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.