If, therefore, the quantity of labor-time required for the production of commodities remained constant, their exchange value would remain the same. But the ease and the difficulty of production are constantly changing. If the productivity of labor increases, the same use-value will be produced in less time. If the productivity of labor declines, more time will be required for the production of the same use-value. Thus, the labor-time contained in a commodity or its exchange-value is a variable quantity, increasing or diminishing in an inverse ratio to the rise and fall of the productivity of labor. The productive power of labor which is applied in the manufacturing industry on a predetermined scale depends in the agricultural and extractive industries also on natural conditions which are beyond human control. _The same labor_ will yield a greater or less output of various metals, according to their more or less close occurrence in the earth’s crust. _The same labor_ may be embodied in two bushels of wheat in a favorable season, and only in one in an unfavorable season. In this case, scarcity or abundance, as natural conditions, seem to determine the exchange value of commodities, because they determine the productivity of certain kinds of labor which depend upon natural conditions. Unequal volumes of different use-value contain the same quantity of labor-time or the same exchange value. The smaller the volume of a use-value containing a certain quantity of labor-time as compared with other use-values, the greater its _specific exchange-value_. If we find that certain use-values, such as, e. g., gold, silver, copper and iron, or wheat, rye, barley and oats, form a series of specific exchange values which, though not retaining exactly the same numerical ratio, still retain through widely remote epochs of civilization the same rough proportion of relatively larger and smaller quantities, we may draw the conclusion that the progressive development of the productive powers of society has equally, or approximately so, affected the labor-time necessary for the production of the various commodities.