innumerable instances: thus Linn us, misled by external appearances,
actually classed an homopterous insect as a moth. We see something of
the same kind even in our domestic varieties, as in the thickened stems
of the common and swedish turnip. The resemblance of the greyhound and
racehorse is hardly more fanciful than the analogies which have been
drawn by some authors between very distinct animals. On my view of
characters being of real importance for classification, only in so far
as they reveal descent, we can clearly understand why analogical or
adaptive character, although of the utmost importance to the welfare of
the being, are almost valueless to the systematist. For animals,
belonging to two most distinct lines of descent, may readily become
adapted to similar conditions, and thus assume a close external
resemblance; but such resemblances will not reveal will rather tend to
conceal their blood-relationship to their proper lines of descent. We
can also understand the apparent paradox, that the very same characters
are analogical when one class or order is compared with another, but
give true affinities when the members of
the same class or order are compared one with another: thus the shape
of the body and fin-like limbs are only analogical when whales are
compared with fishes, being adaptations in both classes for swimming
through the water; but the shape of the body and fin-like limbs serve
as characters exhibiting true affinity between the several members of
the whale family; for these cetaceans agree in so many characters,
great and small, that we cannot doubt that they have inherited their
general shape of body and structure of limbs from a common ancestor. So
it is with fishes.
As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances, to
inhabit for instance the three elements of land, air, and water, we can
perhaps understand how it is that a numerical parallelism has sometimes
been observed between the sub-groups in distinct classes. A naturalist,
struck by a parallelism of this nature in any one class, by arbitrarily
raising or sinking the value of the groups in other classes (and all
our experience shows that this valuation has hitherto been arbitrary),
could easily extend the parallelism over a wide range; and thus the
septenary, quinary, quaternary, and ternary classifications have
probably arisen.
As the modified descendants of dominant species, belonging to the
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