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On the Gait of Animals
By Aristotle


Translated by A. S. L. Farquharson

Part 1

We have now to consider the parts which are useful to animals for
movement in place (locomotion); first, why each part is such as it
is and to what end they possess them; and second, the differences
between these parts both in one and the same creature, and again by
comparison of the parts of creatures of different species with one
another. First then let us lay down how many questions we have to
consider. 

The first is what are the fewest points of motion necessary to animal
progression, the second why sanguineous animals have four points and
not more, but bloodless animals more than four, and generally why
some animals are footless, others bipeds, others quadrupeds, others
polypods, and why all have an even number of feet, if they have feet
at all; why in fine the points on which progression depends are even
in number. 

Next, why are man and bird bipeds, but fish footless; and why do man
and bird, though both bipeds, have an opposite curvature of the legs.
For man bends his legs convexly, a bird has his bent concavely; again,
man bends his arms and legs in opposite directions, for he has his
arms bent convexly, but his legs concavely. And a viviparous quadruped
bends his limbs in opposite directions to a man's, and in opposite
directions to one another; for he has his forelegs bent convexly,
his hind legs concavely. Again, quadrupeds which are not viviparous
but oviparous have a peculiar curvature of the limbs laterally away
from the body. Again, why do quadrupeds move their legs criss-cross?

We have to examine the reasons for all these facts, and others cognate
to them; that the facts are such is clear from our Natural History,
we have now to ask reasons for the facts. 

Part 2

At the beginning of the inquiry we must postulate the principles we
are accustomed constantly to use for our scientific investigation
of nature, that is we must take for granted principles of this universal
character which appear in all Nature's work. Of these one is that
Nature creates nothing without a purpose, but always the best possible
in each kind of living creature by reference to its essential constitution.
Accordingly if one way is better than another that is the way of Nature.
Next we must take for granted the different species of dimensions
which inhere in various things; of these there are three pairs of
two each, superior and inferior, before and behind, to the right and
to the left. Further we must assume that the originals of movements
in place are thrusts and pulls. (These are the essential place-movements,
it is only accidentally that what is carried by another is moved;
it is not thought to move itself, but to be moved by something else.)

Part 3

After these preliminaries, we go on to the next questions in order.

Now of animals which change their position some move with the whole
body at once, for example jumping animals, others move one part first
and then the other, for example walking (and running) animals. In
both these changes the moving creature always changes its position
by pressing against what lies below it. Accordingly if what is below
gives way too quickly for that which is moving upon it to lean against
it, or if it affords no resistance at all to what is moving, the latter
can of itself effect no movement upon it. For an animal which jumps
makes its jump both by leaning against its own upper part and also
against what is beneath its feet; for at the joints the parts do in
a sense lean upon one another, and in general that which pushes down
leans upon what is pushed down. That is why athletes jump further
with weights in their hands than without, and runners run faster if
they swing their arms; there is in extending the arms a kind of leaning
against the hands and wrists. In all cases then that which moves makes
its change of position by the use of at least two parts of the body;
one part so to speak squeezes, the other is squeezed; for the part
that is still is squeezed as it has to carry the weight, the part
that is lifted strains against that which carries the weight. It follows
then that nothing without parts can move itself in this way, for it
has not in it the distinction of the part which is passive and that
which is active. 

Part 4

Again, the boundaries by which living beings are naturally determined
are six in number, superior and inferior, before and behind, right
and left. Of these all living beings have a superior and an inferior
part; for superior and inferior is in plants too, not only in animals.
And this distinction is one of function, not merely of position relatively
to our earth and the sky above our heads. The superior is that from
which flows in each kind the distribution of nutriment and the process
of growth; the inferior is that to which the process flows and in
which it ends. One is a starting-point, the other an end, and the
starting-point is the superior. And yet it might be thought that in
the case of plants at least the inferior is rather the appropriate
starting-point, for in them the superior and inferior are in position
other than in animals. Still they are similarly situated from the
point of view of function, though not in their position relatively
to the universe. The roots are the superior part of a plant, for from
them the nutriment is distributed to the growing members, and a plant
takes it with its roots as an animal does with its mouth.

