In eastern Washington the Wisconsin Glacier advanced south to the northern edge of the Columbian Plateau. Northern mice advancing before it probably did not survive on the treeless plateau but existed in forested areas of northern Idaho and driftless areas of northern Washington. No natural barriers separated the northern mice from the pale, short-tailed forms. The nonuniform topography perhaps allowed more mingling of the two types where climatic conditions permitted. Intergradation in some places as well as competition and elimination of one form or another in other places occurred. Following the withdrawal of the Vashon ice and the establishment of soil and forests on the deglaciated land, the long-tailed mice of western Washington (_P. m. oreas_) apparently spread northward, unhindered by competition, until they reached southern Alaska. The deep coniferous forests of western Washington presented conditions acceptable to _oreas_ and it persisted there despite postglacial changes in climate. 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.