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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,
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.
↑
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| We can understand why a species or a group of species may
→from its allies, in
several of its most important characteristics,
→OMIT
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,
if these extreme forms are connected together by a chain of intermediate
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,
in all probability descended from the same parents. |
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Analogical
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| We can understand, on
views, the very important distinction between real affinities and analogical or adaptive resemblances. Lamarck first called attention to this
and he has been ably followed by Macleay and others. The
in the shape of the body and in the fin-like anterior
between
→and
and between
these
→two orders of mammals
and fishes,
analogical. So is the resemblance between a mouse and a shrew-mouse (Sorex), which belong to different orders; and the still closer resemblance, insisted on by Mr. Mivart, between the mouse and a small marsupial animal (Antechinus) of Australia. These latter resemblances may be accounted for, as it seems to me, by adaptation for similarly active movements through thickets and herbage, together with concealment from enemies. Amongst insects there are innumerable
thus Linnæus,
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