The range of the pocket mouse of southeastern Washington, _Perognathus parvus parvus_, is continuous with the range of the race in Oregon. This same race occurs in the Yakima Valley, whence it probably arrived from Oregon in relatively recent time. The distribution of pocket mice on the Columbian Plateau, in eastern Oregon and in the Yakima Valley resembles that of the least chipmunk in those areas. It is also similar to, but of more recent origin than, that of the ground squirrels, _Citellus washingtoni_, and _townsendii_. DIPODOMYS ORDII.--This kangaroo rat enters the desert area of southeastern Washington from Oregon. It may be expected eventually to cross the Columbia River to the Yakima Valley and the Snake River to the Columbian Plateau. CASTOR CANADENSIS.--Two races of beavers occur in Washington. One, found in southwestern Washington and northwestern Oregon, is dark with a short, wide skull. The other, ranging over most of the state, is paler with a longer, narrower skull. The form now found in southwestern Washington and adjacent Oregon (_idoneus_) was probably isolated there by the Vashon glaciation and developed its characters while isolated. The other race, _leucodonta_, was probably widely spread in Wisconsin Time. Beavers are present in Moses Lake, in almost the center of the Columbian Plateau. Beavers might well have lived in the streams of melt water that emerged from the Wisconsin Glacier. The beavers of western Washington, save those in the extreme southwest, are like the beavers of eastern Washington. It seems likely that the race _leucodonta_ originated north of the state of Washington and was forced southward by the Vashon-Wisconsin glaciers. This northern race, adapted to boreal conditions, competed with the resident coastal race, _idoneus_, and occupied much of its range. The distribution of the races of muskrat in Washington closely resembles that of the beavers. ONYCHOMYS LEUCOGASTER.--The desert-dwelling grasshopper mouse has doubtless entered eastern Washington and the Yakima Valley from eastern Oregon at a relatively recent time. REITHRODONTOMYS MEGALOTIS.--The harvest mouse, like the grasshopper mouse, seems to have entered Washington from Oregon at a relatively recent date. Within the last ten years it has extended its range into the Okanogan Valley in British Columbia.