North America is customarily divided into nine areas of material culture, and though this is convenient, a more correct method, as C. Wissler points out[805], is to locate the respective groups of typical tribes as culture centres, classifying the other tribes as intermediate or transitional. The geographical stability of the material culture centres is confirmed by archaeological evidence which suggests that the striking individuality they now possess resulted from a more or less gradual expansion along original lines. The material cultures of these centres possess great vitality and are often able completely to dominate intrusive cultural unity. Thus tribes have passed from an intermediate state to a typical, as when the Cheyenne were forced into the Plains centre, and the Shoshonian Hopi adopted the typical Pueblo culture. Wissler comes to the conclusion that "the location of these centres is largely a matter of ethnic accident, but once located and the adjustments made, the stability of the environment doubtless tends to hold each particular type of material culture to its initial locality, even in the face of many changes in blood and language." It is from his valuable paper that the material culture traits of the following areas have been obtained. I. Eskimo Area. The fact that the Eskimo live by the sea and chiefly upon sea food does not differentiate them from the tribes of the North Pacific coast, but they are distinguished from the latter by the habit of camping in winter upon sea ice and living upon seal, and in the summer upon land animals. The kayak and "woman's boat," the lamp, harpoon, float, woman's knife, bowdrill, snow goggles, trussed-bow, and dog traction are almost universal. The type of winter shelter varies considerably, but the skin tent is general in summer and the snow house, as a more or less permanent winter house, prevails east of Point Barrow.