strate their
variation. The fact of difference, then, is surely not inconsistent with
even the closest ties of blood, and we do not need to go beyond the scope
of daily observation to find that this is true in nature wherever we look.

Should we extend our observations so as to include the cats of Boston and
Philadelphia and San Francisco, the animals would probably vary over a
wider range, but they would be so similar to New York cats in their
make-up that we would have no difficulty in regarding them and all the
others of the United States as the descendants of a single pairs of
ancestors, perhaps brought over in the "Mayflower." But why does this view
seem justified? Because experience has taught us that the living things
which resemble each other most closely are those which are most intimately
bound by ties of blood and common heritage. It is "natural" for relatives
to resemble one another more than persons not related, and for brothers and
sisters to be more alike than cousins. Science does not refer to something
outside everyday observation when it states that _the possession by two
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
anc