The commodity-supply must be of a certain volume in order to satisfy the
 demand during a given period. A continual extension of the circle of buyers
 is counted upon. For instance, in order to last for one day, a part of
 the commodities in the market must constantly remain in the commodity-form
 while the remainder is fluent, turns into money. True, the part which stagnates
 while the rest is fluent decreases steadily, just as the size of the supply
 itself decreases until it is all sold. The stagnation of commodities thus
 counts as a requisite condition of their sale. The volume must furthermore
 be larger than the average sale or the average demand. Otherwise the excess
 over these averages could not be satisfied. On the other hand the supply
 must constantly be renewed, because it is constantly being drawn on. This
 renewal cannot come from anywhere in the last instance except from production,
 from a supply of commodities. It is immaterial whether this comes from
 abroad or not. The renewal depends on the periods required by the commodities
 for their reproduction. The commodity-supply must last the whole time. The
 fact that it does not remain in the hands of the original producer but
 passes through various reservoirs, from the wholesaler to the retailer,
 changes merely the appearance and not the nature of the thing. From the
 point of view of society, a part of the capital retains in both instances
 the form of a commodity-supply until the commodities enter productive or
 individual consumption. The producer tries to keep a stock corresponding
 to his average demand in order not to depend directly on production and
 to ensure for himself a steady clientele. Purchase periods corresponding
 to the periods of production are formed and the commodities constitute
 supplies for longer or shorter time, until they can be replaced by new
 commodities of the same kind. Constancy and continuity of the process of
 circulation, and therefore of the process of reproduction, which includes
 the process of circulation, are safeguarded only by the formation of such
 supplies.