The manner in which increasing population advances rent, as explained and illustrated in current treatises, is that the increased demand for subsistence forces production to inferior soil or to inferior productive points. Thus, if, with a given population, the margin of cultivation is at 30, all lands of productive power over 30 will pay rent. If the population be doubled, an additional supply is required, which cannot be obtained without an extension of cultivation which will cause lands to yield rent that before yielded none. If the extension be to 20, then all the land between 20 and 30 will yield rent, and have a value, and all land over 30 will yield increased rent and have increased value.

It is here that the Malthusian doctrine receives from the current elucidations of the theory of rent the support of which I spoke when enumerating the causes that have combined to give that doctrine an almost undisputed sway in current thought. According to the Malthusian theory, the pressure of population against subsistence becomes progressively harder as population increases, and although two hands come into the world with every new mouth, it becomes, to use the language of John Stuart Mill, harder and harder for the new hands to supply the new mouths.