Aristotle · a new plain-English translation from the original language
1| ON THE PROGRESSION OF ANIMALS Concerning the parts that are useful to animals for motion 1 from place to place, we must examine for what cause each of them is such as it is, and for the sake of what it belongs to them, and further, concerning the differences these parts have both from one another within the same single animal and from the corresponding parts of animals differing in kind. Let us first take up the points we must examine. One is: with how few points of contact at the minimum do animals move? Next, why do the blooded animals move with four such points and the bloodless ones with more? And, generally, for what cause some animals are footless, some two-footed, some four-footed, and some many-footed; and why all of them have an even number of feet, in every case where they have feet at all, and in general, wherever an animal moves by points of contact, these are even in number. Further, for what cause a human being and a bird are both two-footed, while fish are footless; and why the human being and the bird, though both two-footed, bend their legs in opposite ways (for the human being bends the leg toward the convex side, the bird toward the concave side); and why a human being himself bends his own legs and arms in opposite ways to each other —
2| for he bends the arms toward the concave side, but the knees toward the convex side. And the four-footed live-bearing animals bend their limbs in the opposite way both to human beings and to themselves — for they bend the front legs toward the convex side of the curve, but the hind legs toward the concave side. Further, among the four-footed animals, those that are not live-bearing but egg-laying bend their legs in a peculiar way, sideways.) Beyond these questions, for what cause the four-footed animals move diagonally — concerning all these matters, and whatever others are akin to them, we must study the causes. That these things happen so is plain from the researches on nature; why they happen so we must now examine.
3| 2 The starting point of the inquiry is to lay down, as we are accustomed often to use for the method of natural science, the principles that hold in this way throughout all the works of nature. Of these, one is that nature does nothing in vain, but always, out of what is possible for the substance concerning each kind of animal, does what is best; hence if it is better thus, then it is also so by nature. Further, we must grasp the dimensions of magnitude — how many there are and which belong to which things. For the dimensions are six, forming three pairs: one is the above and the below, a second is the front and the back, and a third is the right and the left. Beyond this, the starting points of motions from place to place are pushing and pulling. These, then, are motions in their own right; but what is carried along by something else moves accidentally — for what is carried along by something else does not seem to move itself, but to be moved by another. 3 Having distinguished these points, let us go on to what follows from them. Of animals that change place, some change with the whole body all at once, as those that leap,
4| while others change by parts, as each of the animals that walk does. In both these kinds of change, what is moved always changes by bracing itself against what underlies it. Hence, if that underlying thing shifts away faster than allows what is producing the motion upon it to get a purchase, or if it offers the moving thing no resistance at all, nothing is able to move itself upon it. For even the leaping animal makes its leap by bracing itself, both upward against itself and downward against what lies beneath its feet — for the parts have a certain mutual resistance to one another at the joints, and, generally, the pressing part has resistance against the part pressed. That is why pentathletes leap farther holding weights than not holding them, and runners run faster swinging their arms — for a certain resistance arises in the tension against the hands and wrists. And what is moved always effects its change using at least two organic parts as instruments, the one as it were pressing, the other pressed. For the part that stays still is pressed because it bears the weight, while the part that is lifted is stretched by the part that carries the load. Hence nothing without parts can be moved in this way — for it does not contain within itself the distinction between the part that will be acted upon and the part that will act.
5| Since the dimensions by which animals are naturally marked off are six in number — the above and below, the front and back, and further the right and the left — all living things have the above and below part. For the above and below is found not only in animals but also in plants. And it is distinguished by function, not merely by position relative to the earth and the heavens. For that from which the distribution of nourishment and growth proceeds for each thing, this is above; that at which this distribution finally ends, this is below. For the one is a kind of starting point, the other an end point; and the starting point is above. And yet it might seem that the below is more proper to plants — for the above and below do not hold in the same way by position for plants as for animals. Relative to the whole, they are not alike, but by function they are alike. For the roots are the above for plants — since it is from there that nourishment is distributed to what is growing, and the plant takes in its nourishment by these, just as animals do by their mouths. And whatever things not only live but are also animals, in such things there belongs both the front and the back.
