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  plant is to produce seed and fruit. But still as, in the case of

  nutriment, animals with straight intestines are more violent in

  their desire for food, so those which have not testes but only

  ducts, or which have them indeed but internally, are all quicker in

  accomplishing copulation. But those which are to be more temperate

  in the one case have not straight intestines, and in the other have

  their ducts twisted to prevent their desire being too violent and

  hasty. It is for this that the testes are contrived; for they make the

  movement of the spermatic secretion steadier, preserving the folding

  back of the passages in the vivipara, as horses and the like, and in

  man. (For details see the Enquiries about Animals.) For the testes

  are no part of the ducts but are only attached to them, as women

  fasten stones to the loom when weaving; if they are removed the

  ducts are drawn up internally, so that castrated animals are unable to

  generate; if they were not drawn up they would be able, and before now

  a bull mounting immediately after castration has caused conception

  in the cow because the ducts had not yet been drawn up. In birds and

  oviparous quadrupeds the testes receive the spermatic secretion, so

  that its expulsion is slower than in fishes. This is clear in the case

  of birds, for their testes are much enlarged at the time of

  copulation, and all those which pair at one season of the year have

  them so small when this is past that they are almost indiscernible,

  but during the season they are very large. When the testes are

  internal the act of copulation is quicker than when they are external,

  for even in the latter case the semen is not emitted before the testes

  are drawn up.

  5

  Besides, quadrupeds have the organ of copulation, since it is

  possible for them to have it, but for birds and the footless animals

  it is not possible, because the former have their legs under the

  middle of the abdomen and the latter have no legs at all; now the

  penis depends from that region and is situated there. (Wherefore also

  the legs are strained in intercourse, both the penis and the legs

  being sinewy.) So that, since it is not possible for them to have

  this organ, they must necessarily either have no testes also, or at

  any rate not have them there, as those animals that have both penis

  and testes have them in the same situation.

  Further, with those animals at any rate that have external testes,

  the semen is collected together before emission, and emission is due

  to the penis being heated by its movement; it is not ready for

  emission at immediate contact as in fishes.

  All the vivipira have their testes in front, internally or

  externally, except the hedgehog; he alone has them near the loin. This

  is for the same reason as with birds, because their union must be

  quick, for the hedgehog does not, like the other quadrupeds, mount

  upon the back of the female, but they conjugate standing upright

  because of their spines.

  So much for the reasons why those animals have testes which have

  them, and why they are sometimes external and sometimes internal.

  6

  All those animals which have no testes are deficient in this part,

  as has been said, not because it is better to be so but simply because

  of necessity, and secondly because it is necessary that their

  copulation should be speedy. Such is the nature of fish and

  serpents. Fish copulate throwing themselves alongside of the females

  and separating again quickly. For as men and all such creatures must

  hold their breath before emitting the semen, so fish at such times

  must cease taking in the sea-water, and then they perish easily.

  Therefore they must not mature the semen during copulation, as

  viviparous land-animals do, but they have it all matured together

  before the time, so as not to be maturing it while in contact but to

  emit it ready matured. So they have no testes, and the ducts are

  straight and simple. There is a small part similar to this connected

  with the testes in the system of quadrupeds, for part of the reflected

  duct is sanguineous and part is not; the fluid is already semen when

  it is received by and passes through this latter part, so that once it

  has arrived there it is soon emitted in these quadrupeds also. Now

  in fishes the whole passage resembles the last section of the

  reflected part of the duct in man and similar animals.

  7

  Serpents copulate twining round one another, and, as said above,

  have neither testes nor penis, the latter because they have no legs,

  the former because of their length, but they have ducts like for on

  account of their extreme length the seminal fluid would take too

  long in its passage and be cooled if it were further delayed by

  testes. (This happens also if the penis is large; such men are less

  fertile than when it is smaller because the semen, if cold, is not

  generative, and that which is carried too far is cooled.) So much for

  the reason why some animals have testes and others not. Serpents

  intertwine because of their inaptitude to cast themselves alongside of

  one another. For they are too long to unite closely with so small a

  part and have no organs of attachment, so they make use of the

  suppleness of their bodies, intertwining. Wherefore also they seem

  to be slower in copulation than fish, not only on account of the

  length of the ducts but also of this elaborate arrangement in uniting.

  8

  It is not easy to state the facts about the uterus in female

  animals, for there are many points of difference. The vivipara are not

  alike in this part; women and all the vivipara with feet have the

  uterus low down by the pudendum, but the cartilaginous viviparous fish

  have it higher up near the hypozoma. In the ovipara, again, it is

  low in fish (as in women and the viviparous quadrupeds), high in

  birds and all oviparous quadrupeds. Yet even these differences are

  on a principle. To begin with the ovipara, they differ in the manner

  of laying their eggs, for some produce them imperfect, as fishes whose

  eggs increase and are finally developed outside of them. The reason is

  that they produce many young, and this is their function as it is with

  plants. If then they perfected the egg in themselves they must needs

  be few in number, but as it is, they have so many that each uterus

  seems to be an egg, at any rate in the small fishes. For these are the

  most productive, just as with the other animals and plants whose

  nature is analogous to theirs, for the increase of size turns with

  them to seed.

