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, 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 larv 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, now
supposed to be represented in many embryos, has not been obliterated,
either by the successive variations in a long course of modification
having supervened
at a very early age, or by the variations having been inherited at an
earlier period than that at which they first appeared. It should also
be borne in mind, that the supposed law of resemblance of ancient forms
of life to the embryonic stages of recent forms, 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.
Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many
descendants from some one ancient progenitor, at a very early period in
the life of each, though perhaps caused at the earliest, and being
inherited at a corresponding not early period. Embryology rises greatly
in interest, when we thus look at the embryo as a picture, more or less
obscured, of the common parent-form of each great class of animals.
_Rudimentary, atrophied, or aborted organs_. Organs or parts in this
strange condition, bearing the stamp of inutility, are extremely common
throughout nature. For instance, rudimentary mamm are very general in
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