In immediate pre-Vashon-Wisconsin Time, the dark race streatori probably occurred in western Washington. The race murica probably occurred in the Blue Mountains then, as it does today, but probably occurred also in the Cascades of Washington. The descent of the Vashon glaciers probably displaced streatori from the northern part of its range, at least temporarily. In the Cascades, murica was likewise forced southward. Ermines related to the northern richardsonii were forced into northern Washington and Idaho by the Wisconsin ice. They probably were unable to live on the barren, unglaciated plains of eastern Washington but persisted in Idaho.
The ranges of the three forms at the maximum extent of the Vashon-Wisconsin may be reconstructed as follows: streatori in southwestern Washington; murica in the southern Cascades and the Blue Mountains; invicta stock in northern Idaho. While so isolated, the ermines of the southern Cascades probably mingled, to a certain extent, with streatori and developed the characters that now separate gulosa from both murica and streatori. The intermediate nature of gulosa has been mentioned by [Hall] (1945: 85).
The retreat of the ice allowed streatori to move north and invicta to move north and east into Washington and the northeastern Cascades. To a lesser extent, gulosa may have moved north. The poorly-marked race olympica probably evolved from streatori in the Recent. It is difficult to account for the dark race fallenda.
It must have evolved from streatori in the Recent but the origin of such a strongly marked race in such a short time is surprising. It might be mentioned that a similarly differentiated race of chipmunk, Eutamias amoenus felix, occupies much the same range.
Mustela frenata.—The long-tailed weasels of the Pacific Coast behave as a plastic group and clearly show the effect of the Vashon-Wisconsin Divergence. The range of the coastal race, altifrontalis, indicates that it was isolated in southwestern Washington during Vashon Time. In that period, or shortly after, it extended its range southward but only along the extreme, coastal area of Oregon (see [Hall], 1936: 101). Following the retreat of the ice it extended its range northward to the deglaciated area of western Washington.
Also following the retreat of the ice, a Great Basin subspecies (nevadensis) extended its range northward. This race seems to have been more adaptable and successful than other kinds of Great Basin mammals, for it extended its range farther northward, eastward and westward than most.
A third race, washingtoni, was isolated in the southern Cascade Mountains during Vashon Time and became differentiated from both altifrontalis and nevadensis. It is now found in the Cascades from central Oregon north to Mount Rainier. It is difficult to see why it did not extend its range to include the northern Cascades when the glacial ice left, but it did not. Instead altifrontalis entered the northern Cascades from the west and nevadensis did the same from the east. Weasels obtained in habitats north of Mt. Rainier are intergrades between altifrontalis and nevadensis.
One is reminded here of the douglasii group of Thomomys talpoides in which subspecies did not move north of Mt. Rainier in postglacial time. The area north of Mt. Rainier was populated instead by gophers of the fuscus group, subspecies of which invaded the area from the east. Perhaps Mt. Rainier itself served as a barrier to alpine mammals in the immediate post-Pleistocene. Perhaps Mustela f. washingtoni will eventually extend its range northward, displacing the altifrontalis-nevadensis intergrades from the habitats to which washingtoni may be better adapted.
The Blue Mountains of southeastern Washington are occupied by a weasel (effera) that has a more extensive range in eastern Oregon. The range of this race has probably not changed materially for a long period of time.