Things that are not only alive but are animals have both a front and
a back, because they all have sense, and front and back are distinguished
by reference to sense. The front is the part in which sense is innate,
and whence each thing gets its sensations, the opposite parts are
the back. 

All animals which partake not only in sense, but are able of themselves
to make a change of place, have a further distinction of left and
right besides those already enumerated; like the former these are
distinctions of function and not of position. The right is that from
which change of position naturally begins, the opposite which naturally
depends upon this is the left. 

This distinction (of right and left) is more articulate and detailed
in some than in others. For animals which make the aforesaid change
(of place) by the help of organized parts (I mean feet for example,
or wings or similar organs) have the left and right distinguished
in greater detail, while those which are not differentiated into such
parts, but make the differentiation in the body itself and so progress,
like some footless animals (for example snakes and caterpillars after
their kind, and besides what men call earth-worms), all these have
the distinction spoken of, although it is not made so manifest to
us. That the beginning of movement is on the right is indicated by
the fact that all men carry burdens on the left shoulder; in this
way they set free the side which initiates movement and enable the
side which bears the weight to be moved. And so men hop easier on
the left leg; for the nature of the right is to initiate movement,
that of the left to be moved. The burden then must rest on the side
which is to be moved, not on that which is going to cause movement,
and if it be set on the moving side, which is the original of movement,
it will either not be moved at all or with more labour. Another indication
that the right is the source of movement is the way we put our feet
forward; all men lead off with the left, and after standing still
prefer to put the left foot forward, unless something happens to prevent
it. The reason is that their movement comes from the leg they step
off, not from the one put forward. Again, men guard themselves with
their right. And this is the reason why the right is the same in all,
for that from which motion begins is the same for all, and has its
natural position in the same place, and for this reason the spiral-shaped
Testaceans have their shells on the right, for they do not move in
the direction of the spire, but all go forward in the direction opposite
to the spire. Examples are the murex and the ceryx. As all animals
then start movement from the right, and the right moves in the same
direction as the whole, it is necessary for all to be alike right-handed.
And man has the left limbs 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. 

Part 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. 

Part 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. 

Part 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.

Part 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.

Part 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 since limbed creatures stand and take their weight alternately
on one or other of the opposite legs, if one be thrust forward the
other of necessity must be bent. For the opposite limbs are naturally
of equal length, and the one which is under the weight must be a kind
of perpendicular at right angles to the ground. 

When then one leg is advanced it becomes the hypotenuse of a right-angled
triangle. Its square then is equal to the square on the other side
together with the square on the base. As the legs then are equal,
the one at rest must bend either at the knee or, if there were any
kneeless animal which walked, at some other articulation. The following
experiment exhibits the fact. If a man were to walk parallel to a
wall in sunshine, the line described (by the shadow of his head>
would be not straight but zigzag, becoming lower as he bends, and
higher when he stands and lifts himself up. 

It is, indeed, possible to move oneself even if the leg be not bent,
in the way in which children crawl. This was the old though erroneous
account of the movement of elephants. But these kinds of movements
involve a flexion in the shoulders or in the hips. Nothing at any
rate could walk upright continuously and securely without flexions
at the knee, but would have to move like men in the wrestling schools
who crawl forward through the sand on their knees. For the upper part
of the upright creature is long so that its leg has to be correspondingly
long; in consequence there must be flexion. For since a stationary
position is perpendicular, if that which moves cannot bend it will
either fall forward as the right angle becomes acute or will not be
able to progress. For if one leg is at right angles to the ground
and the other is advanced, the latter will be at once equal and greater.
For it will be equal to the stationary leg and also equivalent to
the hypotenuse of a right-angled triangle. That which goes forward
therefore must bend, and while bending one, extend the other leg simultaneously,
so as to incline forward and make a stride and still remain above
the perpendicular; for the legs form an isosceles triangle, and the
head sinks lower when it is perpendicularly above the base on which
it stands. 

