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[2]."

[2] _Origin of Species_, p. 372.

It is true that even a single character, if found common to a large
number of forms, while uniformly absent from others, is also regarded by
naturalists as of importance for purposes of classification, although
they recognise it as of a value subordinate to that of aggregates of
characters. But this also is what we should expect on the theory of
descent. If even any one structure be found to run through a number of
animals presenting different habits of life, the readiest explanation of
the fact is to be found in the theory of descent; but this does not
hinder that if several such characters always occur together, the
inference of genetic relationship is correspondingly confirmed. And the
fact that before this inference was ever drawn, naturalists recognised
the value of single characters in proportion to their constancy, and the
yet higher value of aggregates of characters in proportion to their
number--this fact shows that in their work of classification naturalists
empirically observed the effects of a cause which we have now
discovered, to wit, hereditary transmission of characters through
ever-widening groups of changing species.

There is another argument which appears to tell strongly in favour of
the theory of descent. We have just seen that non-adaptive structures,
not being required to change in response to change of habits or
conditions of life, are allowed to persist unchanged through many
generations, and thus furnish exceptionally good guides in the science
of classification--or, according to our theory, in the work of tracing
lines of pedigree. But now, the converse of this statement holds equally
true. For it often happens that adaptive structures are required to
change in different lines of descent in analogous ways, in order to meet
analogous needs; and, when such is the case, the structures concerned
have to assume more or less close resemblances to one another, even
though they have severally descended from quite different ancestors. The
paddles of a whale, for instance, most strikingly resemble the fins of a
fish as to their outward form and movements; yet, on the theory of
descent