66 University of California Publications in Zoology [Vol.12 



may be present under duress, since their greatest abundance is known 

 to be in adjacent zones or faunas. Thus Lynx and Eptesicus are much 

 more plentiful in the Upper Sonoran zone or even in the Transition 

 zone; and, on the other hand, Sigmodon and Pyrocephalus are genera 

 of subtropical abundance. The presence of elements of the latter 

 category was probably what led Merriam at one time (1894, p. 233, 

 footnote, pi. 14) to refer the lower Colorado River Valley to the 

 Tropical zone. All students who employ the life-zone system, now 

 unhesitatingly agree in referring the area in question to the Lower 

 Sonoran zone. 



The "western desert tract," of Mearns (1897, pi. 2), is probably 

 similar to the "Colorado Desert fauna" of the present paper. The 

 former term is not considered apropos in the system adopted by the 

 present writer on the ground that an inanimate area is designated 

 thereby, rather than an assemblage of living things inhabiting the 

 area. 



ASSOCIATIONAL AREAS OF THE REGION 



Since the entire region under consideration all belongs to one zone 

 and all to one fauna, according to the definitions of these distributional 

 terms given in the preceding chapter, the study of local distribution 

 in the Colorado River region pertains chiefly to associations. Perhaps 

 nowhere else in America can one find the degree of associational con- 

 trast which is presented in the region under consideration. A stream 

 of large volume, with paralleling strips of well-watered bottom land, 

 maintains its course to the sea through what is considered the hottest 

 and most arid desert in the world. 



There is nothing to show that the atmosphere is appreciably more 

 humid in the vicinity of the bottom lands or the river itself than 

 upon the open desert. The evaporated moisture is quickly dissipated ; 

 that is, it becomes diluted to an imperceptible proportion in the desert 

 air currents. It appears, therefore, that the great floral differences 

 observed between the extreme associations are due primarily to differ- 

 ence in amount of soil water available. There are, of course, such 

 additional factors as varying alkalinity, and shade (see Spaulding, 

 1909). 



It must be understood that the associations here defined are recog- 

 nized by the writer primarily because of their service in the treatment 

 of animal distribution. Botanists have found it useful to make much 



