| Vulpes fulva | Phenacomys albipes | |
| Urocyon cinereoargenteus | Microtus canicaudus | |
| Neotoma fuscipes | Thomomys bulbivorus | |
| Phenacomys silvicola | Lepus californicus | |
| Phenacomys longicaudus | Sylvilagus bachmani |
Several of these mammals which occur south of the river but not north of it are common on the south bank, a few miles from favorable but uninhabited territory on the north.
Seemingly the pre-Vashon faunas of western Oregon and Washington were similar. Some species became extinct in Washington in the course of Vashon isolation. Others persisted. The very close relationship of the mammals of the first group indicates some crossing of the river. The best known of such crossings was that of the Beechey ground squirrel which, previous to 1915, was unknown in Washington. In 1915, when there was no man-made bridge at White Salmon, it crossed the river and since has spread over an area of at least 50 square miles. The distribution of the mountain beavers is unusual in that the form in the lowlands of Washington is indistinguishable from the subspecies in the Cascades of Oregon.
The mammals that are racially distinct on the two sides of the Columbia River merit careful scrutiny. The Peromyscus of the two sides more closely resemble one another than those of southern Oregon resemble those of northern Oregon or than those of southern Washington resemble those of northern Washington. For Peromyscus maniculatus, the Columbia River is simply a convenient boundary for the separation of two slightly different races. The Oregon race of the bushy-tailed wood rat is a coastal type but the Washington form is the same as that of eastern Washington. Seemingly the more eastern race spread to an unoccupied habitat in western Washington. Other races that differ on the two sides of the Columbia probably developed while separated by the river.
Fig. 20. Rocky bluff along north bank of the Columbia River near Lyle. Washington. March 20. 1939. Habitat of Beechey ground squirrel and yellow-bellied marmot. (Fish and Wildlife Service photo by Victor B. [Scheffer]. No. 640.)
The San Juan Islands now possess a limited mammalian fauna. Unfortunately the activities of man have somewhat changed the native populations, especially by the introduction of the domestic rabbit which is now a serious pest in the islands. The Douglas squirrel, present on Blakeley Island, is said to have been introduced and one resident claims to have first brought it to the island. Two different persons claim credit for introducing the Townsend chipmunk on Orcas Island but do not account for its presence on Lopez Island. The three mammals most abundant and widely distributed in the islands are Sorex vagrans, Peromyscus maniculatus and Microtus townsendii. These species, at least, probably reached the islands at an early time. The two last named are now subspecifically distinct from their mainland relatives. Other mammals which probably were established before the arrival of the white man include the mink, otter, beaver, muskrat, raccoon and black-tailed deer.
The Great Basin Fauna of eastern Washington exists as three units, one on the Columbian Plateau, another in southeastern Washington and the third in the Yakima Valley area. The desert species of the Yakima Valley are more closely related to the species of eastern Oregon than they are to those of the Columbian Plateau. In a number of respects the Columbian Plateau gives indications of age. The ground squirrel, Citellus washingtoni, is related to, but specifically distinct from Citellus townsendii of the Yakima Valley and eastern Oregon. Perognathus parvus lordi is a well-marked race, as is Microtus pennsylvanicus kincaidi and Thomomys talpoides devexus. We suppose that these species were present on the Columbian Plateau at least through the Recent and probably through all of Wisconsin Time. The loess deposits of eastern Washington seem to have been laid down in Wisconsin and Recent times. These indicate an arid climate which, although probably cool, was probably not so cold as to exterminate these species. On the other hand, some species that are now abundant on the Columbian Plateau seem to have arrived there relatively recently. The black-tailed jack rabbit, for example, was unknown in eastern Washington before 1870 when it appeared in Walla Walla County. In 1905 it crossed the Snake River on ice and invaded the Columbian Plateau where it rapidly spread over the whole area. In January, 1920, it crossed the Columbia in two places and spread over the Yakima Valley.