The Descent of Man

similarity or dissimilarity in a few points. If two
languages were found to resemble each other in
a multitude of words and points of construction,
they would be universally recognised as
having sprung from a common source, notwithstanding
that they differed greatly in some
few words or points of construction. But with
organic beings the points of resemblance must
not consist of adaptations to similar habits of
life: two animals may, for instance, have had
their whole frames modified for living in the
water, and yet they will not be brought any
nearer to each other in the natural system. Hence
we can see how it is that resemblances in
several unimportant structures, in useless and
rudimentary organs, or not now functionally
active, or in an embryological condition, are by
far the most serviceable for classification; for
they can hardly be due to adaptations within a
late period; and thus they reveal the old lines of
descent or of true affinity.