6| For all these have perception, and it is by this that the back and the front are marked off — for that toward which perception is naturally directed, and the point from which it comes for each of them, this is front, and the parts opposite to these are back. But whatever animals share not only in perception but are also able to produce change of place by themselves through their own agency, in these, besides what has been said, the left and the right too are marked off in the same way as what was said before, each of the two marked off by some function and not by position. For that part of the body from which the starting point of change of place naturally comes, this is right for each of them, and the part opposite to this, which is naturally disposed to follow it, is left. This distinction is more sharply articulated in some animals than in others. For whatever animals produce the said change of place by using organic parts — I mean, for example, feet or wings or some other such part — in these the point stated is more sharply articulated; but whatever animals advance not by such parts but by making divisions with the body itself, as some of the footless animals do, for instance snakes and the family of caterpillars, and besides these what people call earthworms, in these too what has been said holds, yet it is not equally clearly marked out.
7| That the starting point of motion is on the right side is shown also by the fact that everyone carries loads on the left side — for in this way what carries the load is able to move, since what will do the moving is left free. That is why people also hop more easily on the left leg — for the right is naturally disposed to move, the left to be moved; so that the load too must be placed not on the part that will do the moving but on the part that will be moved. And if it is placed on the part that moves and on the starting point of the motion, either the animal will not move at all, or it will move with greater difficulty. A further sign that the starting point of motion, and thrusting forward, come from the right side is this: everyone thrusts the left parts forward, and even when standing still they hold the left parts more forward, unless it happens otherwise by chance. For they do not move by the part that has stepped forward, but by the part that has stepped off; and they defend themselves with the right parts. For this same cause the right side is the same for all — for that from which the starting point of motion comes has, in all of them, the same position by nature; and the right is that from which the starting point of motion is. And for this reason the spiral-shelled testaceans are all right-handed. For they do not move toward the spiral coil, but all advance toward the opposite side, for example purple-shellfish and whelks.
8| Since, then, everything moves starting from the right, and the right parts move in the same direction as themselves, it is necessary that all be alike right. Of all animals, human beings have their left side most set free, because they, more than any other animal, are constituted according to nature; and by nature the right, once separated from the left, is better. That is why in human beings the right parts are right in the fullest sense. And once the right is thus marked off, it is reasonable that the left parts are less mobile, and it is in human beings that they are most set free. The other principles, too, exist in the human being most according to nature and most clearly marked off — namely the upper and the front.
9| 5 Now the animals in which the upper and the front are marked off as distinct, as in human beings and birds, are bipeds. (Of the four points, two are wings in some and hands and arms in others.) Those that have the front in the same place as the upper are four-footed, many-footed, or footless. For I call a foot a part situated at a point that touches the ground and is capable of moving the animal from place to place; indeed feet seem to have gotten their name from the ground. Some have the front and the back in the same place, such as the cephalopods and the spiral-shelled testaceans; these have been discussed earlier elsewhere. Since there are three regions — up, middle, and down — bipeds have their up oriented toward the up of the universe, many-footed and footless animals toward the middle, and plants toward the down. The reason is that plants are immobile, while nourishment is upward for them, and nourishment comes from the earth. Four-footed animals are oriented toward the middle, as are many-footed and footless ones, because they are not upright; bipeds are oriented toward the up because they are upright, and the human being most of all, since it is by nature most fully a biped. It is reasonable, too, that the principles derive from these parts; for a principle is honorable, and the upper is more honorable than the lower, the front than the back, and the right than the left. It is also fine to state the matter the other way around: that because the principles are in these parts, these parts are more honorable than the opposite parts.
