DISTRIBUTION OF THE ANIMALS 9 



boreal there is not only a well-marked belt of overlapping (comprising 

 the Transition Zone) but in this belt there are numerous species closely 

 restricted to it. On the eastern slope, however, Canadian and Upper 

 Sonoran are jammed so closely together by reason of the steepness that 

 the belt of intermingling of elements is very narrow or at best indistinct ; 

 there is scarcely if any room for the existence of restrictedly Transition 

 Zone species. 



Although presenting a strongly Great Basin aspect, the Mono basin, 

 doubtless because of its high altitude, does not show a pure representation 

 of austral life. It does contain a number of elements (that is, species) 

 which from a study of their entire ranges we know to belong predominantly 

 to the upper division of the austral, namely Upper Sonoran. But there 

 are also present about as many, or as dominant, boreal elements. 



Frankly, we found difficulty in assigning some parts of the Mono 

 portion of the Yosemite section to one life zone rather than to another. 

 This was particularly true of the south, sun-facing slope of Williams Butte 

 (pi. 19&), which is clothed with piiion. This tree to the southward along 

 the Sierras forms a belt which through Walker Pass is continuous with the 

 digger pine belt of the west slope of the Sierras ; and as a rule we can safely 

 diagnose this belt by reason of this one plant indicator as Upper Sonoran. 

 But on Williams Butte the piilons are mixed with western junipers, Jeffrey 

 pines, mountain mahogany, and certain shrubby plants which are accepted 

 as diagnostic of Transition, Canadian, or even Hudsonian. We found in 

 this anomalous assemblage of plants such 'good' Upper Sonoran birds as 

 bush-tits and Woodhouse jays in close association with mountain chickadees 

 and Clark nutcrackers. This was after the breeding season ; and, of course, 

 there was a chance that in the case of the last-named species, at least, the 

 individuals observed had moved down from the higher altitudes but a very 

 few miles to the westward. In the case of small mammals, which are 

 incapable of quickly traversing considerable stretches of territory, we 

 found, on Williams Butte, True white-footed mice, which are typically 

 Upper Sonoran, in the same trap-line with Mono chipmunks, which find 

 their metropolis in the Canadian life zone. 



Another tract in the Mono country which was for similar reasons 

 perplexing occupies the lower slope down toward the lake shore from 

 Mono Mills. There, pale-faced kangaroo rats, Stephens soft-haired ground 

 squirrels, and desert jack rabbits were found, species which belong to 

 groups whose habitats lie chiefly within the Upper Sonoran Zone, but here 

 were found in company with animals and plants of more northern, 

 Transition or even Canadian, predilections. The sage-hen, to cite one of 

 these latter, is a 'good' Transition bird. 