Of limbless animals, some progress by undulations (and this happens
in two ways, either they undulate on the ground, like snakes, or up
and down, like caterpillars), and undulation is a flexion; others
by a telescopic action, like what are called earthworms and leeches.
These go forward, first one part leading and then drawing the whole
of the rest of the body up to this, and so they change from place
to place. It is plain too that if the two curves were not greater
than the one line which subtends them undulating animals could not
move themselves; when the flexure is extended they would not have
moved forward at all if the flexure or arc were equal to the chord
subtended; as it is, it reaches further when it is straightened out,
and then this part stays still and it draws up what is left behind.

In all the changes described that which moves now extends itself in
a straight line to progress, and now is hooped; it straightens itself
in its leading part, and is hooped in what follows behind. Even jumping
animals all make a flexion in the part of the body which is underneath,
and after this fashion make their leaps. So too flying and swimming
things progress, the one straightening and bending their wings to
fly, the other their fins to swim. Of the latter some have four fins,
others which are rather long, for example eels, have only two. These
swim by substituting a flexion of the rest of their body for the (missing)
pair of fins to complete the movement, as we have said before. Flat
fish use two fins, and the flat of their body as a substitute for
the absent pair of fins. Quite flat fish, like the Ray, produce their
swimming movement with the actual fins and with the two extremes or
semicircles of their body, bending and straightening themselves alternately.

Part 10

A difficulty might perhaps be raised about birds. How, it may be said,
can they, either when they fly or when they walk, be said to move
at four points? Now we did not say that all Sanguinea move at four
points, but merely at not more than four. Moreover, they cannot as
a fact fly if their legs be removed, nor walk without their wings.
Even a man does not walk without moving his shoulders. Everything
indeed, as we have said, makes a change of place by flexion and straightening,
for all things progress by pressing upon what being beneath them up
to a point gives way as it were gradually; accordingly, even if there
be no flexion in another member, there must be at least in the point
whence motion begins, is in feathered (flying) insects at the base
of the 'scale-wing', in birds at the base of the wing, in others at
the base of the corresponding member, the fins, for instance, in fish.
In others, for example snakes, the flexion begins in the joints of
the body. 

In winged creatures the tail serves, like a ship's rudder, to keep
the flying thing in its course. The tail then must like other limbs
be able to bend at the point of attachment. And so flying insects,
and birds (Schizoptera) whose tails are ill-adapted for the use in
question, for example peacocks, and domestic cocks, and generally
birds that hardly fly, cannot steer a straight course. Flying insects
have absolutely no tail, and so drift along like a rudderless vessel,
and beat against anything they happen upon; and this applies equally
to sharded insects, like the scarab-beetle and the chafer, and to
unsharded, like bees and wasps. Further, birds that are not made for
flight have a tail that is of no use; for instance the purple coot
and the heron and all water-fowl. These fly stretching out their feet
as a substitute for a tail, and use their legs instead of a tail to
direct their flight. The flight of insects is slow and frail because
the character of their feathery wings is not proportionate to the
bulk of their body; this is heavy, their wings small and frail, and
so the flight they use is like a cargo boat attempting to make its
voyage with oars; now the frailty both of the actual wings and of
the outgrowths upon them contributes in a measure to the flight described.
Among birds, the peacock's tail is at one time useless because of
its size, at another because it is shed. But birds are in general
at the opposite pole to flying insects as regards their feathers,
but especially the swiftest flyers among them. (These are the birds
with curved talons, for swiftness of wing is useful to their mode
of life.) The rest of their bodily structure is in harmony with their
peculiar movement, the small head, the slight neck, the strong and
acute breastbone (acute like the prow of a clipper-built vessel, so
as to be well-girt, and strong by dint of its mass of flesh), in order
to be able to push away the air that beats against it, and that easily
and without exhaustion. The hind-quarters, too, are light and taper
again, in order to conform to the movement of the front and not by
their breadth to suck the air. 

