Mr. Lankester also adduces the close
resemblance of the parts on the right and left sides of the body, and in
the successive segments of the same individual animal; and here we have
parts commonly called homologous which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures are the
same with those which I have classed, though in a very imperfect manner, as
analogous modifications or resemblances. Their formation may be attributed
in part to distinct organisms, or to distinct parts of the same organism,
having varied in an analogous manner; and in part to similar modifications,
having been preserved for the same general purpose or function, of which
many instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistils
in flowers as metamorphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind--vertebrae in the one case and legs in the other--have
actually been converted into skulls or jaws.
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