Only two of these are racially distinct from their relatives in the Cascades. Of these the chipmunk is a plastic species and breaks down into many races in Washington. The chipmunks of the Olympics and of Mt. Rainier are so similar that Howell (1929: 77) considered them as identical and mapped Mount Rainier as an isolated part of the range of the Olympic form (see account of _T. a. caurinus_). The relationship of the red-backed mice, also, is close, but has been obscured by the usual assumption of relationship between _californicus_ (_occidentalis_) and _gapperi_. The principal difference between the alpine forms is the pallor of _nivarius_. This pallor of mammals in general from the Olympic Mountains is noteworthy, but in the red-backed mouse is exceptionally noticeable. This pallor is discussed beyond in the paragraphs dealing with differentiation. Mention should be made here of _Myotis keenii_. This is a species which seems to have extended its range to Washington from the north. The power of flight, of course, removes it from consideration in attempting to reconstruct routes followed by terrestrial mammals. The route of the pocket gopher (_Thomomys_) in emigrating from the Cascades to the Olympics (Dalquest and Scheffer, 1944: 310), was over the outwash train of the Mount Rainier Glaciers, especially the Nisqualli Glacier, to the extensive outwash aprons of the Vashon Glacier around southern Puget Sound, and thence into the Olympic Mountains. Under the conditions in early postglacial time this invasion route, hereinafter termed the Puget Bridge, around the Pleistocene Lake Russell (present Puget Sound), is thought to have been mainly an alpine meadow. Indeed, the isolated prairies remaining today are the unforested remnants of the outwash aprons (see Dalquest and Scheffer, 1942: 69) and possess several species of alpine plants, notably the shooting star, camas, and bear grass. If the Vashon Glacier remained in place considerably longer than the Wisconsin Glacier, these Rocky Mountain species may have invaded the Cascades from northeastern Washington and travelled around the southern edge of the Puget Glacier or of Lake Russell. The close relationship of the races involved, however, suggests that the emigration took place much more recently. The barriers to such movement even today are slight, consisting principally of narrow areas of forest. For the water shrew, an almost continuous water habitat still exists, by way of the Nisqualli River, streams in the Puget Sound area, and the Satsop River in the Olympics. Tree-living forms such as the fisher and marten might easily travel the intervening distance today, and, by going along the forests north of the Chehalis River, reach the Olympics without crossing more than small streams and virtually without descending to the ground. Chipmunks and mice probably utilized the prairie or meadow area of the Puget Bridge, as did the gophers.