The Bendire water shrew, Sorex bendirii albiventer, is not restricted to alpine habitat but occurs throughout the Olympic Peninsula. Its nearest relative is S. b. bendirii of the rest of western Washington. S. b. albiventer differs from bendirii only in possessing a partially white ventral surface. We can only conclude that the white belly of albiventer is a mutation that the local environment has favored and that the characters have, therefore, spread through the population on the Olympic Peninsula. Occasional specimens are taken with dark bellies characteristic of bendirii ([Jackson], 1928: 199).
The Olympic marmot, Marmota olympus, specifically distinct, and apparently the only preglacial relic species of alpine mammal in the Olympics, is most nearly related to Marmota vancouverensis of the unglaciated mountains of Vancouver Island, British Columbia. Both olympus and vancouverensis are close relatives of Marmota caligata which ranges southward into the Cascades of Washington.
The Columbia River in its course westward through the Cascade Mountains, might be expected to act as a highway for the movement of mammals, but the extent to which it has done so seems to be slight, at least in postglacial time. The pocket gopher of southwestern Washington reached the area about Vancouver from the southern Cascades by way of meadows on the gravel terraces of Wisconsin glacial drift. No other mammal seems to have extended this far. Several Great Basin species, such as the cottontail, extend westward in the valley of the Columbia to the vicinity of Bingen.
The mammals of western Oregon and southwestern Washington are closely similar as are the plants and climate, despite the fact that the broad Columbia River courses through the area and did so all through Recent and Pleistocene times. Many species would be expected to have crossed this barrier by swimming and rafting, and that they did so is indicated by the large number of mammals which are identical or very closely related on the two sides of the river.
Mammals which seem not to differ on the two sides of the river include:
| Scapanus townsendii | Citellus beecheyi | |
| Scapanus orarius | Tamias townsendii | |
| Sorex trowbridgii | Sciurus griseus | |
| Sorex vagrans | Glaucomys sabrinus | |
| Canis lupus | Castor canadensis | |
| Felis concolor | Microtus townsendii | |
| Lynx rufus | Microtus oregoni | |
| Mephitis mephitis | Ondatra zibethicus | |
| Spilogale gracilis | Zapus princeps | |
| Procyon lotor | Odocoileus hemionus | |
| Ursus americanus |
The following mammals are subspecifically distinct in western Washington and western Oregon:
| Washington | Oregon | |
| Sorex bendirii bendirii | Sorex bendirii palmeri | |
| Sorex obscurus setosus | Sorex obscurus bairdi | |
| Neotoma cinerea occidentalis | Neotoma cinerea fusca | |
| Peromyscus maniculatus austerus | Peromyscus maniculatus rubidus | |
| Clethrionomys californicus occidentalis | Clethrionomys californicus californicus | |
| Microtus longicaudus macrurus | Microtus longicaudus abditus | |
| Aplodontia rufa rufa | Aplodontia rufa pacifica |
The following species are found in western Oregon but do not occur in western Washington: