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 the physiology of man is tolerably well known, that of other animals is less understood the farther we get away from man and his allies.  For this reason most of our knowledge of the living body as a machine must be derived from the study of man.  This is, however, fortunate rather than otherwise.  In the first place, it enables us to proceed from the known to the unknown; and in the second place, more interest attaches to the problem as connected with human physiology than along any other line.  In our discussion, therefore, we shall refer chiefly to the physiology of man.  If we find that the functions of human life are amenable to a mechanical explanation we cannot hesitate to believe that this will be equally true of the lower orders of nature.  For similar reasons little reference will be made to the mechanism of plant life.  The structure of the plant is simpler and its activities are much more easily referable to mechanical principles than are those of animals.  For these reasons it will only be necessary for us to turn our attention to the life activities of the higher animals.