The Complete Works of Aristotle

by Barnes, Jonathan, Aristotle

  The beaks of birds also vary with their modes of life. For in some the beak is straight, in others crooked; straight, in those who use it merely for eating; crooked, in those that live on raw flesh. For a crooked beak is an advantage in fighting; and these birds must, of course, get their food from the bodies of other animals, and in [15] most cases by violence. In such birds, again, as live in marshes and are herbivorous the beak is broad and flat, this form being best suited for digging and cropping, and for pulling up plants. In some of these marsh birds, however, the beak is elongated, as too is the neck, the reason for this being that the bird gets its food from some depth below the surface. For most birds of this kind, and most of those whose feet [20] are webbed, either in their entirety or each part separately, live by preying on some of the smaller animals that are to be found in water, and their neck serves as a fishing-rod, the beak representing the line and hook.

  The upper and under sides of the body, that is of what in quadrupeds is called the trunk, present in birds one unbroken surface, and they have no arms or forelegs [693b1] attached to it, but in their stead wings, which are a distinctive peculiarity of these animals; and that is why the ends of the wings lie on the back in the place of a shoulder-blade.

  The legs are two in number, as in man; not however, as in man, bent outwards, [5] but bent inwards like the legs of a quadruped. The wings are bent like the forelegs of a quadruped, having their convexity turned outwards. That the feet should be two in number is a matter of necessity. For a bird is essentially a sanguineous animal, and at the same time a winged animal; and no sanguineous animal has more than four points for motion. In birds, then, as in those other sanguineous animals that live and move upon the ground, the limbs attached to the trunk are four in number. But, while in all the rest these four limbs consist of a pair of arms and a pair of legs, or of [10] four legs as in quadrupeds, in birds the arms or forelegs are replaced by a pair of wings, and this is their distinctive character. For it is part of the substance of a bird that it shall be able to fly; and it is by the extension of wings that this is made possible. Of all arrangements, then, the only possible, and so the necessary, one is that birds shall have two feet; for this with the wings will give them four points for [15] motion. The breast in all birds is sharp-edged, and fleshy. The sharp edge is to minister to flight, for broad surfaces move with considerable difficulty, owing to the large quantity of air which they have to displace; while the fleshy character acts as a protection, for the breast, owing to its form, would be weak, were it not amply covered.

  Below the breast lies the belly, extending, as in quadrupeds and in man, to the vent and to the place where the legs are jointed to the trunk. [20]

  Such, then, are the parts which lie between the wings and the legs. Birds like all other animals, whether produced viviparously or from eggs, have an umbilicus during their development, but, when the bird has attained to fuller growth, no signs of this remain visible. The cause of this is plainly to be seen during the process of development; for in birds the umbilical cord unites with the intestine, and is not a [25] portion of the blood vessels as is the case in viviparous animals.

  Some birds, again, are well adapted for flight, their wings being large and strong. Such, for instance, are those that have talons and live on flesh. For their [694a1] mode of life renders the power of flight a necessity, and it is on this account that their feathers are so abundant and their wings so large. Besides these, however, there are also other genera of birds that can fly well; all those, namely, that depend [5] on speed for security, or that are of migratory habits. On the other hand, some kinds of birds have heavy bodies and are not constructed for flight. These are birds that are frugivorous and live on the ground, or that are able to swim and get their living in watery places. In those that have talons the body, without the wings, is small; for the nutriment is consumed in the production of these wings, and of the weapons and defensive appliances; whereas in birds that are not made for flight the contrary [10] obtains, and the body is bulky and so of heavy weight. In some of these heavy-bodied birds the legs are furnished with what are called spurs, which replace the wings as a means of defence. Spurs and talons never co-exist in the same bird. For nature never makes anything superfluous; and if a bird can fly, and has talons, [15] it has no use for spurs; for these are weapons for fighting on the ground, and on this account belong to certain heavy-bodied birds. These latter, again, would find the possession of talons not only useless but actually injurious; for the claws would stick into the ground and interfere with progression. This is the reason why all birds with talons walk so badly, and why they never settle upon rocks. For the character of [20] their claws is ill-suited for either action.

  All this is the necessary consequence of the process of development. For the earthy matter in the body issuing from it is converted into parts that are useful as weapons. That which flows upwards gives hardness or size to the beak; and, should [25] any flow downwards, it either forms spurs upon the legs or gives size and strength to the claws upon the feet. But it does not at one and the same time produce both these results, one in the legs, the other in the claws; for such a dispersion of this residual matter would destroy all its efficiency. In other birds this earthy residue furnishes [694b1] the legs with the material for their elongation; or sometimes, in place of this, fills up the interspaces between the toes. Thus it is a matter of necessity that such birds as swim shall either be actually web-footed, or shall have a kind of broad blade-like [5] margin running along the whole length of each distinct toe. The forms, then, of these feet are the necessary results of the causes that have been mentioned. Yet at the same time they are intended for the animal’s advantage. For they are in harmony with the mode of life of these birds, who, living on the water, where their wings are useless, require that their feet shall be such as to serve in swimming. For [10] these feet are so developed as to resemble the oars of a boat, or the fins of a fish; and if the fins of the one or the webbing of the other is destroyed, they can no longer swim.

