92 



University of California Publications in Zoology ["Vol.12 



Attention is here called to our records of the capture of pocket mice. 

 An unexpectedly large number of species of the genus Pcrognathus 

 wa.s found to occur in the region traversed. As many as three species 

 were taken in one night in traps placed close together. But by testing 

 many localities and comparing the results we soon came to know where 

 to expect each separate species. The diagrams herewith presented 

 (figs. C-F) show in statistical form the associational preferences of 

 four species of Perognathus. Providing the same number of trap- 

 nights (counting one trap set one night as one "trap-night") was 



Fig. C. Diagram showing frequency of capture of Fero;inathus formosus in 

 the several associations. There were fifty-three individuals trapped, of which 

 the associational occurrence was satisfactorily recorded. 



devoted to each association, this method should be fairly accurate. 

 There will here occur to the reader ways of securing much greater 

 precision in results of field-work in the future. But in fact, as our 

 experience grew, far more effort was expended on ground promising 

 additional specimens of rare species, than on ground already thor- 

 oughly exploited. 



While a mammal or bird may be closely confined to a narrow 

 territory characterized by certain conspicuous plants, the critical 

 factor or factors of its environment may be quite apart from food 

 requirements. As an illustration, the case of Dipodomys descrti 



