ONDATRA ZIBETHICUS.--The distribution of the two races of muskrat that occur in Washington is almost exactly that of the beavers. Probably one form was isolated in southwestern Washington and northwestern Oregon during Vashon Time and another, more adaptable, race occurred in eastern Washington. After the retreat of the ice the adaptable race spread widely but the muskrats of the lower Columbia River changed their range little or not at all. APLODONTIA RUFA.--The race of _Aplodontia_ found in the Cascades of Washington was probably confined to the southern Cascades in Vashon Time and has since spread northward to reoccupy the range as far north as southern British Columbia. The presence of a mountain beaver in western Washington that is indistinguishable from the race _rufa_, found in the Cascades of Oregon, is most surprising. In Oregon, _pacifica_ occurs in the western lowlands and _rufa_ in the mountains to the east. In Washington _rufa_ occurs in the western lowlands and _rainieri_ in the mountains to the east. The offset in range of _rufa_ seems best explained by assuming that all of western Washington was once occupied by mountain beavers (_rufa_) that spread from the Cascades of Oregon to the Cascades of Washington and thence to the western Washington lowlands. This must have occurred early in the interglacial cycle preceding the Vashon Glaciation. While confined to the southern Cascades, _rainieri_ developed the larger size that now separates it from the ancestral _rufa_ that occurs to the west and south of it. ZAPUS PRINCEPS.--The coastal race of the jumping mouse was probably distinct from the more inland population before Vashon-Wisconsin Times. The differences between them were probably accentuated while the Vashon-Wisconsin glaciers separated their ranges. The western race (_trinotatus_) was isolated in the southern Cascades and southwestern Washington. After the retreat of the glaciers, _trinotatus_ moved northward through western Washington and the Cascades into British Columbia. The race _kootenayensis_ of the adjacent Rocky Mountains moved eastward through northeastern Washington to the Cascades. The race _oregonus_, found in the Blue Mountains, has probably been resident there since it evolved from the populations to the east. ERETHIZON DORSATUM.--I have inadequate basis for speculation concerning the historical distribution of the porcupine.