goods will distribute itself among products in such a manner that these products will have exchange values in proportion to their marginal pain costs. This result is brought about solely by control of the relative supplies of these products, the exchange values of which are all derived from utility solely after the method described in the utility theory. When we introduce the question of skill into the problem, we find that the supplies of many kinds of labor are limited not with reference to the disutility of the tasks performed, but are limited solely because the requisite brain-power, ingenuity or strength are scarce. The marginal product of such labor is raised by the limitation of its supply. Thus, it is a truism that many occupations of the lowest disutility afford very high wages, and that in the vast majority of cases high wages are not caused by high disutility, but by scarcity of competent persons. Suppose the commodity A is scarce, is of high exchange value, and is the marginal product of a skilled labor day. Article B is the product of a day of the lowest kind of labor. One A may well exchange for three or four B. Yet the marginal labor cost of A is, in the typical case, even less than that of B, for the skilled laborer ordinarily suffers less pain cost per day than the unskilled. Hence, the exchange values of these products are quite out of proportion to their comparative marginal disutility costs. The existence of non-competing groups, first emphasized and named such by Cairnes, is then a fatal obstacle in the way of the adjustment of exchange values to comparative marginal costs.