Part 11

So much then for these questions. But why an animal that is to stand
erect must necessarily be not only a biped, but must also have the
superior parts of the body lighter, and those that lie under these
heavier, is plain. Only if situated like this could it possibly carry
itself easily. And so man, the only erect animal, has legs longer
and stouter relatively to the upper parts of his body than any other
animal with legs. What we observe in children also is evidence of
this. Children cannot walk erect because they are always dwarf-like,
the upper parts of their bodies being longer and stouter than the
lower. With advancing years the lower increase disproportionately,
until the children get their appropriate size, and then and not till
then they succeed in walking erect. Birds are hunchbacked yet stand
on two legs because their weight is set back, after the principle
of horses fashioned in bronze with their forelegs prancing. But their
being bipeds and able to stand is above all due to their having the
hip-bone shaped like a thigh, and so large that it looks as if they
had two thighs, one in the leg before the knee-joint, the other joining
his part to the fundament. Really this is not a thigh but a hip, and
if it were not so large the bird could not be a biped. As in a man
or a quadruped, the thigh and the rest of the leg would be attached
immediately to quite a small hip; consequently the whole body would
be tilted forward. As it is, however, the hip is long and extends
right along to the middle of the belly, so that the legs are attached
at that point and carry as supports the whole frame. It is also evident
from these considerations that a bird cannot possibly be erect in
the sense in which man is. For as it holds its body now the wings
are naturally useful to it, but if it were erect they would be as
useless as the wings of Cupids we see in pictures. It must have been
clear as soon as we spoke that the form of no human nor any similar
being permits of wings; not only because it would, though Sanguineous,
be moved at more than four points, but also because to have wings
would be useless to it when moving naturally. And Nature makes nothing
contrary to her own nature. 

Part 12

We have stated above that without flexion in the legs or shoulders
and hips no Sanguineous animal with feet could progress, and that
flexion is impossible except some point be at rest, and that men and
birds, both bipeds, bend their legs in opposite directions, and further
that quadrupeds bend their in opposite directions, and each pair in
the opposite way to a man's limbs. For men bend their arms backwards,
their legs forwards; quadrupeds their forelegs forwards, their back
legs backwards, and in like manner also birds bend theirs. The reason
is that Nature's workmanship is never purposeless, as we said above,
but everything for the best possible in the circumstances. Inasmuch,
therefore, as all creatures which naturally have the power of changing
position by the use of limbs, must have one leg stationary with the
weight of the body on it, and when they move forward the leg which
has the leading position must be unencumbered, and the progression
continuing the weight must shift and be taken off on this leading
leg, it is evidently necessary for the back leg from being bent to
become straight again, while the point of movement of the leg thrust
forward and its lower part remain still. And so the legs must be jointed.
And it is possible for this to take place and at the same time for
the animal to go forward, if the leading leg has its articulation
forwards, impossible if it be backwards. For, if it be forwards, the
stretching out of the leg will be while the body is going forwards,
but, if the other way, while it is going backwards. And again, if
the flexion were backwards, the placing of the foot would be made
by two movements and those contrary to one another, one, that is,
backwards and one forwards; for in the bending together of the limb
the lower end of the thigh would go backwards, and the shin would
move the foot forwards away from the flexion; whereas, with the flexion
forwards, the progression described will be performed not with contrary
motions, but with one forward motion. 

Now man, being a biped and making his change of position in the natural
way with his two legs, bends them forward for the reasons set forth,
but his arms bend backwards reasonably enough. If they bent the opposite
way they would be useless for the work of the hands, and for taking
food. But quadrupeds which are also viviparous necessarily bend their
front legs forwards. For these lead off first when they move, and
are also in the forepart of their body. The reason that they bend
forward is the same as in the case of man, for in this respect they
are like mankind. And so quadrupeds as well as men bend these legs
forward in the manner described. Moreover, if the flexion is like
this, they are enabled to lift their feet high; if they bent them
in the opposite way they would only lift them a little way from the
ground, because the whole thigh and the joint from which the shin-bone
springs would lie under the belly as the beast moved forward. If,
however, the flexion of the hind legs were forwards the lifting of
these feet would be similar to that of the forefeet (for the hind
legs, too, would in this case have only a little room for their lifting
inasmuch as both the thigh and the knee-joint would fall under the
position of the belly); but the flexion being backwards, as in fact
it is, nothing comes in the way of their progression with this mode
of moving the feet. Moreover, it is necessary or at least better for
their legs to bend thus when they are suckling their young, with a
view to such ministrations. If the flexion were inwards it would be
difficult to keep their young under them and to shelter them.

