Considering the long existence of the Puget Bridge, it is surprising that such forms as the pika, water rat and golden-mantled ground squirrel did not cross to the Olympics. These forms are, however, species of the higher or eastern slopes of the Cascades. The third group of Olympic mammals includes the white-bellied water shrew and the Olympic marmot, both indigenous forms. 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: