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 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.