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  into the male). For if a hen-bird is in process of producing

  wind-eggs and is then trodden by the cock before the egg has begun

  to whiten and while it is all still yellow, then they become fertile

  instead of being wind-eggs. And if while it is still yellow she be

  trodden by another cock, the whole brood of chicks turn out like the

  second cock. Hence some of those who are anxious to rear fine birds

  act thus; they change the cocks for the first and second treading, not

  as if they thought that the semen is mingled with the egg or exists in

  it, or that it comes from all parts of the cock; for if it did it

  would have come from both cocks, so that the chick would have all

  its parts doubled. But it is by its force that the semen of the male

  gives a certain quality to the material and the nutriment in the

  female, for the second semen added to the first can produce this

  effect by heat and concoction, as the egg acquires nutriment so long

  as it is growing.

  The same conclusion is to be drawn from the generation of

  oviparous fishes. When the female has laid her eggs, the male spinkles

  the milt over them, and those eggs are fertilized which it reaches,

  but not the others; this shows that the male does not contribute

  anything to the quantity but only to the quality of the embryo.

  From what has been said it is plain that the semen does not come

  from the whole of the body of the male in those animals which emit it,

  and that the contribution of the female to the generative product is

  not the same as that of the male, but the male contributes the

  principle of movement and the female the material. This is why the

  female does not produce offspring by herself, for she needs a

  principle, i.e. something to begin the movement in the embryo and to

  define the form it is to assume. Yet in some animals, as birds, the

  nature of the female unassisted can generate to a certain extent,

  for they do form something, only it is incomplete; I mean the

  so-called wind-eggs.

  22

  For the same reason the development of the embryo takes place in the

  female; neither the male himself nor the female emits semen into the

  male, but the female receives within herself the share contributed

  by both, because in the female is the material from which is made

  the resulting product. Not only must the mass of material exist

  there from which the embryo is formed in the first instance, but

  further material must constantly be added that it may increase in

  size. Therefore the birth must take place in the female. For the

  carpenter must keep in close connexion with his timber and the

  potter with his clay, and generally all workmanship and the ultimate

  movement imparted to matter must be connected with the material

  concerned, as, for instance, architecture is in the buildings it

  makes.

  From these considerations we may also gather how it is that the male

  contributes to generation. The male does not emit semen at all in some

  animals, and where he does this is no part of the resulting embryo;

  just so no material part comes from the carpenter to the material,

  i.e. the wood in which he works, nor does any part of the

  carpenter's art exist within what he makes, but the shape and the form

  are imparted from him to the material by means of the motion he sets

  up. It is his hands that move his tools, his tools that move the

  material; it is his knowledge of his art, and his soul, in which is

  the form, that moves his hands or any other part of him with a

  motion of some definite kind, a motion varying with the varying nature

  of the object made. In like manner, in the male of those animals which

  emit semen Nature uses the semen as a tool and as possessing motion in

  actuality, just as tools are used in the products of any art, for in

  them lies in a certain sense the motion of the art. Such, then, is the

  way in which these males contribute to generation. But when the male

  does not emit semen, but the female inserts some part of herself

  into the male, this is parallel to a case in which a man should

  carry the material to the workman. For by reason of weakness in such

  males Nature is not able to do anything by any secondary means, but

  the movements imparted to the material are scarcely strong enough when

  Nature herself watches over them. Thus here she resembles a modeller

  in clay rather than a carpenter, for she does not touch the work she

  is forming by means of tools, but, as it were, with her own hands.

  23

  In all animals which can move about, the sexes are separated, one

  individual being male and one female, though both are the same in

  species, as with man and horse. But in plants these powers are

  mingled, female not being separated from male. Wherefore they generate

  out of themselves, and do not emit semen but produce an embryo, what

  is called the seed. Empedocles puts this well in the line: 'and thus

  the tall trees oviposit; first olives...' For as the egg is an embryo,

  a certain part of it giving rise to the animal and the rest being

  nutriment, so also from a part of the seed springs the growing

  plant, and the rest is nutriment for the shoot and the first root.

  In a certain sense the same thing happens also in those animals

  which have the sexes separate. For when there is need for them to

  generate the sexes are no longer separated any more than in plants,

  their nature desiring that they shall become one; and this is plain to

  view when they copulate and are united, that one animal is made out of

  both.