10| That the principle of motion is from the right, then, is clear from what has been said. 6 Now since it is necessary, in the case of anything continuous, one part of which moves while the other is at rest — the whole being able to move while one part stands still, and both moving in opposite motions — that there be some common point at which these parts are continuous with one another, and that here too lies the principle of the motion of each of the parts, and likewise of course of their rest; and since, for as many of the stated oppositions as have a motion peculiar to each of the opposite parts, all of these have a common principle, corresponding to the natural union of the parts mentioned — I mean the right and the left, the up and the down, the front and the back — then, in the case of front and back, there is no such distinction concerning what moves itself, because no natural motion toward the back exists for anything, nor does the moving thing have any determination by which it makes a change toward either of these directions; but in the case of right and left, and up and down, such a distinction does exist. That is why, of the animals that progress by using organic parts, those parts are not marked off as distinct by the difference of front and back, but are marked off by the remaining two differences — both of them, but primarily by the one that distinguishes according to right and left — because the latter distinction must exist immediately in animals with two points of movement, while the former exists first in those with four.
11| Since, then, the up and the down, and the right and the left, are bound up with one and the same common principle in relation to themselves (I mean the principle that governs motion), and since, in everything that is going to produce motion from each part properly, it is necessary that this motion be somehow determined and ordered by its distances from the stated principles (both the corresponding and the paired parts among those in these parts), the cause of all the motions mentioned (and this is the common principle in the animal from which comes both the motion of the right and the left, and likewise that of the up and down) must stand in a similar relation for each animal, in proportion to
12| 7 each of the principles in the parts mentioned. It is clear, then, that locomotion belongs either solely or chiefly to those animals that effect their change of place by means of two or four points. So since this holds most of all, roughly speaking, for the blooded animals, it is clear that no blooded animal can move by more than four points, and that if anything is naturally constituted to move by four points only, it must be blooded. What actually happens with animals agrees with what has been said. For none of the blooded animals, when divided into several pieces, can live for any time worth mentioning, so to speak, nor can it share in the locomotion by which it moved while it was continuous and undivided; but some of the bloodless and many-footed animals, when divided, can live a long time in each of their parts, and can move with the very same motion they had before being divided — for example the creatures called centipedes, and
13| other elongated insects. For in all of these the hind part travels along the same path as the front. The reason they can live when divided is that each of them is constituted just as a single continuous thing would be if it were composed of many animals joined together. This is clear from what was said before, because that is the way the matter stands. For the things most fully constituted according to nature are naturally suited to move by two or four points, and likewise this holds for as many of the blooded animals as are footless. For these too move by four points, through which they effect their motion. For using two points, they progress by bendings; for the right and left, front and back, lie side by side in width at each bend — at the part near the head the front point is right and left, and at the part near the tail the hind points are right and left. It seems to move by two points, by the forward point of contact and the rear one. The reason is that it is narrow in width, since here too the right leads, and the response comes on the back side, just as in four-footed animals. The length is the cause of the bendings;
14| for just as tall men walk swaybacked, and when the right shoulder leads forward (for the left hip inclines further backward, and the middle becomes hollow and swaybacked), so too one must think of snakes moving on the ground as swaybacked. A sign that they move in the same way as four-footed animals is that they alternate the hollow and the convex in turn. For whenever the left of the front pair leads again, the hollow occurs again on the opposite side; for the right in turn becomes the inner side. Let the right front point be marked A, the left B, the left hind point C, the right D. Snakes on land move in this way, and among water creatures the eels, conger eels, and morays do too, along with all the others that have a more snake-like shape. Except that some of these water creatures have no fin at all, such as the morays, but make use of the sea just as snakes make use of both the land and the sea (for snakes swim in the same way they move on land); others have only two fins, such as the conger eels, the eels, and a certain kind of mullet that occurs in the lake at Siphae.
15| And for this reason animals accustomed to live on land move with fewer bends in water than on land, as with the eel kind. But mullets, which have two fins, make up for the four points of support by bending, in the water. 8 The cause of snakes' footlessness is, first, that nature does nothing in vain, but in every case looks to what is best among the possibilities available, preserving for each thing its own substance and its essence; and further, what we said earlier, that none of the blooded animals can move with more than four points. From this it is clear that none of the blooded animals whose length is disproportionate to the rest of their bodily nature, as with snakes, can have feet. For they cannot have more than four feet (since then they would be bloodless), while if they had two feet or four they would be virtually unable to move at all; the motion would necessarily be so slow and useless.
