any new system of organization. Hence it is that the history of the
living machine has shown a tendency toward development along a few
well-marked lines, and although this complication becomes greater, we
still see the same fundamental scheme of organization running through
the whole. As the ages have progressed the machines have become more
perfect in the adjustment of their parts, i.e., they have become more
perfect machines, but the history has been simply that of perfecting
the early machines rather than the production of new types.

==Evidence for this History.==--As just outlined, we see that the living
machines have been gradually brought into their present condition by a
process which has been called organic evolution. But we must pause for a
moment to ask what is our evidence that such has been the history of the
living machine. The whole possibility of understanding living nature
depends upon our accepting this history and finding an explanation of
it. At the outset we have the question of fact, and we must notice the
grounds upon which we stand in assuming this history to be as outlined.

This problem is the one which has occupied such a prominent place in the
scientific world during the last forty years, and which has contributed
so largely toward making modern biology such a different subject from
the earlier studies of natural history. It is simply the evidence for
organic evolution, or the theory of descent. The subject has for forty
years been thoroughly sifted and tested by every conceivable sort of
test. As a result of the interest in the question there has been
disclosed an immense mass of evidence, relevant and irrelevant. As the
evidence has accumulated it has become more and more evident that the
evolution theory must be recognized as the only one which is in accord
with the facts, and the outcome has been a practical unanimity among
thinkers that the theory of descent must be the foundation of our
further study. The evidence which has forced this conclusion upon
scientists we must stop for a moment to consider, since it bears very
directly upon the subject we are studying.

==Historical.==--The first source of evidence is naturally a historical
one. This long history of the construction of the living machine has
left its record in the rocks which form the earth's surface. During this
long period the rocks of the earth's crust have been deposited, and in
these rocks have been left samples of many