Therefore the evolution of an organic
system in material respects involves its functional or dynamic evolution
as an inseparable correlate; the two proceed in unity, and they cannot be
regarded as entirely distinct without violating common-sense.
The fin of a fish is used as an organ of locomotion in water; from some
such organ have evolved the walking limbs of amphibia and reptiles,
constructed for progression upon land. Among the mammalia the fore limbs
have become structurally adapted so as to be such diverse organs of
locomotion as the stilt-like leg of a horse, the flipper of a seal, the
whale's paddle, and the bat's wing, while among the birds the wing may
change into a flipper like that of the penguin, or become reduced to a
vestige as in _Apteryx_. We may focus our attention upon the material
likenesses and differences in such a series of locomotory organs, but an
inevitable accompaniment of their physical changes in the transformation
of species has been an evolution in the functional matter of locomotion.
The most complex and differentiated tracts of even the highest animals
have evolved from a simple sac like that of a polyp or jellyfish, as we
know from the independent testimony of comparative anatomy and embryology;
in this case also the evolution of alimentary functions is no less
inseparable from the transformations in structural respects.
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