83} _Origin_, Ed. i. p. 441, vi. p. 607.

_Importance of embryology in classification._

We are now prepared to perceive why the study of embryonic forms is of
such acknowledged importance in classification{484}. For we have seen
that a variation, supervening at any time, may aid in the modification
and adaptation of the full-grown being; but for the modification of the
embryo, only the variations which supervene at a very early period can
be seized on and perpetuated by selection: hence there will be less
power and less tendency (for the structure of the embryo is mostly
unimportant) to modify the young: and hence we might expect to find at
this period similarities preserved between different groups of species
which had been obscured and quite lost in the full-grown animals. I
conceive on the view of separate creations it would be impossible to
offer any explanation of the affinities of organic beings thus being
plainest and of the greatest importance at that period of life when
their structure is not adapted to the final part they have to play in
the economy of nature.

{484} _Origin_, Ed. i. p. 449, vi. p. 617.

_Order in time in which the great classes have first appeared._

It follows strictly from the above reasoning only that the embryos of
(for instance) existing vertebrata resemble more closely the embryo of
the parent-stock of this great class than do full-grown existing
vertebrata resemble their full-grown parent-stock. But it may be argued
with much probability that in the earliest and simplest condition of
things the parent and embryo must have resembled each other, and that
the passage of any animal through embryonic states in its growth is
entirely due to subsequent variations affecting _only_ the more mature
periods of life. If so, the embryos of the existing vertebrata will
shadow forth the full-grown structure of some of those forms of this
great class which existed at the earlier periods of the earth's
history{485}: and accordingly, animals with a fish-like structure ought
to have preceded birds and mammals; and of fish, that higher organized
division with the vertebrae extending into one division of the tail ought
to have preceded the equal-tailed, because the embryos of the latter
have an unequal tail; and of Crustacea, entomostraca ought to have
preceded the ordinary crabs and barnacles--polypes ought to have
preceded jelly-fish, and infusorial animalcules to have existed before
both.