  In some birds the legs are very long, the cause of this being that they inhabit marshes—and nature makes the organs for the function, and not the function for [15] the organs. It is, then, because these birds are not meant for swimming that their feet are without webs, and it is because they live on ground that gives way under the foot that their legs and toes are elongated, and that these latter in most of them have an extra number of joints. Again, though all birds have the same material composition, they are not all made for flight; and in these, therefore, the nutriment [20] that should go to their tail-feathers is spent on the legs and used to increase their size. This is the reason why these birds when they fly make use of their legs as a tail, stretching them out behind, and so rendering them serviceable, whereas in any other position they would be simply an impediment.

  In other birds, where the legs are short, these are held close against the belly [25] during flight. In some cases this is merely to keep the feet out of the way, but in birds that have talons the position has a further purpose, being the one best suited for rapine. Birds that have a long and a thick neck keep it stretched out during flight; but those whose neck though long is slender fly with it coiled up. For in this position it is protected, and less likely to get broken, should the bird fly against any obstacle.

  [695a1] In all birds there is an ischium, but in such a way that it would scarcely be taken for one, but rather for a second thigh-bone because of its length; for it extends as far as to the middle of the belly. The reason for this is that the bird is a biped, and yet is unable to stand erect. For if its ischium extended but a short way from the [5] fundament, and then immediately came the leg, as is the case in man and in quadrupeds, the bird would be unable to stand up at all. For while man stands erect, and while quadrupeds have their heavy bodies propped up in front by the forelegs, birds can neither stand erect owing to their dwarf-like shape, nor have anterior legs [10] to prop them up, these legs being replaced by wings. As a remedy for
this nature has given them a long ischium, and brought it to the centre of the body, fixing it firmly; and she has placed the legs under this, so that the weight on either side may be equally balanced, and standing or progression rendered possible. Such then is the reason why a bird, though it is a biped, does not stand erect. The reason why its legs are destitute of flesh is the same as in the case of quadrupeds, about which we have already spoken.

  In all birds alike, whether web-footed or not, the number of toes in each foot is [15] four. For the Libyan ostrich may be disregarded for the present, and its cloven hoof and other discrepancies of structure as compared with the tribe of birds will be considered further on. Of these four toes three are in front, while the fourth points backwards, serving as a heel, to give steadiness. In the long-legged birds this fourth [20] toe is much shorter than the others, as is the case with the corncrake, but the number of their toes is not increased. The arrangement of the toes is such as has been described in all birds with the exception of the wryneck. Here only two of the toes are in front, the other two behind; and the reason for this is that the body of the wryneck is not inclined forward so much as that of other birds. All birds have [25] testicles; but they are inside the body. The reason for this will be given in the treatise on the Generation of Animals.

  13 · Thus then are fashioned the parts of birds. But in fishes a still further [695b1] stunting has occurred in the external parts. For here, for reasons already given, there are neither legs nor hands nor wings, the whole body from head to tail [5] presenting one unbroken surface. This tail differs in different fishes, in some approximating in character to the fins,20 while in some of the flat kinds, it is spinous and elongated, because the material which should have gone to the tail has been diverted thence and used to increase the breadth of the body. Such, for instance, is the case with the torpedos, the sting-rays, and whatever other Selachia there may be of like nature. In such fishes, then, the tail is spinous and long; while in some others [10] it is short and fleshy, for the same reason which makes it spinous and long in the torpedo. For to be short and fleshy comes to the same thing as to be long and less amply furnished with flesh.

  The opposite has occurred in the fishing-frog; for here the anterior and broad part of the body is not of a fleshy character, and so all the fleshy substance which [15] has been thence diverted has been placed by nature in the tail and hinder portion of the body.

  In fishes there are no limbs attached to the body. For in accordance with their essential substance they are swimming animals; and nature never makes anything superfluous or in vain. Now fish are sanguineous in their substance; and since they [20] are made for swimming they have fins, and as they are not made for walking they are without feet; for feet are attached to the body that they may be of use in progression on land. Moreover, fishes cannot have feet, or any other similar limbs, as well as four fins; for they are sanguineous animals. The cordylus, though it has [25] gills, has feet, for it has no fins but has its tail flattened out and loose in texture.

  Fishes, unless, like the ray and the sting-ray they are broad and flat, have four [696a1] fins, two on the upper and two on the under side of the body; and no fish ever has more than these. For, if it had, it would be a bloodless animal.

