uch accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth. But
there is no obvious reason why, for instance, the wing of a bat, or the fin
of a porpoise, should not have been sketched out with all the parts in
proper proportion, as soon as any structure became visible in the embryo.
And in some whole groups of animals and in certain members of other groups,
the embryo does not at any period differ widely from the {442} adult: thus
Owen has remarked in regard to cuttle-fish, "there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the embryo
are completed;" and again in spiders, "there is nothing worthy to be called
a metamorphosis." The larvae of insects, whether adapted to the most diverse
and active habits, or quite inactive, being fed by their parents or placed
in the midst of proper nutriment, yet nearly all pass through a similar
worm-like stage of development; but in some few cases, as in that of Aphis,
if we look to the admirable drawings by Professor Huxley of the development
of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology,--namely the
very general, but not universal difference in structure between the embryo
and the adult;--of parts in the same individual embryo, which ultimately
become very unlike and serve for diverse purposes, being at this early
period of growth alike;--of embryos of different species within the same
class, generally, but not universally, resembling each other;--of the
structure of the embryo not being closely related to its conditions of
existence, except when the embryo becomes at any period of life active and
has to provide for itself;--of the embryo apparently having sometimes a
higher organisation than the mature animal, into which it is developed? I
believe that all these facts can be explained, as follows, on the view of
descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the
embryos at a very early period, that slight variations necessarily appear
at an equally early period. But we have little evidence on this
head--indeed the evidence rather points the other way; for it is notorious
that breeders of cattle, horses, and vario