This brief sketch of the anthropology of the Plains naturally raises a few quite fundamental questions: How did these tribes come to be here? How long have they been here? What was the origin of their cultures? While final answers cannot be given for these, some progress toward their solution has been made. Taking the cultural classification as our point of view, we see that Plains Indians are not peculiar in stature or head form, yet seem to fall into a group distinct from other parts of the continent. These differences are, however, slight and give us no insight into the origins of the tribal groups. For example, the shorter western tribes ranging from 165 to 170 cm. fall into a large group of low statures including most of the Californian, Plateau, North Pacific Coast, and South eastern areas. The Comanche, who speak a language of Shoshonean stock widely distributed over the Plateau area, are also relatively short. The greater part of the typical and Village tribes, however, range from 170 to 175 cm., including the Yuma, Mohave, and Pima of the Southwest, the Iroquois and most Algonkin of the Woodland area. As to head form, the moderately long head of the Plains does not hold for the Osage and Wichita of the south and the Nez Perce of the north west, but extends over the Plateau area on the west and into the Woodland area of the east. Hence, in a general way, the tall, somewhat long-headed, typical tribes seem to have relatives to the east in the Woodlands through Indiana, Ohio, and New York. Possibly this represents the influence of some older parent group whose blood gradually worked its way across the continent through many languages and several varieties of culture. On the other hand, the shorter, less long-headed tribes were massed around the Plains in the Southwest, the Plateaus, and part of the Woodlands almost engulfing the taller group. Now, while it seems clear that migrations of blood are in evidence, there is, as yet, no satisfactory means of determining the point of origin and the direction of movement for these types. Turning from physical type to language, we have several large masses impinging upon the Plains and while it seems most likely that the parent speech for each stock arose somewhere outside the Plains, we are not yet clear as to the impossibility of their arising in the Plains and spreading to other cultures. It does not seem probable that all of them would arise within this small area, but, on the other hand, it is impossible to give satisfactory proof for any particular tribe. Thus, language gives us but a presumption in favor of migrations into the Plains of the Siouan, Caddoan, and Shoshonean speaking tribes. It is true that many tribes have migration legends some of which are consistent with a few details of culture; but as these nearly always take the forms of other myths, they cannot be given much historical weight. The plain fact is that the moment we get beyond the period of exploration in the Plains, historical data fail us. We know where the tribes were when discovered and most of their movements since that date, but beyond that we must proceed by inference and the interpretation of anthropological data.
Not being able to discover how the various tribes came to be in the Plains, we can scarcely expect to tell how long they have been there. The archaeological method may be brought into play here; but as yet we lack sufficient data. Mounds and earthworks have been discovered in the Dakotas and southward along the Missouri, apparently the fringe of the great mound area in the Woodlands to the east, but in the open plains, we have so far only evidence of states of culture similar to those we have just described, from which we infer that no other culture preceded this one. Yet for all we know, its origin may date back several thousand years. Certain it is that in 1540 all the typical Plains traits of culture were in function, and since the wheels of primitive progress move slowly we can safely assume a remote origin.
Anyway when we consider the culture of the Plains since 1540, it appears that so many of the traits enumerated in these pages are almost entirely peculiar to the area that we are constrained to conclude that they developed within it. This is strengthened by the peculiar adaptation of many of these traits to the geographical conditions, suggesting that they were in vented or discovered by a Plains people. It seems, therefore, that while the origin of the blood and languages of the Plains cannot be determined, its cultural problem is in a fair way to be solved. Among the most distinctive traits are the sun dance, a camp circle band system, the soldier societies, highly developed ritualistic bundles, a peculiar geometric decorative art, the use of the horse and travois, the skin-covered tipi, the earth-lodge, and economic dependence upon the buffalo. Some of these are absolutely confined to the area and though others are found elsewhere they occur as secondary rather than as primary traits. We may safely conclude, therefore, that the tribes of the Plains at least developed these traits to their present form, if they did not actually invent them.
Perhaps the most interesting phase of Plains anthropology is the general diffusion of traits among the many political and linguistic units found therein. Miss Semple favors the theory that a Plains region is the most favorable environment for the diffusion of cultural traits. Whatever may be the fate of this hypothesis, it is clear that among the Indians of the Plains there has been sufficient diffusion to carry many traits over the greater part of the area. That diffusion rather than independent development or convergent evolution is the most satisfactory explanation of this case, may be seen from noting that the various tribes were acquainted with many of their neighbors, that in the sign language they had a ready means of inter communication, and that since their discovery the actual diffusion of several traits has been observed by anthropologists.