16| Every footed animal necessarily has an even number of feet. For those that effect their change of place only by leaping have no need of feet at all for that kind of motion; but those that do use leaping, yet for which this motion is not by itself sufficient but which also need walking, clearly for some walking is the better way, while for others it is altogether impossible otherwise. For since this kind of change of place occurs part by part, and not with the whole body at once as in leaping, it is necessary that while some feet are moving, others remain still, and that the opposite feet do each of these in turn, the weight shifting from the feet that are moving to those that remain still. That is why nothing can walk using three feet, nor using one; for with one foot there is no support at all on which the body's weight can rest, while with three there is support only on one side of the opposition, so that anything attempting to move that way must necessarily fall. But animals with many feet, such as centipedes, can even walk with an odd number of feet, as is actually seen happening now, if one maims one of their feet, because the loss of the paired foot is made good by the remaining number of feet on each side; for in these animals the maimed part is, as it were, dragged along by the others. Nevertheless it is clear that even these animals would carry out the change of place better if they had an even number of feet, with none missing, but had their feet matched in pairs; for in this way they would be able to balance their own weight and not tip more to one side than the other, provided they had matching points of support and the corresponding place on the other side were not left empty. And in walking it advances from each of the two sides alternately; for in this way its posture returns to the same shape it had at the start.
17| 9 It has been stated, then, that all footed animals have an even number of feet, and for what cause. That, if nothing were at rest, there would be neither bending nor straightening, is clear from the following. Bending is a change from straight to either curved or angled, and straightening is a change from either of these to straight. And in all the changes mentioned, the bending or straightening must occur with reference to one fixed point. But indeed, if there were no bending, there would be neither walking nor swimming nor flying. For footed animals, since they stand in turn on each of the two opposite legs and take the weight on it, it is necessary that, while one advances, the other bends. For the paired limbs are naturally equal in length, and the one supporting the weight must be upright, like a perpendicular to the ground. But when it advances, there arises the hypotenuse, whose square is equal to the fixed length together with the interval between. And since the limbs are equal, the one at rest must bend, either at the knee or at whatever joint of flexion it has, as would be the case even if some walking animal had no knee. A sign that this is so: if someone were to walk on the ground alongside a wall, the line traced would not be
18| straight but wavy, because the traced line becomes lower while the leg is bending, and higher while it is planted and the body is raised. It is possible, however, to move even without the leg having a bend, as infants crawl. And there was an old story of this kind told about elephants, which is not true. Such creatures too move by a bending that occurs at the shoulder blades or the hips. But nothing could walk upright continuously and securely without a bending joint; it could move only in the way that, in the wrestling schools, those who advance through the dust on their knees do; for the upper part of the body is large, so the limb must be long, and if so, bending is necessary. For since it stands upright, if the part moving forward were to remain unbent, then either it will fall as the upright angle becomes smaller, or it will not advance at all. For if, while one leg remains upright, the other is to have advanced, that leg would have to be longer while still being equal in length; for it would have to equal both the leg at rest and the hypotenuse together. The advancing limb, then, must necessarily bend, and having bent, must at the same time straighten the other, and lean to the side, and stand astride, and remain along the perpendicular; for the limbs form an isosceles triangle, and the head is lower whenever the perpendicular falls on the leg on which it stands.
