from causes of which we are wholly ignorant, at a very early period of
life, or each step might be inherited at an earlier period than that at
which it first appeared. In either case (as with the short-faced
tumbler) the young or embryo would closely
resemble the mature parent-form. We have seen that this is the rule of
development in certain whole groups of animals, as with cuttle-fish and
spiders, and with a few members of the great class of insects, as with
Aphis. With 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 larv 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 larv , in the first
stage, might differ greatly from the larv 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
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