According to Simpson (1947), the occurrence of the same group of mammals on two different land masses is to be taken as prima facie evidence that migration has occurred. Keeping in mind then the present geographic distribution, unspecialized condition of the dentition of _Eozapus_, and its resemblance to the extinct _Pliozapus_ known from North America but not from Asia, it may be that _Eozapus_ descended from primitive stock of a North American jumping mouse that was forced to the periphery (across the Asiatic North American land bridge) by the more specialized zapidine stock. Subsequently or perhaps during the migration of the pre-_Eozapus_ stock the zapidine stock may have dispersed transcontinentally, occupying most of northern North America. The unprogressive _Macrognathomys_ and _Pliozapus_ line which remained in North America may have become extinct. Any such period of dispersal and climatic equilibrium ended when glaciers came to cover most of the northern part of the continent and the mammals living there were forced southward by the ice or remained in ice-free refugia within the glaciated area. Later, with melting and retreat of the ice, the jumping mice could have again spread enough to occupy the northern part of the continent. Such glaciation isolated segments of the population and aided their evolution into distinct species. If it be assumed, as Matthew (1915) did and as Hooper (1952:200) later on the generic level did, that the region of origin and center of dispersal for a given group of animals is characterized by the presence of the most progressive forms, then southeastern Canada and the northeastern United States make up the area of origin and center of dispersal in relatively late time of the subfamily Zapodinae. This area is inhabited by _Zapus hudsonius_ and _Napaeozapus_, the most progressive members of the subfamily. As I visualize it, the evolution of the Zapodinae occurred in two stages: the first stage involved the movement of the primitive pre-_Eozapus_ stock to Asia and the second stage involved the dispersal, isolation, and specialization in North America of the more progressive basic zapidine stock into the present genera _Zapus_ and _Napaeozapus_. Status of the genera _Eozapus_, _Zapus_, and _Napaeozapus_ The genus _Zapus_ is one of three living genera in the subfamily Zapodinae. These genera _Zapus_ and _Napaeozapus_ from North America and _Eozapus_ from China have been variously considered as subgenera of the genus _Zapus_ (Preble, 1899) or as three separate genera (Ellerman, 1940). [Illustration: FIGS. 4-15. Three views of the skull and a lateral view of the left lower jaw of each of the Recent genera of the subfamily Zapodinae. × 1.5. FIGS. 4-7. _Eozapus s. vicinus_, adult, male, No. 240762 USNM, Lanchow, Kansu, China. FIGS. 8-11. _Zapus h. pallidus_, adult, male, No. 240762 KU, 5-1/2 mi. N, 1-3/4 mi. E Lawrence, Douglas County, Kansas.