The Doctrine of Evolution | Page 8

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 cooerdinated, 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.
All organisms must perform these eight functions in one way or
another. The bacterium, the simplest animal, the lowest plant, the
higher plants and animals,--all of these have a biological problem to
solve which comprises eight terms or parts, no more and no less. This
is surely an astonishing agreement when we consider the varied forms
of living creatures. And perhaps when we see that this is true we may
understand why adaptation is a characteristic of all organisms, for they
all have similar biological problems to solve, and their lives must
necessarily be adjusted in somewhat similar ways to their surroundings.
Carrying the analysis of organic structure one step further, it is found
that the various organisms are themselves complex, being composed of
tissues. A frog's leg as an organ of locomotion is composed of the
protecting skin on the outside, the muscles, blood vessels, and nerves
below, and in the center the bony supports of the whole limb. Like the
organs, these tissues are differentiated, structurally and functionally,
and they also are so placed and related as to exhibit the kind of
mechanical adjustment which we call adaptation. The tissues, then, in
their relations to the organs are like the organs in their relations to the
whole creature, i.e. adapted to specific situations where they may most
satisfactorily perform their tasks.
Finally, in the last analysis, all organisms and organs and tissues can be
resolved into elements which are called cells. They are not little hollow

cases, it is true, although for historical reasons we employ a word that
implies such a condition. They are unitary masses of living matter with
a peculiar central body or nucleus, and every tissue of every living
thing is composed of them.
The cells of bone differ from those of cartilage mainly in the different
consistency of the substances secreted by the cells to lie between them;
skin cells are soft-walled masses lying close together; even blood is a
tissue, although it is fluid and its cells are the corpuscles which float
freely