We have talked about the necessity of power development in all living things, and have seen that power development depends on the oxidation of fuel. Of course, our fuel is the food that we eat. No substance is suitable for fuel unless it contains power which can be gotten out by oxidation, and unless, in addition, it is suited to the particular kind of oxidation that goes on in the body, and can be handled by the body. Wood is excellent fuel for some purposes, but as a food for man it has no value, even when ground fine and mixed with flour as was done in some European countries during the Great War, because wood cannot be handled by the body in the way in which a usable fuel must be. Although wood is not good food, closely related materials are, and in fact make up the bulk of it. All fuel food is either vegetable or animal. All animal food traces back finally to the vegetable world, and it is an interesting fact that we do not usually care to eat flesh that is more than one remove from the vegetable kingdom. Animals that are flesh eaters are not considered fit for food, except in the case of fish and birds, and the flesh that these eat is not commonly thought of as being such, since it consists mostly of the flesh of insects, frogs, and fish themselves. The real sources of food, then, are in the vegetable world. Of the countless thousands of kinds of plants that exist a few dozen have proven to be of enough use for human food, or for food for the animals on which human beings feed, to justify us in taking the trouble to raise them on our farms and in our gardens. There must be something about these particular plants to make us prefer them. If we look into the reason for the preference we shall understand something of the qualities which make plants good for food in the first place. At the beginning of the chapter were set down the things which make a substance fit for food. These are: the ability to yield power by oxidation, and a composition suitable to be used by the body. The ability to yield power involves the possession of a store of it. Power, or energy, which means the same thing in our present use of the words, is never present anywhere except as the result of an earlier exhibition of power. It is not made out of nothing. The sun is a reservoir of energy on which the earth draws, and we do not know with any certainty from whence the sun got its power. The heated center of the earth itself is a reservoir of power on which we may draw at some time in the future, when cheaper sources are used up. Except for energy from these sources and for trifling amounts that may be brought in by meteorites, there is none on the earth’s surface that has not always been here. On the other hand, the earth is constantly losing energy into space. The amount that reaches us from the sun balances our losses into space, so that the total energy present holds fairly steady. The energy that comes to us