As pointed out earlier in this discussion, the separation between the progenitors of Z. trinotatus and Z. princeps probably occurred when the present Cascade Range and the Sierra Nevada were being formed. From this time until Pleistocene glaciation an incomplete geographic isolation was in effect between the populations of the Pacific coast and the intermountain populations. Perhaps in the region north of the present Cascade Range there was moderate interbreeding between these populations and the transcontinental form. There may have been a similar zone of interbreeding along the crest of the present Cascades where the intermountain and coastal populations conceivably could have met. At least incipient characters probably were present when in Pleistocene time, continental glaciation further isolated the two populations. Since the retreat of the last ice (Wisconsin) the unprogressive coastal Z. trinotatus has expanded its range only slightly, reaching as far as southwestern British Columbia. It seems that ecological difference rather than the barrier formed by the higher elevations is responsible for the limited expansion of range. The population of princeps has extended its range northward to the southern part of the Yukon Territory but does not occur in coastal southern British Columbia because that area already was occupied by Zapus trinotatus. The ranges of the two species meet and overlap in southwestern British Columbia. The species occur sympatrically in Manning Park where, according to Carl et al. (1952:77), they occupy the same range in the region of Allison Pass, Pinewoods, and Timberline Valley. These workers remark that no intergradation was apparent between individuals of the two species obtained in the same trap line.
I have examined material of both species from Allison Pass. There the species differ in color, in the shape of the skull, and in the size and shape of the baculum. Material from Timberline Valley, an area in which Carl et al. (loc. cit.) reported both species, here is assigned to Z. princeps. Where bacula have been preserved the identity of the species is instantly possible.
In summary: First, a population of jumping mice, possibly a monotypic genus, occurred over most of North America; then this population partly divided into Pacific northwest, intermountain (from the east slopes of the present Rocky Mountains to the east slopes of the present Cascade Range and the Sierra Nevada), and transcontinental (eastern and northern) groups with the least progressive groups peripheral; a further reduction or possibly a complete isolation of these populations followed owing to Pleistocene glaciation (especially in the Wisconsin period); and, finally, the present day contacts were established between these populations which by now have differentiated into species. Conceivably, Z. burti (Blancan age) and Z. rinkeri (mid Pleistocene) may represent stages in the development of Z. hudsonius.
ANNOTATED LIST OF SPECIFIC AND SUBSPECIFIC NAMES
(Applied to the genus Zapus since 1899)
Edward A. Preble’s (1899) early revisionary account of the genus Zapus provides an annotated list of the names which had been proposed for American jumping mice to that date. The present account supplies in chronological order the names proposed (including the new kinds described by Preble) in the 54 years since Preble’s revision. Detailed synonymies are given for each kind under the accounts of the subspecies.
1899 campestris (Zapus hudsonius) Preble, N. Amer. Fauna, 15:20, August 8, 1899, applies to the jumping mouse of southeastern Montana, and the Black Hills region of Wyoming and South Dakota.
1899 minor (Zapus princeps) Preble, N. Amer. Fauna, 15:23, August 8, 1899, originally applied to the jumping mouse of the prairies of Saskatchewan, but now includes populations of this species from the plains of Canada (southern Manitoba to Canadian Rockies) and northern United States (Montana, North and South Dakota).
1899 oregonus (Zapus princeps) Preble, N. Amer. Fauna, 15:24, August 8, 1899, originally applied to the jumping mouse of eastern Oregon, but now applies also to populations from southeastern Idaho, eastern and central Nevada, and extreme northeastern California.