In North and South America, great simplicity of continental build gave rise to a corresponding simplicity of native ethnic and cultural condition.  There is only one marked contrast throughout the length of this double continent, that between its Atlantic and Pacific slopes.  On the Atlantic side of the Cordilleras, a vast trough extends through both land-masses from the Arctic Ocean to Patagonia; this has given to migration in each a longitudinal direction and therefore constantly tended to nullify the diversities arising from contrasted zonal conditions.  On the Pacific side of North America, there has been an unmistakeable migration southward along the accessible coast from Alaska to the Columbia River, and down the great intermontane valleys of the western highlands from, the Great Basin to Honduras;[775] while South America shows the same meridional movement for 2,000 miles along the Pacific coast and longitudinal valleys of the Andes system.  There was little encouragement to cut across the grain of the continents.  The eastern range of the Cordilleras drew in general a dividing line between the eastern and western tribes.[776] Though Athapascans from the east overstepped it at a few points in North America, the Great Divide has served effectually to isolate the two groups from one another and to draw that line of linguistic cleavage which Major Powell has set down in Ms map of Indian linguistic stocks.  Consequently, Americanists recognize a distinct resemblance among the members of the North Atlantic group of Indians, as among those of the South Atlantic group; but they note an equally distinct contrast between each of them and its corresponding Pacific group.  Nor is this contrast superficial; it extends to physical traits, temperament and culture,[777] and appears in the use of the vigesimal system of enumeration in primitive Mexico, Central America, among the Tlingits of the Northwest coast and the Eskimo as also among the Chukches and Ainus of Asia, while in the Atlantic section of North America the decimal system, with one doubtful exception, was alone in use.[778]

[Sidenote:  Cultural superiority of the Pacific slope Indians.]

To the anthropo-geographer, the significant fact is that all the higher phases of native civilization are confined to the Pacific slope group of Indians, which includes the Mexican and Isthmian tribes.  From the elongated center of advanced culture stretching from the Bolivian highlands northward to the Anahuac Plateau, the same type shades off by easy transitions through northern Mexico and the Pueblo country, vanishes among the lower intrusive stocks of Oregon and California, only to reappear among the Haidas and Tlingits of British Columbia and Alaska, whose cultural achievements show affinity to those of the Mayas in Yucatan.[779] Dall found certain distinguishing customs or characteristics spread north and south along the western slope of the continent in a natural geographical line of migration.