In this classification, North America falls in part in two realms, the _Arctogaeic_ and _Neogaeic_, the former embracing the table-land of north-central Mexico and all of that portion of the continent lying to the northward, while the lowland of Mexico, together with Central America and the West Indies, falls in the latter realm. The Arctogaeic includes also nearly the whole of the eastern hemisphere. The relationship expressed in this classification of both the living and extinct mammalia of North America to that of Eurasia, is supposed to be due to a former land connection between the Old and the New World at Bering Strait, and is most clearly marked by northern species, the intercontinental bridge being too far north to be available for southern forms. The mammals and many of the other animals of the low, hot borders of Mexico and of Central America are a northward extension of the fauna of South America--that continent constituting nearly the entire Neogaeic realm. The mammals of the West Indies are few in species, and have their nearest relationship with the fauna of the continent to the southward. LIFE-REGIONS AND LIFE-ZONES The detailed study of the zoology of North America is far from complete, but the voluminous results reached have led to several attempts at broad generalization in reference to geographical distribution. Important and highly instructive memoirs have been presented in this connection by J. A. Allen, Angelo Heilprin, E. D. Cope, and others, who have in the main attempted to correlate the distribution of animal species, but principally the mammals, with variations in mean annual temperature. Among the latest of these contributions, and marking the advance made at the close of the nineteenth century, is the classification proposed by C. Hart Merriam, already referred to in the sketches that have been given of the climate and of the flora of the continent. The basis for this classification is the seemingly well-determined law that the northward distribution of terrestrial animals and plants is controlled by the sum of the positive temperatures for the entire season of growth and reproduction, and that the southward distribution is governed by the mean temperature of a brief period during the hotter portion of the year. By "positive temperatures" is meant the sum of the mean daily temperature above that which determines the period of physiological activity in plants and of reproductive activity in animals, assumed to be 6 deg. C. or 43 deg. F. The exact length of the period to be taken as the hottest portion of the year has not been definitely determined, but must be short enough to fall within the hottest part of the summer in high northern latitudes, and probably increases in length from north to south; the time assumed is the six hottest consecutive weeks of the year. On the basis just stated, Merriam has divided North America into the following life-regions and life-zones: