by Aristotle
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 [5] once, for example jumping animals, others with their parts, for example walking 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 [10] 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 [15] 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 [20] 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 [25] it has not in it the distinction of the part which is passive and that which is active.
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 [30] 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 the earth and the sky. The superior is that from which flows in each kind the distribution of nutriment and the process of growth; the inferior is [705b1] 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 [5] 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, [10] because they all have sense, and front and back are distinguished by reference to sense. The front is the part in which sense is naturally located, and whence each thing gets its sensations, the opposite parts are the back.
[15] 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 [20] 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 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 [25] parts, but make the differentiation in the body itself and so progress, like some footless animals (for example snakes and caterpillars, 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 [30] 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 [706a1] 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 source 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 [5] 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 [10] 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 the origin of motion is from the right. And for this reason the spiral-shaped testaceans have their shells on the right, for they do not move in the direction of the spiral but all go forward in the direction [15] opposite to the spiral. Examples are the murex and the trumpet-shell. 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 better than the left, being separate [20] 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 less movable, and most detached. In man, too, the other starting-points are found most naturally and clearly distinct, the superior part that [25] is and the front.
5 · Animals which, like men and birds, have the superior part distinguished from the front are two-footed (biped). In them, of the four points of motion, two are wings in the one, hands and arms in the other. Animals which have the superior and the front parts identically situated are four-footed, many-footed, or footless. I use [30] the term foot for a member connected with a point on the ground which produces movement; for the feet appear to have got their name from the ground under our feet.1
Some animals, too, have the front and back parts identically situated, for [706b1] example cephalopods 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 superior part of the [5] universe; 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 [10] 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 [15] well that these parts are more honourable than their opposites just because the starting-points are in them.
6 · The above discussion has made it clear that the source 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 [20] 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 source of the motion (and similarly of the absence of motion) of each of the parts must lie.
Clearly2 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 [25] them has a common source of its movements at the juncture of the parts in question.
Now before and behind are not distinctions relatively to that which sets up its [3
0] 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. [707a1] 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 by those of the remaining two pairs; the prior difference dividing these members into right and left—for this difference must appear as soon as you have division into two, [5] the other difference appearing of necessity where there is division into four.
Since then the superior and inferior and the right and left are linked to one another by the same common source (by which I mean that which controls their movement), and further, everything which is going 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 said sources [10] (and these sources in the individual parts are in pairs arranged co-ordinately or diagonally, and the common centre is the source from which the animal’s movements of right and left, and similarly of superior and inferior, start), and since each animal must have this source at a point where it is similarly related to each of [15] the sources 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. Thus, 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 [20] so to progress at four points it must of necessity be sanguineous.
7 · What happens 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 [25] 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 the [707b1] fore-part.
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 [5] 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 [10] 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 [15] them the right is the leading 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 backs hollowed and with their right shoulder leading in a forward direction (for their left hip is rather inclined backwards, so that their [20] middle becomes hollowed), so we ought to conceive snakes as moving with hollowed backs 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, [25] the left hind C, the right 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, [708a1] 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 mullet which breeds in the lake [5] 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 mullet which has two fins, by its flexion in a fluid makes up the four points.
8 · The reason why snakes are limbless is first that nature makes nothing [10] without purpose, but always regards what is the best possible for each individual, preserving the peculiar substance of each and its essence, 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 [15] 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 [20] their movement necessarily be.
But every footed animal has necessarily an even number of such feet. For those which only jump and so move from place to place do not need feet for this movement at least, but those which not only jump but also need to walk, finding that movement [25] not sufficient for their purposes, evidently either are better able to progress with even feet or cannot otherwise progress at all;3 [for every animal which has limbs must have an even number]4 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 [30] the feet 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 [708b1] 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 [5] 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, [10] however, drag the wounded limb after them with the remainder, and do not properly speaking walk.5 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 [15] 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.
[20] 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 straight line to an arc or an angle, straightening a change from either of these to a straight line. Now in all such changes the flexion or the straightening must be [25] 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 must of necessity be bent. For the opposite limbs are naturally of equal length, and [30] 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. Its square then is [709a1] equal to the square on the side at rest together with the square on the line between the legs. 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 exhibits the fact. If a man were to walk parallel to a wall …6 the line [5] desc
ribed 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 [10] of elephants. But these kinds of movements involve a flexion in the shoulders or in the hips. Nothing 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 [15] flexion. For since a stationary position is perpendicular, if that which moves forward cannot bend it will either fall 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. That which goes forward [20] 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 [25] ways, either they undulate on the ground, like snakes, or up and down, like caterpillars), and undulation is a flexion; others by crawling, 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. [30] 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 [709b1] extended they would not have moved forward at all if the flexure were equal; 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.