The rule that goods which have the same cost of production have also equal value in exchange, is applicable only to the extent that it is possible to transfer the factors of production at will from one branch of production to another. Where this really free competition does not exist, the price depends entirely on the quantity of the supply, compared with the solvability or capacity to pay of the purchaser; and hence, it may sometimes rise far above the cost of production (monopoly-price), and sometimes sink far below it (forced price, or under-price).(662) Such hindrances to competition depend, in part, upon natural causes. Thus, in the case of the works of art of a deceased artist, which cannot be increased in number;(663) or in that of living celebrities who cannot extend their mental activity in the same degree that their reputation has grown. So, also, in the case of precious stones, which are sometimes found free, and therefore cost nothing, but which, at the same time, have a high price.(664) Many valuable agricultural products are, together with their production, limited to a definite and sometimes very small district.(665) It is to be regarded as a modification of such natural monopolies when substitutes for a kind of goods which diminish, at least in part, the demand for them, are found, at a cheaper price; for instance, ordinary table-wines in the stead of fine wines. The rule applies much more strictly to those goods which, on account of their greater quantity, can replace inferior ones,(666) than it does to those where this is not possible.