2. Population pressure results, with a tendency of individuals to emigrate outwards, to the border of the range of the subspecies, where the population pressure is less.
3. The outward moving tendency keeps the center of the range of the subspecies genetically "pure."
4. The peripheral wave continues, as long as favorable habitat is encountered, until an oppositely directed wave of another race is encountered.
5. Areas of intergradation represent local mingling of genetic factors and do not affect the "pure" individuals of the central part of the range of the subspecies.
Certain aspects of this hypothesis are strongly supported by the distribution of mammals in Washington. Witness the rapid invasion of Citellus beecheyi and Lepus californicus in Washington, and the eastern cottontail in western Washington and the domestic rabbit in the San Juan Islands. The volume of the "wave of population pressure" where no opposing force is met, is scarcely believable. In seven years the eastern cottontails released in southwestern Washington multiplied from a maximum of 12 individuals to a minimum of 40,000.
Competition between subspecies where their ranges come into contact seems to be exceptional. Peromyscus maniculatus oreas and P. m. austerus seem to afford an example of this. However, in a few cases subspecies seem to be determined in part by adaptation to restricted environments; each race lives only where local conditions favor its respective adaptations.
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.
The greatest changes, other than the extinctions, to occur in the mammalian fauna of Washington since the late Pleistocene, are changes in distribution. The interglacial cycle preceding the Vashon-Wisconsin glaciation was of far greater duration than the Recent. Presumably the mammalian fauna had, from a distributional standpoint, reached a relatively stable condition. The descent of the Vashon-Wisconsin ice destroyed the stability and set parts of the fauna in motion. Probably no stability was reached before the ice began to recede, and when it did so the previous movements of the various species were, at least in part, reversed. Stability has not yet been reached by the mammalian fauna of Washington. Great changes have occurred in historic times and other changes probably are under way at present.
In the following pages an attempt has been made to interpret the probable late Pleistocene and Recent distributional history of the species of mammals occurring in Washington. The interpretations are made in the light of what is known of the physical history of the state and are to be accepted as such rather than as evidence for the conclusions made concerning the physical history of the state of Washington and adjacent areas.
Scapanus townsendii.—Probably this animal was confined to the Humid Transition Life-zone of the Pacific Coast since the Pliocene.