Likewise, the theory of net product, so highly social, progressive, and conservative, has been individualized, if I may say so, by monopoly, and the principle which ought to secure society's welfare causes its ruin. The monopolist, always striving for the greatest possible net product, no longer acts as a member of society and in the interest of society; he acts with a view to his exclusive interest, whether this interest be contrary to the social interest or not. This change of perspective is the cause to which M. de Sismondi attributes the depopulation of the Roman Campagna. From the comparative researches which he has made regarding the product of the agro romano when in a state of cultivation and its product when left as pasture-land, he has found that the GROSS product would be twelve times larger in the former case than in the latter; but, as cultivation demands relatively a greater number of hands, he has discovered also that in the former case the NET product would be less. This calculation, which did not escape the proprietors, sufficed to confirm them in the habit of leaving their lands uncultivated, and hence the Roman Campagna is uninhabited. "All parts of the Roman States," adds M. de Sismondi, "present the same contrast between the memories of their prosperity in the Middle Ages and their present desolation. The town of Ceres, made famous by Renzo da Ceri, who defended by turns Marseilles against Charles V. and Geneva against the Duke of Savoy, is nothing but a solitude. In all the fiefs of the Orsinis and the Colonnes not a soul. From the forests which surround the pretty Lake of Vico the human race has disappeared; and the soldiers with whom the formidable prefect of Vico made Rome tremble so often in the fourteenth century have left no descendants. Castro and Ronciglione are desolated."--Studies in Political Economy. In fact, society seeks the greatest possible gross product, and consequently the greatest possible population, because with it gross product and net product are identical. Monopoly, on the contrary, aims steadily at the greatest net product, even though able to obtain it only at the price of the extermination of the human race. Under this same influence of monopoly, interest on capital, perverted in its idea, has become in turn a principle of death to society. As we have explained it, interest on capital is, on the one hand, the form under which the laborer enjoys his net product, while utilizing it in new creations; on the other, this interest is the material bond of solidarity between producers, viewed from the standpoint of the increase of wealth. Under the first aspect, the aggregate interest paid can never exceed the amount of the capital itself; under the second, interest allows, in addition to reimbursement, a premium as a reward of service rendered. In no case does it imply perpetuity.