The variety of flora and fauna, and their fitness for man's needs, largely condition the possible production. If, in the course of evolution, it had chanced that wheat and corn, the horse and the cow, had been crowded out in the struggle for existence, we should have had a very different civilization. The possibilities of civilization in Peru, and those of all the Indians on the American continent, were limited for lack of domestic animals. Animals that are fit for domestication are a necessary intermediate agent by aid of which man can appropriate and turn to his use the fertile qualities of the soil. Not content with the material world about him, even when it is at its best, man alters it in many ways. He enriches the soil, improves the varieties of animals, he even in some slight degree affects the climate, and by the use of a multitude of artificial bits of matter called tools, works profound changes in the world in which he lives. [Sidenote: By motion and energy] 2. _A large part of the utility of goods is conditioned on motion and energy._ It has been said that man's power in production is limited to moving things. The outer world is to man the sole source of motive forces. He can bring things together and they produce the result. Further, it may be said that nearly every kind of utility is conditioned on motion. It is man's aim to secure a constant inflow of goods. To secure this either he must move to get the goods, or he must cause goods to move toward him. The law of "conservation of energy" helps to explain economic action; the supply of energy in the universe cannot be increased or diminished, but may take on new forms. So a limited supply in man's control may take on various forms and so have different effects on gratifications. One and the same source of energy may be converted into the different forms of heat, light, motion, electricity, etc. But there must be some source. Man's desire is directed to getting force at the right place and in the right degree. If light or heat is too intense, it causes pain; the glare of the sun blinds instead of giving keener vision. A moderate force applied to any of the senses gives the maximum clearness or pleasure. Man is constantly endeavoring to secure forces from the outer world and to adjust motion so that it will directly or indirectly best serve his purposes. [Sidenote: By food, animals, and fuel] 3. _Among the main sources of power used by men are food, domestic animals, and fuel._ In eating food man stores up force in his own body. When he draws the bow he puts force into it to lie latent until liberated at the right moment. There must be a source of energy likewise that mental action may go on, and the power of sunbeams, stored for a time in food, is liberated in the processes of thought.