The differences separating the race _oregonensis_ from other subspecies found in Washington are relatively great. This lowland race is smaller and richer in color. The other races exhibit slight but relatively constant differences. The relatively great difference between _oregonensis_ and the other races indicates that _oregonensis_ was isolated from the remainder of the species for a considerable time. Presumably _oregonensis_ was a strongly differentiated coastal race in pre-Vashon Time and occupied most of western Washington and Oregon. The descent of the Vashon ice restricted the range of _oregonensis_ to southwestern Washington and western Oregon. The descent of the ice forced a northern race, _fuliginosus_, southward into the range of _oregonensis_. The northern race, adapted to boreal conditions, was able to compete successfully with the established _oregonensis_ only in mountainous areas. In the Cascade Mountains, _fuliginosus_ extended its range southward to southern Oregon. The descent of the Wisconsin ice in eastern Washington forced the flying squirrels of adjacent British Columbia southward into the Rocky Mountains. These squirrels were probably closely related to _fuliginosus_, or to _bangsi_, which latter race already may have been established farther south in the Rocky Mountains. The Blue Mountains of southeastern Washington were probably inhabited by _bangsi_ in Wisconsin times, or even earlier. The retreat of the Vashon-Wisconsin glaciers allowed the flying squirrels to extend their ranges northward. In western Washington _oregonensis_ moved to southern British Columbia. In the Cascade Mountains the more boreal _fuliginosus_ moved much farther northward and, north of the Okanogan Valley, spread eastward to the arid, interior valleys of British Columbia. Subsequent differentiation in the population of the arid, interior valleys developed the slightly differentiated race _columbiensis_. Farther east, flying squirrels from the northern Rocky Mountains moved northward. Northeastern Washington and adjacent British Columbia were occupied by _latipes_, derived from _bangsi_. THOMOMYS TALPOIDES.--Views as to the probable historical distribution of this plastic group have been presented in an earlier report (Dalquest and Scheffer, 1944: 308-333). This may be briefly summarized as follows.