II. ARCTOGÆIC REALM. { 1. _Malagasy Region._—Madagascar. { 2. _Ethiopian Region._—Africa south of the { Sahara Desert. { 3. _Oriental Region._—Southern peninsulas of { Asia, Malay Archipelago. { 4. _Holarctic Region._—N. Africa, Europe, Asia, { (except southern part), boreal N. America. { 5. _Sonoran Region._—Remainder of N. America { (except lowlands of Mexico). North America, as is expressed by this scheme, is zoölogically composite; the northern half, including nearly all of Canada, belongs to the vast Holarctic Region, which also comprises Europe, Africa north of the Sahara and Asia north of the Himalaya Mountains. The remainder of the continent, exclusive of the Mexican coastal lowlands, is set off as the Sonoran Region. Inasmuch as we have here to do with broadly continuous land-areas, not demarcated by great physical features, and as the genera and species of mammals differ greatly in regard to their ability to withstand a wide range of climatic variations, it is not to be expected that the boundaries between the regions which make up North America should be very sharply drawn. It is not surprising, therefore, to find a transition zone, extending all across the continent, in which the Holarctic and Sonoran faunas mingle, or that Central America should, in considerable measure, be transitional to South America, though zoölogically a part of the latter. [Illustration: FIG. 53.—Zoölogical Divisions of North America. (After Merriam.)] Dr. Merriam’s arrangement, which deals only with North America without reference to the Old World, divides the land into a series of transcontinental zones, which he calls the Arctic, Boreal, Upper and Lower Sonoran and Tropical. These zones have very irregular and sinuous boundaries, which follow lines of equal temperature (isothermal lines) during the breeding season, May, June and July, the tortuous boundaries being conditioned by topographical features, which deflect the isothermal lines. [Illustration: FIG. 54.—Polar Bear (_Thalarctus maritimus_).—By permission of the N.Y. Zoölog. Soc.] The Arctic zone is part of a circumpolar area, which is very much the same in North America, Asia and Europe; and in any of these continents the fauna differs much more from that of the contiguous zone to the south than from the Arctic fauna of another continent. There are some local differences, but the characteristic mammals of this Arctic zone are the Polar Bear, Arctic Fox, Musk Ox, Barren-ground Caribou, Lemming, Arctic Hare, and a marmot. Most, if not all, of these forms are of Old World origin. [Illustration: FIG. 55.—Musk Ox (_Ovibos wardi_) female; the males have much larger horns.—By permission of the N.Y. Zoölog. Soc.]