Those who think this answer an easy and satisfactory one must be unaware of the unsettling of many problems since Mill's day. Mill had no less than three laws of value--that of the Equation of Supply and Demand, that of Cost of Production, and that of Differential Cost of Production. The former law, he said, applied to goods of which the quantity was naturally, or artificially, or temporarily, limited, and it was, besides, the sole determinant of the value of labour. But then Mill was assuming a definite Wage Fund--a fixed portion of the circulating capital of the country predestined for the payment of wages. This definite sum, and no more, was to employ all the workers, however many they might be. If, then, wages fell, the reason was obvious--there were too many workers. Wherever Mill touches on low wages we have a sermon on the evils of over-population, and his favourite explanation did not fail him here. "Where employers take full advantage of competition, the low wages of women are a proof that the employments are overstocked." But this is logical only if "overstocking" is the sole possible cause of low wages--which might be doubted even under a Wage Fund theory. But the Wage Fund is now one of the antiquities of political economy. Since Jevons we have looked for the measure of value in marginal utility; for the value of productive goods in their marginal utility as instruments of production; and for the value of labour in the value of its products, and not in any predetermined fund divided out among a variable number of workers by the action of supply and demand. And where invention is constantly widening and strengthening our power over natural resources and increasing the productiveness of labour, the presumption is against the idea that over-population is even a strong factor in modern wages.