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7
It is clear then how locomotion belongs to those animals only
which make their changes of place by means of two or four points in
their structure； or to such animals par excellence。 Moreover； since
this property belongs almost peculiarly to Sanguineous animals； we see
that no Sanguineous animal can progress at more points than four；
and that if it is the nature of anything so to progress at four points
it must of necessity be Sanguineous。
What we observe in the animal world is in agreement with the above
account。 For no Sanguineous animal if it be divided into more parts
can live for any appreciable length of time； nor can it enjoy the
power of locomotion which it possessed while it was a continuous and
undivided whole。 But some bloodless animals and polypods can live a
long time； if divided； in each of the severed parts； and can move in
the same way as before they were dismembered。 Examples are what is
termed the centipede and other insects that are long in shape； for
even the hinder portion of all these goes on progressing in the same
direction as before when they are cut in two。
The explanation of their living when thus divided is that each of
them is constructed like a continuous body of many separate living
beings。 It is plain； too； from what was said above why they are like
this。 Animals constructed most naturally are made to move at two or
four points； and even limbless Sanguinea are no exception。 They too
move by dint of four points； whereby they achieve progression。 They go
forward by means of two flexions。 For in each of their flexions
there is a right and a left； both before and behind in their flat
surface； in the part towards the head a right and a left front
point； and in the part towards the tail the two hinder points。 They
look as if they moved at two points only； where they touch before
and behind； but that is only because they are narrow in breadth。 Even。
in them the right is the sovereign part； and there is an alternate
correspondence behind； exactly as in quadrupeds。 The reason of their
flexions is their great length； for just as tall men walk with their
spines bellied （undulated） forward； and when their right shoulder is
leading in a forward direction their left hip rather inclined
backwards； so that their middle becomes hollow and bellied
（undulated）； so we ought to conceive snakes as moving in concave
curves （undulations） upon the ground。 And this is evidence that they
move themselves like the quadrupeds； for they make the concave in
its turn convex and the convex concave。 When in its turn the left of
the forward parts is leading； the concavity is in its turn reversed；
for the right becomes the inner。 （Let the right front point be A；
the left B； the right hind C； the left D。）
Among land animals this is the character of the movement of
snakes； and among water animals of eels； and conger…eels and also
lampreys； in fact of all that have their form snakelike。 However； some
marine animals of this shape have no fin； lampreys for example； but
put the sea to the same use as snakes do both land and water （for
snakes swim precisely as they move on the ground）。 Others have two
fins only； for example conger…eels and eels and a kind of cestreus
which breeds in the lake of Siphae。 On this account too those that are
accustomed to live on land； for example all the eels； move with
fewer flexions in a fluid than on land； while the kind of cestreus
which has two fins； by its flexion in a fluid makes up the remaining
points。
8
The reason why snakes are limbless is first that nature makes
nothing without purpose； but always regards what is the best
possible for each individual； preserving the peculiar essence of
each and its intended character； and secondly the principle we laid
down above that no Sanguineous creature can move itself at more than
four points。 Granting this it is evident that Sanguineous animals like
snakes； whose length is out of proportion to the rest of their
dimensions； cannot possibly have limbs； for they cannot have more than
four （or they would be bloodless）； and if they had two or four they
would be practically stationary； so slow and unprofitable would
their movement necessarily be。
But every limbed animal has necessarily an even number of such
limbs。 For those which only jump and so move from place to place do
not need limbs for this movement at least； but those which not only
jump but also need to walk； finding that movement not sufficient for
their purposes； evidently either are better able to progress with even
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
relative to one point。 Moreover； without flexion there could not be
walking or swimming or flying。 For s