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detached more than any other animal because he is natural in a
higher degree than the other animals； now the right is naturally
both better than the left and separate from it； and so in man the
right is more especially the right； more dextrous that is； than in
other animals。 The right then being differentiated it is only
reasonable that in man the left should be most movable； and most
detached。 In man； too； the other starting…points are found most
naturally and clearly distinct； the superior part that is and the
front。
5
Animals which； like men and birds； have the superior part
distinguished from the front are two…footed （biped）。 In them； of the
four points of motion； two are wings in the one； hands and arms in the
other。 Animals which have the superior and the front parts identically
situated are four…footed； many…footed； or footless （quadruped；
polypod； limbless）。 I use the term foot for a member employed for
movement in place connected with a point on the ground； for the feet
appear to have got their name from the ground under our feet。
Some animals； too； have the front and back parts identically
situated； for example； Cephalopods （molluscs） and spiral…shaped
Testaceans； and these we have discussed elsewhere in another
connexion。
Now there is in place a superior； an intermediate； and an
inferior； in respect to place bipeds have their superior part
corresponding to the part of the universe； quadrupeds； polypods； and
footless animals to the intermediate part； and plants to the inferior。
The reason is that these have no power of locomotion； and the superior
part is determined relatively to the nutriment； and their nutriment is
from the earth。 Quadrupeds； polypods； and footless animals again
have their superior part corresponding to the intermediate； because
they are not erect。 Bipeds have theirs corresponding to the superior
part of the universe because they are erect； and of bipeds； man par
excellence； for man is the most natural of bipeds。 And it is
reasonable for the starting points to be in these parts； for the
starting…point is honourable； and the superior is more honourable than
the inferior； the front than the back； and the right than the left。 Or
we may reverse the argument and say quite well that these parts are
more honourable than their opposites just because the
starting…points are in them。
6
The above discussion has made it clear that the original of movement
is in the parts on the right。 Now every continuous whole； one part
of which is moved while the other remains at rest must； in order to be
able to move as a whole while one part stands still； have in the place
where both parts have opposed movements some common part which
connects the moving parts with one another。 Further in this common
part the original of the motion （and similarly of the absence of
motion） of each of the parts must lie。
Clearly then if any of the opposite pairs of parts （right and
left； that is； superior and inferior； before and behind） have a
movement of their own； each of them has for common original of its
movements the juncture of the parts in question。
Now before and behind are not distinctions relatively to that
which sets up its own motion； because in nature nothing has a movement
backwards； nor has a moving animal any division whereby it may make
a change of position towards its front or back； but right and left；
superior and inferior are so distinguished。 Accordingly； all animals
which progress by the use of distinct members have these members
distinguished not by the differences of before and behind； but only of
the remaining two pairs； the prior difference dividing these members
into right and left （a difference which must appear as soon as you
have division into two）； and the other difference appearing of
necessity where there is division into four。
Since then these two pairs； the superior and inferior and the
right and left； are linked to one another by the same common
original （by which I mean that which controls their movement）； and
further； everything which is intended to make a movement in each
such part properly must have the original cause of all the said
movements arranged in a certain definite position relatively to the
distances from it of the originals of the movements of the
individual members （and these centres of the individual parts are in
pairs arranged coordinately or diagonally； and the common centre is
the original from which the animal's movements of right and left；
and similarly of superior and inferior； start）； each animal must
have this original at a point where it is equally or nearly equally
related to each of the centres in the four parts described。
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 c