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 fo