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
_Classification_; Comparative Anatomy, or _Morphology_; Comparative
Development, or _Embryology; _Palaeontology_, which comprises the facts
provided by fossil relics of animals and plants of earlier geological
ages; and _Geographical Distribution_. Each of these divisions includes a
descriptive and analytical series of facts, whose characteristics are
"explained" or summarized in the form of the general principles of the
respective divisions. Such principles, taken singly and collectively,
constitute the evidences of evolution.

The particular nature of any one of these categories, evolved in the
development of science practically in the order stated, depends upon the
special quality of an animal which it selects for comparison and
organization in connection with other similar facts, and also in its own
mode of viewing its facts. One and the same organism may present materials
for two, three, or even all five of these divisions, for they are by no
means mutually exclusive. For example, a common cat possesses certain
definite characteristics which give it a particular place when animals
more or less like it are grouped or classified according to their degrees
of resemblance and difference, in small _genera_ of very similar forms, in
larger _tribes_ or _orders_ of similar genera, and in more and more
inclusive groups of these lesser divisions, such as the _classes_ and
_phyla_, or main branche