Indeed, for centuries the sharper features of the contrast have been steadily softened. The money economy of the city gradually spread to the rural districts, but never entirely displaced barter, which lingers everywhere. Important steps toward a money economy were the commuting of forced or customary labor of the serfs into a money payment to the lord, and at the same time the substitution of money payments for payments in kind (use of lands, specified goods, etc.) to the peasants. Thus arose a free peasant class receiving wages. But land continued to be rented and landed estates to be hereditary throughout Europe. As they did not pass from hand to hand as a commercial or marketable form of wealth, their value was rarely, if ever, expressed in terms of money and as a ratio to the rent they bore. The result was the fixing of the erroneous idea that agricultural wealth is essentially different in the character of its service and yield from wealth used in manufactures. One phase of the error was the idea held by the physiocratic writers and by Adam Smith that in agriculture "nature labors along with man," while in manufacture "nature does nothing, man does all." This view was corrected by later critics (Buchanan, Ricardo, and others), but the main portion of the fallacy persisted in the supposed contrast between the characters of the services performed by natural resources and by artificially produced wealth. § II. THE CONCEPT OF CAPITAL IN MODERN BUSINESS [Sidenote: Extension of the use of the money loan and of the capital concept] 1. _The development of the use of money and credit has led to the expression of the value of all indirect agents, without distinction, in terms of money._ This is a capitalistic age. The development of a class of money-lenders has led to a transfer of all sorts of wealth from owners to users by means of money. As in medieval Europe city wealth was bought and sold, and measured and expressed, so in twentieth century America are the farm, the waterfall, and the mine. Every purchase with money owned or borrowed is to-day called an investment of capital. To invest means to clothe, and an investment of capital is clothing money in any kind of wealth, whether it be a ship, a factory, or a farm. Interest on money is the contractual form in which more and more the use of wealth is paid for. The borrower does not ask the wealthy man to buy for him a factory and to rent it to him. It is not impossible for the transaction to take that form; but in practice it is inconvenient. The capital concept, the expression of wealth in the form of money, spreads over almost the whole face of the economic world. In promissory notes, mortgages, capital stock, bonds, and many other forms, are expressed the obligations of borrowers bound to pay regularly a sum called interest for the use of the multifarious wealth they have chosen to employ. [Sidenote: Definition of capital]