incipient differences probably were present and possibly the animals approached in character those found in these areas today in that the ecology of the region was much the same as now. In the region between the Rocky Mountains and the present Cascade Range and the Sierra Nevada, the flora (in late Pliocene) became semidesert, which presumably made most of this region uninhabitable for jumping mice. The aridity probably induced local concentration into boreal montane islands, thus possibly displacing the populations of the two species that were in contact. In Pleistocene times continental glaciation must have interrupted the contacts between the coastal, intermontane (the area between the present Rocky Mountains and the present Cascade Range and the Sierra Nevada), and northern and eastern groups of _Zapus_ or mammals of any genus that occurred over all of this vast region. The advance of the ice southward would have increased opportunity for evolution by interposing barriers that isolated some populations. The populations possibly were re-established in interglacial periods and then were isolated again by another descent of glacial ice. If a population occupied the unglaciated coastal region of Oregon and Washington it may have been separated from other populations to the north and east by an ice cap which covered most of the Cascade Range. The population occupying the intermountain region probably was isolated from the population to the north and west. The formation of glaciers presumably reduced the size of areas available to the populations occupying eastern North America, Alaska, and Canada with the result that they persisted only in areas south of the ice or in ice-free refugia (central and western Alaska) within the glaciated area. According to Axelrod (1948), the flora in the eastern United States during the Pleistocene furnished most of the stock for the revegetation of southern and subarctic Canada east of the Rocky Mountains. Eastern populations of _Z. hudsonius_ (or its progenitors) probably followed the spread of this vegetation and, thus, extended their range into Canada where they crossbred with populations advancing south and east from the refugia in Alaska. Western montane floras, which extended north along the Rocky Mountains and the Cascade and Coast ranges, probably paved the path for a northward migration of populations of the intermountain _Z. princeps_ (or its progenitors). Populations of _Z. princeps_ moved eastward from the present Rocky Mountains, inhabiting the high plains of southern Canada and the north-central United States. In general, _Zapus hudsonius_ occupies the region to the north and to the east of that inhabited by _Zapus princeps_; however, the ranges of the two meet and overlap in central and northern British Columbia and in the high plains area of southern Alberta, Saskatchewan, eastern Manitoba, eastern Montana, North Dakota, and northern South Dakota. In these places of