On Vancouver Island, British Columbia, a short-tailed, _austerus_-type of mouse occurs in the lowlands and a long-tailed, _oreas_-type of mouse in the mountains. Vancouver Island is a large island (16,400 square miles). Apparently a stock of _austerus_ from the mainland reached Vancouver Island and were able to find suitable habitat and compete with and drive out the established _oreas_ in the manner that they probably did in the western Washington lowlands. The large Vancouver Island offers far more variety of habitat and opportunity for establishment of emigrating mice than do the smaller San Juan Islands, the largest of which is Orcas (57 square miles). Following the withdrawal of the Wisconsin ice from eastern Washington, numerous minor movements and adjustments of deer mice seem to have taken place. Long-tailed, _oreas_-type of mice were exterminated on the Columbian Plateau if, indeed, they ever became established there. Long-tailed mice did apparently become established in driftless areas and forested areas to the south of the drift border. With the establishment of soil and forests on the deglaciated land, the short-tailed _gambelii_ and the long-tailed mice became thoroughly intermixed. In some areas, especially near the type locality of _artemisiae_, the _gambelii_ characters of the mixture predominate at the present time. Deer mice from the Okanogan Valley differ from _gambelii_ only in slightly larger size and darker color. In other areas, notably near Metaline, Pend Oreille County, the _oreas_ characters of the mixture dominate at the present time. Mice from here are large and red and differ from typical _oreas_ in having relatively short tails. Other less marked segregations of characters are numerous, in Idaho and British Columbia as well as in Washington. Cowan (1937) has described _Peromyscus m. alpinus_, isolated in a mountainous area of British Columbia. This seems to be a form derived from _oreas_-type stock. With the reëstablishment of soil and forests on the deglaciated land of British Columbia north of Washington, a northward extension of the _gambelii-oreas_ mixture occurred. Another invasion was taking place at this time, however. _Peromyscus m. arcticus_ spread onto the deglaciated land from the north or east, ahead of artemisiae (the _gambelii-oreas_ mixture). This new invader intergraded with _artemisiae_ as well as with _oreas_. Further collecting and studies are required in this area before the relationships of the three subspecies can be completely understood.