The difficulties in the way of the desired increase of capital are here of great importance. The smaller the surplus over and above their absolutely necessary wants, which the people produce, the less their tendency to make savings, the less the inclination to capitalization; and the less the security afforded by the law is, the higher must the rate of interest be to induce people to face these difficulties. We may very well transfer the idea of cost of production to this condition.[183-2] The demand for capital depends, on the one hand, on the number and the solvability of borrowers, especially of non-capitalists like landowners and workmen; and, on the other hand, on the value in use of the capital itself. Hence the growth of population is, other circumstances being the same, a means to raise the rate of interest; because it infallibly increases the competition of borrowers of capital, even if the increased rate must take place at the expense of wages. The solvability or paying capacity of the land-owning class as contrasted with the capitalists can, in the last analysis, depend only on the extent and fertility of their lands and on the quality of their agricultural husbandry; the solvability or paying capacity of the working class, only on their skill and industry. Where these have grown, an increase of the rate of interest may be found in connection with an absolute growth of the rate of wages and of rent, because the aggregate income of the nation has become greater. The value in use of capital, which is more homogeneous in proportion as it has the character of circulating capital (_res fungibiles_) is, in most instances, synonymous with the skill of the working class, and the richness of the natural forces connected with it. The deciding element, therefore, is the yield of the least productive investment of capital which must be made to employ all the capital seeking employment. This least productive employment of capital must determine the rate of interest customary in a country precisely as cost of production on the most unfavorable land determines the price of corn (§§ 110, 150), and as the result of the work of the laborer last employed does the rate of wages. (§ 165.)