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 [Sidenote: 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. Exchange is seen to arise out of the differences in the situations of men with reference to goods. The different subjective valuations give rise to demand, and demand leads to exchange. In early societies differences in natural products were the most usual causes of exchange. Salt, though so essential to life, is found in few places. The metals early became indispensable for weapons of defense or for the chase, and were sought far and wide. Rare shells, feathers, jewels, and the precious metals appealed in early times to a universal desire for ornament. Products like these are the objects of a rude sort of exchange in the first simple efforts made to adjust possessions to wants. Within the tribe, differences in the skill and ability of men to produce arrow heads or weapons or ornaments, bring about the exchange of goods. [Sidenote: Mutual advantage in exchange]