Sciurus griseus.—This species of the Pacific Coastal Fauna probably entered Washington from Oregon since the retreat of the Vashon Glacier. It has probably entered the state in relatively recent times.
Tamiasciurus hudsonicus.—The two species of red squirrels, T. hudsonicus and T. douglasii, are specifically distinct and probably became differentiated in the Pleistocene when southward moving glaciers cut in two the range of the ancestral stock. The morphological differences are too great, comparatively, to have occurred during the Vashon-Wisconsin Divergence. T. hudsonicus probably occupied a range in pre-Wisconsin Time that included the Rocky Mountains and areas to the north. Glacial ice probably restricted the range of hudsonicus in Wisconsin Time but after the retreat of the ice hudsonicus moved northward to reoccupy its former range. It also moved westward across northern Washington to the Cascades, where it met the range of douglasii. Farther north, it moved westward to the Pacific, thus occupying an area that, in pre-Vashon time, probably was occupied by douglasii.
Tamiasciurus douglasii.—The Douglas squirrel probably occupied the coastal region of Oregon, Washington and British Columbia in pre-Vashon Time. The descent of the ice restricted its range to southwestern Washington and areas to the south. After the retreat of the ice it moved northward somewhat but, like other coastal species, the movement was slow. Meanwhile hudsonicus from the Rocky Mountain Fauna, had spread to the coast of British Columbia.
Glaucomys sabrinus.—This flying squirrel is a plastic species. It inhabits all of the forested parts of Washington. The distributional picture presented by the 5 races (Fig. 92) which occur in Washington is complicated. The ranges of 3 of these lie principally outside the state of Washington.
The race oregonensis occupies Washington and Oregon west of the Cascades; fuliginosus occupies the Cascades of Washington, Oregon and southern British Columbia; columbiensis occupies the interior valleys of British Columbia and adjoining Washington; latipes occupies the northern Rocky Mountains of British Columbia, northern Idaho and extreme northeastern Washington; and bangsi occupies the Blue Mountains of Washington and Oregon and a wide range in Idaho and eastward.
The differences separating the race oregonensis from other subspecies found in Washington are relatively great. This lowland race is smaller and richer in color. The other races exhibit slight but relatively constant differences. The relatively great difference between oregonensis and the other races indicates that oregonensis was isolated from the remainder of the species for a considerable time. Presumably oregonensis was a strongly differentiated coastal race in pre-Vashon Time and occupied most of western Washington and Oregon. The descent of the Vashon ice restricted the range of oregonensis to southwestern Washington and western Oregon. The descent of the ice forced a northern race, fuliginosus, southward into the range of oregonensis. The northern race, adapted to boreal conditions, was able to compete successfully with the established oregonensis only in mountainous areas. In the Cascade Mountains, fuliginosus extended its range southward to southern Oregon.
The descent of the Wisconsin ice in eastern Washington forced the flying squirrels of adjacent British Columbia southward into the Rocky Mountains. These squirrels were probably closely related to fuliginosus, or to bangsi, which latter race already may have been established farther south in the Rocky Mountains. The Blue Mountains of southeastern Washington were probably inhabited by bangsi in Wisconsin times, or even earlier. The retreat of the Vashon-Wisconsin glaciers allowed the flying squirrels to extend their ranges northward. In western Washington oregonensis moved to southern British Columbia. In the Cascade Mountains the more boreal fuliginosus moved much farther northward and, north of the Okanogan Valley, spread eastward to the arid, interior valleys of British Columbia. Subsequent differentiation in the population of the arid, interior valleys developed the slightly differentiated race columbiensis. Farther east, flying squirrels from the northern Rocky Mountains moved northward. Northeastern Washington and adjacent British Columbia were occupied by latipes, derived from bangsi.
Thomomys talpoides.—Views as to the probable historical distribution of this plastic group have been presented in an earlier report ([Dalquest] and [Scheffer], 1944: 308-333). This may be briefly summarized as follows.
Previous to Vashon-Wisconsin Times, pocket gophers occupied at least the Cascade Mountains and the Columbian Plateau of Washington. The race occupying the Columbian Plateau, devexus, was probably racially distinct in pre-Wisconsin time. The descent of the Vashon-Wisconsin glaciers isolated gophers in the southern Cascades. Three racial stocks developed there: shawi in the Mount Rainier area; limosus in the Columbia River Valley south of the Cascades; and immunis in the mountainous area between the other two races. At the maximum extent of the Wisconsin ice, gophers from the Columbia River Valley (limosus) were able to cross the Simcoe Bridge and reach the Blue Mountains. With the retreat of the Wisconsin ice, the Simcoe Bridge was closed. Gophers isolated in the Blue Mountains developed the racial characters of acqualidens and those between the Blue Mountains and the Columbia River intergraded with the desert race, devexus. These intergrades, which have, also, some characters of their own, bear the name columbianus. Gophers in the southern Cascades (limosus) moved westward on prairie-like river terraces to Clark County where, isolated, they became racially distinct (douglasii).