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ON THE GAIT OF ANIMALS.........................................................................................................................1

by Aristotle..............................................................................................................................................1

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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

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

We have to examine the reasons for all these facts, and others cognate to them; that the facts are such is clear

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 placemovements, 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.)

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

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 startingpoint, the other an end, and the startingpoint is the superior. And yet

it might be thought that in the case of plants at least the inferior is rather the appropriate startingpoint, 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

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

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

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 earthworms), 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 spiralshaped 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 righthanded. 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 startingpoints are found most naturally and clearly

Animals which, like men and birds, have the superior part distinguished from the front are twofooted

(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 fourfooted, manyfooted, 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 spiralshaped 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 startingpoint 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 startingpoints are in them.

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

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

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.

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

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

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

Among land animals this is the character of the movement of snakes, and among water animals of eels, and

congereels 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

congereels 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.

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

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

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

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

When then one leg is advanced it becomes the hypotenuse of a rightangled 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

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

rightangled 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

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

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.

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 'scalewing', 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 illadapted 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 scarabbeetle 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

waterfowl. 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 clipperbuilt vessel, so as to be wellgirt, 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

hindquarters, too, are light and taper again, in order to conform to the movement of the front and not by

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 dwarflike,

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 hipbone shaped like a thigh, and so large that it looks as if they had

two thighs, one in the leg before the kneejoint, 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

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

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 shinbone 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

kneejoint 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.

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

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.

This is the way then the limbs bend, and for the reasons given. But the hind limbs move crisscross 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 crisscross fashion, and neither of these

two is satisfactory, they must move crisscross; for moving in the way we have said they cannot possibly

experience either of these untoward results. And this is why horses and suchlike animals stand still with

their legs put forward crisscross, 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 crisscross 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 crisscross 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

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

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

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 featherlike 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

We have already stated the fact that nonsanguineous 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 (bandyshape) 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

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 holesfor 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.

Now the rest have bandy legs because they are softskinned, but the crayfish is hardskinned 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 nonsanguineous polypods, because their limbs have a hard and

shelllike 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 shelllike 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

Fishes of the flat kind swim with their heads twisted, as oneeyed men walk; they have their natural shape

distorted. Webfooted 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

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 nonsanguineous. 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

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