actorily answer these
questions. In the vertebrata, we see a series of internal vertebrae
bearing certain processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external appendages;
and in flowering plants, we see a series of successive spiral whorls of
leaves. An indefinite repetition of the same part or organ is the common
characteristic (as Owen has observed) of all low or little-modified
forms; therefore we may readily believe that the unknown progenitor of
the vertebrata possessed many vertebrae; the unknown progenitor of
the articulata, many segments; and the unknown progenitor of flowering
plants, many spiral whorls of leaves. We have formerly seen that
parts many times repeated are eminently liable to vary in number and
structure; consequently it is quite probable that natural selection,
during a long-continued course of modification, should have seized on
a certain number of the primordially similar elements, many times
repeated, and have adapted them to the most diverse purposes. And as
the whole amount of modification will have been effected by slight
successive steps, we need not wonder at discovering in such parts or
organs, a certain degree of fundamental resemblance, retained by the
strong principle of inheritance.

In the great class of molluscs, though we can homologise the parts of
one species with those of another and distinct species, we can indicate
but few serial homologies; that is, we are seldom enabled to say that
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
vertebrae: 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 vertebrae, 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--vertebrae in the one case and legs in the
other--have actual