388 University of California Publications in Zoology t VoL - 10 



SONORAN BIOTA OF THE SAN DIEGAN DISTRICT 



COMPARED WITH THAT OF THE 



ADJACENT DESERT 



The San Jacinto area includes a section of the divide between 

 the desert and Pacific slopes of southern California. Only a small 

 portion of this divide, however, is taken up by the Boreal zone 

 see pis. 6, 7). Over a large part of the area the Sonoran zones 

 are continuous over the divide from the desert to the Pacific 

 side. It is almost altogether the upper division of the Sonoran 

 zone which tops the watershed, a narrow and diluted strip of 

 Lower Sonoran extending over the low San Gorgonio Pass. 



While the Sonoran zone is thus continuous from the Pacific 

 side over upon the desert, the fauna undergoes marked and 

 rather abrupt change. To refer to basic causes, we may say that 

 while aestival temperature is similar on the two slopes, degree 

 of humidity is widely different. The great difference in dryness 

 on the two sides of the San .Jacinto Mountains is impressed upon 

 the traveler in many sensible ways, so that recourse to a hygro- 

 meter is unnecessary to prove to him the nature of the change. 

 Species and gt n< ra replace one another up the mountain, that is, 

 from zone to zone, being segregated by temperature conditions; 

 species and subspecies replace one another across the divide, from 

 faunal district to faunal district, being controlled in their re- 

 spective distribution by atmospheric humidity. 



That the metamorphosis in biotic complexion from the Pacific 

 to the desert is correlated with decrease in relative humidity is 

 proved by the fact that replacement of species occurs on an 

 average along the line where the obviously humid and dry areas 

 meet. In the San Jacinto area studies of faunal behavior can 

 be prosecuted under particularly favorable conditions, because 

 this line of mergence between humid and arid faunal districts 

 takes place within such a very short distance, and further be- 

 cause, while the Pacific side is not so very humid, as compared 

 with the northwest coast belt of the United States, the desert side 

 possesses an extraordinarily less percentage of humidity. The 



