“…But in different stages of the society, the proportions of the whole produce of the earth which will be allotted to each of these classes, under the names of rent, profits, and wages, will be essentially different; depending mainly on the actual fertility of the soil, on the accumulation of capital and population, and on the skill, ingenuity, and the instruments in agriculture." Since the publication of the Principles, many attempts have been done to extend or generalize the system (Barkai, 1959(Barkai, , 1966Pasinetti, 1960Pasinetti, , 1974Cochrane, 1970;Brems, 1970;Caravale and Tosato, 1980;Casarosa, 1985;Negish, 1989;Morishima, 1989). Nevertheless, what Ricardo (1821: preface) observed long time ago is still relevant today: "To determine the laws which regulate this distribution, is the principal problem in Political Economy: much as the science has been improved by the writings of Turgot, Stuart, Smith, Say, Sismondi, and others, they afford very little satisfactory information respecting the natural course of rent, profit, and wages."…”