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.

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