limbs or cannot otherwise progress at all every animal which has limbs

must have an even us for as this kind of movement is effected by

part of the body at a time, and not by the whole at once as in the

movement of leaping, some of the limbs must in turn remain at rest,

and others be moved, and the animal must act in each of these cases

with opposite limbs, shifting the weight from the limbs that are being

moved to those at rest. 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.

Now in all such changes the flexion or the straightening must be