The materials, forces and relations or conditions of external nature, immovably connected with parts of the land, even when in themselves exhaustless, either allow only of a definite amount of economic utilization, as, for instance, the mechanical force of a given waterfall, which can drive only a definite number of mills of a definite size;(209) or their increased utilization is accompanied by difficulties which increase with still greater rapidity. This last is the case, especially in the employment of land for agricultural purposes. It is, according to Senior, one of the four fundamental axioms of Political Economy, that additional labor, spent on a given quantity of land, produces, as a rule, a relatively smaller yield; assuming, of course, that the art of agriculture remains the same. It is not possible to determine either generally, or in particular cases, the precise point at which agriculture should stop, to prevent relatively smaller returns from increased expenditure of labor and capital. Improvements in the art of agriculture may remove it a great distance. But, that there is such a point admits of no doubt. No one will believe that an acre of land can be made to produce a quantity of the means of subsistence sufficient to support all Europe, no matter what the amount of seed used, or of manure etc. employed.(210) This is most apparent in forest-economy, where the absolute increase of the so-called wood-capital becomes, after a certain time, smaller from year to year.(211) Section XXXV. External Nature.—Elements Of Agricultural Productiveness. In treating of the agricultural productiveness of a piece of land, it is necessary to distinguish three things,—its bearing-capacity, its capacity for cultivation, and its direct capacity to afford food to plants.(212) Plants grow by drawing a part of the elements which enter into their composition from the atmosphere, and a part from the earth through the agencies of sunlight and of water. While the air, the sun’s heat, and in most parts of the world, water, are free and inexhaustible goods, the earth’s supply of food for plants must be considered as analogous, so far as its exhaustibility and capacity to be appropriated are concerned, to the beds of coal and of ore etc. which occur in mining districts. This is certainly true, with a few important differences, however, as for instance, that, as a rule, it is impossible, except through the cultivation of plants, to obtain from the earth the stores of plant food which it contains;(213) and that it is possible to husbandry to replace the portion of these stores taken from the earth by the harvest, through the agency of manures.(214)