His third cause is that good land is scarce. Lands differ in fertility, and there is not, as in a new country, enough of the most fertile to supply all our wants. When the produce of the inferior begins to be absolutely necessary, the inferior will be cultivated at a price enough to repay cost and give ordinary profits to the farmer. But what is simply enough to do that for him will do much more than that for all the holders of superior lands, and all that is much more can be taken by a landlord as rent without placing the tenant at any disadvantage as compared with his neighbours. As soon as this happens in a country, the extra profits, which are called by economists rent, will appear in it; and the growth of population, by leading to an increased demand for food and to an increased price of it, will cause the cultivation of inferior lands, or else a more expensive cultivation of the old ones; and again, since the necessary new supplies cannot be permanently kept up without one or other of these two resources, the price, and with it the rent, will, in the absence of inventions, remain permanently higher. In other words, this third cause is the “law of diminishing returns.” It is this law of diminishing returns which bulks most largely in the tract of Sir Edward West, written in the same year as that of Malthus. West’s theory of rent is simply, “that in the progress of the improvement of cultivation the raising of rude produce becomes progressively more expensive, or in other words, the ratio of the net produce of land to the gross produce is continually diminishing.”[529] He sees how near Adam Smith came to it when he said, that in the progress of cultivation the total amount of rent increased, but the proportion of it to the produce diminished, so that from being _e. g._ half the produce it became one-third.[530] He sees, as even in 1798 Malthus had seen,[531] that but for this law population might increase indefinitely on a few fertile lands instead of spreading over the globe (West, p. 13), whereas because of this law inventions in agriculture are not able to remove “the necessity of having recourse to inferior land, and of bestowing capital with diminished advantage on land already in tillage” (p. 50). He pushes the principle so far as to say broadly that whatever increases agricultural production increases cost, while whatever increases manufacturing production diminishes cost (p. 48), inferring that the former must tend abroad and the latter at home to prevent the displacement of English agriculture by foreign competition. As he had little or no influence on Malthus, his tract need not be noticed in detail; it is enough to say that, while West is superior in style and arrangement, Malthus is the more comprehensive. West is clearer and simpler because he includes less.