19| Footless animals advance in some cases by a wave-like motion (this happens in two ways: some make their bends along the ground, as snakes do, others upward, as caterpillars do), and this waving is a form of bending; others use a looping motion, as with the creatures called earthworms and leeches. For these advance with the leading part, and draw the whole of the rest of the body up to it, and in this way change from place to place. It is clear that if the two lengths were not greater than the one, wave-moving animals could not move at all. For when the bend is straightened out, if it then occupied an equal length as before, they would not advance at all; but as it is, once straightened it extends beyond that length, and once this part comes to rest, it draws the rest of the body after it. In all the changes described, the moving creature advances at one time by stretching out straight, at another by bending together, becoming straight in its leading parts and bent in the parts that follow. All jumping creatures, too, make their bend in the underlying part of the body, and it is in this posture that they leap. And flying and swimming animals likewise: some fly by straightening and bending their wings, others swim with fins, and of these some use four fins, others two — namely those of more elongated shape, such as the eel kind;
20| and for the rest of their motion, in place of the two fins they lack, they swim by bending the rest of the body, as has been said before. Flat fish use the breadth of their body in place of some fins, and for the rest use two fins. Those that are altogether flat, such as the ray, swim by straightening and bending the fins themselves together with the outer curves of the body. 10 One might perhaps raise the difficulty how birds move with four points, whether flying or walking, on the supposition that it was said that all blooded animals move with four. But that is not what was said — rather, that they move with no more than four. Nevertheless, they could neither fly if their legs were taken away, nor walk if their wings were... (text breaks off here)
21| removed — since not even a man can walk without moving his shoulders somewhat. But all of them, as has been said, produce their change of place by bending and extending; for all of them advance to some extent into what lies beneath them, as into something yielding, so that it is necessary, even if the bending does not also occur at some other part, that at least the starting-point be there: for the whole-winged insects, of the wing; for birds, of the wing; for others, of the analogous part, as with fish. For others, like snakes, the starting-point of the bending is in the bendings of the body. The rump serves flying creatures for steering their flight straight, just as rudders serve ships. And this too must necessarily bend at its point of attachment. That is why the whole-winged insects, and among the split-winged those whose rump is ill-suited for the use just described — as with peacocks and cocks, and in general the birds that are not strong flyers — do not fly in a straight course; for none of the whole-winged insects has a rump at all, so that each is carried along like a rudderless ship, and each of them comes down wherever it happens to; and likewise the sheath-winged insects, such as dung-beetles and cockchafers, and the uncased-winged, such as bees and wasps.
22| And for those that are not strong flyers the rump is useless too, as with purple coots and herons and all the swimming birds; instead, they fly by stretching out their feet, and use their legs in place of a rump for steering their flight straight. The flight of the whole-winged insects is slow and weak, because the nature of their wings is not proportioned to the weight of the body, but the body is large while the wings are small and weak. So it is just as if a merchant freighter should try to make its voyage by oars — that is how these creatures manage their flight. And the weakness both of the wings themselves and of their point of growth contributes something to what has been said. Among birds, the peacock's rump is at one time useless because of its size, and at another does no good because the bird sheds it. Birds have the nature of their wings opposite to that of the whole-winged insects, and this is most true of those birds that fly fastest. Such are the crook-taloned birds; for these, speed of flight is useful for their way of life. And the rest of their bodily parts seem to be consistent with this, suited to swift motion: in all of them the head is small and the neck not thick, while the breast is strong and sharp — sharp so as to be well set for cutting through, like the prow of a light boat, and strong in the nature of its flesh, so that it can push away the air that strikes against it;
23| and it does this easily and without effort; while the hind parts are light and narrow again to a point, so that they may follow after the front parts without dragging the air because of their breadth. 11 Let this, then, be our determination of these matters. As for why an animal that is going to walk upright must necessarily be two-footed, and must have the upper parts of its body lighter and the parts standing beneath them heavier, this is clear: for only if constituted in this way could it easily carry itself. That is why man, being the only animal that stands upright, has legs proportionate to the upper part of the body — the largest and strongest among footed animals. What happens with small children makes this clear too: for they cannot walk upright, because they are all dwarf-like, and have the upper parts of the body larger and stronger than is proportionate to the parts below.
