4. _As industry developed, the renting contract remained almost wholly confined to cases of renting lands and houses._ The materials and appliances needed for manufacture and commerce are so manifold and varying in quality that the rent-form of contract is very cumbersome and difficult for exchangers to enforce. If a merchant about to embark on a trading journey wished to rent a ship and a stock of goods, the renting contract became most difficult to interpret. He must agree to repay the loan in goods of the same kind and quality as those received, a contract most difficult to execute, and giving occasion to costly tests and countless disagreements. It was much easier for the merchant to get his loan under the interest contract, _i.e._, a money loan, with which to buy the goods. With the growth of industry and commerce, wealth increased in towns, taking many forms, as those of ships, wagons, tools, and stocks of goods, that could not conveniently be rented. [Sidenote: The thought of it remains associated with a rural economy] In England, the country which developed its industrial system earliest, the idea of rent, therefore, gradually became disassociated almost entirely from the use or hire of any wealth but land and real property. Because in the Middle Ages rent was associated almost entirely with natural resources, they being the only important forms of wealth which men rented from others, there was fostered the idea that the essential mark of rent is the connection with natural resources. It is a simple example of the association of ideas. In the transfer or loan of movable goods, the rent contract was quite overshadowed by the other form of contract, that of a money loan. According to this explanation the essential and primary difference between renting wealth and borrowing money at interest is not in the kind of wealth whose use is thus temporarily transferred, but in the nature of the contract. But as forms of wealth differ in their fitness for transfer under the two forms of contract, there goes on a competition between them, as a result of which each becomes associated with certain groups of goods. In the Middle Ages the renting contract was the dominant form, but it has been progressively displaced by loans in the money form, and its importance is still declining. [Sidenote: Renting contracts most used with land]