4. _An increase of population more rapid than that of the artificial industrial agents would reduce marginal productivity._ Labor makes use of many kinds of agents besides the so-called natural resources. If population is stationary while tools are allowed to wear out or if an increasing population, while opening up a proportionate supply of land for food, fails to accumulate a proportionate stock of other tools, the marginal productivity of labor must diminish. Labor would be more imperfectly equipped with spades, hoes, wagons, horses, cattle, machinery. These artificial agents help in getting not only manufactured products, but food products. The equipment of labor must keep pace with the number of workers or they will be forced to the lower, or less effective, uses in the tools. On the other hand, the growth of science and invention, and the growth of wealth faster than the population, equipping labor as it does with more efficient implements, cause the marginal productivity of labor to rise, and hence also the wages. [Sidenote: The wage-fund theory explained] 5. _The "wage-fund theory" was an imperfect perception of this truth that wages are influenced by the efficiency of the industrial equipment._ As the subsistence theory took a partial view, looking at agricultural land alone as the determinant of wages, so the wage-fund theory looked alone at a portion of the capital in the hands of employers which was the fund from which wages were paid. The large part played in discussion by this doctrine and the strong hold it had on thought is somewhat puzzling now; for if one begins to doubt its entire truth it is difficult to be quite just to its merits or to state it in a form that is plausible. The theory was that wages depended on the amount of capital that, in some way not clearly seen, was set apart by employers for the payment of wages. The capital making up the fund out of which wages were supposed to be paid, was only a very small part of all capital, even in the narrow sense in which that term was then used. It was assumed that this wage fund, once set aside, was necessarily paid out to laborers, wages being therefore determined by simple division: laborers were the divisor, the wage fund the dividend, and the average wage the result. When the theory is thus baldly expressed, it appears to begin and end on the surface of the facts; and the wage fund appears to be rather the arithmetic sum of variously determined payments than, in any sense, the cause of wages. [Sidenote: The wage-fund theory a partial truth]