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Show iSoo] ANIMAL LIFE 55 more potent than moisture, and naturalists have been able to demonstrate a remarkably close correlation of life zones to isothermal lines. 1 It is important to recall the close geographic proximity of Alaska to the northeastern part of the great continents of Europe and Asia, called by geographers Eurasia. Indeed, the two continents were probably united in the Tertiary period, as appears from the general continuity of the circum-polar frozen region, and the similarity in physical features and climate of the northern halves of both land masses, ^ o facilitate the comparative study of animal life, naturalists have divided the earth's surface into so- called zoogeographic areas, ^. pf the various schemes proposed the most logical and most convenient for our purposes recognizes the close similarity of the Eurasian and North American faunas. 2 xIt includes in an arctic realm the entire land area north of the annual isotherm of 320 F.; the area between the isotherms of 320 F. and 700 F. in the north temperate realm. \. The portions of these realms falling within North America are designated respectively as the North American arctic region and the North American temperate region. The latter comprises almost the entire 1 Merriam, " The Geographic Distribution of Life in North America" ( Biological Society, Proceedings, VII.); " Laws of Temperature Control of the Geographic Distribution of Terrestrial Animals and Plants" ( National Geog. Soc., Magazine, VI.) * Allen, Geographical Distribution of North American Animals, 206. |