Two objections to the employment of these analogies will present themselves at once. The definition may be all very well as far as the machines are concerned, but, it may be asked, should a living thing like a horse or a dog be compared with the steamship or the locomotive? Can we look upon the living thing as a mechanism in the proper sense of the word? A second objection will be that human invention and ingenuity have controlled the evolution of the steamship and engine by the perfection of newer and more efficient parts. It is certainly true that organic evolution cannot be controlled in the same way by men, and that science has not yet found out what all the factors are. And yet we are going to learn in a later discussion that nature's method of transforming organisms in the course of evolution is strikingly similar to the human process of trial and error which has brought the diverse modern mechanisms to their present conditions of efficiency. This matter, however, must remain for the time just as it stands. The first objection, namely, that an organism ought not to be viewed as a machine, is one that we must meet immediately, because it is necessary at the very outset to gain a clear idea of the essentially mechanical nature of living things and of their relations to the conditions under which they live. It is only when we have such a clear understanding that we can profitably pursue the further inquiries into the evidence of evolution. Our first real task, therefore, is an inquiry into certain fundamental questions about life and living things, upon which we shall build as we proceed.
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All living things possess three general properties which seem to be unique; these are a peculiar chemical constitution, the power of repairing themselves as their tissues wear out, and the ability to grow and multiply. The third property is so familiar that we fail to see how sharply it distinguishes the creatures of the organic world. To realize this we have only to imagine how strange it would seem if locomotives and steamships detached small portions of themselves which could grow into the full forms of the parent mechanisms. Equally distinctive is the marvelous natural power which enables an animal to re-build its tissues as they are continually used up in the processes of living; for no man-made, self-sustaining mechanism has ever been perfected. The property of chemical composition is believed by science to be the basis of the second and the third; but this matter of chemical constitution must take its proper place in the series of structural characters, which we shall discuss further on as we develop the conception of organic mechanism.
Whatever definition we may employ for a machine or an engine, we cannot exclude the living organism from its scope. As a "device for transforming and utilizing energy" the living organism differs not at all from any "dead" machine, however complex or simple. The greatest lesson of physiological science is that the operations of the different parts of the living thing, as well as of the whole organism itself, are mechanical; that is, they are the same under similar circumstances. The living creature secures fresh supplies of matter and energy from the environment outside of itself; these provide the fuel and power for the performance of the various tasks demanded of an efficient living thing, and they are the sources upon which the organism draws when it rebuilds its wasted tissues and replenishes its energies. The vital tasks of all organisms must be considered in due course, but at first it is necessary to justify our analogies by analyzing the structural characteristics of animals and plants, just as we might study locomotives in a mechanical museum before we should see how they work upon the rails.
Among the familiar facts which science reveals in a new light are the peculiarly definite qualities of living things as regards size and form. There is no general agreement in these matters among the things of the inorganic world. Water is water, whether it is a drop or the Pacific Ocean; stone is stone, whether it is a pebble, a granite block, or a solid peak of the Rocky Mountains. It is true that there is a considerable range in size between the microscopic bacterium at one extreme and the elephant or whale at the other, but this is far less extensive than in the case of lifeless things like water and stone. In physical respects, water may be a fluid, or a gas in the form of steam, or a solid, as a crystal of snow or a block of ice. But the essential materials of living things agree throughout the entire range of plant and animal forms in having a jellylike consistency.
But by far the most striking and important characteristic of living things is their definite and restricted chemical composition. Out of the eighty and more chemical elements known to science, the essential substance of living creatures is formed by only six to twelve. These are the simple and obvious characteristics of living things which are denoted by the word "organic." Everyone has a general idea of what this expression signifies, but it is important to realize that it means, in exact scientific terms,—constituted in definite and peculiar ways.
The living thing, then, possesses a definite constitution, which is a mechanical characteristic, while furthermore it is related to its surroundings in a hard and fast way. Just as locomotives are different in structure so that they may operate successfully under different conditions, so the definite characteristics of living things are exactly what they should be in order that organisms may be adjusted or fitted into the places in nature which they occupy. This universal relation to the environment is called adaptation. It is only too obvious when our attention is directed to it, but it is something which may have escaped our notice because it is so natural and universal. The trunk of a tree bears the limbs and branches and leaves above the ground, while the roots run out into the surrounding soil from the foot of the trunk; they do not grow up into the air. An animal walks upon its legs, the wings of a bird are just where they should be in order that they may be useful as organs of flight. And these mechanical adjustments in the case of living creatures occur for the same reason as in mechanisms like the steamship, which has the propeller at its hinder end and not elsewhere, and which bears its masts erect instead of in any other way.
The next step in the analysis of organisms reveals the same wonderful though familiar characteristics. The living organism is composed of parts which are called organs, and these differ from one another in structural and functional respects. Each of them performs a special task which the others do not, and each differentiated organ does its part to make the whole creature an efficient mechanism. The leg of the frog is an organ of locomotion, the heart is a device for pumping blood, the stomach accomplishes digestion, while the brain and nerves keep the parts working in harmony and also provide for the proper relation of the whole creature to its environment. So rigidly are these organs specialized in structure and in function that they cannot replace one another, any more than the drive wheels of the locomotive could replace the smokestack, or the boiler be interchanged with either of these. All of the organs are thus fitted or adjusted to a particular place in the body where they may most efficiently perform their duties. Each organ therefore occupies a particular place in an organic environment, so to speak. Thus the principle of adaptation holds true for the organs which constitute an organism, as well as for organisms themselves in their relations to their surroundings.
The various organs of living things are grouped so as to form the several organic systems. There are eight of these, and each performs a group of related tasks which are necessary for complete life. The alimentary system concerns itself with three things: it gets food into the body, or ingests; it transforms the insoluble foods by the intricate chemical processes of digestion; and it absorbs or takes into itself the transformed food substances, which are then passed on to the other parts of the body. It is hardly necessary to point out that the ingestive structures for taking food and preparing it mechanically lie at and near the mouth, while the digesting parts, like the stomach, come next, because chemical transformation is the next thing to be done; while finally the absorbing portions of the tract, or the intestines, come last. The second group of organs, like gills and lungs, supplies the oxygen, which is as necessary for life as food itself; this respiratory system also provides for the passage from the body of certain of the waste gases, like carbonic acid gas and water vapor. The excretory system of kidneys and similar structures collects the ash-waste produced by the burning tissues, and discharges this from the whole mechanism, like the ash hoist of a steamship. The circulatory system, made up of smaller and larger vessels, with or without a heart, transports and propels the blood through the body, carrying the absorbed foods, the supplies of oxygen, and the waste substances of various kinds. All of these four systems are concerned with "commissary" problems, so to speak, which every individual must solve for and by itself.
Another group of systems is concerned with wider relations of the individual and its activities. For example, the motor system accomplishes the movements of the various organs within the body, and it also enables the organism to move about; thus it provides for motion and locomotion. Systems of support, comprising bones or shells, occur in many animals where the other organs are soft or weak. Perhaps the most interesting of the individual systems of relation is the nervous system. The strands of its nerve fibers and its groups of cells keep the various organs of the body properly coördinated, whereas in the second place, through the sensitive structures at the surface of the body, they receive the impressions from the outside world and so enable the organism to relate itself properly to its environment. The last organic system differs from the other seven in that the performance of its task is of far less importance to the individual than it is to the race as a whole. It is the reproductive system, with a function that must be always biologically supreme. We can very readily see why this must be so; it is because nature has no place for a species which permits the performance of any individual function to gain ascendency over the necessary task of perpetuating the kind. Nature does not tolerate race suicide.