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  like an insect, has a body that admits of being curled up, its

  vertebrae being cartilaginous and easily bent. The faculty in question

  belongs then to serpents simply as a necessary consequence of this

  character of their vertebrae; but at the same time it has a final

  cause, for it enables them to guard against attacks from behind. For

  their body, owing to its length and the absence of feet, is ill-suited

  for turning round and protecting the hinder parts; and merely to

  lift the head, without the power of turning it round, would be of no

  use whatsoever.

  The animals with which we are dealing have, moreover, a part which

  corresponds to the breast; but neither here nor elsewhere in their

  body have they any mammae, as neither has any bird or fish. This is

  a consequence of their having no milk; for a mamma is a receptacle for

  milk and, as it were, a vessel to contain it. This absence of milk

  is not peculiar to these animals, but is common to all such as are not

  internally viviparous. For all such produce eggs, and the nutriment

  which in Vivipara has the character of milk is in them engendered in

  the egg. Of all this, however, a clearer account will be given in

  the treatise on Generation. As to the mode in which the legs bend, a

  general account, in which all animals are considered, has already been

  given in the dissertation on Progression. These animals also have a

  tail, larger in some of them, smaller in others, and the reason for

  this has been stated in general terms in an earlier passage.

  Of all oviparous animals that live on land there is none so lean

  as the Chamaeleon. For there is none that has so little blood. The

  explanation of this is to be found in the psychical temperament of the

  creature. For it is of a timid nature, as the frequent changes it

  undergoes in its outward aspect testify. But fear is a

  refrigeration, and results from deficiency of natural heat and

  scantiness of blood. We have now done with such sanguineous animals as

  are quadrupedous and also such as are apodous, and have stated with

  sufficient completeness what external parts they possess, and for what

  reason they have them.

  12

  The differences of birds compared one with another are differences

  of magnitude, and of the greater or smaller development of parts. Thus

  some have long legs, others short legs; some have a broad tongue,

  others a narrow tongue; and so on with the other parts. There are

  few of their parts that differ save in size, taking birds by

  themselves. But when birds are compared with other animals the parts

  present differences of form also. For in some animals these are hairy,

  in others scaly, and in others have scale-like plates, while birds are

  feathered.

  Birds, then, are feathered, and this is a character common to them

  all and peculiar to them. Their feathers, too, are split and

  distinct in kind from the undivided feathers of insects; for the

  bird's feather is barbed, these are not; the bird's feather has a

  shaft, these have none. A second strange peculiarity which

  distinguishes birds from all other animals is their beak. For as in

  elephants the nostril serves in place of hands, and as in some insects

  the tongue serves in place of mouth, so in birds there is a beak,

  which, being bony, serves in place of teeth and lips. Their organs

  of sense have already been considered.

  All birds have a neck extending from the body; and the purpose of

  this neck is the same as in such other animals as have one. This

  neck in some birds is long, in others short; its length, as a

  general rule, being pretty nearly determined by that of the legs.

  For long-legged birds have a long neck, short-legged birds a short

  one, to which rule, however, the web-footed birds form an exception.

  For to a bird perched up on long legs a short neck would be of no

  use whatsoever in collecting food from the ground; and equally useless

  would be a long neck, if the legs were short. Such birds, again, as

  are carnivorous would find length in this part interfere greatly

  with their habits of life. For a long neck is weak, and it is on their

  superior strength that carnivorous birds depend for their subsistence.

  No bird, therefore, that has talons ever has an elongated neck. In

  web-footed birds, however, and in those other birds belonging to the

  same class, whose toes though actually separate have flat marginal

  lobes, the neck is elongated, so as to be suitable for collecting food

  from the water; while the legs are short, so as to serve in

  swimming. The beaks of birds, as their feet, 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 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 get its food from some depth below the surface. For most birds of

  this kind, and most of those whose feet 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 use these parts for

  their capture, the neck acting as a fishing-rod, and 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 attached to it, but in their stead wings,

  which are a distinctive peculiarity of these animals; and, as these

  wings are substitutes for arms, their terminal segments 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, 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 essentially 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 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 of the essence 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

&n
bsp; the wings will give them four points for 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.

  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 portion of the vascular system, 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 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 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 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, it has no use for spurs; for these are

  weapons for fighting on the ground, and on this account are an

  appanage of 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

  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 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 the legs with

  the material for their elongation; or sometimes, in place of this,

  fills up the interspaces between the toes. Thus it is simply a

  matter of necessity, that such birds as swim shall either be

  actually web-footed, or shall have a kind of broad blade-like margin

  running along the whole length of each distinct toe. The forms,

  then, of these feet are simply 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 these feet are so developed as to resemble the oars of a

  boat, or the fins of a fish; and the destruction of the foot-web has

  the same effect as the destruction of the fins; that is to say, it

  puts an end to all power of swimming.

  In some birds the legs are very long, the cause of this being that

  they inhabit marshes. I say the cause, because nature makes the organs

  for the function, and not the function for 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 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 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.

  In all birds there is an ischium, but so placed and of such length

  that it would scarcely be taken for an ischium, but rather for a

  second thigh-bone; 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 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 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 central point, 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. Why its legs are destitute of flesh has also already been

  stated; for the reasons are the same as in the case of quadrupeds.

  In all birds alike, whether web-footed or not, the number of toes in

  each foot is four. For the Liby
an 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

  backward, serving, as a heel, to give steadiness. In the long-legged

  birds this fourth toe is much shorter than the others, as is the

  case with the Crex, 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 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 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 presenting one unbroken surface. This

  tail differs in different fishes, in some approximating in character

  to the fins, while in others, namely 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 Trygons, and whatever other Selachia there may be of

  like nature. In such fishes, then, the tail is spinous and long; while

  in some others 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.

  What has occurred in the Fishing-frog is the reverse of what has

  occurred in the other instances just given. For here the anterior

  and broad part of the body is not of a fleshy character, and so all

  the fleshy substance which has been thence diverted has been placed by