animals of a great body of similar characters beneath their minor
differences is an indication of their common ancestry_.

Thus at the very outset our simple illustration establishes the most
fundamental principle of comparative anatomy. Let us see how it works
further. The Manx cat possesses an abbreviated tail, although in other
respects it is practically the same as the familiar long-tailed form; the
Angora and the Persian differ in having long hair. All of these animals
are so much alike in so many respects, and so closely resemble the wild
cats, that it is not unreasonable to regard them all as the descendants of
the same original wild ancestors, and as the varying products of lines
which branched out from the same stock in different directions and at
different times. It is, in a word, their "cat-_ness_" which demonstrates
their relationships. But common sense need not stop here. Guided by the
facts of anatomical similarity, it convinces us that the dun-colored lion
and puma, the striped tiger and the spotted leopard are simply cats of a
larger growth whose remoter ancestry is one with that of the previously
cited forms. Not until we explore and compare their several systems do we
see how thoroughgoing is their uniformity in structural plan. And because
reason justifies the view regarding the origin of domestic cats from wild
ancestors, the evolution of all the various members of the cat tribe must
be acknowledged. These animals exhibit a fundamental likeness, which, to
employ a musical analogy, is the "theme" of "cat-_ness_," and they are so
many variations of this theme.

The members of another tribe of the familiar carnivora display in their
own way the same kind of evidences of relationship. The varieties of
domesticated dogs differ far more widely among themselves than do common
cats, yet their community of ancestry is demonstrated not only by
structural resemblances, but also by the striking fact that forms as