Difference of Fertility. — We will suppose four different tiers of land, of unequal fertility. The first will yield forty bushels of corn; the second, with the same labor, thirty; the third, twenty; the fourth, ten.

Now, while there was enough land of the first to produce all the corn wanted, nobody would give any rent for the first tier on account of its fertility; but when, by the increase of population, it became necessary to cultivate No. 2, which would only yield thirty, No. 1 would command a rental of ten bushels, because a man might as well give ten bushels rent for No. 1 as to cultivate No. 2 without rent.

When, again, necessity compelled the cultivation of No. 3, No. 2 would pay a rent of ten bushels, and No. 1 of twenty bushels. And further, when tier No. 4 must be brought under culture to produce the quantity of corn needed for consumption, then, as it would with equal labor produce but ten bushels, No. 1 would yield a rent of thirty, No. 2 of twenty, and No. 3 of ten; while the last, or No. 4, would afford no rent.*

* Mr. Ricardo, we believe, first brought out this principle clearly in his "Political Economy," London, 1819.