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 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 zooelogy and how the evolutionary teachings of each body of fact support and supplement those of the other.

Palaeontology 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 zooelogy 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 and putting together what we know already about the commonest animals, or noting what can be observed in a visit to a zooelogical garden and aquarium.  On account of the present limitations of time, the subject of classification will be combined with comparative anatomy; embryology will be taken up together with these subjects; palaeontology will be the main subject of the next discussion, which will include also a brief statement of the meaning of distribution.  Then we will be prepared to study nature to see how evolution works.

II

THE STRUCTURE AND DEVELOPMENT OF ANIMALS AS EVIDENCE OF EVOLUTION