1914] Grinnell: Mammals and Birds of the Colorado Valley 91 



showed the presence there of P. e. eremicus only, while field-work 

 among the rocky hills in the vicinity of Victorville, on the Mohave 

 Desert, showed P. c. stephensi only. 



As another illustration, along- the Colorado River. Mt lospiza tin lodia 

 saltonis and Pipilo aierti were often found on common ground, 

 although evidently averaging differently in associational preference. 

 To prove beyond doubt what is the true ecologic niche of each, a 

 knowledge of the distribution of each species elsewhere in their respec- 

 tive ranges becomes necessary. In the extensive arrowweed tracts 

 around the west end of Salton Sea, Melospiza melodia saltonis is an 

 abundant species while the other bird is absent. In the mesquite belt 

 not far distant to the west, in the vicinity of Martinez and Torres, 

 Pipilo aberti is prevalent, and the song sparrow absent. Hence the 

 species of towhee in question may be confidently assigned to the mes- 

 quite association, and the subspecies of song sparrow to the arrowweed 

 association. 



It is not to be inferred that all species behave in this clearcut 

 fashion associationally, any more than that all do so zonally or faunally 

 (see Grinnell and Swarth, 1913, p. 220). At the same time the writer 

 feels fairly sure of adequate grounds for proposing a general law in 

 this regard, namely, that where the faunist happens to meet with a 

 heterogeneous assemblage of biotic elements, not subject to clear asso- 

 ciational diagnosis in the restricted locality of first observation, assign- 

 ment of the species each to a well-defined association becomes possible 

 by tracing out their ranges severally into the adjacent areas. 



A concurrent axiom is that if associational analysis is carried far 

 enough, no two species of birds or mammals will be found to occupy 

 precisely the same ecologic niche, though they may apparently do so 

 where their respective associations are represented fragmentarily and 

 in intermixture. 



In determining the associational status of mammals we have to deal 

 chiefly with elusive animals, of nocturnal habits, which are hidden 

 away during the day for the most part beyond reach. Trapping is 

 not an altogether certain index to association; for individuals may be 

 caught repeatedly in a trap-line which may not happen to intersect 

 at all the regular forage ground or breeding home of the species. Indiv- 

 iduals forage far and wide beyond the limits of their home territory 

 and at the close of the breeding season wander in similar fashion. 

 Some species, including nearly all mammals except xerophilous rodents, 

 regularly travel far for water. 



