explanation, and therefore the fundamental problem is to explain
how this machine came into existence. The second problem is simpler, for
it is simply to explain the running of the machine after it is made. If
the organism is really a machine, we ought to be able to find some way
of explaining its actions as we can those of a steam engine.

Of these two problems the first is the more fundamental, for if we fail
to find an explanation for the existence of the machine, our explanation
of its method of action is only partly satisfactory. But the second
question is the simpler, and must be answered first. We cannot hope to
explain the more puzzling matter of the origin of the machine unless we
can first understand how it acts. In our treatment of the subject,
therefore, we shall divide it into two parts:

I. _The Running of the Living Machine_.

II. _The Origin of the Living Machine_.

PART I.

_THE RUNNING OF THE LIVING MACHINE._

* * * * *

CHAPTER I.

IS THE BODY A MACHINE?

The problem before us in this section is to find out to what extent
animals and plants are machines. We wish to determine whether the laws
and forces which regulate their activities are the same as the laws and
forces with which we experiment in the chemical and physical laboratory,
and whether the principles of mechanics and the doctrine of the
conservation of energy apply equally well in the living machine and the
steam engine.

It might be inferred that the proper method of study would be to confine
our attention largely to the simplest forms of life, since the problems
would be here less complicated, and therefore of easier solution. This,
however, has not been nor can it be the method of study. Our knowledge
of the processes of life have been derived largely from the most rather
than the least complex forms. We have a better knowledge of the
physiology of man and his allies than any other animals. The reason for
this is plain enough. In the first place, there is a value in the
knowledge of the life activities of man entirely apart from any
theoretical aspects, and hence human physiology has demanded attention
for its own sake. The practical utility of human physiology has
stimulated its study for centuries; and in the last fifty years of
scientific progress it has been human physiology and that of allied
animals that has attracted the chief attention of physiologists. The
result is that while