90 University of California Publications in Zoology [Vol.12 



particularly along the river bottom. Only two insectivorous birds 

 were restricted to the Encelia- association, namely, the wrens, Salpinctes 

 obsoletus obsoletus and Catherpes mexicanus eonspcrsus. Both are by 

 structure adapted to gleaning from crevices of rock surfaces after the 

 manner of nuthatches on tree trunks. 



General Discussion op Associational Restriction 



From the preceding description of the conditions in the region 

 studied, it is obvious that there are two groupings into which all the 

 designated associations can be classed, namely, riparian and desert. 

 The riparian set of associations includes those which owe their pres- 

 ence to the existence of the river, and is delimited outwardly at the 

 mesquite association (see figs. A and B). The Colorado River appar- 

 ently exerts no influence beyond the immediate bottom lands, which 

 are affected by the underground water supply. All of the desert set 

 of associations are represented in varying proportion over the vast 

 arid tracts stretching away to the east and west of the river. Often 

 they are discontinuous, but recur again and again in the same fauna 

 with the same constitution. 



The ripariau associations are thus narrow strips of varying width 

 closely paralleling the river from north to south and persisting prac- 

 tically continuously from the point of emergence of the Colorado River 

 from the Grand Canon to the Gulf of California. In the broad delta 

 region the riparian associations spread out so that there are great 

 areas of each, doubtless sufficient to be computed by the square mile. 

 It is thus possible to trace the elements severally, of each association, 

 to places of prevalence over considerable areas, even though those ele- 

 ments are, on the upper river, scattered sparsely along a narrow strip. 

 Associational diagnosis of species thus often becomes possible when a 

 knowledge of local conditions alone would be inconclusive. This prin- 

 ciple deserves enlarging upon. 



There were caught in the same trap-line opposite The Needles 

 both Peromyscus eremicus eremicus and Peromyscus crinitus stephensi. 

 It might have been impossible to say from the data gathered at that 

 particular point, where the adjacent associations were complexly inter- 

 mixed, just what sort of ground each species preferred. But trapping 

 previously done in the salt-bush association in the Imperial Valley 



