actions follow; and if we can find an explanation of these, we have explained the living machine.  If we grant that certain parts of the body can assimilate food and multiply, having the power of contraction when irritated, we can readily explain the other functions of the living machine by the application of these properties to the complicated machinery of the body.  But these properties are fundamental, and unless we can grasp them we have failed to reach the centre of the problem.

As we pass from the more to the less complicated animals we find a gradual simplification of the machinery until the machinery apparently disappears.  With this simplification of the machinery we find the animals provided with less varied powers and with less delicate adaptations to conditions.  But withal we find the fundamental powers of the living organisms the same.  For the performance of these fundamental activities there is apparently needed no machinery.  The simple types of living bodies are simple in number of parts, but they possess essentially the same powers of assimilation and growth that characterize the higher forms.  It is evident that in our attempt to trace the vital properties to their source we may proceed in two ways.  We may either direct our attention to the simplest organisms where all secondary machinery is wanting, or to the smallest parts into which the tissues of higher organisms can be resolved and yet retain their life properties.  In either way we may hope to find living phenomena in its simplest form independent of secondary machinery.

But the fact is, when we turn our attention in these two directions, we find the result is the same.  If we look for the lowest organisms we find them among forms that are made of a single cell, and if we analyze the tissues of higher animals we find the ultimate parts to be cells.  Thus, in either direction, the study of the cell is forced upon us.

Before beginning the study of the cell it will be well for us to try to get a clear notion of the exact nature of the problems we are trying to solve.  We wish to explain the activities of life phenomena in such a way as to make them intelligible through the application of natural forces.  That these processes are fundamentally chemical ones is evident enough.  A chemical oxidation of food lies at the basis of all vital activity, and it is thus through the action of chemical forces that the vital powers are furnished with their energy.  But the real problem is what it is in the living machine that controls these chemical processes.  Fat and starch may be oxidized in a chemist’s test tubes, and will there liberate energy; but they do not, under these conditions, manifest vital phenomena.  Proteid may be brought in contact with oxygen without any oxidation occurring, and even if it is oxidized no motion or assimilation or reproduction occurs under ordinary conditions.  These phenomena occur only when the oxidation takes place in the living machine.  Our problem is then to determine, if possible, what it is in the living machine that regulates the oxidations and other changes in such a way as to produce from them vital activities.  Why is it that the oxidation of starch in the living machine gives rise to motion, growth, and reproduction, while if the oxidation occurs in the chemist’s laboratory, or even in a bit of dead protoplasm, it simply gives rise to heat?