d in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, all under the so-called natural system? I believe it
has been unconsciously used; and thus only can I understand the several
rules and guides which have been followed by our best systematists.
As we have no written pedigrees, we are forced to trace community
of descent by resemblances of any kind. Therefore, we choose those
characters which are the least likely to have been modified, in relation
to the conditions of life to which each species has been recently
exposed. Rudimentary structures on this view are as good as, or even
sometimes better than other parts of the organisation. We care not how
trifling a character may be--let it be the mere inflection of the angle
of the jaw, the manner in which an insect's wing is folded, whether the
skin be covered by hair or feathers--if it prevail throughout many and
different species, especially those having very different habits of
life, it assumes high value; for we can account for its presence in so
many forms with such different habits, only by inheritance from a
common parent. We may err in this respect in regard to single points of
structure, but when several characters, let them be ever so trifling,
concur throughout a large group of beings having different habits, we
may feel almost sure, on the theory of descent, that these characters
have been inherited from a common ancestor; and we know that such
aggregated characters have especial value in classification.

We can understand why a species or a group of species may depart from
its allies, in several of its most important characteristics, and yet be
safely classed with them. This may be safely done, and is often done,
as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet, if these extreme forms
are connected together by a chain of intermediate groups, we may at
once infer their community of descent, and we put them all into the same
class. As we find organs of high physiological importance--those
which serve to preserve life under the most diverse conditions of
existence--are generally the most constant, we attach especial value to
them; but if these sa