FIG. 32. _Eozapus s. vicinus_, adult, male, No. 240762 USNM, Lanchow, Kansu, China. FIG. 33. _Zapus p. princeps_, adult, male, No. 32858 KU, Medicine Wheel Ranch, 28 mi. E Lovell, Big Horn County, Wyoming. FIG. 34. _Napaeozapus i. insignis_, adult, male, No. 9544 KU, 3 mi. W Base Station, Coos County, New Hampshire.] Distribution of and Speciation in the Genus _Zapus_ Many of the described kinds of the genus _Zapus_ were initially named as distinct species (see Preble, 1899). Subsequently (see Hall, 1931), some of the nominal species were reduced to the rank of subspecies. Only three species in the genus _Zapus_ are recognized in the following account. The concept of species adopted here is, in Mayr's (1942:120) words, this: "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." The three species are _Z. trinotatus_, _Z. princeps_, and _Z. hudsonius_. No hybridization is known where two occur together or where their ranges are adjacent. Each of these species has several geographically contiguous subspecies. The three species of _Zapus_ are closely related but are not equally progressive. If eastern North America is considered to be the region of origin and center of dispersal of _Zapus_ (see pp. 368-369) the geographically distant species would be expected to be the least progressive, and such seems to be the case. _Zapus trinotatus_ is geographically farthest removed and structurally least progressive. _Zapus hudsonius_ occurs at the center of dispersal and is the most progressive structurally whereas _Z. princeps_ is geographically and structurally intermediate. Structural progressiveness is postulated for the species that has the simplest (in this instance specialized) baculum and smallest fourth upper premolar. The phyletic branches of the genus _Zapus_ possibly developed from geographic segments of a population radiating from the centrally located progressive group. On continental areas where a species with a wide and continuous range gives rise to several daughter species, geographic isolation is thought to be important in bringing about the formation of species. The unspecialized populations conceivably occupied an area west of the present Rocky Mountains and south of latitude 50°. From later Miocene times on, climatic and geological differentiation occurred in this area, and with the growth of geological barriers and differentiation of habitat these unspecialized populations may have been separated into two ecological groups, one inhabiting the more arid area between the present Rocky Mountains and the present Cascade Range and Sierra Nevada and the other group inhabiting the Pacific coastal region. Isolation of each of these groups probably was not complete. How far differentiation might have proceeded with incomplete isolation can only be guessed, but at least