218 University of California Publications in Zoology [Vol.10 



defined than are zones, but their existence is readily appreciable 

 when one considers large areas, and the recognition of them in 

 dealing with problems of distribution is expedient as a measure 

 leading to relative accuracy of treatment. 



In the San Jacinto region two areas of very different fauna] 

 complexion meet. But only the Lower and Upper Sonoran 

 zones are involved in this junction. Thus, to be absolutely cor- 

 rect, four faunas should be distinguished by name, one for each 

 faunal division of each zone. We have, however, found it more 

 convenient to combine the two Sonoran sections on the arid side 

 under one name. Colorado Desert fauna, calling the two on the 

 Pacific side together the San DiegaD fauna or district. The 

 inter-relations of these two faunas are discussed at length in 

 another chapter (see pp. 388-392). 



"While the positions of life zones are determined chiefly by 

 latitude and altitude, faunas are delimited by factors dependent 

 upon proximity of oceans, and air currents in connection with 

 topographic features. Faunas in North America tend towards 

 a longitudinal, or north and south position, and hence transect 

 life zones, cutting them up into lesser life areas. 



Associations 



The term "association" as applied to animals is allied in 

 meaning to the "formation" of some botanists. Each association 

 is inclusive of one or more of the botanist's "formations"; but 

 it also includes the animal life present, even when, as happens 

 with carnivorous forms, there may be no direct dependence upon 

 plants. 



The names employed in designating associations are taken 

 from conspicuous or dominant permanent elements. Thus con- 

 venience is the sole criterion in selecting names. As with zones 

 and faunas, associations are often capable of subdivision ; in fact 

 such splitting may be carried logically to the point where but one 

 species occupies each its own niche. However, at the present 

 stage of study, the larger groupings, or major associations, are 

 most serviceable, with, in a few cases, recourse to smaller or 

 minor associations. 



