Beginning with eastern Asia, we find that the Americans, or in some instances their civilization only, are supposed to have come originally from China, Japan, India, Tartary, Polynesia. Three principal routes are proposed by which they may have come, namely: Bering Strait, the Aleutian Islands, and Polynesia. The route taken by no means depends upon the original habitat of the emigrants; thus the people of India may have emigrated to the north of Asia, and crossed Bering Strait, or the Chinese may have passed from one to the other of the Aleutian Islands until they reached the western continent. Bering Strait is, however, the most widely advocated, and perhaps most probable, line of communication. The narrow strait would scarcely hinder any migration either east or west, especially as it is frequently frozen over in winter. At all events it is certain that from time immemorial constant intercourse has been kept up between the natives on either side of the strait; indeed, there can be no doubt that they are one and the same people. Several writers, however, favor the Aleutian route.[I-64] [Sidenote: DIFFUSION OF ANIMALS.] But there is a problem which the possibility of neither of these routes will help to solve: How did the animals reach America? It is not to be supposed that ferocious beasts and venomous reptiles were brought over by the immigrants, nor is it more probable that they swam across the ocean. Of course such a question is raised only by those who believe that all living creatures are direct descendants of the animals saved from the flood in Noah's ark; but such is the belief of the great majority of our authors. The easiest way to account for this diffusion of animals is to believe that the continents were at one time united, though this is also asserted, with great show of probability, by authors who do not think it necessary to find a solid roadway in order to account for the presence of animals in America, or even to believe that the fauna of the New World need ever in any way have come from the Old World. Again, some writers are inclined to wonder how the tropical animals found in America could have reached the continent via the polar regions, and find it necessary to connect America and Africa to account for this.[I-65]