Between the Boreal subregion and the Sonoran region is the Transition zone, which follows all the complex windings of the boundary lines. It covers most of New England, New York, Pennsylvania and southern Ontario; passing through southern Michigan and Wisconsin, it bends northward over Minnesota and covers most of North Dakota, Manitoba and the plains of the Saskatchewan, then turns abruptly southward and includes eastern Montana and parts of South Dakota and Nebraska. Crossing Wyoming, it follows around the northern edge of the Great Basin to the plains of the Columbia. The three great mountain-systems carry the zone far to the south and arms of it extend along the Appalachians to northern Georgia, along the Rockies to New Mexico, and it follows the Sierras to southern California. “The Transition zone, as its name indicates, is a zone of overlapping Boreal and Sonoran types. Many Boreal genera and species here reach the extreme southern limits of their distribution and many Sonoran genera and species their northern limits. But a single mammalian genus (Synaptomys) [one of the field mice] is restricted to the Transition zone.... A number of species, however, seem to be nearly or quite confined to this zone” (Merriam).
Fig. 61.—Upper figure, European Bison (Bison bonasus). Lower figure, American Bison (B. bison).—By permission of the N.Y. Zoölog. Soc.
Fig. 62.—Wolverene (Gulo luscus).—By permission of the N.Y. Zoölog. Soc.
Fig. 63.—Wapiti or “Elk” (Cervus canadensis).—By permission of the N.Y. Zoölog. Soc.