We interpret the mixture of faunas in the Cascades as follows: The Vashon-Wisconsin ice sheet was in place for a long period of time, longer, probably, than the Recent. During this time, forest mammals of the Pacific Coast were isolated from forest mammals farther east by glaciers to the north and desert to the east. Changes took place in both of the separated forest faunas. Certain species, perhaps, such as the mammoth, became extinct. Other forms were exterminated then or at an earlier time in one fauna or the other. If _Aplodontia_, _Neurotrichus_ or _Scapanus_ occurred in the Rocky Mountain faunal area, it lived in an inland area of rigorous climate, and disappeared there because it was unable to adapt itself to the cold. In the mild climate caused by proximity of the ocean, mild even in Vashon-Wisconsin time to judge from evidence yielded by study of fossil pollens, primitive forms such as moles, the Bendire shrew, and mountain beaver persisted along the coast, where there were no boreal conditions. Some alpine forms, such as _Marmota olympus_, _Ochotona princeps brunnescens_ and _Citellus saturatus_ persisted in the Olympic or Cascade mountains as relic species. On the whole, however, the glacial divergence resulted in a boreal forest fauna and a temperate forest fauna. In addition to change in component species, there were evolutionary changes in the species themselves. In some these were considerable, as shown by the differences between related forms of the two faunas. In most species, however, evolutionary changes have resulted in only subspecific differences.