[Hinton] (1926: 215) separated the American Clethrionomys into three groups, suggesting that Old World counterparts of each group exist. [Davis] (1939: 308) gives an excellent analysis of the emigrational history of the American species, and points out the close relationship of the Siberian and Arctic-American forms. He suggests that the rutilus group invaded Arctic-America from Siberia at the close of the last glaciation.
Two other groups are recognized by [Davis], who assumes that both arose from a common ancestral stock and divided into two stems, one of which (gapperi group) followed the main Rocky Mountain course southward and worked eastward across Canada while the other (californicus group) followed down the Pacific Coast. [Davis] states, "This southward extension of range in America probably took place in the Pleistocene, but almost certainly the present southern range of the genus in the West was not reached until late in, or after, that period."
Our studies of the Clethrionomys of Washington indicate the essential correctness of [Davis]' conclusions. The separation of the two groups, however, probably was caused by southward moving glaciers and the separation of the mice into two stocks closely paralleled the divergence of other groups, such as that of the snowshoe rabbits. The two groups of red-backed mice are more sharply separated than was thought by [Davis]. The intermediate color of the back of C. g. idahoensis, mentioned by [Davis], seems to be merely coincidental. The essential difference in the two groups is the sharply marked red band of the gapperi group as opposed to the general red area on the dorsal surface of the californicus group. Clethrionomys gapperi nivarius has been considered a derivative of occidentalis because the range of the latter surrounds the Olympic Mountains, where nivarius is found. Considering the immediate post-Pleistocene movements of mammals from the Cascades to the Olympic Mountains, so clearly illustrated in Thomomys, Tamias amoenus, and Phenacomys, a means is indicated by which Clethrionomys of the gapperi group might have reached the Olympics. The apparent lack of intergradation between occidentalis and nivarius gives proof of their relatively distant relationship. C. nivarius seems not to have been derived from occidentalis, and apparently does not belong to the californicus group. It belongs instead to the gapperi group, and I consider it to be a subspecies of gapperi. In no sense is it intermediate between the two groups. The other form considered by [Davis] to be intermediate between the two groups is caurinus. This mouse has not yet been taken in Washington, although it may eventually be found on Point Roberts, on the Fraser River delta. Its distribution is paralleled by that of many other mammals that are definitely not of the Pacific Coastal Fauna.
The californicus group, I feel, contains only the races of Clethrionomys californicus, while the gapperi group contains C. gapperi and its races, including caurinus, and possible other species.
Possibly intergradation occurs between C. californicus occidentalis and C. gapperi. Nevertheless, I fail to find evidence of such intergradation. I have taken C. gapperi nivarius and C. californicus occidentalis within ten miles of each other, but each retained its distinctive characters with no evidence of intergradation. In the case of C. gapperi saturatus and C. californicus occidentalis the proof is less conclusive. In spite of numerous attempts to trap Clethrionomys in the area geographically intermediate between their ranges, I have taken none. Though common along the coast, occidentalis becomes progressively scarcer to the east, being rare in the vicinity of Seattle and apparently absent from the western base of the Cascades. So far as is known, the ranges of caurinus and occidentalis are separated by the Fraser River.
Microtus pennsylvanicus and MONTANUS.—The Pennsylvania meadow mouse is closely related to Microtus montanus. Certain races of montanus, notably those from southern Oregon, California, and northern Nevada, closely resemble pennsylvanicus externally and cranially. From the central part of its range northward, montanus becomes progressively less like pennsylvanicus. The races nanus and canicaudus are quite different from pennsylvanicus both externally and cranially, and in addition the anterior loop of the second molar is less constricted; often it is not constricted at all. Microtus pennsylvanicus and Microtus montanus occur together over parts of the northern Rocky Mountains. Where the two species came together, pennsylvanicus occurs with races of montanus that are most unlike it.
Microtus pennsylvanicus kincaidi closely resembles races of Microtus montanus that occur in southern Oregon, California, and Nevada. It is larger, darker, and longer-furred than Microtus pennsylvanicus funebris from Washington. Seemingly pennsylvanicus and montanus diverged previous to Vashon-Wisconsin Time. The stock that gave rise to montanus spread over the Great Basin while pennsylvanicus ranged farther east. Some of the montanus stock worked northward in the Rocky Mountains. Microtus pennsylvanicus had meanwhile moved westward to the Rocky Mountains. The two stocks met and behaved as full species.
Microtus pennsylvanicus probably occupied northeastern Washington in the interglacial cycle preceding the Recent. The advance of the Wisconsin ice eliminated most of these mice. The glacier dammed the Columbia River and caused it to turn southward from its basalt-marginal course and take a path over the Columbian Plateau. Along this glacial river a population of Microtus persisted to become kincaidi.
While the Wisconsin ice was at its maximum extent, Microtus montanus from the Blue Mountains crossed the Simcoe Bridge to the Yakima Valley and the eastern Cascade Mountains. The closing of the Simcoe Bridge isolated these mice, which subsequently became slightly differentiated, as canescens. Another stock moved westward along the Columbia River to western Oregon. This stock is now called Microtus canicaudus but is probably racially rather than specifically distinct from Microtus montanus nanus.
With the retreat of the Wisconsin ice, montanus extended its range northward from the Yakima Valley along the eastern Cascade Mountains to extreme southern British Columbia. Microtus pennsylvanicus funebris entered northeastern Washington with other elements of the Rocky Mountain Fauna.