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 other and distinct species, we can indicate but few
serial homologies; that is, we are seldom enabled to say that one {438}
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, &c.,--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 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 {439} circumstance mentioned
by Agassiz, namely, that having forgotten to ticket the embryo of some