The value of the money material may fall so greatly as a result of greater production, as in the case of iron, tin, copper, that it becomes unsuitable. Again, as wealth grows, as exchanges increase, as the use of money develops, as commerce extends to more distant lands, the heavier, less precious metals fail to serve the money need, especially in the larger transactions. Thus, in a sense, different commodities compete, each trying to prove its fitness to be a medium of exchange; but only one, or two, or three at the most, can at one time hold such a place. Silver and gold, step by step, often making little progress in a century, have displaced other commodities, and are the staple and dominant forms of money in the world to-day. Every community has witnessed some stage of this evolution. Now nations are divided into two great groups, silver- and gold-using, in accordance with the metals they use as standards. The gold-using countries are the most advanced industrially, requiring the most valuable money metal. Many countries have passed in the last century from the silver to the gold standard, and in an intermediate period have tried to use both standards. The Asiatic and South American countries mainly use silver, while most of those in North America and Europe use gold. While industrial changes thus affect the choice of money, in turn money reacts upon the other industrial conditions. If a new and more convenient material is found, or the value of the money metal changes to a degree that affects the generalness of its use, industry is greatly affected. The discovery of mines in America brought into Europe, in the sixteenth century, a great supply of the precious metals, and this change in the use of money reacted powerfully on industry. Money being itself one of the most important of the industrial conditions, is affected by and in turn affects all others. § II. NATURE OF THE USE OF MONEY [Sidenote: Money is an indirect agent, a tool to effect exchanges]