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