y possible arrangement, would be genealogical; descent being the
hidden bond of connexion which naturalists have been seeking under the
term of the Natural System. On this view we can understand how it is
that, in the eyes of most naturalists, the structure of the embryo is
even more important for classification than that of the adult. In two or
more groups of animals, however much they may differ from each other
in structure and habits in their adult condition, if they pass through
closely similar embryonic stages, we may feel assured that they all are
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 stage 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 larvae will not resemble any still more ancient
form in its adult