We have investigated the laws which determine the value of two classes of commodities—the small class which, being limited to a definite quantity, have their value entirely determined by demand and supply, save that their cost of production (if they have any) constitutes a minimum below which they can not permanently fall; and the large class, which can be multiplied _ad libitum_ by labor and capital, and of which the cost of production fixes the maximum as well as the minimum at which they can permanently exchange [if there be free competition]. But there is still a third kind of commodities to be considered—those which have, not one, but several costs of production; which can always be increased in quantity by labor and capital, but not by the same amount of labor and capital; of which so much may be produced at a given cost, but a further quantity not without a greater cost. These commodities form an intermediate class, partaking of the character of both the others. The principal of them is agricultural produce. We have already made abundant reference to the fundamental truth that in agriculture, the state of the art being given, doubling the labor does not double the produce; that, if an increased quantity of produce is required, the additional supply is obtained at a greater cost than the first. Where a hundred quarters of corn are all that is at present required from the lands of a given village, if the growth of population made it necessary to raise a hundred more, either by breaking up worse land now uncultivated, or by a more elaborate cultivation of the land already under the plow, the additional hundred, or some part of them, at least, might cost double or treble as much per quarter as the former supply.