account for living phenomena. If the supernatural must be
brought in here and there to account for living phenomena, then
evolution ceases to have much meaning. It is undoubtedly a fact that the
rapidly developing ideas along the above mentioned lines of dynamical
biology have, been potent factors in bringing about the adoption of
evolution. Certain it is that, had it been found that no correlation
could be traced between vital and non-vital forces, the doctrine of
evolution could not have stood, and even now the special significance
which we shall in the end give to evolution will depend upon how we
succeed in answering the questions above outlined. The fact is that this
problem of the mechanical explanation of vital phenomena forms the
capstone of the arch, the sides of which are built of the doctrines of
the conservation of energy and the theory of evolution. To the
presentation of these problems the following pages will be devoted. The
fact that both the doctrine of the conservation of energy and that of
evolution are practically everywhere accepted indicates that the
mechanical nature of vital forces is regarded as proved. But there are
still many questions which are not so easily answered. It will be our
purpose in the following discussion to ascertain just what are these
problems in dynamical biology and how far they have been answered. Our
object will be then in brief to discover to what extent the conception
of the living organism as a machine is borne out by the facts which have
been collected in the last quarter century, and to learn where, if
anywhere, limits have been found to our possibility of applying the
forces of chemistry and physics to an explanation of life. In other
words, we shall try to see how far we have been able to understand
living phenomena in terms of natural force.

==Outline of the Subject==.--The subject, as thus presented, resolves
itself at once into two parts. That the living organism is a machine is
everywhere recognized, although some may still doubt as to the
completeness of the comparison. In the attempt to explain the phenomena
of life we have two entirely different problems. The first is manifestly
to account for the existence of this machine, for such a completed piece
of mechanism as a man or a tree cannot be explained as a result of
simple accident, as the existence of a rough piece of rock might be
explained. Its intricacy of parts and their purposeful interrelation
demands