no metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed; and again in spiders, there is
nothing worthy to be called a metamorphosis. The larv of insects,
whether adapted to the most diverse and active habits, or quite
inactive, being fed by their parents or placed in the midst of proper
nutriment, yet nearly all pass through a similar worm-like stage of
development; but in some few cases, as in that of Aphis, if we look to
the admirable drawings by Professor Huxley of the development of this
insect, we see no trace of the vermiform stage.
How, then, can we explain these several facts in embryology, namely the
very general, but not universal difference in structure between the
embryo and the adult; of parts in the same individual embryo, which
ultimately become very unlike and serve for diverse purposes, being at
this early period of growth alike; of embryos of different species
within the same class, generally, but not universally, resembling each
other; of the structure of the embryo not being closely related to its
conditions of existence, except when the
embryo becomes at any period of life active and has to provide for
itself; of the embryo apparently having sometimes a higher organisation
than the mature animal, into which it is developed. I believe that all
these facts can be explained, as follows, on the view of descent with
modification.
It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily
appear at an equally early period. But we have little evidence on this
head indeed the evidence rather points the other way; for it is
notorious that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after the animal has been born,
what its merits or form will ultimately turn out. We see this plainly
in our own children; we cannot always tell whether the child will be
tall or short, or what its precise features will be. The question is
not, at what period of life any variation has been caused, but at what
period it is fully displayed. The cause may have acted, and I believe
generally has acted, even before the embryo is formed; and the
variation may be due to the male and female sexual elements having been
affected by the conditions to which either parent, or their ancestors,
have been exposed. Nevertheless an effect thus caused at a very early
period, even before the formation of the embryo, may appear late in
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