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.
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