10 ANIMAL LIFE IX THE 10 SEMITE 



111 the nature of the case, as regards these exceptional localities, we 

 trust that the reader will understand why it is impossible for us to make, 

 positive statements witli regard to their zonal complexion. Two persons, 

 with some difference in perspective — that is, witli a different understanding 

 of the 'importance' of indicators — would very probably weight tlieir find- 

 ings differently. Our conclusion, as shown on our map and in our life-zone 

 table, namely, to call the western part of the Mono Lake basin, that part, 

 included within the Yosemite 'section,' Transition, is therefore presented 

 tentatively. The margin of determination is so small, with regard especially 

 to Williams Butte and the tract immediately south of Mono Lake, that 

 someone else, working the territory more intensively and listing the critical 

 species statistically (by individual composition, w'hich we did not), might 

 find adequate grounds for mapping it as Upper Sonoran. 



Returning to the Sierran divide: The Hudsonian Zone is found to be 

 well characterized on the east slope down to an average of about the 9500- 

 foot contour. This zone simply mantles the Sierras, save for the Arctic- 

 Alpine 'islands' which rise above timber line. Below the Hudsonian, good 

 Canadian is represented, with marked resemblance florally to that on the 

 western slope, in the lower part of Bloody Caiion. Moisture conditions 

 are there more exactly as they are on the west flank of the Sierras. Else- 

 where, Canadian is rather different in aspect from what it looks like on 

 the western slope, because of the prevailing aridity. Jeffrey pines and 

 mountain mahogany predominate in the place of red firs and aspens. The 

 steepest declivities, close to the Sierran divide, involve a lowering of 

 altitude to about the 8000-foot contour; thence eaiit to Mono Lake the 

 slopes involved in the long, lateral moraines are gentle, and the blending 

 of Canadian through Transition with 'austral' takes place gradually over 

 several miles of territory. Here is where most trouble wa-s experienced 

 in fixing upon a boundary between Canadian and Transition — and for the 

 same reasons as given above with respect to the Transition-Upper-Sonoran 

 boundary. Good Canadian extends east along the cold streams, where it 

 is marked conspicuously by thickets of aspen, well down toward the shores 

 of Mono Lake — to as low as 7000 feet ; Transition extends west up toward 

 the foot of the east Sierran face, especially along the south-facing slopes 

 of glacial ridges, to 9000 feet. Thus at Walker Lake one finds the interest- 

 ing situation of the Canadian Zone occupying the cool, shaded bed of the 

 glacial groove, with Transition on tiie south, sun-facing Avail above it : 

 the usual zonal relationship is reversed. Pacts such as this strengthen 

 our belief that the prime i)hysical factor accounting for zonation is not 

 altitude, or moisture, or soil, per se, but temperature. 



