4. Demand thus varies at different ratios of exchange between goods, and may be expressed graphically by a demand curve. This would show for any one man the decline of the marginal utility of each added portion of a good, and these individual demand curves may be united into a demand curve for a group of men. The demand curve expresses graphically what a man would be willing to pay at each particular stage in the increase of goods. We have here come to the very threshold of the subject of markets and exchange.
Elasticity of demand
5. Elasticity of demand, in the case of any good, expresses the degree in which a change in its ratio to other goods will increase the demand. Elasticity varies for different classes of men according to their wealth and to the cost of the goods. If strawberries are a dollar a box in the city market, a slight fall in the price, say to seventy-five cents, will increase the demand but slightly. But if the price is fifteen cents and falls to ten, the increase in the demand will be marked, for the number of consumers to whom a difference of five cents is important is then very great. The demand for the staples is comparatively inelastic. A certain amount of simple food is necessary to support life; an increase in its price will not quickly check the demand. On the other hand, if the price of staple foods falls, no very great increase will take place in the demand.
CHAPTER 5
EXCHANGE IN A MARKET
§ I. EXCHANGE OF GOODS RESULTING FROM DEMAND
Reciprocal demand becomes exchange
1. Exchange in the usual economic sense is the transfer of two goods by two owners, each of whom deems the good taken more than a value-equivalent for the one given. The comparison of goods that has been discussed above is a kind of exchange. When a person chooses one thing rather than another, one form of gratification may be said to be mentally exchanged for another. This is exchange in that person's mind, or subjective exchange. But the word "exchange" as usually employed means an exchange of goods between persons. It is objective exchange, and when the word is used without modification, it is to be understood in the objective sense. In the last chapter were analyzed the motives of the individual man. Robinson Crusoe on his desert island would in very many ways be acted upon by the same motives in reference to economic goods that men are in society. Yet, it is exchange in society and the complicated problems arising from this transfer of goods from person to person that constitute nearly the whole of the subject-matter of political economy.