Ii)l2 ['nivcrsity of California Publications in Zoology [Vol.12 



The fact is apparent that only members of the strictly desert asso- 

 ciations are stopped at the river. And of these the species of the 

 most remote associational position are, with one exception, Xeotoma 

 intermedia dcsertorum. most effectively delimited. Also degree of 

 isolation is in a measure commensurate witli amount of difference 

 between forms of the same genus. 



It is pertinent to inquire how the Colorado River acts as a barrier 

 to those species affected. It appears that in every one of the eleven 

 cases the animal in question has no need to visit any water-supply. All 

 are species capable of maintaining successful existence without a drop 

 of water other than that obtained by chemical elaboration from their 

 food. In our three months' experience we did not once find evidence 

 that any individual of any of the eleven species in question had visited 

 the river's edge. 



Furthermore, to the best of our knowledge, all the species are of 

 limited foraging range. In the ease of the two diurnal chipmunks, 

 Ammospermophihis harrisi harrisi and Ammospermophilus Icucurus 

 leucurus. which could be seen, it was seldom that an individual was 

 come upon more than fifty yards from its burrow. In the case of 

 Pcrognatltus. which carefully closes the mouths of its burrows for the 

 day, after its night's activity abroad, it was impossible to secure 

 definite information on this score except as afforded by trapping; 

 but the writer's impression is that it. too, does not ordinarily venture 

 many rods from its retreat. Indi^^duals doubtless travel farther at 

 times of rutting, but it is likely that even then the limits of the 

 native association would not be far transgressed. 



It is further to be noted that those species finding an insuperable 

 check at the river are all closely confined to one general kind of 

 associational environment, even though two minor associations, as here 

 defined, be occupied. The river plus intervening associations of an 

 unfavorable nature constitutes the total barrier to the rodents in 

 question. 



It is true that the element of distance here implied is reduced to 

 a negligible quantity where hills closely abut upon the river channel. 

 But the major part of the river's course, probably four-fifths of it 

 below the lower end of the Grand Caiion, is through valleys of vary- 

 ing width, occupied by riparian associations most adverse in essential 

 ecological particulars to the species of the upland deserts adjacent. 



Along the remaining fifth of the river's course, where the banks 

 rise abruptly and are continuous with the adjacent hill slopes, with 



