adjustment to the conditions of life which is the universal characteristic
of plants and animals. It is the history of these creatures and the origin
of their adapted conditions that we are called upon to study. We must
scrutinize the nature of to-day to see if we can find evidence that
evolution is true, and if we can discern the forces which, acting upon the
living mechanism as man has dealt with machines, might bring the various
species of the present day to their modern forms.

* * * * *

We have now learned that evolution means a common ancestry of living forms
that have come to differ in the course of time; our common reason has
shown us also that organisms are in a true sense complicated chemical
mechanisms adapted to meet the conditions under which they must operate.
We come now to the evidences offered by the organic world that evolution
is true and that natural forces control its workings. Clearly the
examination of the matter of _fact_ is independent of the question of
_method_. For just as the chemist may experiment with various substances
to see if they will dissolve in water and not in alcohol before it is
necessary or desirable for him to take up the further studies of the laws
of solution, so reasonable grounds must be found for regarding evolution
as true before passing to its method of accomplishment. And in the
following discussions, the animals will be used almost exclusively, not
because the study of plants fails to discover the same relations and
principles, but because the better known animal series is more varied and
extensive, and above all for the reason that the human organism arrays
itself as the highest term of the animal series.

In the complete scheme adopted by most naturalists, five categories
include the evidences bearing upon the fact of evolution. These are