[Sidenote: Desire for better agents impels men to improvements] 1. _While man destroys some agents of production he multiplies many others._ We have noted many kinds of depreciation, destruction, and wearing out of wealth; but the normal thing in a healthy society is an increase, on the whole, of rent-bearers. The increase of rents is due to two causes: changes in the agents by which they become more efficient technically, or more numerous; and changes taking place outside of the agents, affecting the utility of the products. The first of these will be considered in this section. The increase of the efficiency of agents is usually the aim of the individual producer, and thus is brought about an increase of the stock of wealth. In some cases, however, improvements such as the dredging of harbors or as the protecting of forests, are made by men collectively through the agency of governments. Somewhere, however, the desire for these changes must arise in the minds of individuals. Increase of most things involves "cost" or sacrifice, in the psychological sense; that is, man must strive, perhaps suffer, to get a certain result. This end, therefore, must be in itself desirable, and social organization must be such as to present a motive to the men to make the needed effort. [Sidenote: Improvements by adaptation of natural resources] 2. _Rent-bearers may be increased in quantity and improved in quality by the adaptation of natural resources to man's purposes._ To get food, men use the tracts of land that under the conditions give the largest product. Other tracts less fertile, or for some reason less available, are ditched, tiled, and diked, and fertilizers are carried up steep hillsides to make a soil upon the very crags. In commerce and transportation, new ways are opened by canals, railroads, and tunnels. An isthmian canal will raise the efficiency of ships plying between New York and San Francisco, enabling them to carry a greater amount of freight within a year. The tolls will represent to the users an expenditure only partially offsetting the increased efficiency of the agents of transportation. By the building of wharves, the dredging of harbors, and by many other methods, indirect agents are constantly growing in number and efficiency. [Sidenote: Machinery is an adaptation of natural resources]