24| But as the age advances the lower parts grow more, until they attain their proper size, and then their bodies achieve upright walking. Birds, being light, are two-footed because their weight lies toward the back, just as craftsmen make bronze horses that have their forelegs raised. The chief cause of their being able to stand while two-footed is that they have a hip-bone like a thigh, and so large that they seem to have two thighs — the one in the leg before the joint, and the other running from the seat toward that part; but it is not a thigh, it is a hip-bone. For if it were not so large, a bird could not be two-footed. For as with men and four-footed animals, if the hip-bone were short, the thigh and the rest of the leg would begin right from it; and then the whole body would be far too far forward, tipping. As it is, being long, it extends as far as under the middle of the belly, so that from that point the legs, planted firmly beneath, carry the whole body. From this it is also clear that a bird cannot be upright as a man is. For the nature of their wings is useful to them as their body is now constituted, but if they were upright it would be useless — like the way painters paint the Loves as having wings. For together with what has been said it is clear that neither a man, nor anything else of that shape, could possibly be winged — not only because, being blooded, it would then move by more than four points, but because the possession of wings would be useless to them while moving according to their nature; and nature does nothing contrary to nature.
25| That, then, if there were no bending in the legs or in the 12 shoulder-blades and hip-bones, none of the blooded and footed animals could move forward, has been stated earlier; and that there could be no bending if nothing remained at rest; and that men, though both two-footed, and birds bend their legs oppositely to one another, and further that the four-footed animals bend theirs oppositely both to themselves and to men. For men bend their arms toward the hollow side, but their legs toward the convex side, while the four-footed animals bend their front legs toward the convex side, and their hind legs toward the hollow side; and birds likewise. The cause is that nature makes nothing in vain, as has been said before, but does everything for the best among the possibilities. So, since for all those to whom change of place by means of the two legs belongs by nature, when each stands still the weight is in both legs, but when moving forward the foot that leads in position must be light, while as the advance continues it must in turn take up the weight again, it is clear that the leg must necessarily become straight again after having been bent, while the point at the foot that was thrust forward, and the shin, remain fixed.
26| And this can happen at the same time as the animal moves forward if the leading leg has its bend toward the front, but it is impossible if the bend is toward the back. For in the one case the extension of the leg will occur as the body is carried forward, but in the other, as the body is carried back. Further, if the bend were toward the back, the placing of the foot would come about through two motions opposite to each other, the one toward the back and the other toward the front; for in the bending of the leg it would be necessary for the end of the thigh to move forward toward the back, while the shin, from the bend, moves the foot toward the front — toward the front
27| ...and since the bend is where it is, the progression we have described will not occur through movements that are opposite to one another while both are directed toward the same forward motion. Now man, being two-footed and making his change of place with his legs in accordance with nature for the reason stated, bends his legs forward, and his arms toward the concave side, reasonably so; for they would be useless if bent the opposite way, both for the use of the hands and for taking food. As for the four-footed and live-bearing animals, their front legs — since these lead their progression and are in the front part of the body — must bend toward the convex side for the same reason as in humans; for in this respect they are alike. That is why four-footed animals too bend forward in the manner described. For if their bend occurs this way, they will be able to raise their feet well clear of the ground; but if they bent the opposite way, they would raise them only a little off the ground, because as the animal advances the whole thigh, and the bend from which the shin grows, would come to be under the belly. As for the hind legs, if the bend were forward, the raising of the feet would be the same for them as for the front ones (for it would likewise be only slight, given the raising of the legs, since both the thigh and the bend would fall under the region of the belly); but if it is backward, as indeed they now bend, nothing hinders their progression in such a movement of the feet. Further, for those of them that suckle their young, it is necessary, or at least better, that the legs be bent this way for that function too; for it is not easy, if the bend is made inward beneath themselves, to hold and shelter their young.
