At present, pennsylvanicus occurs in northeastern Washington while montanus is found in southeastern Washington and the eastern Cascade Mountains. The Okanogan River Valley separates their ranges. Eventually montanus may extend its range to northeastern Washington and pennsylvanicus to the Cascades, the two forms occurring together as they do in Montana, Idaho and Colorado. The isolation of kincaidi on the Columbian Plateau seems complete and the probability of its range reaching that of its related species seems slight.
Microtus longicaudus.—The later distributional history of the long-tailed meadow mouse is not yet clear.
Microtus townsendii.—This Pacific Coastal species probably lived west of the Sierra Nevada-Cascade Mountains since the early Pleistocene. It has extended its range northward since the retreat of the Vashon ice and has reached some of the islands in Puget Sound and the Strait of Georgia. This species seems to be related to Microtus richardsoni but the two species probably separated at an early time.
Microtus richardsoni.—The water rat of the Cascade Mountains seems to be have been forced southward into Washington and Oregon by the descent of the Vashon-Wisconsin glaciers where it became isolated from the water rat of the northern Rocky Mountains. After the retreat of the ice, both forms have moved northward. [Bailey] (1900) records macropus from Bonner County, Idaho, not far from northeastern Washington but in Washington up to this time, macropus has been recorded only from the Blue Mountains of the southeastern part of the state.
Microtus oregoni.—The later historical distribution of the creeping mouse was probably the same as that of townsendii.
Lagurus curtatus.—The distribution of this species of the Great Basin Fauna has probably changed little if any since the late Pleistocene.
Ondatra zibethicus.—The distribution of the two races of muskrat that occur in Washington is almost exactly that of the beavers. Probably one form was isolated in southwestern Washington and northwestern Oregon during Vashon Time and another, more adaptable, race occurred in eastern Washington. After the retreat of the ice the adaptable race spread widely but the muskrats of the lower Columbia River changed their range little or not at all.
Aplodontia rufa.—The race of Aplodontia found in the Cascades of Washington was probably confined to the southern Cascades in Vashon Time and has since spread northward to reoccupy the range as far north as southern British Columbia.
The presence of a mountain beaver in western Washington that is indistinguishable from the race rufa, found in the Cascades of Oregon, is most surprising. In Oregon, pacifica occurs in the western lowlands and rufa in the mountains to the east. In Washington rufa occurs in the western lowlands and rainieri in the mountains to the east.