  It is the nature of those creatures which do not emit semen to

  remain united a long time until the male element has formed the

  embryo, as with those insects which copulate. The others so remain

  only until the male has discharged from the parts of himself

  introduced something which will form the embryo in a longer time, as

  among the sanguinea. For the former remain paired some part of a

  day, while the semen forms the embryo in several days. And after

  emitting this they cease their union.

  And animals seem literally to be like divided plants, as though

  one should separate and divide them, when they bear seed, into the

  male and female existing in them.

  In all this Nature acts like an intelligent workman. For to the

  essence of plants belongs no other function or business than the

  production of seed; since, then, this is brought about by the union of

  male and female, Nature has mixed these and set them together in

  plants, so that the sexes are not divided in them. Plants, however,

  have been investigated elsewhere. But the function of the animal is

  not only to generate (which is common to all living things), but

  they all of them participate also in a kind of knowledge, some more

  and some less, and some very little indeed. For they have

  sense-perception, and this is a kind of knowledge. (If we consider

  the value of this we find that it is of great importance compared with

  the class of lifeless objects, but of little compared with the use

  of the intellect. For against
the latter the mere participation in

  touch and taste seems to be practically nothing, but beside absolute

  insensibility it seems most excellent; for it would seem a treasure to

  gain even this kind of knowledge rather than to lie in a state of

  death and non-existence.) Now it is by sense-perception that an

  animal differs from those organisms which have only life. But since,

  if it is a living animal, it must also live; therefore, when it is

  necessary for it to accomplish the function of that which has life, it

  unites and copulates, becoming like a plant, as we said before.

  Testaceous animals, being intermediate between animals and plants,

  perform the function of neither class as belonging to both. As

  plants they have no sexes, and one does not generate in another; as

  animals they do not bear fruit from themselves like plants; but they

  are formed and generated from a liquid and earthy concretion. However,

  we must speak later of the generation of these animals.

  Book II

  1

  THAT the male and the female are the principles of generation has

  been previously stated, as also what is their power and their essence.

  But why is it that one thing becomes and is male, another female? It

  is the business of our discussion as it proceeds to try and point

  out (1) that the sexes arise from Necessity and the first efficient

  cause, (2) from what sort of material they are formed. That (3) they

  exist because it is better and on account of the final cause, takes us

  back to a principle still further remote.

  Now (1) some existing things are eternal and divine whilst others

  admit of both existence and non-existence. But (2) that which is noble

  and divine is always, in virtue of its own nature, the cause of the

  better in such things as admit of being better or worse, and what is

  not eternal does admit of existence and non-existence, and can partake

  in the better and the worse. And (3) soul is better than body, and

  living, having soul, is thereby better than the lifeless which has

  none, and being is better than not being, living than not living.

  These, then, are the reasons of the generation of animals. For since

  it is impossible that such a class of things as animals should be of

  an eternal nature, therefore that which comes into being is eternal in

  the only way possible. Now it is impossible for it to be eternal as an

  individual (though of course the real essence of things is in the

  individual)- were it such it would be eternal- but it is possible

  for it as a species. This is why there is always a class of men and

  animals and plants. But since the male and female essences are the

  first principles of these, they will exist in the existing individuals

  for the sake of generation. Again, as the first efficient or moving

  cause, to which belong the definition and the form, is better and more

  divine in its nature than the material on which it works, it is better

  that the superior principle should be separated from the inferior.

  Therefore, wherever it is possible and so far as it is possible, the

  male is separated from the female. For the first principle of the

  movement, or efficient cause, whereby that which comes into being is

  male, is better and more divine than the material whereby it is

  female. The male, however, comes together and mingles with the

  female for the work of generation, because this is common to both.

  A thing lives, then, in virtue of participating in the male and

  female principles, wherefore even plants have some kind of life; but

  the class of animals exists in virtue of sense-perception. The sexes

  are divided in nearly all of these that can move about, for the

  reasons already stated, and some of them, as said before, emit semen

  in copulation, others not. The reason of this is that the higher

  animals are more independent in their nature, so that they have

  greater size, and this cannot exist without vital heat; for the

  greater body requires more force to move it, and heat is a motive

  force. Therefore, taking a general view, we may say that sanguinea are

  of greater size than bloodless animals, and those which move about

  than those which remain fixed. And these are just the animals which

  emit semen on account of their heat and size.

