Many different commodities have served mankind as money. In the Homeric poems oxen served as a measure of value. Where slavery has prevailed slaves have often served as the medium of exchange and the standard of value. The American Indian used strings of beads called wampum as money; in Massachusetts in 1649 wampum was made legal tender among the settlers in the payment of debts to the amount of forty shillings. In the East Indies cowry shells have been used for small change, while among the Fijians whales' teeth have served the same purpose. Wheat, lumps of salt, cubes of tea, cacao beans, and birds' heads are a few of the commodities which at different times and in different places have served as money. Professor Carl Bucher in speaking of the variety of kinds of money among primitive peoples writes, "The money of each tribe is that trading commodity which it does not itself produce but which it regularly acquires from other tribes by way of exchange." Of the commodities which have already been mentioned as money some served the purpose well and some poorly, but none of them would prove satisfactory in an advanced commercial state.