Postglacial changes in climate did, however, permit the dark, short-tailed mice (rubidus) to cross the Columbia River and to become established in suitable habitats, namely the deciduous jungles of the river valleys. From these points the mice spread northward through the lowlands of western Washington, infiltrating the range of oreas, competing with it, and driving it from the lowlands. This movement is in progress today. By logging and clearing of lands for agriculture man has considerably speeded the invasion of the southern mice. Slight differentiation of the short-tailed mice north of the Columbia River (austerus) separates them from their parent race, rubidus.
In the dense forests along the Pacific Coast of Washington, austerus did not become established. This area includes most of the land west of Puget Sound. There is a narrow band of austerus that extends along the eastern and part of the northern edge of the Olympic Peninsula, where they have probably invaded in relatively recent time.
On the Oregon side of the Columbia River the range of rubidus is continuous from the Pacific to the Cascades. On the Washington side of the river, oreas extends from the Pacific to eastern Wahkiakum County, where the range of austerus starts. At the border of the ranges of the two subspecies, ecological conditions determine the ranges; oreas occurs in the Douglas fir and hemlock forests and austerus occurs in the jungles of alder and salmonberry in the stream valleys. The range of austerus extends from eastern Wahkiakum County east along the Columbia, to western Klickitat County. In a north-south strip across the Columbia, through the ranges of rubidus in Oregon and austerus in Washington, normal intergradation is apparent. By gradual degrees rubidus changes to austerus. In a north-south strip farther west, through the ranges of rubidus and oreas, the same transition of the Oregon rubidus is seen, namely an approach toward austerus. The cline is, of course, abruptly terminated by the range of the dissimilar oreas.
On Puget Island, a small island lying in the Columbia River in western Wahkiakum County and politically within the state of Washington (see [Scheffer], 1942) a population of deer mice occurs that resembles rubidus more closely than austerus. This island lies in the river between the ranges of rubidus and oreas. The lack of intergradation between these two subspecies has probably kept the Puget Island population pure rubidus. Puget Island is a junglelike lowland locally known as "tideland."
The San Juan Islands of northern Puget Sound were thoroughly glaciated in Wisconsin Time and probably were under thousands of feet of ice when the Vashon Glacier was at its maximum extent. The subspecies of deer mouse occurring there now is hollisteri, a race similar to oreas in color, body size and cranial characters and probably derived from oreas. P. m. hollisteri differs from both oreas and austerus in its much shorter tail. Shortness of tail is apparently a product of insularity in northwest coastal mice. The transition from long-tailed mainland mice to short-tailed island mice is shown by [Hall] (1938A: 461). When oreas first occupied the area about Puget Sound (this area is occupied by austerus today) some individuals probably reached the San Juan Islands soon thereafter. Contact between oreas and hollisteri is now prevented by the presence of austerus between their ranges. Of course, a water barrier separates austerus from hollisteri but austerus does occur in pure form on some islands in Puget Sound (not on any of the San Juan Islands), for instance, on Whidby Island. One wonders why austerus has not established itself on some of the San Juan Islands but considering the degree of difference between hollisteri and austerus, I doubt that the two would interbreed if they did occur together.
On Vancouver Island, British Columbia, a short-tailed, austerus-type of mouse occurs in the lowlands and a long-tailed, oreas-type of mouse in the mountains. Vancouver Island is a large island (16,400 square miles). Apparently a stock of austerus from the mainland reached Vancouver Island and were able to find suitable habitat and compete with and drive out the established oreas in the manner that they probably did in the western Washington lowlands. The large Vancouver Island offers far more variety of habitat and opportunity for establishment of emigrating mice than do the smaller San Juan Islands, the largest of which is Orcas (57 square miles).
Following the withdrawal of the Wisconsin ice from eastern Washington, numerous minor movements and adjustments of deer mice seem to have taken place. Long-tailed, oreas-type of mice were exterminated on the Columbian Plateau if, indeed, they ever became established there. Long-tailed mice did apparently become established in driftless areas and forested areas to the south of the drift border. With the establishment of soil and forests on the deglaciated land, the short-tailed gambelii and the long-tailed mice became thoroughly intermixed. In some areas, especially near the type locality of artemisiae, the gambelii characters of the mixture predominate at the present time. Deer mice from the Okanogan Valley differ from gambelii only in slightly larger size and darker color. In other areas, notably near Metaline, Pend Oreille County, the oreas characters of the mixture dominate at the present time. Mice from here are large and red and differ from typical oreas in having relatively short tails. Other less marked segregations of characters are numerous, in Idaho and British Columbia as well as in Washington. [Cowan] (1937) has described Peromyscus m. alpinus, isolated in a mountainous area of British Columbia. This seems to be a form derived from oreas-type stock.
With the reëstablishment of soil and forests on the deglaciated land of British Columbia north of Washington, a northward extension of the gambelii-oreas mixture occurred. Another invasion was taking place at this time, however. Peromyscus m. arcticus spread onto the deglaciated land from the north or east, ahead of artemisiae (the gambelii-oreas mixture). This new invader intergraded with artemisiae as well as with oreas. Further collecting and studies are required in this area before the relationships of the three subspecies can be completely understood.
If geographic ranges are assigned to the named subspecies of deer mice that occur in Washington, it must, in part, be done arbitrarily. The deer mice of the San Juan Islands are all referred to the subspecies hollisteri. Only Puget Island, Wahkiakum County, is considered to be inhabitated by rubidus. The boundary between the ranges of austerus and oreas is sharply defined and further collecting by resident naturalists should result in detailed mapping of the boundary.
The ranges and distribution of the deer mice of eastern Washington are less clearly defined than those of western Washington. Peromyscus m. gambelii exists in its purest state on the Columbian Plateau. Here the Columbia River makes a convenient boundary to the west. Pure Peromyscus m. oreas exist in the Cascade Mountains. In the area between the Columbian Plateau and the Cascades the deer mice are variously intermediate between gambelii and oreas and, over most of the area, are nearest to gambelii. This might be considered a natural area of intergradation such as commonly occurs between the ranges of subspecies. These intergrades, however, are similar to Peromyscus m. artemisiae and the area occupied by these intermediates is connected on the north with the range of artemisiae. Thus [Osgood] (1909: 61) regards specimens from Easton and Wawawai, in the area of intergradation, as artemisiae or intergrades between gambelii and artemisiae. [Taylor] and [Shaw] (1929: 22) include the entire area of intergradation in the range of artemisiae. This treatment, although convenient to the student of geographic variation, is apt to conceal the evolutionary and historical influences. These influences may be appreciated better if the intergrades from each locality are referred to the subspecies they most closely resemble.