_Classification_; Comparative Anatomy, or _Morphology_; Comparative
Development, or _Embryology; _Palæontology_, 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 branches of the animal tree. The common cat and its
relatives are even earlier to be regarded as anatomical subjects, and
their thorough analysis belongs to comparative anatomy,--a name which
explains itself. The purpose of this department of natural history is to
explore the entire range of animal forms and animal structures, and to
determine the degree of resemblance and difference exhibited by the
general characters of entire organisms and by the special qualities of
their several systems of organs. It provides the data from which
classification selects those which indicate mutual affinities with