Exchange of goods and transportation went hand in hand in their development, very slowly and surely. After trade had become pretty well established, it became necessary to have a medium of exchange. Some well-known article whose value was very well recognized among the people who were trading became the standard for fixing prices in exchange. Thus, in early Anglo-Saxon times the cow was the unit of the measure of value. Sometimes a shell, as a _cowrie_ of India or the wampum of the American Indian, was used for this purpose. Wheat has been at one time in America, and tobacco in another, a measure of exchange because of the scarcity of money. Gradually, as the discovery and use of precious metals became common and desirable because of their brightness {106} and service in implement and ornament, they became the medium of exchange. Thus, copper and gold, iron and bronze have been used as metallic means of exchange--that is, as money. So from the beginning of trade and swapping article for article, it came to be common eventually to swap an article for something called money and then use the money for the purchase of other desirable articles. This made it possible for the individual to carry about in a small compass the means of obtaining any article in the market within the range of the purchasing power of his money. Trade, transportation, and exchange not only had a vast deal to do with economic progress but were of tremendous importance in social development. They were powerful in diffusion, extension, and promotion of culture. _The Struggle for Existence Develops the Individual and the Race_.--The remnants and relics of the arts and industries of man give us a fair estimate of the process of man's mind and the accomplishment of his physical labor. It is through the effort involved in the struggle for existence that he has made his various steps forward. Truly the actual life of primitive man tends to verify the adage that "necessity is the mother of invention." It was this tremendous demand on him for the means of existence that caused him to create the things that protected and improved his life. It was the insistent struggle which forced him to devise means of taking advantage of nature and thus led to invention and discovery. Every new invention and every new discovery showed the expansion of his mind, as well as gave him the means of material improvement. It also added to his bodily vigor and added much to the development of his physical powers. Upon this economic foundation has been built a superstructure of intellectual power, of moral worth and social improvement, for these in their highest phases of existence may be traced back to the early beginnings of life, where man was put to his utmost effort to supply the simplest of human wants. {107} SUBJECTS FOR FURTHER STUDY 1. The change in social life caused by the cultivation of the soil. 2. The effect of the discovery and use of fire on civilization.