  So much for the cause of the existence of the two sexes. Some

  animals bring to perfection and produce into the world a creature like

  themselves, as all those which bring their young into the world alive;

  others produce something undeveloped which has not yet acquired its

  own form; in this latter division the sanguinea lay eggs, the

  bloodless animals either lay an egg or give birth to a scolex. The

  difference between egg and scolex is this: an egg is that from a

  part of which the young comes into being, the rest being nutriment for

  it; but the whole of a scolex is developed into the whole of the young

  animal. Of the vivipara, which bring into the world an animal like

  themselves, some are internally viviparous (as men, horses, cattle,

  and of marine animals dolphins and the other cetacea); others first

  lay eggs within themselves, and only after this are externally

  viviparous (as the cartilaginous fishes). Among the ovipara some

  produce the egg in a perfect condition (as birds and all oviparous

  quadrupeds and footless animals, e.g. lizards and tortoises and most

  snakes; for the eggs of all these do not increase when once laid).

  The eggs of others are imperfect; such are those of fishes,

  crustaceans, and cephalopods, for their eggs increase after being

  produced.

  All the vivipara are sanguineous, and the sanguinea are either

  viviparous or oviparous, except those which are altogether

  infertile. Among bloodless animals the insects produce a scolex, alike

  those that are generated by copulation and those that copulate

  themselves though not so generated. For there are some insects of this

  sort, which though they come into being by spontaneous generation

  are yet male and female; from their union something is produced,

  only it is imperfect; the reason of this has been previously stated.

  These classes admit of much cross-division. Not all bipeds are

  viviparous (for birds are oviparous), nor are they all oviparous

  (for man is viviparous), nor are all quadrupeds oviparous (for

  horses, cattle, and countless others are viviparous), nor are they

  all viviparous (for lizards, crocodiles, and many others lay eggs).

  Nor does the presence or absence of feet make the difference

  between them, for not only are some footless animals viviparous, as

  vipers and the cartilaginous fishes, while others are oviparous, as

  the other fishes and serpents, but also among those which have feet

  many are oviparous and many viviparous, as the quadrupeds above

  mentioned. And some which have feet, as man, and some which have

  not, as the whale and dolphin, are internally viviparous. By this

  character then it is not possible to divide them, nor is any of the

  locomotive organs the cause of this difference, but it is those

  animals which are more perfect in their nature and parti
cipate in a

  purer element which are viviparous, for nothing is internally

  viviparous unless it receive and breathe out air. But the more perfect

  are those which are hotter in their nature and have more moisture

  and are not earthy in their composition. And the measure of natural

  heat is the lung when it has blood in it, for generally those

  animals which have a lung are hotter than those which have not, and in

  the former class again those whose lung is not spongy nor solid nor

  containing only a little blood, but soft and full of blood. And as the

  animal is perfect but the egg and the scolex are imperfect, so the

  perfect is naturally produced from the more perfect. If animals are

  hotter as shown by their possessing a lung but drier in their

  nature, or are colder but have more moisture, then they either lay a

  perfect egg or are viviparous after laying an egg within themselves.

  For birds and scaly reptiles because of their heat produce a perfect

  egg, but because of their dryness it is only an egg; the cartilaginous

  fishes have less heat than these but more moisture, so that they are

  intermediate, for they are both oviparous and viviparous within

  themselves, the former because they are cold, the latter because of

  their moisture; for moisture is vivifying, whereas dryness is furthest

  removed from what has life. Since they have neither feathers nor

  scales such as either reptiles or other fishes have, all which are

  signs rather of a dry and earthy nature, the egg they produce is soft;

  for the earthy matter does not come to the surface in their eggs any

  more than in themselves. This is why they lay eggs in themselves,

  for if the egg were laid externally it would be destroyed, having no

  protection.

  Animals that are cold and rather dry than moist also lay eggs, but

  the egg is imperfect; at the same time, because they are of an

  earthy nature and the egg they produce is imperfect, therefore it

  has a hard integument that it may be preserved by the protection of

  the shell-like covering. Hence fishes, because they are scaly, and