The Doctrine of Evolution | Page 7

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