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You have probably noticed that the vertebrate differs greatly from
all his predecessors. The gulf between him and them is indeed wide
and deep. His origin and ancestry are yet far from certain. But an
attempt to decipher his past history, though it may lead to no sure
conclusions, will yet be of use to us. Practically all aquatic
vertebrates lead a swimming life, neither sessile nor creeping. The
embryonic development of our appendages leads to the same
conclusion. We must never forget that the embryonic development of
the individual recapitulates briefly the history of the development
of the race. Now the legs and arms, or fore- and hind-legs, of
higher vertebrates and the corresponding paired fins of fish develop
in the embryo as portions of a long ridge extending from front to
rear of the side of the body.

This justifies the inference that the primitive vertebrate ancestor
had a pair of long fins running along the sides of the body, but
bending slightly downward toward the rear so as to meet one another
and continue as a single caudal fin behind the anal opening. Such
fins, like the feathers of an arrow, could be useful only to keep
the animal "on an even keel" as it was forced through the water by
the lateral sweeps of the tail. They would have been useless for
creeping.

But there is another piece of evidence that he was a free swimming
form. All vertebrates breathe by gills or lungs, and these are
modified portions of the digestive system, of the walls of the
oesophagus, from which even the lung is an embryonic outgrowth.
Now practically all invertebrates breathe through modified portions