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 proto