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, I think, clearly see why
embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play
in classing large and widely-distributed genera, because all the
species of the same genus, inhabiting any distinct and isolated region,
have in all probability descended from the same parents.
We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and he has been
ably followed by Macleay and others. The resemblance, in the shape of
the body and in the fin-like anterior limbs, between the dugong, which
is a pachydermatous animal, and the whale, and between both these
mammals and fishes, is analogical. Amongst insects there are
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