and putting together what we know already about the commonest animals, or
noting what can be observed in a visit to a zoölogical 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; palæontology 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


In order to become acquainted with the way the structures of animals
provide evidences of evolution, it is by no means necessary to review the
entire range of their forms, because research has discovered that the
principles of relationship are universal among animals, and that any group
of examples will demonstrate what is taught by comparative anatomy as a
whole. The commonest creatures may serve us best in order that we may come
to view evolution as a process that involves each and every living thing
that we know, and not as something which belongs only to the remote and
unknown past.

Let us begin with the common cat and the group of carnivora or
flesh-eating animals to which it belongs. As we pass along the streets of
the city, we will see many cats which differ in some details, though they