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.
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