read this passage i found in this website:http: //www.publicbookshelf.com/public_html/The_Great_Republic_By_the_Master_Historians_Vol_I/nativeame_c.html

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 immigrants: thus, the people of India may have immigrated 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 hardly hinder any migration either east or west, especially as it is frequently frozen over in winter.