29| 13 There being four ways of bending at the joints (for it is necessary to bend either toward the concave side both in front and behind, as in the cases marked A, or the opposite way, toward the convex side, as in the cases marked B, or in reverse, not in the same direction, but the front toward the convex side and the hind toward the concave, as in the case marked Γ, or the opposite of these, the convex sides toward each other and the concave sides outward, as is the case with what is marked ∠) — in the way of A or of B, nothing bends, neither among the two-footed nor among the four-footed; in the way of Γ, the four-footed animals bend; but in the way of ∠, none of the four-footed animals bends except the elephant, but man bends both his arms and his legs: for he bends the arms toward the concave side, and the legs toward the convex side. And in humans the limbs always have their bends alternating in opposite directions: for instance the elbow toward the concave side, but the wrist toward the convex side, and again the shoulder toward the convex side; and likewise with the legs, the thigh toward the concave side, the knee toward the convex side, but the foot, on the contrary, toward the concave side. And it is plain that the lower limbs stand opposite to the upper ones as well; for the starting point is opposite in each: the shoulder toward the convex side, the thigh toward the concave side; and that is why the foot is toward the concave side, while the wrist of the hand is toward the convex side.
30| The bends of the legs, then, occur in this way and for the reasons stated. The hind legs move relative to the front legs along the diagonal: for after the right front leg, they move the left hind leg, then the left front leg, and after this the right hind leg. The reason is that if the front legs moved together and first, the gait would be pulled apart, or would become pitched forward, and with the hind legs, as it were, dragged along behind. Moreover, such a thing is not walking but leaping; and it is difficult to keep making the change continuously by leaping. Here is a sign of this: even now, among horses, those that move in this manner soon give out — for example those used in processions. So it is for these reasons that they do not move the front and hind legs separately in this way. And if both right legs moved first, the animals would fall outside their points of support and would fall over. If, then, it is necessary to move either in one of these ways or along the diagonal, and neither of the other ways is possible, it is necessary to move along the diagonal;
31| for when moving as described, neither of these mishaps can befall them. And this is why horses and animals of that kind, when they come to a stop after advancing, stand with their legs positioned diagonally, and not with both right legs or both left legs together. In the same way, too, all animals that have more than four feet make their movement; for always, among each four feet taken in sequence, the hind pair moves relative to the front pair along the diagonal — this is clear in animals that move slowly. Crabs also move in this same way, for they belong to the many-footed animals; for they too always move along the diagonal, in whatever direction they make their progress. For this animal makes its motion in a peculiar way: it is the only animal that does not move forward, but sideways. But since the front is marked out by the eyes, nature has made the eyes able to follow the limbs; for the crabs' limbs move sideways for them, so that in a certain way the crabs too move forward on this account.
32| 15 Birds bend their legs just as the four-footed animals do. For their nature is, in a certain way, similar: for birds, the wings serve in place of the front legs. That is why they are bent in the same way as the front legs are bent in those animals, since for birds the natural starting point of the change involved in walking comes from the wings; for flight is their proper motion. That is why, if the wings are removed, no bird would be able either to stand or to go forward. Further, since a bird is two-footed and not upright, and has the front parts of its body lighter, it is either necessary or better, for the sake of being able to stand, that the thigh be positioned underlying the body in the way it now is — I mean, growing toward the back. But indeed, if it must be positioned this way, the bend of the leg must necessarily occur toward the concave side, just as with four-footed animals in the case of the hind legs, for the same reason we stated in the case of the four-footed and live-bearing animals. In general, both birds and the fully-winged among flying creatures, and the swimming creatures in water — those of them that make their progress through the water by means of organs — it is not hard to see that it is better for the attachment of the aforementioned parts to be from the side, just as indeed it now appears to be the case for them, both in birds and in the fully-winged creatures.
33| The same holds also for fish: for birds have wings, water-creatures have fins. For in this way, contracting and expanding most quickly and most forcefully, one group would produce its motion against the air, the other against the water; for the front and hind parts of the body would follow along, being carried as the medium yields, one in the water, the other in the air. As for the burrowing creatures among the four-footed and egg-laying animals, such as crocodiles, lizards, geckos, freshwater turtles, and tortoises, all of them have their legs attached from the side and stretched out flat on the ground, and they bend them sideways, because this arrangement is useful for ease in burrowing and for sitting on and guarding their eggs. Since their legs are positioned outward, they must draw their thighs in and place them beneath themselves in order to raise the whole body. And when this happens, it is not possible for them to bend their legs in any way other than outward.
