We will let another figure represent the entire product of the same amount of labor and the same amount of capital that were represented in the former case. We will assume that there is at the outset a complete force of laborers, and that no men are added to it or taken from it; but we will gradually introduce units of capital instead of units of labor as in the former case. The amount of capital is now represented by the line _A'E'_ and the product of the first unit of it by the line _A'C'_. The product of the successive units declines along the curve _C'D'_. The final unit of capital then brings into existence the amount of wealth represented by _E'D'_. As every other unit now produces the same amount, the capital as a whole creates the quantity represented by _A'B'D'E'_ and every unit of it makes its own separate contribution to that amount. In this we have simply applied to capital and its earnings the principle we formerly applied to labor and its earnings. [Illustration] _General Form of the Law of Final Productivity._--This principle is the law of final productivity, one of those universal principles which govern economic life in all its stages of evolution. Either one of the two agents of industry, used in increasing quantities in connection with a fixed amount of the other agent, is subject to a law of diminishing returns. The final unit of the increasing agent produces less than did the earlier units in the series. This does not mean that at any one time one unit produces less than another, for at any one time all are equally productive. It means that the tenth unit produces less than the ninth did _when there were only nine in use_, and that the ninth unit formerly produced less than the eighth did in that still earlier stage of the process _in which there were only eight in use, etc._ If the productive wealth of the United States were only five hundred dollars per capita instead of more than twice that amount, interest would be higher than it is, because the productive power of every dollar's worth of capital would be more than the productive power of each dollar's worth is now; and, on the other hand, if we continue to pile up fortunes, great and small, till there are in the country two thousand dollars for every man, woman, and child of the population, interest will fall, because the productive power of a dollar's worth will become less than it now is.