The cost of production of these commodities tends to increase.
This is not a property inherent in the commodities themselves. If population were stationary, and the produce of the earth never needed to be augmented in quantity, there would be no cause for greater cost of production.295 The only products of industry which, if population did not increase, would be liable to a real increase of cost of production, are those which, depending on a material which is not renewed, are either wholly or partially exhaustible, such as coal, and most if not all metals; for even iron, the most abundant as well as most useful of metallic products, which forms an ingredient of most minerals and of almost all rocks, is susceptible of exhaustion so far as regards its richest and most tractable ores.
When, however, population increases, as it has never yet failed to do, then comes into effect that fundamental