ual 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
embryo 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 various fancy animals,
cannot positively tell, until some time after the animal has been born,
what its merits or form will ultimately turn out. We see this plainly in
our own children; we cannot always tell whether the child will be tall
or short, or what its precise features will be. The question is not, at
what period of life any variation has been caused, but at what period
it is fully displayed. The cause may have acted, and I believe generally
has acted, even before the embryo is formed; and the variation may be
due to the male and female sexual elements having been affected by
the conditions to which either parent, or their ancestors, have been
exposed. Nevertheless an effect thus caused at a very early period, even
before the formation of the embryo, may appear late in life; as when
an hereditary disease, which appears in old age alone, has been
communicated to the offspring from the reproductive element of one
parent. Or again, as when the horns of cross-bred cattle have been
affected by the shape of the horns of either parent. For the welfare of
a very young animal, as long as it remains in its mother's womb, or in
the egg, or as long as it is nourished and protected by its parent,
it must be quite unimportant whether most of its characters are fully
acquired a little earlier or later in life. It would not signify, for
instance, to a bird which obtained its food best by having a long beak,
whether or not it assumed a beak of this particular length, as long as
it was fed b