208 It is easy to see that it is the most important substances needed in industry which are mentioned in this section. Many political economists have considered the principal difference between agriculture and the industries and economies of towns to lie in the contrast here referred to. Thus, _A. Sena_, Sulle Cause che possono far abbondare li Regni d’oro e d’argento, dove non sono miniere, 1613, I, 3. See the description of the difference between land and machines in _Malthus_, Principles, III, 5; _Senior_, Outlines, 86. But it is nothing more than a difference of gradation. Even in the most active of businesses there is a limit which the accumulation of means of production cannot pass without a relative diminution of the income. This boundary is imposed by the limited nature of those organic beings which must contribute to production either actively or passively. Thus, for instance, a manufacturing establishment or commercial business can be enlarged with advantage only so long as it is still possible for one superintendent to conduct it. And so, when cattle are furnished with very abundant and substantial food, a pound of meat costs the producer a much higher price than when they are more moderately supplied: sometimes in the ratio of 1.95:0.98. _Boussingault_, Economie rurale, II. Where there is absolute over-feeding, the producer must suffer loss. But, even inorganic nature imposes its own limits here; as, for instance, when ships, machines etc., on account of the insufficient strength of the materials of which they are made, cannot be constructed beyond a certain size. But all these limits are much narrower than those imposed by the quality of immovability. _ 209 Senior_, Outlines, 26, 81 ff. See _Stewart_, Principles, II, ch. 11; _Ortes_, E. N., I, 18, II, 18 ff. This most important principle in Political Economy is thus illustrated by _John Stuart Mill_, Principles, book I, ch. 12. “The limitation to production from the properties of the soil is not like the obstacle opposed by a wall, which stands immovable in one particular spot, and offers no hindrance to motion short of stopping it entirely. We may rather compare it to a highly elastic and extendible band, which is hardly ever so violently stretched, that it could not possibly be stretched any more, yet the pressure of which is felt long before the final limit is reached, and felt more severely the nearer that limit is approached.” This is, if possible, more obvious in building than in agriculture, both as to the construction of new stories and the excavation of deeper cellars.