respect to the final cause
of the young in these cases not undergoing any metamorphosis, or closely
resembling their parents from their earliest age, we can see that this
would result from the two following contingencies; firstly, from the
young, during a course of modification carried on for many generations,
having to provide for their own wants at a very early stage of
development, and secondly, from their following exactly the same habits
of life with their parents; for in this case, it would be indispensable
for the existence of the species, that the child should be modified at
a very early age in the same manner with its parents, in accordance with
their similar habits. Some further explanation, however, of the embryo
not undergoing any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to 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, etc., 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; and in so far it reveals the structure of its progenitor. In
two groups of animal, however much they may at present differ from each
other in structure and habits, if they pass through the same or similar
embryonic stages