Hence, although in a purely descriptive ethnography it would be advisable to take the tribes of California in immediate succession to those of Oregon, and those of Mexico next in order to the Californian, the present arrangement will be different, and the transition will be from the Oregon Indians to the Indians on the east of the Rocky Mountains. This departure from the strict line of ethnological continuity, is demanded in the present volume; because the question as to the origin of the American population, being considered of so much more importance than the mere description of different tribes, the arrangement follows the order, in which the reader requires facts as a basis for his reasoning, rather than the absolute sequence of ethnological relationship. This accounts for certain departures, which may possibly have been noticed, from the form and method of description adopted in the ethnology of Asia; it also is a reason for sometimes placing together groups on the score of _difference_ rather than _likeness_. Such is the case here. The classes about to be noticed follow those that have already been considered, not because they are closely related, but because they present marks of disconnection which are necessary to be known and appreciated previous to any argument upon subjects like the unity or _non_-unity of the American population, or its connexion or _non_-connexion with the population of the Old World. In other words, as the nearest affinities of the Oregon tribes are with the Californian, the present order of sequence is artificial rather than natural. As to the line itself which thus diverts our inquiries from the true ethnological sequence, it is the area of a family already[120] mentioned--the area of the _Paduca_ tribes. Of this the peculiarity is as follows. It begins with the country of the Wihinast, is separated from the Pacific by the comparatively small areas of the Wailatpu, Molele, Kalapuya, and Yakon, and extends in a south-east direction as far as the Gulf of Mexico. Hence, with the exception of a narrow tract on the Lower Columbia, it runs from sea to sea; so separating all the numerous sections of the Indians of the United States and Canada from those of Spanish America, _i.e._ from those of Mexico wholly, and from those of California partially. This gives us a limit for the parts about to be noticed, which, roughly speaking, constitute-- _Politically._--the United States and Canada-- _Physically._--the river-systems of the St. Lawrence, the Red River, and the Mississippi, and also of those rivers which, like the Potomac, fall into the Atlantic-- _Ethnologically._--the country included between the Eskimo, Athabaskan, Kútani, Salish, Sahaptin, and Paduca areas.