The causes of its triumph, the sources of its strength, are not obscure. Seemingly backed by an indisputable arithmetical truth—that a continuously increasing population must eventually exceed the capacity of the earth to furnish food or even standing room, the Malthusian theory is supported by analogies in the animal and vegetable kingdoms, where life everywhere beats wastefully against the barriers that hold its different species in check—analogies to which the course of modern thought, in leveling distinctions between different forms of life, has given a greater and greater weight; and it is apparently corroborated by many obvious facts, such as the prevalence of poverty, vice, and misery amid dense populations; the general effect of material progress in increasing population without relieving pauperism; the rapid growth of numbers in newly settled countries and the evident retardation of increase in more densely settled countries by the mortality among the class condemned to want. The Malthusian theory furnishes a general principle which accounts for these and similar facts, and accounts for them in a way which harmonizes with the doctrine that wages are drawn from capital, and with all the principles that are deduced from it. According to the current doctrine of wages, wages fall as increase in the number of laborers necessitates a more minute division of capital; according to the Malthusian theory, poverty appears as increase in population necessitates the more minute division of subsistence. It requires but the identification of capital with subsistence, and number of laborers with population, an identification made in the current treatises on political economy, where the terms are often converted, to make the two propositions as identical formally as they are substantially.[15] And thus it is, as stated by Buckle in the passage previously quoted, that the theory of population advanced by Malthus has appeared to prove decisively the theory of wages advanced by Smith. Ricardo, who a few years subsequent to the publication of the “Essay on Population” corrected the mistake into which Smith had fallen as to the nature and cause of rent, furnished the Malthusian theory an additional support by calling attention to the fact that rent would increase as the necessities of increasing population forced cultivation to less and less productive lands, or to less and less productive points on the same lands, thus explaining the rise of rent. In this way was formed a triple combination, by which the Malthusian theory has been buttressed on both sides—the previously received doctrine of wages and the subsequently received doctrine of rent exhibiting in this view but special examples of the operation of the general principle to which the name of Malthus has been attached—the fall in wages and the rise in rents which come with increasing population being but modes in which the pressure of population upon subsistence shows itself.