Possibly, also, the whole of the Pacific Coastal Fauna possessed an inherent sluggishness resulting from their long residence in the uniform climate and habitat of the Pacific Coast. Certainly the species show today great habitat specialization as compared with species of the Rocky Mountain Fauna. Also, there are fewer individual mammals per unit of area in western Washington than in northeastern Washington. The persistence of the Vashon Glacier, an inherent lack of incentive to emigrate, or retention of a favorable environment, may account for the relatively small area invaded by the Pacific Coastal Fauna. 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_