Lepus americanus.—Snowshoe rabbits were probably spread over forested parts of Washington in pre-Wisconsin time. A coastal race, washingtonii, was probably distinct from the remainder of the species at an early time. It, and its kindred races klamathensis and tahoensis, are very different from races derived later from the Rocky Mountain Fauna. From this it is deduced that washingtonii was a member of the Pacific Coastal Fauna before the advance of the last continental glaciers.
The snowshoe rabbits now found in the Cascade Mountains of Washington are racially distinct from populations occurring to the north and east, but are more closely related to the northern and eastern rabbits than they are to washingtonii. This race, cascadensis, probably lived in the area north of the state of Washington in pre-Wisconsin interglacial time. The snowshoe rabbit of northeastern Washington was probably a race having an extensive range in the Rocky Mountains from which pineus in the Blue Mountains developed. The descent of the Vashon-Wisconsin glaciers found washingtonii restricted to western Oregon. It might have persisted also in southwestern Washington but, had this been the case, we would expect a broader zone of intergradation between washingtonii and cascadensis than actually exists.
The glaciers forced cascadensis southward into the Cascades of southern Washington. The difference between washingtonii and cascadensis and the narrow zone of intergradation between them, indicate that the two races were not in contact in pre-Vashon Time. The presence in the Cascades of Oregon of a race related to washingtonii rather than cascadensis also supports this view. The snowshoe rabbits of northeastern Washington were probably eliminated from areas where the ice was in place. The effect of the glaciers in nearby areas was probably less on snowshoe rabbits than upon other species.
Following the retreat of the glaciers, washingtonii reinvaded western Washington and spread northward through the lowlands to the Fraser River. In the Cascades, cascadensis moved even farther north. L. a. pineus of the Rocky Mountain Fauna invaded northeastern Washington from adjacent areas to the south. Another race, columbiensis, spread from the interior valleys of British Columbia to the western part of northeastern Washington in probably relatively recent times. The post-Wisconsin history of the distribution of this race is uncertain.
Lepus californicus.—The black-tailed jack rabbit occurs throughout the sagebrush areas of eastern Washington, except in the Okanogan Valley. It invaded Washington from the south recently ([Couch], 1927: 313). The first blacktails were thought to have entered Walla Walla County about 1870. Here they were checked by the Snake River. About 1905 they crossed the Snake River, on ice, and by 1920 had spread over much of the Columbian Plateau. In January, 1920, they crossed the Columbia River in two places, on ice jams, and by 1927 had spread north and west over the sagebrush areas between the Columbia River and the Cascade Mountains. When [Couch], in 1927, published his account, he predicted that the blacktail would eventually spread to the Okanogan Valley. This has not yet occurred although it is still to be expected.
It should be noted that the migrational history of the black-tailed jack rabbit is known only because the animal is large, spectacular, and diurnal, and therefore noticed by resident ranchers, and because the facts came to the notice of a competent biologist. Migration of a less conspicuous mammal, for example, a mouse, would rarely be detected.
Sylvilagus nuttallii.—The cottontail might have been resident on the Columbian Plateau during Wisconsin Time but probably it invaded, or perhaps reinvaded, the Plateau from eastern Oregon in the Recent.
Sylvilagus idahoensis.—Too little is known about this rabbit in Washington to hazard a guess as to its recent distributional history.
Cervus canadensis.—The elk emigrated from Siberia to North America in the late Pleistocene but previous, certainly, to Vashon-Wisconsin time. Probably the coastal race was distinct from the interior population in the preceding interglacial cycle. The descent of the last continental glaciers, in Washington, separated the elk of the Pacific Coastal Fauna from the Rocky Mountain Fauna. After the retreat of the ice the coastal race moved northward to southern British Columbia and the Rocky Mountain race moved northward and westward through British Columbia. Northeastern Washington was reoccupied by the Rocky Mountain elk.