one part or organ is homologous with another in the same individual.
And we can understand this fact; for in molluscs, even in the lowest
members of the class, we do not find nearly so much indefinite
repetition of any one part, as we find in the other great classes of
the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebr : the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these cases
probably be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebr , both jaws and legs, etc., as having been
metamorphosed, not one from the other, but from some common element.
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 vertebr in the one case and legs in the
other have actually been modified into skulls or jaws. Yet so strong is
the appearance of a modification of this nature having occurred, that
naturalists can hardly avoid employing language having this plain
signification. On my view
these terms may be used literally; and the wonderful fact of the jaws,
for instance, of a crab retaining numerous characters, which they would
probably have retained through inheritance, if they had really been
metamorphosed during a long course of descent from true legs, or from
some simple appendage, is explained.
_Embryology_. It has already been casually remarked that certain organs
in the individual, which when mature become widely different and serve
for different purposes, are in the embryo exactly alike. The embryos,
also, of distinct animals within the same class are often strikingly
similar: a better proof of this cannot be given, than a circumstance
mentioned by Agassiz, namely, that having forgotten to ticket the
embryo of some vertebrate animal, he cannot now tell whether it be that
of a mammal, bird, or reptile. The vermiform larv of moths, flies,
beetles, etc., resemble each other much more closely than do the mature
insects; but in the case of larv , the embryos are active, and have
been adapted for special lines of life. A trace of the law of embryonic
resemblance, sometimes lasts till a rather late age: thus birds of the
same genus, and of closely allied genera, often resemble each other in
their first and second plumage; as we see in the spotted feathers in
the thrush group. In the cat tribe, most of the species are striped or
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