CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
2. 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. 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.
Among insects there are innumerable instances; thus Linnaeus, misled by
external appearances, actually classed an homopterous insect as a moth. We
see something of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds of the Chinese
and common pig, which are descended from distinct species; and in the
similarly thickened stems of the common and specifically distinct Swedish
turnip. The resemblance between the greyhound and race-horse is hardly
more fanciful than the analogies which have been drawn by some authors
between widely different animals.
On the 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 characters, 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 have become adapted to similar
conditions, and thus have assumed a close external resemblance; but such
resemblances will not reveal--will rather tend to conceal their
blood-relationship. We can thus also understand the apparent paradox, that
the very same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group are
compared together: 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 between the the several
members of the whale family, the shape of the body and the fin-like limbs
offer characters exhibiting true affinity; for as these parts are so nearly
similar throughout the whole family, we cannot doubt that they have been
inherited from a common ancestor. So it is with fishes.