greatest precision and surety. But its materials are _all_ the facts of
animal structure, and because each and every known organism can be and
must be studied, the investigator engaged in formulating the evidence of
evolution has at his disposal all the data referring to the entire realm
of animals. The data of embryology are likewise coextensive with the
territory of the animal world, for we do not know of any form which does
not change in the course of its life history. An adult cat is the product
of a kitten which is itself the result of a long series of changes from
earlier and simpler conditions. In so far as it deals with structures in
the making, embryology is a study of anatomy, but as it is concerned
primarily with all of the plastic remodeling which animals undergo during
the production of their final forms, it is an independent study.
Nevertheless we shall learn how intimate are the relations of these two
divisions of zoölogy and how the evolutionary teachings of each body of
fact support and supplement those of the other.

Palæontology searches everywhere among the deposits of earlier ages for
links to be fitted into their proper sequence of time, from which it
constructs the chain of diverse types leading down to the species of the
present. A cat of to-day is therefore viewed in an entirely different
connection, as the last term in a consecutive series of species. Forming
alliances with geology, and even with physics and chemistry, this
department of zoölogy endeavors to reconstruct the past from what it
learns to-day about organisms and the conditions under which they live.
Finally the observations that cats of various kinds do not occur
everywhere in the world, but only in certain more or less restricted
localities, belong to the subject of geographical distribution, and
illustrate its nature.

Our task is to learn the teachings of these several divisions by recalling