the modification has been greater
in degree, and has taken a longer time to complete? I believe it has
thus been unconsciously used; and only thus can I understand the several
rules and guides which have been followed by our best systematists. We
have no written pedigrees; we have to make out community of descent by
resemblances of any kind. Therefore we choose those characters which,
as far as we can judge, 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 its
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, occur together 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 correlated or aggregated characters have
especial value in classification.

We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, 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 same organs, in another group or section of a
group, are found to differ much, we at once value them less in
our classification. We shall hereafter,