And so nothing can walk on three limbs or on
one; in the latter case it has no support at all on which to rest
the body's weight, in the former only in respect of one pair of
opposites, and so it must necessarily fall in endeavouring so to move.
Polypods however, like the Centipede, can indeed make progress on an
odd number of limbs, as may be seen by the experiment of wounding
one of their limbs; for then the mutilation of one row of limbs is
corrected by the number of limbs which remain on either side. Such
mutilated creatures, however, drag the wounded limb after them with
the remainder, and do not properly speaking walk. Moreover, it is
plain that they, too, would make the change of place better if they
had an even number, in fact if none were missing and they had the
limbs which correspond to one another. In this way they could equalize
their own weight, and not oscillate to one side, if they had
corresponding supports instead of one section of the opposite sides
being unoccupied by a limb. A walking creature advances from each of
its members alternately, for in this way it recovers the same figure
that it had at first.
9
The fact that all animals have an even number of feet, and the
reasons for the fact have been set forth. What follows will explain
that if there were no point at rest flexion and straightening would be
impossible. Flexion is a change from a right line to an arc or an
angle, straightening a change from either of these to a right line.
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