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Hudsonian, but such a distinction is not tenable, foron Mount San 
Gorgonio, where the Boreal Region is best developed, both species 
intermingle from the neighborhood of Dry Lake (altitude 2775 
meters) to the very summit. Ascending the mountain from 
Dry Lake one passes through pure forests of these two pines. 
Even the floor, which is composed of loose broken granitic rock, 
is almost bare of vegetation. Continuing upward the trees be- 
come fewer and more reduced in stature until it is difficult to 
walk beneath their gnarled branches. Finally, as the summit is 
approached, they become prostrate shrubs with such stiff compact 
branches that one may tramp over them with little difficulty. On 
the summit of Mount Pinos, where Pinus Murrayana has not been 
recorded, Pinus flexilis is found intermingling with Abies concolor 
and other upper Transition species. In fact it is impossible to 
detect two distinct belts of these two pines on any of the moun- 
tains. Notwithstanding the fact, therefore, that there are traces 
of Arctic-alpine plants, the only logical treatment demands that 
all the Boreal Region of southern California be placed in the 
Canadian Zone. 
In addition to the localities mentioned there is a fringe of the 
Canadian Zone on the western borders of Bear Valley, and Bluff 
Lake. Pinus Murrayana grows rather abundantly here, the 
lowest altitude (2010 meters) reached by this species in southern 
California. 
The characteristic trees and shrubs are: 
Pinus flexilis Ribes montigenum 
Pinus Murrayana Sericotheca concolor 
Populus tremuloides Phyllodoce Brewert 
Salix glaucops 
Sericotheca concolor and Phyllodoce Breweri extend northward 
through the Sierra Nevada to the southern Cascade Mountains, 
and to the mountains of western Nevada. All the remaining 
species extend to the Rocky Mountains, and one, Populus tremu- 
loides, to the northern Atlantic. The geographical distribution 
of these species demonstrates the intimate relation the flora of 
this zone bears to that of the more northern parts of North America. 
The origin of a boreal flora on these isolated southern mountain 
peaks is traceable to the influence of the Glacial Period. During 
that period central and southern California possessed a climate 
similar to that found to-day in the Puget Sound region. Perpet- 
