There are, then, two subspecies that do not interbreed, each capable of existing in any ecologic niche that will support deer mice. Where the ranges of the two subspecies come together, they compete. The large size, richer color, longer tail, and perhaps arboreal habits of _oreas_ give it an advantage in coniferous forests. The small size and dark color of _austerus_ give it an advantage in other habitats, especially, perhaps, in winter, when the deciduous trees have shed their leaves. The differences separating _austerus_ from _oreas_ would be expected to have developed under complete isolation. That _oreas_ developed from _austerus_ or _austerus_ from _oreas_ seems impossible. A glance at the distribution map (Fig. 107) shows that the range of _austerus_ clearly separates the range of _oreas_ into two segments. The range of _austerus_ tapers out to the north, east and west. On the south _austerus_ intergrades with _rubidus_ from Oregon, a subspecies from which it is but slightly differentiated. The geographic behavior of the four subspecies of deer mice that occupy extensive areas in Washington may be summarized as follows: _P. m. gambelii_ occupies southeastern Washington and intergrades normally with _oreas_ in the eastern Cascade Mountains and with _artemisiae_ at the northern edge of the Columbian Plateau; _artemisiae_, occupying northeastern Washington, closely resembles populations of mice that are known to be intergrades between _oreas_ and _gambelii_ and itself intergrades with both _oreas_ and _gambelii_; _oreas_ occupies most of western Washington, intergrades with its neighbors _artemisiae_ and _gambelii_ to the east, but does not intergrade with _austerus_, its morphologically more similar neighbor in the west; _austerus_ occupies a range in western Washington that is almost surrounded by the range of _oreas_, a subspecies with which it apparently does not interbreed. The relations of these four subspecies appear to be the result of certain great changes in the range of deer mice in the Pacific Northwest that occurred during and since the last Pleistocene glaciation. We reconstruct these changes as follows. In the Puyallup interglacial cycle, just previous to the last (Vashon-Wisconsin) continental glaciation, clines, or gradual transitions existed in the ranges of the deer mice along the Pacific Coast. The small, dark, short-tailed mice of the coastal redwood forests of California and Oregon became gradually larger, redder and longer-tailed to the north, climaxing in a large, red, long-tailed form in the spruce forests of southern Alaska and northern British Columbia. A similar cline existed inland. Small, pale, short-tailed mice of the Great Basin became increasingly larger, darker, and longer-tailed to the north, reaching a maximum in the spruce forests of northern British Columbia.