  The upper pair of fins is present in nearly all fishes, but not so the under pair; for these are wanting in some of those fishes that have long thick bodies, such as the [5] eel, the conger, and a certain kind of mullet that is found in the lake at Siphae. When the body is still more elongated, and resembles that of a serpent as is the case in the muraena, there are absolutely no fins at all; and locomotion is effected by the flexures of the body, the water being put to the same use by these fins as is the ground by serpents. For serpents swim in water exactly in the same way as they [10] glide on the ground. The reason for these serpent-like fishes being without fins is the same as that which causes serpents to be without feet; and what this is has been already stated in the works on the Progression and the Movement of Animals. The reason was this. If the points of motion were four, motion would be effected under difficulties; for either the two pairs of fins would be close to each other, in which case motion would scarcely be possible, or they would be at a very considerable [15] distance apart, in which case the long interval between them would be just as great an evil. On the other hand, to have more than four such motor points would convert the fishes into bloodless animals. A similar explanation applies to the case of those fishes that have only two fins. For here again the body is of great length and like that of a serpent, and its undulations do the office of the two missing fins. It is owing to this that such fishes can even crawl on dry ground, and can live there for a [20] considerable time; and some do not begin to gasp at once, while others, whose nature is akin to that of land-animals, are still less affected. In such fishes as have but two fins it is the upper pair that is present, excepting when the flat broad shape of the body prevents this. The fins in such cases are placed at the head, because in this region there is no elongation, which might serve in the absence of fins as a [25] means of locomotion; whereas in the direction of the tail there is a considerable lengthening out in fishes of this conformation. As for the rays and the like, they use the marginal part of their flattened bodies in place of fins for swimming.

  Fish which are not so flat, e.g. the torpedo and the fishing-frog, have fins: the upper pair are placed further back because of the flatness of the fore parts, and the under pair are placed close to the head (for the flatness does not prevent it from [30] moving), while to compensate for this advancement they are smaller than the upper ones. In the torpedo two fins are placed on the tail, and the fish uses the broad expansion of its body to supply the place of the other two, each lateral half of its circumference serving the office of a fin.

  The head, with its several parts, as also the organs of sense, have already come under consideration.

  There is one peculiarity which distinguishes fishes from all other sanguineous [696b1] animals, namely, the possession of gills. Why they have these organs has been set forth in the treatise on Respiration. These gills are in most fishes covered by opercula, but in the Selachia there are no such coverings. For an operculum requires [5] fish-spine for its formation, and in other fishes the skeleton is made of this substance, whereas in the Selachia it is invariably formed of cartilage. Again, while the motions of spinous fishes are rapid, those of the Selachia are sluggish, inasmuch as they have neither fish-spine nor sinew; but an operculum requires rapidity of motion, seeing that the office of the gills is to minister as it were to expiration. For this reason in Selachia the branchial orifices themselves effect their own closure, [10] and thus there is no need for an operculum to ensure its taking place with due rapidity. In some fishes the gills are numerous, in others few in number; in some again they are double, in others single. The last gill in most cases is single. For a detailed account of all this, reference must be made to the Anatomies and to the [15] History of Animals.

  It is the abundance or the deficiency of the cardiac heat which determines the abundance or deficiency of the gills. For, the greater an animal’s heat, the more rapid and the more forcible does it require the branchial movement to be; and numerous and double gills act with more force and rapidity than such as are few and [20] single. Thus, too, it is that some fishes that have but few gills, and those of comparatively small efficacy, can live out of water for a considerable time; for in them there is no great demand for refrigeration. Such, for example, are the eel and all other fishes of serpent-like form.

  Fishes also present diversities as regards the mouth. For in some this is placed in front, at the very extremity of the body, while in others, as the dolphin and the [25] Selachia, it is placed on the under surface; so that these fishes turn on the back in order to take their food. The purpose of nature in this was apparently not merely to provide a means of salvation for other animals, by allowing them opportunity of escape during the time lost in the act of turning—for all the fi
shes with this kind of mouth prey on living animals—but also to prevent these fishes from giving way too [30] much to their gluttonous ravening after food. For had they been able to seize their prey more easily than they do, they would soon have perished from over-repletion. An additional reason is that their snout is round and small, and therefore cannot admit of a wide opening.

  Again, even when the mouth is not placed on the under surface, there are differences in the extent to which it can open. For in some cases it can gape widely, [697a1] while in others it is set at the point of a small tapering snout; the former being the case in carnivorous fishes, such as those with saw teeth, whose strength lies in their mouth, while the latter is its form in all such as are not carnivorous.

  The skin is in some fishes covered with scales (the scale of a fish is a thin and [5] shiny film, and therefore easily becomes detached from the surface of the body). In others it is rough, as for instance in the angel-fish, the ray, and the like. Fewest of all are those whose skin is smooth. The Selachia have no scales, but a rough skin. This is explained by their cartilaginous skeleton. For the earthy material which has been thence diverted is expended by nature upon the skin.