1914] Grinnell: Mammals and Birds of the Colorado Valley 



95 



area in which it is able to get the food nece&sary for itself by the 

 method which its anatomical structure and psychological equipment 

 prescribe. 





Fig. F. Diagram showing frequency of capture of Perognathus s. spinatus 

 in the several associations. One hundred and ten individuals were trapped, of 

 which associational occurrence was satisfactorily recorded. While the asso- 

 ciational preference of this species is almost identical with that of P. inter- 

 medins, the ranges of the two species are wholly distinct, being separated 

 by the Colorado Eiver. Associational preference is also seen to be similar to 

 that of P. formosus, which occupies apparently identical ground. But P. spin- 

 atus belongs to a different subgenus (Chaetodipus) from P. formosus (subgenus 

 Perognathus), hence the two are not nearly related and probably do not come 

 into close ecological competition. 



Not infrequent are the cases where food-supply is located in one 

 place, and the refuge or breeding site in another. Animals of such 

 habit are necessarily of relatively great powers of locomotion, and 

 may live regularlv in two or more associations situated remotelv from 