Part 13

Now there are four modes of flexion if we take the combinations in
pairs. Fore and hind may bend either both backwards, as the figures
marked A, or in the opposite way both forwards, as in B, or in converse
ways and not in the same direction, as in C where the fore bend forwards
and the hind bend backwards, or as in D, the opposite way to C, where
the convexities are turned towards one another and the concavities
outwards. Now no biped or quadruped bends his limbs like the figures
A or B, but the quadrupeds like C, and like D only the elephant among
quadrupeds and man if you consider his arms as well as his legs. For
he bends his arms concavely and his legs convexly. 

In man, too, the flexions of the limbs are always alternately opposite,
for example the elbow bends back, but the wrist of the hand forwards,
and again the shoulder forwards. In like fashion, too, in the case
of the legs, the hip backwards, the knee forwards, the ankle in the
opposite way backwards. And plainly the lower limbs are opposed in
this respect to the upper, because the first joints are opposites,
the shoulder bending forwards, the hip backwards; wherefore also the
ankle bends backwards, and the wrist of the hand forwards.

Part 14

This is the way then the limbs bend, and for the reasons given. But
the hind limbs move criss-cross with the fore limbs; after the off
fore they move the near hind, then the near fore, and then the off
hind. The reason is that (a) if they moved the forelegs together and
first, the animal would be wrenched, and the progression would be
a stumbling forwards with the hind parts as it were dragged after.
Again, that would not be walking but jumping, and it is hard to make
a continuous change of place, jumping all the time. Here is evidence
of what I say; even as it is, all horses that move in this way soon
begin to refuse, for example the horses in a religious procession.
For these reasons the fore limbs and the hind limbs move in this separate
way. Again, (b) if they moved both the right legs first the weight
would be outside the supporting limbs and they would fall. If then
it is necessary to move in one or other of these ways or criss-cross
fashion, and neither of these two is satisfactory, they must move
criss-cross; for moving in the way we have said they cannot possibly
experience either of these untoward results. And this is why horses
and such-like animals stand still with their legs put forward criss-cross,
not with the right or the left put forward together at once. In the
same fashion animals with more than four legs make their movements;
if you take two consecutive pairs of legs the hind move criss-cross
with the forelegs; you can see this if you watch them moving slowly.
Even crabs move in this way, and they are polypods. They, too, always
move criss-cross in whichever direction they are making progress.
For in direction this animal has a movement all its own; it is the
only animal that moves not forwards, but obliquely. Yet since forwards
is a distinction relative to the line of vision, Nature has made its
eyes able to conform to its limbs, for its eyes can move themselves
obliquely, and therefore after a fashion crabs are no exception but
in this sense move forwards. 

Part 15

Birds bend their legs in the same way as quadrupeds. For their natural
construction is broadly speaking nearly the same. That is, in birds
the wings are a substitute for the forelegs; and so they are bent
in the same way as the forelegs of a quadruped, since when they move
to progress the natural beginning of change is from the wings (as
in quadrupeds from the forelegs). Flight in fact is their appropriate
movement. And so if the wings be cut off a bird can neither stand
still nor go forwards. 

Again, the bird though a biped is not erect, and has the forward parts
of the body lighter than the hind, and so it is necessary (or at least
preferable for the standing posture) to have the thigh so placed below
the body as it actually is, I mean growing towards the back. If then
it must have this situation the flexion of the leg must be backwards,
as in the hind legs of quadrupeds. The reasons are the same as those
given in the case of viviparous quadrupeds. 

If now we survey generally birds and winged insects, and animals which
swim in a watery medium, all I mean that make their progress in water
by dint of organs of movement, it is not difficult to see that it
is better to have the attachment of the parts in question oblique
to the frame, exactly as in fact we see it to be both in birds and
insects. And this same arrangement obtains also among fishes. Among
birds the wings are attached obliquely; so are the fins in water animals,
and the feather-like wings of insects. In this way they divide the
air or water most quickly and with most force and so effect their
movement. For the hinder parts in this way would follow forwards as
they are carried along in the yielding medium, fish in the water,
birds in the air. 

