 which show no distinctive modifications in the two areas (see 

 table A) ; (2) species which are separately confined to one or the 

 other fauna, and so sharply delimited associationally that t hex- 

 do not even invade the strip over which the two faunas blend 

 (see table B) ; (3) species which belong intrinsically to one or 

 the other area but which penetrate across the strip of blending 

 and even into the frontier of the opposite area (see table C) ; 

 (4) species represented in each of the two areas, but by obviously 

 closely related forms, either subspecies, or species but slightly 

 differentiated (see table D). 



The first category consists of species, all birds, of apparent 

 indifference to conditions of humidity. They range for the most 

 part widely over the western United States, and, with the notable 

 exception of the cactus wren, either perform regular and ex- 

 tended migrations, or are of strong flight and vagrant inclination. 



The second category comprises species which occupy very 

 narrow ranges of environmental conditions, so limited that in 

 some cases the confines of a single association are not over- 

 stepped. As examples of such extreme delimitation, Dipodomys 

 deserti, of the aeolian sands, and the erissal thrasher, of the 



