[Illustration: FIG. 56.—Arctic Fox (_Vulpes lagopus_) in winter dress.—By permission of the N.Y. Zoölog. Soc.] [Illustration: FIG. 57.—Arctic Fox in summer dress.—By permission of the N.Y. Zoölog. Soc.] The American portion of the great Holarctic region is called by Mr. Lydekker, who uses Wallace’s term, the “Canadian subregion,” and by Dr. Merriam the “Boreal region.” Not that there is any difference of principle involved in this varying nomenclature, for Dr. Merriam says: “It so happens that the Boreal element in America resembles that of Eurasia so closely that in the judgment of many eminent authorities the two constitute a single primary region—a view in which I heartily concur.” The Canadian or Boreal subregion of the Holarctic is the great belt of coniferous forest, which extends obliquely across North America from Alaska to New England; its frontier with the Arctic zone is the northern limit of trees and it is divided from the Transition zone approximately by the line of latitude 45° N., though with a sinuous course, and it is carried far to the south by the wooded heights of the Appalachian, Rocky and Sierra Nevada Mountains, and along the Pacific coast, the mixed character of which has already been explained; it extends almost to San Francisco. The subregion is further divisible into northern and southern belts, called the Hudsonian and Canadian faunas, the limit between them approximately following the isothermal line of 57° F. The mammals of this subregion are largely of Old World origin, many of them coming in with the great immigrations of the Pliocene and Pleistocene epochs; but there are also native American elements and even one genus of South American origin, the Short-tailed or Canada Porcupine (_Erethizon_). [Illustration: FIG. 58.—Canada Porcupine (_Erethizon dorsatus_).—By permission of the N.Y. Zoölog. Soc.] [Illustration: FIG. 59.—Woodchuck or Marmot (_Marmota monax_).—By permission of the N.Y. Zoölog. Soc.] [Illustration: FIG. 60.—Mink (_Lutreola vison_).—By permission of the N.Y. Zoölog. Soc.]