EQUAL in importance to being alive is the power to go on living; therefore, having described the signs of life, our next task is to consider how that life is maintained. When the primary fact of life was given as continuous power development, the foundation was laid for this topic, for life cannot fail to go on if continuous power development is maintained.

Power development in living animals as in locomotives depends on fuel and oxygen; evidently continuous supplies of these must be provided if life is to go on. The living animal differs from the locomotive in this: that while some one attends to supplying the locomotive with fuel, most living animals, except the very young, have to attend to providing themselves. There are exceptions to this rule. The tapeworms that inhabit the intestines of animals, and sometimes of men, live in a stream of food; they are put to no trouble to obtain it. The same is true of many kinds of parasites. Except for these, however, it holds true that animals must attend to their own wants. We shall now begin to see the utility of the most conspicuous sign of life spoken of in the first chapter, namely, motion, for food must be gotten where it is; only tapeworms and similar animals swim in it. All the rest, including ourselves, must go to where the food is. Even animals like oysters, that are anchored to the rocks, have to use motion in getting food. In their case the motion consists in setting up a current in the sea water into and through their bodies, from which current they sift out the tiny food particles which abound in the ocean.

If an animal happens to live in the ocean, where every drop furnishes its particle or two of foodstuff, and especially if the animal is small, or sluggish, like the oyster, almost any kind of motion will serve to bring the animal all the food it needs. The simplest of the one-celled animals, that must be watched through the microscope to see how they behave, blunder about aimlessly, and in the course of their blundering bump up against food particles often enough to keep themselves fed. If an animal happens to live where food is scarcer, or if it is big and active, and so must have large quantities of food, aimless blundering about will never get it enough to keep it alive. It must have some means of finding out where the food is. Since we ourselves come under the head of animals whose food needs are so large that we must locate food supplies, and not depend on happening onto them, we can identify in ourselves the means which are used for doing this. We all know that our sense organs, the eyes, ears, nose, and finger tips are what we depend on for telling us where food is to be found. The same is true of all animals that are able to hunt for food; they have some sort of sense organs to help in guiding them to where the food is.

The story of the machinery for finding food is not yet quite complete, for the muscles which actually make the movements by which the animal gets about are in one part of the body, while the sense organs which are to furnish the information by which the movements are guided are in quite a different part, and in animals as large as ourselves, some distance away. From our eyes to our leg muscles is quite a space, and it is evident that this space must be bridged somehow if our legs are to move in obedience to information which our eyes bring in. In ourselves and in almost all other animals this space is bridged by means of special machinery for the purpose. We are familiar with it under the name of the nervous system. We may not have been in the habit of thinking of the nervous system in just this way, but at bottom this is exactly what the nervous system does for us, namely, guides our muscles according to the information brought in by our sense organs. There is more to nervous activity than just this, but this is the starting point and groundwork for all the rest, as we shall try to show presently.

Continuous food supplies are the main necessity for continued life, but there are some other things that have to be looked out for in addition. The favorite food for large numbers of animals, and, indeed, in many cases the only food, consists of the bodies of other animals. All the flesh-eating sorts prey on other animals for their food. This places the other animals on the defensive, so that a large part of their activity consists in escaping the attacks of the beasts that wish to eat them. For most kinds of animals the greater part of their waking life is taken up with movements which serve either to get them food or to prevent them from becoming food for others. If we add to these the movements that are necessary to preserve the animals against other kinds of danger than the danger of being eaten, and those connected with the propagation and care of the young, we shall have about covered the list of what we may call the serious activities of animals, and of men as well. Many kinds are active besides in play. This is particularly true of young animals, although grown-ups, both among animals and men, find play both agreeable and beneficial when not overindulged.

Protective motions need to be even more accurately made than those whose purpose is the getting of food, for if the food is missed at one effort another trial may be more successful, but if an attempt to escape fails there will probably be no more chances to try. The sense organs and the nervous system are just as deeply concerned, therefore, in avoiding harm as in finding food materials, and it is as important for them to do their work well in the one case as in the other. When we think of the activities of animals, for whatever purpose they are carried on, we must think of them as made up of the combined actions of the muscles, the nerves, and the sense organs, and not of any of them working by themselves.

These parts of us that are so closely concerned in the maintaining of our life by getting us food and keeping us safe from harm make up, also, the only parts of us which really share in what we may call conscious living. When we come right down to it we could spare our other organs—heart, lungs, stomach, and the rest—and never miss them so far as adding anything to our happiness is concerned. In fact, the less these organs intrude themselves into our attention the better off we are; only when we are ailing do we begin to think about them. Of course, they are absolutely necessary to us, and we should die instantly if one of the more important of them were to stop working, but the part they play is not one which enters actively into our consciousness, as do the muscles, nerves, and sense organs.

Naturally, we will ask what all these other organs are for if they do not share in our conscious life. Why can we not get along with just those that we use for getting food, for avoiding harm, for play, and for the other activities which they carry on? The answer to this question is found in the fact emphasized above that continuous power development is necessary to continued life. By themselves the muscles, nerves, and sense organs cannot carry on power development; they require the aid of a great many other organs to do this. Just how these other organs work will be described later; at present it will be enough to recall that every muscle, every nerve, and every sense organ is actually made up of a great many of the tiny construction units—the cells about which we were talking a few pages back—and that every one of these cells must be developing power all the time if it is to go on living. In order to be able to do this they must, every one, be able to oxidize fuel continuously, and this means that they must receive constant supplies, both of the fuel itself and of the oxygen with which it combines. Some system of delivering these materials must be in operation, and in case the materials have to be prepared for use beforehand this must be provided for also. The heart, the lungs, the stomach, and the various other organs that are useful but not conspicuous, are concerned in these necessary jobs. In an automobile factory we have a similar situation; the men that stand at the machines actually make the parts that go into the finished automobiles, but unless other men are hard at work preparing the castings, and bringing them to where the machine operators can get them, not many automobiles will be turned out. So in the body, unless the various organs are carrying on their work of preparing and delivering materials to the muscles, the nerves, and the sense organs, these latter cannot perform their tasks of getting the food for the whole body and of securing the body against harm, nor can they carry on the pleasant, but not absolutely necessary, activities of play and recreation.