The universal use of certain commodities for money rests on social experience, and not, as many believe, on some fine-spun theory evolved by statesmen and the law courts, that in making exchanges one commodity ought to be preferred over another. Among the early Jewish patriarchs, cattle and sheep were regarded not only as wealth, but also as measures of wealth. Homer, the Greek poet, speaks of the wealth of the Greeks in terms of cattle. Our own American Indians employed shells in making exchanges among themselves; some African tribes use cubes of salt as money; while in the interior of Russia tanned hides at one time circulated as a medium of exchange. Slowly, but obviously not in the same period of time, each independent society, employing the selective process in making its choice, has experimented with various commodities as money.