The Doctrine of Evolution | Page 6

was
in a certain true sense the ancestor of the "Lusitania," with its deep keel
and screw propellers, of the side-wheel steamship for river and harbor
traffic like the "Priscilla," of the stern-wheel flat-bottom boats of the
Mississippi, and of the battleship, and the tug boat. As in the first
instance, we know that each modern type has developed through the
accumulation of changes, which changes are likewise adjustments to
different conditions. The diversity of modern types of steamships may
be attributed therefore to adaptation.
The several kinds are no more interchangeable than are the different
forms of locomotives that we have mentioned. The flat-bottom boat of
the Mississippi would not venture to cross the Atlantic Ocean in winter,
nor would the "Lusitania" attempt to plow a way up the shallow
mud-banked Mississippi. These products of mechanical development
are not efficient unless they run under the circumstances which have
controlled their construction, unless they are fitted or adapted to the
conditions under which they must operate.
Evolution, then, means descent with adaptive modification. We must
examine the various kinds of living creatures everywhere to see if they,
like the machines, exhibit in their make-up similar elements which
indicate their common ancestry in an earlier age, and if we can interpret
their differences as the results of modifications which fit them to
occupy different place in nature.
Two objections to the employment of these analogies will present
themselves at once. The definition may be all very well as far as the
machines are concerned, but, it may be asked, should a living thing like
a horse or a dog be compared with the steamship or the locomotive?
Can we look upon the living thing as a mechanism in the proper sense
of the word? A second objection will be that human invention and
ingenuity have controlled the evolution of the steamship and engine by
the perfection of newer and more efficient parts. It is certainly true that
organic evolution cannot be controlled in the same way by men, and

that science has not yet found out what all the factors are. And yet we
are going to learn in a later discussion that nature's method of
transforming organisms in the course of evolution is strikingly similar
to the human process of trial and error which has brought the diverse
modern mechanisms to their present conditions of efficiency. This
matter, however, must remain for the time just as it stands. The first
objection, namely, that an organism ought not to be viewed as a
machine, is one that we must meet immediately, because it is necessary
at the very outset to gain a clear idea of the essentially mechanical
nature of living things and of their relations to the conditions under
which they live. It is only when we have such a clear understanding
that we can profitably pursue the further inquiries into the evidence of
evolution. Our first real task, therefore, is an inquiry into certain
fundamental questions about life and living things, upon which we
shall build as we proceed.
* * * * *
All living things possess three general properties which seem to be
unique; these are a peculiar chemical constitution, the power of
repairing themselves as their tissues wear out, and the ability to grow
and multiply. The third property is so familiar that we fail to see how
sharply it distinguishes the creatures of the organic world. To realize
this we have only to imagine how strange it would seem if locomotives
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