Perhaps Pliozapus and Eozapus represent one phyletic line and Zapus and Napaeozapus a second line, both of which lines evolved from a pre-zapidine stock in the Miocene. As mentioned earlier, Wilson (1936) thinks that Pliozapus is not directly ancestral to Eozapus. Possibly these two genera diverged at an early date; nevertheless, they are closely related primitive forms.
Zapus and Napaeozapus closely resemble each other and both are structurally advanced; Napaeozapus seems to have differentiated at a more rapid rate.
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).
