This volume outlines in simple terms the basic concepts of ecology. It is not a textbook of ecology, but an introduction to the subject and to the more advanced presentations of special aspects that will appear in succeeding volumes. There are many excellent textbooks of ecology, to some of which the reader is referred and to which the present volume should also serve as a useful primer. I have linked this presentation of ecological theory to a survey of present theories of Earth History. All of earth history is, in a sense, ecology. Earth's environments have evolved throughout the ages from the time when no life existed, just as life forms have evolved from primitive to complex. It seems to me, therefore, that one should understand how these environments have come into being. I have also presented a simple account of the energy systems that are used by living things. These too need to be understood, if we are to appreciate energy problems. Man, like all other living things, requires the energy provided by his food, but unlike other creatures his way of life demands additional ('auxiliary') energy, and this has introduced a new factor to man's ecology. Hitherto, most of this auxiliary energy has been obtained from fossil fuels, coal, oil and uranium. But these are not inexhaustible; they represent capital, not income, and it is still not clear what the sources of the auxiliary energy will be.