All economic goods, he said, have value. They have a physical value, and a value given them by the labor expended on them. Labor is the common factor of economic values. And the common denominator is the time that is consumed by the labor. Labor-time, therefore, is the universal measure of value, the common medium that determines values. But this labor is acquired in the open labor market by the capitalist at the lowest possible price, a price whose utmost limit is the bare cost of living. The reward for his labor is called a wage. This wage does not by any means measure the value of his services. What, then, becomes of the "surplus value," the value over and above wages? The capitalist appropriates it. Indeed, the great aim of the capitalist is to make this surplus value as big as possible. He measures his success by his profits. "Surplus value," or profit, is, then, a species of robbery; it is ill-gotten gain, withholding from the workman that which by right of toil is his. How did it come about that society was so organized as to permit this wholesale wrong upon the largest and most defenseless of its classes? It is in answer to this question that Marx makes his most notable contribution to Socialistic theory. With great skill, and displaying a comprehensive knowledge of economic history, especially of English industrial history, he traces the development of modern industrial society. He follows the evolution of capital from the days of medieval paternalism through the period of commercial expansion when the voyages of discovery opened virgin fields of wealth to the trader, into the period of inventions when the industrial revolution changed the conditions of all classes and gave a sudden and princely power to capital, establishing the reign of "capitalistic production." Always it was the man with capital who could take advantage of every new commercial and industrial opportunity, and the man without capital who was forced to succumb to the stress of new and cruel circumstances. In every stage of development it has been the constant aim of the capitalist to increase his profits and of the workingman to raise his standard of living. Marx then declares that, in order to have a capitalist society, two classes are necessary: a capitalist and a non-capitalist class; a class that dominates, and one that succumbs. There have always been these two classes. Originally labor was slave, then it was serf, and now it is free. But free labor to-day differs from serf-labor and slave-labor only in that it has a legal right to contract. The economic results are the same as they always have been: the capitalist still appropriates the surplus value.