We have given above the reason why its
legs are not turned in under, that is the hardness and shell-like
character of its integument.
For these reasons then it must lead off with more than one limb, and
move obliquely; obliquely, because the flexion is oblique; and with
more than one limb, because otherwise the limbs that were still
would have got in the way of those that were moving.
Fishes of the flat kind swim with their heads twisted, as one-eyed
men walk; they have their natural shape distorted. Web-footed birds
swim with their feet; because they breath the air and have lungs
they are bipeds, but because they have their home in the water they
are webbed; by this arrangement their feet serve them instead of fins.
They have their legs too, not like the rest of birds in the centre
of their body, but rather set back. Their legs are short, and being
set back are serviceable for swimming. The reason for their having
short legs is that nature has added to their feet by subtracting
from the length of their limbs; instead of length she gives
stoutness to the legs and breadth to the feet. Broad feet are more
useful than long for pushing away the water when they are swimming.
18
There is reason, too, for winged creatures having feet, but fish
none. The former have their home in the dry medium, and cannot
remain always in mid air; they must therefore have feet.
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