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 presently see why embryological characters
are of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera, because
all the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
ANALOGICAL RESEMBLANCES.
We can understand, on the above views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, 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 dugongs and whales, and between these two
orders of mammals and fishes, are analogical.
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