The Olympic Mountains, on the Olympic Peninsula, rise above the timber-line and are surrounded by forested lowlands which in a sense isolates this mountain range. Early workers, notably [Elliot], obtained specimens of mammals from the Olympics and described numerous races, principally, it appears, on the supposition that because the range was somewhat isolated it should possess a unique fauna. Subsequent revisions of groups of mammals have indicated that most of the names proposed, on the basis of specimens from the Olympics, were either invalid or pertained to mammals found also in the Cascades.
The mammals of the Olympic Peninsula appear to be divisible into three groups. A majority of them fall within the first group, namely coastal races possessing wide ranges in the lowlands of western Washington. The second group consists of species of the Rocky Mountain Fauna but with close relatives in the Cascades. The third group includes but two forms, both unique and found only on the Olympic Peninsula.
The first group includes nonalpine forms of the lowlands surrounding the Olympic Mountains. For the most part these are identical with races of the Puget Sound area. A few are slightly differentiated from the mammals of the Puget Sound area but are the same as mammals from southwestern Washington. As will be shown later, some differentiation in the Pacific Coastal Fauna has occurred. This is thought to be evolution in situ, rather than the result of mass movements. Many nonalpine Coastal mammals occur in alpine habitat in the Olympics.
The second group consists of species of the Rocky Mountain Fauna. Their relationship to the mammals of the Cascades is indicated in the two parallel columns below.
| Olympics | Cascades | |
| Sorex palustris navigator | Sorex palustris navigator | |
| Martes caurina caurina | Martes caurina caurina | |
| Martes pennanti | Martes pennanti | |
| Tamias amoenus caurinus | Tamias amoenus ludibundus | |
| Phenacomys intermedius oramontis | Phenacomys intermedius oramontis | |
| Clethrionomys gapperi nivarius | Clethrionomys gapperi saturatus |
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.
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.