ly 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 larvae of moths, flies, beetles, etc.,
resemble each other much more closely than do the mature insects; but
in the case of larvae, 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 spotted
in lines; and stripes can be plainly distinguished in the whelp of
the lion. We occasionally though rarely see something of this kind in
plants: thus the embryonic leaves of the ulex or furze, and the first
leaves of the phyllodineous acaceas, are pinnate or divided like the
ordinary leaves of the leguminosae.

The points of structure, in which the embryos of widely different
animals of the same class resemble each other, often have no direct
relation to their conditions of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
course of the arteries near the branchial slits are related to similar
conditions,--in the young mammal which is nourished in the womb of its
mother, in the egg of the bird which is hatched in a nest, and in the
spawn of a frog under water. We have no more reason to believe in suc