Of oviparous quadrupeds all those that live in holes, like crocodiles,
lizards, spotted lizards, freshwater tortoises, and turtles, have
their legs attached obliquely as their whole body sprawls over the
ground, and bend them obliquely. The reason is that this is useful
for ease in creeping into holes, and for sitting upon their eggs and
guarding them. And as they are splayed outwards they must of necessity
tuck in their thighs and put them under them in order to achieve the
lifting of the whole body. In view of this they cannot bend them otherwise
than outwards. 

Part 16

We have already stated the fact that non-sanguineous animals with
limbs are polypods and none of them quadrupeds. And the reason why
their legs, except the extreme pairs, were necessarily attached obliquely
and had their flexions upwards, and the legs themselves were somewhat
turned under (bandy-shape) and backwards is plain. In all such creatures
the intermediate legs both lead and follow. If then they lay under
them, they must have had their flexion both forwards and backwards;
on account of leading, forwards; and on account of following, backwards.
Now since they have to do both, for this reason their limbs are turned
under and bent obliquely, except the two extreme pairs. (These two
are more natural in their movement, the front leading and the back
following.) Another reason for this kind of flexion is the number
of their legs; arranged in this way they would interfere less with
one another in progression and not knock together. But the reason
that they are bandy is that all of them or most of them live in holes,
for creatures living so cannot possibly be high above the ground.

But crabs are in nature the oddest of all polypods; they do not progress
forwards except in the sense explained above, they are the only animals
which have more than one pair of leading limbs. The explanation of
this is the hardness of their limbs, and the fact that they use them
not for swimming but for walking; they always keep on the ground.
However, the flexion of the limbs of all polypods is oblique, like
that of the quadrupeds which live in holes-for example lizards and
crocodiles and most of the oviparous quadrupeds. And the explanation
is that some of them in their breeding periods, and some all their
life, live in holes. 

Part 17

Now the rest have bandy legs because they are soft-skinned, but the
crayfish is hard-skinned and its limbs are for swimming and not for
walking (and so are not bandy). Crabs, too, have their limbs bent
obliquely, but not bandy like oviparous quadrupeds and non-sanguineous
polypods, because their limbs have a hard and shell-like skin, although
they don't swim but live in holes; they live in fact on the ground.
Moreover, their shape is like a disk, as compared with the crayfish
which is elongated, and they haven't a tail like the crayfish; a tail
is useful to the crayfish for swimming, but the crab is not a swimming
creature. Further, it alone has its side equivalent to a hinder part,
because it has many leading feet. The explanation of this is that
its flexions are not forward nor its legs turned in under (bandy).
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. 

Part 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. Fish on the contrary live
in the wet medium, and take in water, not air. Fins are useful for
swimming, but feet not. And if they had both they would be non-sanguineous.
There is a broad similarity between birds and fishes in the organs
of locomotion. Birds have their wings on the superior part, similarly
fish have two pectoral fins; again, birds have legs on their under
parts and near the wings; similarly, most fish have two fins on the
under parts and near the pectorals. Birds, too, have a tail and fish
a tail-fin. 

Part 19

A difficulty may be suggested as to the movements of molluscs, that
is, as to where that movement originates; for they have no distinction
of left and right. Now observation shows them moving. We must, I think,
treat all this class as mutilated, and as moving in the way in which
limbed creatures do when one cuts off their legs, or as analogous
with the seal and the bat. Both the latter are quadrupeds but misshapen.
Now molluscs do move, but move in a manner contrary to nature. They
are not moving things, but are moving if as sedentary creatures they
are compared with zoophytes, and sedentary if classed with progressing
animals. 

As to right and left, crabs, too, show the distinction poorly, still
they do show it. You can see it in the claw; the right claw is larger
and stronger, as though the right and left sides were trying to get
distinguished. 

The structure of animals, both in their other parts, and especially
in those which concern progression and any movement in place, is as
we have now described. It remains, after determining these questions,
to investigate the problems of Life and Death. 

THE END

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