Thus far the matter has been comparatively simple. We have looked at a concatenation of successive groups with one final product, and with, of course, one marginal utility and one value. But we have now to face the fact that productive groups may pass into a great number of final products, each with a different marginal utility and value. The more industry is divided, the more is this the case. Productive goods, such as coal, oil, labour, go more or less to the making of millions of products. It is this that gives the Supply side its almost overwhelming weight in modern economic science. And it is here that we find the _raison d’être_ of the law of cost as a convenient abbreviated expression of a deeper law. Let us follow the matter out methodically. A stock of productive goods, which we shall call X, is capable of producing finished products A, B, and C. The value of these products for the time is, respectively, 100, 110, and 120. Which product will determine the value of the productive unit of X?—It will be the least of the three. For, suppose so many units of the stock X get lost that it is impossible to make A, B, _and_ C, the one given up will, of course, be A,—the employment of X which produces the least valuable product. Any other choice would be contrary to economic conduct. When we say, then, that means of production get their value from their product, we must be understood as meaning the value of their final or Marginal Product. But, again, if B and C are articles of large common manufacture, they cannot long retain their value of 110 and 120; it is merely a question of time till their value falls to 100. Here we begin to see the plausibility of the idea that cost of production determines value. To put this concretely. A man has a farm of 90 acres divided among three crops, which, in the circumstances of the market, give him three different returns. On 30 acres, he grows wheat, which, we shall suppose, yields him a value represented by 100; on another 30 acres, he grows potatoes, which yield him, say, 110; on another 30 acres, he grows barley, which yields him 120. What is the value of the productive group made up of his labour and one third of his land? (We leave out of account, for simplicity’s sake, the other co-operating factors.) If the value were given to land and labour by the _actual_ returns there would be three different values, and this really is the case where competition has not its full play. But, if there is no monopolist factor, these three values cannot be maintained. The value of the first product, 100, determines the value of the means of production, the labour and land, and it is only a question of time and competition till this value of the means of production has imposed itself on the potatoes and the barley, and reduced their price to the same comparative level as that of wheat.