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.