34| 16 We have already said that the bloodless footed animals are many-footed and that none of them is four-footed. But why it was necessary for their legs — except for the outermost ones — to be attached from the side, to have their bends turned upward, and to be somewhat bowed backward, is now clear. For in all such animals the middle legs must be both leading and following. If, then, they were set directly under the body, they would have to bend both forward and backward — forward because they lead, backward because they follow. But since both must happen to them at once, for this reason they are bowed and have their bends turned to the side, except for the outermost legs. These outermost legs are bent more in the natural way, some as following legs, others as leading ones. Further, they are bent this way also because of the multitude of their legs; for bent in this manner the legs would be less of a hindrance to one another in walking and would knock together less. Their bowleggedness comes from the fact that all, or most, of them are burrow-dwellers; for animals that live this way cannot stand high off the ground. Crabs, among the many-footed animals, are constituted the most unusually of all: they do not make their progression forward except in the way said before, and they alone among animals have many leading legs. The cause of this is the hardness of their feet, and the fact that they do not use them for the sake of swimming
35| but for walking; for they spend their lives going about on foot. In all many-footed animals, then, the bends are to the side, just as in those four-footed animals that are burrow-dwellers. Such are lizards, for example, and crocodiles, and most egg-laying animals. The cause is that some of them burrow only for breeding, while others do so for their whole life. But while in the others the limbs 17 are bowed because they are soft, in the case of crayfish, since they are hard-skinned, the feet are for swimming and not for the sake of walking. In crabs the bend is to the side, but they are not bowed the way the egg-laying four-footed animals are, or the bloodless many-footed ones, because their limbs are hard-skinned and shell-like, since it is not a swimmer but a burrow-dweller; for its life is spent close to the ground. It is also round in shape, and has no tail-flap as the crayfish has; for the tail-flap is useful to crayfish for swimming, but the crab is not a swimmer. And the crab alone has its side like its back, because it has many leading legs. The cause of this is that it does not bend forward and is not bowed. The cause of its not being bowed has been stated already: the hardness and shell-like character of its skin. For these reasons, then, it must go forward with all its legs leading, and to the side: to the side, because the bend is to the side; with all its legs, because resting feet would get in the way of the moving ones. Flatfish-like fish swim the way one-eyed people walk;
36| for their nature is twisted. Web-footed birds swim with their feet: they are two-footed because they take in air and breathe, but web-footed because their life is spent in water. For feet of this kind serve them in place of fins. They have their legs not in the middle, as other birds do, but further back; for since they are short-legged, having the legs set back is useful for swimming. Such birds are short-legged because nature has taken from the length of the legs and added it to the feet, and in place of length has given the legs thickness and the feet breadth. For feet that are broad rather than long are more useful for forcing back the water when they swim. 18 It is also reasonable that winged animals have feet while fish are footless. For the life of the former is on dry land, and it is impossible to remain always aloft, so they must have feet; but the life of fish is in water, and they take in water, not air. Fins, then, are useful for swimming, but feet would be useless. And if they had both, they would have to be bloodless. Birds are, in a way, in a similar condition to fish. For while fish have two fins on the prone (upper) side; and in birds the feet are on the supine part, while in fish, in most cases, there are fins both on the supine part and near the prone part; and the one kind has a tail-flap, the other a tail-fin.
37| 19 Concerning the shell-covered animals one might raise the difficulty of what their motion is, and, if they do not have a right and a left, where their motion comes from; yet they are observed to move. Or rather, this whole class must be regarded as maimed, and as moving the way a footed animal would move if someone cut off its legs — as the seal and the bat move. For these too are four-footed animals, but badly so. The shell-covered animals do move, but they move contrary to their nature. For they are not naturally capable of progression: as compared with stationary, rooted things they can move, but as compared with animals capable of walking, they are stationary. Crabs too have a poor right side, though they do have one. The claw shows this: the right claw is larger and stronger, as though nature wanted the left and the right to be distinguished. This, then, is how matters stand concerning the parts — both the other parts of animals and those concerned with their progression — and concerning every change of place. Now that these matters have been determined, the next thing is to examine the soul.