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life, while other such elements remained inorganic ; there
must have been a point at which some vegetables became
animals, while other vegetables remained vegetable ; or
perhaps there were some common forms, neither animal
nor vegetable, which at certain points became distinctively
vegetable on the one hand, or distinctively animal on the
other. There must have been a point at which some
invertebrates began to be vertebrates, while other inverte
brates remained invertebrate. There must have been a
point at which, following one line of descent, some fish
became reptiles, some reptiles amphibians, some amphibians
mammals; and when, following another line of descent, some
reptiles became birds. And if all this took place under
the reign of law, there must have been mechanical,
chemical, vital laws by which Organic Evolution has been
brought about. But, at present, they are too dimly dis
cerned to enable us to found any strong argument upon
them in favour of the theory of transformation, as opposed
to the theory of Natural Selection ; and, with one notable
exception, I believe that no attempt has been made to
prove that Natural Selection has been the determining
cause of those new departures which produce such different
results.
But if the discussion of these topics has not proceeded
far enough for the purposes of our present argument, the
chasm which once appeared to divide the inorganic from
the living, the simple forms from the more complex, the
invertebrate from the vertebrate, the fish from the reptile,
the reptile from the bird, or the reptile from the am
phibian, and the amphibian from the mammalian, has
been narrowed very considerably, and the whole process
of Organic Evolution, represented by a tree-like classi
fication, becomes no more difficult to conceive than the
transmutation of one species into another.