MUSTELA FRENATA.--The long-tailed weasels of the Pacific Coast behave as a plastic group and clearly show the effect of the Vashon-Wisconsin Divergence. The range of the coastal race, _altifrontalis_, indicates that it was isolated in southwestern Washington during Vashon Time. In that period, or shortly after, it extended its range southward but only along the extreme, coastal area of Oregon (see Hall, 1936: 101). Following the retreat of the ice it extended its range northward to the deglaciated area of western Washington. Also following the retreat of the ice, a Great Basin subspecies (_nevadensis_) extended its range northward. This race seems to have been more adaptable and successful than other kinds of Great Basin mammals, for it extended its range farther northward, eastward and westward than most. A third race, _washingtoni_, was isolated in the southern Cascade Mountains during Vashon Time and became differentiated from both _altifrontalis_ and _nevadensis_. It is now found in the Cascades from central Oregon north to Mount Rainier. It is difficult to see why it did not extend its range to include the northern Cascades when the glacial ice left, but it did not. Instead _altifrontalis_ entered the northern Cascades from the west and _nevadensis_ did the same from the east. Weasels obtained in habitats north of Mt. Rainier are intergrades between _altifrontalis_ and _nevadensis_. One is reminded here of the _douglasii_ group of _Thomomys talpoides_ in which subspecies did not move north of Mt. Rainier in postglacial time. The area north of Mt. Rainier was populated instead by gophers of the _fuscus_ group, subspecies of which invaded the area from the east. Perhaps Mt. Rainier itself served as a barrier to alpine mammals in the immediate post-Pleistocene. Perhaps _Mustela f. washingtoni_ will eventually extend its range northward, displacing the _altifrontalis-nevadensis_ intergrades from the habitats to which _washingtoni_ may be better adapted. The Blue Mountains of southeastern Washington are occupied by a weasel (_effera_) that has a more extensive range in eastern Oregon. The range of this race has probably not changed materially for a long period of time. One would expect the weasels from extreme northeastern Washington to be referable to the race _oribasa_, of the Rocky Mountain Fauna. Instead they are intermediate between that race and the Great Basin race, _nevadensis_. Apparently _nevadensis_ was so dynamic and adaptable that it actually entered the geographic ranges of surrounding races for some distance. In view of Sumner's theory for the retention of subspecies, one might say that the population pressure of _nevadensis_ on the periphery of its range is stronger than the opposing pressure of some surrounding races.