438 
MOEPHOLOGY. 
Chap. XIII. 
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 ani¬ 
mal and vegetable kingdoms. 
Naturalists frequently speak of the skull as formed of 
metamorphosed vertebrae : the jaws of crabs as meta¬ 
morphosed legs; the stamens and pistils of flowers as 
metamorphosed leaves; but it would in these cases pro¬ 
bably 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—verte¬ 
brae 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 oc¬ 
curred, 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 nume¬ 
rous characters, which they would probably have retained 
through inheritance, if they had really been metamor¬ 
phosed 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 
