We have noted elsewhere the variations in human capacity and productiveness. Superficial critics still frequently charge Marx with having overlooked this very obvious fact, whereas it has not only been fully treated by him, but was actually covered by Smith and Ricardo before Marx! With these writers and their followers it is the law of averages which solves the difficulties arising from variations in individual capacity and productivity. It is the _average_ amount of labor expended in killing the beaver which counts, not the actual individual labor in a specified case. Nor did these writers overlook the important differentiation between simple, unskilled labor and labor that is highly skilled. If A in ten hours' labor produces exactly double the amount of exchange-value which B produces in the same time devoted to labor of another kind, it is obvious that the labor of B is not equal in value to that of A. Quantity of labor cannot, therefore, be measured, in individual cases, by time units. Despite a hundred passages which, detached from their context, seem to imply the contrary, Adam Smith recognized this very clearly, and attempted to solve the riddle by a differentiation of skilled and unskilled labor in which he likens skilled labor to a machine; and insists that the labor and time spent in acquiring the skill which distinguishes skilled labor must be reckoned.[170] Another frequent criticism of the Marxian theory has not only been answered by Marx himself--is, in fact, ruled out by the terms of the theory itself--but was amply replied to by Ricardo.[171] The criticism in question consists in the selection of what may be called "unique values," or scarcity values, articles which cannot be reproduced by labor, and whose value is wholly independent of the quantity of labor originally necessary to produce them. Such articles are unique specimens of coins and postage stamps, autograph letters, rare manuscripts, Stradivarius violins, Raphael pictures, Caxton books, articles associated with great personages--such as Napoleon's snuffbox--great auks' eggs, and so on _ad infinitum_. No possible amount of human labor could reproduce these articles, reproduce, that is to say, the exact utilities in them. Napoleon's snuffbox might be exactly duplicated so far as its physical properties are concerned, but the association with Napoleon's fingers, the sentimental quality which gives it its special utility, is not reproducible. But the trade of capitalist society does not consist in the manufacture and sale of these things, which, after all, form a very insignificant part of the exchange-values of the world. III