The conclusion is that the culture of the Wind River Shoshoni exhibits a strange conflicting situation. It belongs neither to the foodgatherers of the west nor to the hunting cultures of the east--it is something sui generis. To ascribe it to anyone of its bordering cultures is to lose the dynamic aspect of the cultural evolution of the tribe. He thus sees the eastern Shoshone as synthesizers and transformers of cultural material derived from both eastern and western sources; their culture is a blend of the two, but it is not a simple compound of them. The present work does not attempt to answer the question of the relation of the content of the culture of the equestrian Shoshone to that of peoples to the east and west but will focus attention upon social structure. And it will do this, not through a consideration of outside influences upon the Shoshone, but by analysis of their social institutions and the relationship of them to economic life. Our note on the length of involvement of the Shoshone in Plains Indian culture leads us to inquire briefly into the time depth of this culture and its relation to the expansion of the American frontier. Although it is quite probable that certain cultural traits and social institutions characteristic of later Plains life had antecedents in the pre-equestrian period, the possession of herds of horses was the basis of later patterns of amalgamation and incipient stratification. It also had much to do with the intensification of warfare. The horse, according to Haines (1938), spread from the Spanish Southwest to more northerly areas along both sides of the Rocky Mountains. Its diffusion along the western slope of the range was presumably the more rapid, and Haines gives evidence indicating that the Shoshone had horses about 1690 to 1700, at which time the animal was found no farther north than the Arkansas-Oklahoma border in the Plains (ibid., p. 435). From this Shoshone center, the use of the horse spread north to the Columbia River, the Plateau, and the northern plains. It followed an independent route north from Texas to the Missouri River and the fringe of the woodlands.