Two other groups are recognized by Davis, who assumes that both arose from a common ancestral stock and divided into two stems, one of which (_gapperi_ group) followed the main Rocky Mountain course southward and worked eastward across Canada while the other (_californicus_ group) followed down the Pacific Coast. Davis states, "This southward extension of range in America probably took place in the Pleistocene, but almost certainly the present southern range of the genus in the West was not reached until late in, or after, that period." Our studies of the _Clethrionomys_ of Washington indicate the essential correctness of Davis' conclusions. The separation of the two groups, however, probably was caused by southward moving glaciers and the separation of the mice into two stocks closely paralleled the divergence of other groups, such as that of the snowshoe rabbits. The two groups of red-backed mice are more sharply separated than was thought by Davis. The intermediate color of the back of _C. g. idahoensis_, mentioned by Davis, seems to be merely coincidental. The essential difference in the two groups is the sharply marked red band of the _gapperi_ group as opposed to the general red area on the dorsal surface of the _californicus_ group. _Clethrionomys gapperi nivarius_ has been considered a derivative of _occidentalis_ because the range of the latter surrounds the Olympic Mountains, where _nivarius_ is found. Considering the immediate post-Pleistocene movements of mammals from the Cascades to the Olympic Mountains, so clearly illustrated in _Thomomys_, _Tamias amoenus_, and _Phenacomys_, a means is indicated by which _Clethrionomys_ of the _gapperi_ group might have reached the Olympics. The apparent lack of intergradation between _occidentalis_ and _nivarius_ gives proof of their relatively distant relationship. _C. nivarius_ seems not to have been derived from _occidentalis_, and apparently does not belong to the _californicus_ group. It belongs instead to the _gapperi_ group, and I consider it to be a subspecies of _gapperi_. In no sense is it intermediate between the two groups. The other form considered by Davis to be intermediate between the two groups is _caurinus_. This mouse has not yet been taken in Washington, although it may eventually be found on Point Roberts, on the Fraser River delta. Its distribution is paralleled by that of many other mammals that are definitely not of the Pacific Coastal Fauna. The _californicus_ group, I feel, contains only the races of _Clethrionomys californicus_, while the _gapperi_ group contains _C. gapperi_ and its races, including _caurinus_, and possible other species.