be constructed in a slightly different manner, then, on the
principle of inheritance at corresponding ages, the active young or larvae
might easily be rendered by natural selection different to any conceivable
extent from their parents. Such differences might, also, become correlated
with successive stages of development; so that the larvae, in the first
stage, might differ greatly from the larvae in the second stage, as we have
seen to be the case with cirripedes. The adult might become fitted for
sites or habits, in which organs of locomotion or of the senses, &c., would
be useless; and in this case the final metamorphosis would be said to be
retrograde.

As all the organic beings, extinct and recent, which have ever lived on
this earth have to be classed together, and as all have been connected by
the finest gradations, the best, or indeed, if our collections were nearly
perfect, the only possible arrangement, would be genealogical. Descent
being on my view 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. For the embryo is the animal in its less modified state; {449} and
in so far it reveals the structure of its progenitor. In two groups of
animals, however much they may at present differ from each other in
structure and habits, if they pass through the same or similar embryonic
stages, we may feel assured that they have both descended from the same or
nearly similar parents, and are therefore in that degree closely related.
Thus, community in embryonic structure reveals community of descent. It
will reveal this community of descent, however much the structure of the
adult may have been modified and obscured; we have seen, for instance, that
cirripedes can at once be recognised by their larvae as belonging to the
great class of crustaceans. As the embryonic state of each species and
group of species partially shows us the structure of their less modified
ancient progenitors, we can clearly see why ancient and extinct forms of
life should resemble the embryos of their descendants,--our existing
species. Agassiz believes this to be a law of nature; but I am bound to
confess that I only hope to see the law hereafter proved true. It can be
proved true in those cases alone in which the ancient state,