In the pocket gophers, where restricted habitat and fossorial habits cause numerous microgeographic races, these microgeographic races may be potential subspecies. This is especially true in the Puget Sound area, where six races occur in a small area. These races meet all the requirements of subspecies and are recognized as such. It should be pointed out, however, that these races and probably many other races produced by isolation, may represent degenerative mutations of the type mentioned by Wright (in Huxley, 1940). The principal differences of such races seem to have resulted from the loss of factors of original multiple factor series, with resultant homogeneity of the race. Inherent variability is another thing that has to be taken into account when considering the differentiation of the mammals of Washington into subspecies. The pocket gopher is an extremely plastic species, especially in Washington, whereas the Douglas squirrel is less so. The flying squirrels, the yellow-pine chipmunk and the snowshoe rabbit are the other plastic species. These species are not so likely to break up into numerous subspecies over all of their ranges as they are in Washington where in a small area the topography is highly varied. The range of the one subspecies, _Tamias amoenus amoenus_, to the southeast of Washington is larger than the combined ranges of all six races occurring in Washington but, so far as I can see, the topography and environment are no more varied in Washington than in the mentioned area to the southeast of it. The range of one subspecies, _Lepus americanus americanus_, in Canada is several times larger than the entire state of Washington, in which four races are found. The shrews are poor subjects for a study of differentiation, principally because their small size makes it difficult to see morphological variations that may be present. The difficulty is increased because cranial sutures become ossified at an early age. Although it is difficult to evaluate the differentiation in them, there is some. The bats, especially the _Myotis_, are less restricted by geographic barriers than are terrestial mammals. Nevertheless, obvious differentiation exists. The larger predatory mammals and the artiodactyls are able to move over large areas, at least in the breeding season, but in these animals also, some differentiation has occurred.