256 Part III. Chapter 3. 
The advent of the later glacial advances inaugurated a change of climate 
and a more general refrigeration of the northern part of North America. Not 
only were the mountain valleys filled with glaciers descending from the moun- 
tain tops, but the larger valleys themselves with the lower mountain peaks 
between had their temperature so reduced as to permit the migration of boreal 
plants into and across them. It was during the period of maximum glaciation, 
that a second contingent of boreal plants reached the cordilleran system 
mountains south of the extremity of the ice sheet. These plants, however, 23 
in reaching the Sierra Nevada Mountains evidently did not descend through 
the Cascade ranges (although some of them did move down these mountain, 
but not from them into the Sierras), but through the Rocky Mountains. To 3 
explain the distribution of groups c and d, we must assume a means of com 
munication between the floras of the Sierra Nevada and those of the southern 
Rocky Mountains shortly before their final separation. It is believed that this 
communication may easily have taken place across Nevada and Utah by way 
of the numerous mountain ranges which traverse those regions. Under their 
present conditions of isolation, the migration of these species would be im- 
possible; but when the present boreal flora was depressed nearly or quite t0 
the base level of the mountains an excellent route for migration was presente n 
This explanation is rendered still more probable by the occurrence of nearly 3 
all the species of group € upon one or more of the higher desert rang 
such as the White, East Humboldt and Trinity mountains of Nevada, and te = 
Uinta and Wahsatch mountains of Utah '). ae | 
The plants of group d also advanced during the period of maximum 
glaciation reaching simultaneously, the Cascade Mountains, and the Rocky 7 
Mountains and from the latter by the route mentioned above to the Sierra | 
Nevada Mountains proper. ne 
The explanation of the circuitous route taken by the boreal plants of the ne 
second migration to the Sierra Nevada Mountains is afforded by a study a 
the distribution of species on Mount Shasta. MERRIAM*) has pol“ | 
out that the Klamath River Valley is the efficient barrier that must be MEZ 
sidered in the distribution of boreal species. Klamath Gap represents & break 
of less than 50 miles separating the boreal fauna and flora of Shasta from 
that of the Cascades. In studying this gap compared with the breadth of 
the combined Pitt River and Feather River gaps, about 100 miles separating 
Mount Shasta from the boreal elements of the Sierra Nevada northwest 
Honey Lake, one might expect Shasta to share more species with the 14 
cades than with the Sierras. The contrary is true. Of the distinctively Cascade 
species 25 per cent are derived from the mountains farther north, 12 PF Be: 
are local types, ı2 per cent belong to transcontinental boreal types; ” 
; National f 
1) CoviLLE, F. V.: Botany of the Death Valley Expedition. Contributions U. 
Herbarium IV: 30. 1893. rth 
2) MERRIAM, C. HART: Results of a biological Survey of Mount Shasta California. Bir 
American Fauna, No. 16: 86. 1899. 
