A little thought will suffice to show that not only are all food-stuffs derived from the earth, but that also every usable resource which constitutes wealth is also drawn from the same source. The same is also pretty nearly true of the various forms of energy, for although the sun is the real source of light and heat, and probably of electricity, these agents are usable only when they have been transformed into earth energies. Thus, the physical energy generated by falling water is merely a transformed portion of solar heat; so also the coal-beds contain both the chemical and physical energy of solar heat and light converted into potential energy--that is, into force that can be used at the will of intelligence. Indeed, the physical being of mankind is an organism born of the earth, and adapted to the earth; and when that physical form dies, it merely is transformed again to ordinary earth substances. The chief activities of living beings are those relating to the maintenance of life. In other words, animals must feed, and they must also protect themselves against extermination. In the case of all other animals this is a very simple matter, they simply live in immediate contact with their food, migrating or perishing if the supply gives out. In the case of mankind the conditions are different and vastly more elaborate. Savage peoples excepted, man does not live within close touch of the things he requires; indeed, he cannot, for he depends upon all the world for what he uses. In a less enlightened state many of these commodities were luxuries; in a civilized state they have become necessities. Moreover, nearly everything civilized man employs has been prepared by processes in which heat is employed. Therefore one may specify several classes of human activities and employments: (_a_) The production of food-stuffs and other commodities by the cultivation of the soil--_Agriculture_. (_b_) The preparation of food-stuffs and things used for shelter, protection, or ornament--_Manufacture_. (_c_) The production of minerals for the generation of power, such as coal, or those such as iron, copper, stone, etc., required in the arts and sciences--_Mining_. (_d_) The exchange of food stuffs and commodities--_Commerce_. (_e_) The transfer of commodities--_Transportation_. It is evident that the prosperity and happiness of a people depend very largely on the condition of their surroundings--that is, their environment. If a country or an inhabited area produces all the food-stuffs and commodities required by its people, the conditions are very fortunate. A very few nations, notably China and the United States, have such diverse conditions of climate, topography, and mineral resources, that they can, if necessary, produce within their national borders everything needed by their peoples.