embryonic stages, we may feel assured that they are all
descended from one parent-form, and are therefore closely related.
Thus, community in embryonic structure reveals community of descent; but
dissimilarity in embryonic development does not prove discommunity of
descent, for in one of two groups the developmental stages may have been
suppressed, or may have been so greatly modified through adaptation to
new habits of life as to be no longer recognisable. Even in groups, in
which the adults have been modified to an extreme degree, community of
origin is often revealed by the structure of the larvae; we have seen,
for instance, that cirripedes, though externally so like shell-fish,
are at once known by their larvae to belong to the great class of
crustaceans. As the embryo often shows us more or less plainly the
structure of the less modified and ancient progenitor of the group, we
can see why ancient and extinct forms so often resemble in their
adult state the embryos of existing species of the same class. Agassiz
believes this to be a universal law of nature; and we may hope hereafter
to see the law proved true. It can, however, be proved true only in
those cases in which the ancient state of the progenitor of the group
has not been wholly obliterated, either by successive variations having
supervened at a very early period of growth, or by such variations
having been inherited at an earlier age than that at which they first
appeared. It should also be borne in mind, that the law may be true,
but yet, owing to the geological record not extending far enough back
in time, may remain for a long period, or for ever, incapable of
demonstration. The law will not strictly hold good in those cases in
which an ancient form became adapted in its larval state to some special
line of life, and transmitted the same larval state to a whole group
of descendants; for such larval state will not resemble any still more
ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, which
are second to none in importance, are explained on the principle of
variations in the many descendants from some one ancient progenitor,
having appeared at a not very early period of life, and having been
inherited at a corresponding period. Embryology rises greatly in
interest, when we look at the embryo as a picture, more or less
obscured, of the progenitor, either in its adult or larval state, of all
the members of the