In all the changes described that which moves now extends itself
in a straight line to progress, and now is hooped; it straightens
itself in its leading part, and is hooped in what follows behind. Even
jumping animals all make a flexion in the part of the body which is
underneath, and after this fashion make their leaps. So too flying and
swimming things progress, the one straightening and bending their
wings to fly, the other their fins to swim. Of the latter some have
four fins, others which are rather long, for example eels, have only
two. These swim by substituting a flexion of the rest of their body
for the (missing) pair of fins to complete the movement, as we have
said before. Flat fish use two fins, and the flat of their body as a
substitute for the absent pair of fins. Quite flat fish, like the Ray,
produce their swimming movement with the actual fins and with the
two extremes or semicircles of their body, bending and straightening
themselves alternately.
10
A difficulty might perhaps be raised about birds. How, it may be
said, can they, either when they fly or when they walk, be said to
move at four points? Now we did not say that all Sanguinea move at
four points, but merely at not more than four. Moreover, they cannot
as a fact fly if their legs be removed, nor walk without their
wings. Even a man does not walk without moving his shoulders.
Everything indeed, as we have said, makes a change of place by flexion
and straightening, for all things progress by pressing upon what being
beneath them up to a point gives way as it were gradually;
accordingly, even if there be no flexion in another member, there must
be at least in the point whence motion begins, is in feathered
(flying) insects at the base of the 'scale-wing', in birds at the base
of the wing, in others at the base of the corresponding member, the
fins, for instance, in fish.
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