  But the eggs of birds and the quadrupedal ovipara are perfect when

  produced. In order that these may be preserved they must have a hard

  covering (for their envelope is soft so long as they are increasing

  in size), and the shell is made by heat squeezing out the moisture

  for the earthy material; consequently the place must be hot in which

  this is to happen. But the part about the hypozoma is hot, as is shown

  by that being the part which concocts the food. If then
the eggs

  must be within the uterus, then the uterus must be near the hypozoma

  in those creatures which produce their eggs in a perfect form.

  Similarly it must be low down in those which produce them imperfect,

  for it is profitable that it should be so. And it is more natural

  for the uterus to be low down than high up, when Nature has no other

  business in hand to hinder it; for its end is low down, and where is

  the end, there is the function, and the uterus itself is naturally

  where the function is.

  9

  We find differences in the vivipara also as compared with one

  another. Some produce their young alive, not only externally, but also

  internally, as men, horses, dogs, and all those which have hair, and

  among aquatic animals, dolphins, whales, and such cetacea.

  10

  But the cartilaginous fish and the vipers produce their young

  alive externally, but first produce eggs internally. The egg is

  perfect, for so only can an animal be generated from an egg, and

  nothing comes from an imperfect one. It is because they are of a

  cold nature, not hot as some assert, that they do not lay their eggs

  externally.

  11

  At least they certainly produce their eggs in a soft envelope, the

  reason being that they have but little heat and so their nature does

  not complete the process of drying the egg-shell. Because, then,

  they are cold they produce soft-shelled eggs, and because the eggs are

  soft they do not produce them externally; for that would have caused

  their destruction.

  The process is for the most part the same as in birds, for the egg

  descends and the young is hatched from it near the vagina, where the

  young is produced in those animals which are viviparous from the

  beginning. Therefore in such animals the uterus is dissimilar to

  that of both the vivipara and ovipara, because they participate in

  both classes; for it is at once near the hypozoma and also

  stretching along downwards in all the cartilaginous fishes. But the

  facts about this and the other kinds of uterus must be gathered from

  inspection of the drawings of dissections and from the Enquiries.

  Thus, because they are oviparous, laying perfect eggs, they have the

  uterus placed high, but, as being viviparous, low, participating in

  both classes.

  Animals that are viviparous from the beginning all have it low,

  Nature here having no other business to interfere with her, and

  their production having no double character. Besides this, it is

  impossible for animals to be produced alive near the hypozoma, for the

  foetus must needs be heavy and move, and that region in the mother

  is vital and would not be able to bear the weight and the movement.

  Thirdly, parturition would be difficult because of the length of the

  passage to be traversed; even as it is there is difficulty with

  women if they draw up the uterus in parturition by yawning or anything

  of the kind, and even when empty it causes a feeling of suffocation if

  moved upwards. For if a uterus is to hold a living animal it must be

  stronger than in ovipara, and therefore in all the vivipara it is

  fleshy, whereas when the uterus is near the hypozoma it is membranous.

  And this is clear also in the case of the animals which produce

  young by the mixed method, for their eggs are high up and sideways,

  but the living young are produced in the lower part of the uterus.

  So much for the reason why differences are found in the uterus of

  various animals, and generally why it is low in some and high in

  others near the hypozoma.

  12

  Why is the uterus always internal, but the testes sometimes

  internal, sometimes external? The reason for the uterus always being

  internal is that in this is contained the egg or foetus, which needs

  guarding, shelter, and maturation by concoction, while the outer

  surface of the body is easily injured and cold. The testes vary in

  position because they also need shelter and a covering to preserve

  them and to mature the semen; for it would be impossible for them,

  if chilled and stiffened, to be drawn up and discharge it.

  Therefore, whenever the testes are visible, they have a cuticular

  covering known as the scrotum. If the nature of the skin is opposed to

  this, being too hard to be adapted for enclosing them or for being

  soft like a true 'skin', as with the scaly integument of fish and

  reptiles, then the testes must needs be internal. Therefore they are

  so in dolphins and all the cetacea which have them, and in the

  oviparous quadrupeds among the scaly animals. The skin of birds also

  is hard so that it will not conform to the size of anything and

  enclose it neatly. (This is another reason with all these animals for

  their testes being internal besides those previously mentioned as

  arising necessarily from the details of copulation.) For the same

  reason they are internal in the elephant and hedgehog, for the skin of

  these, too, is not well suited to keep the protective part separate.

  [The position of the uterus differs in animals viviparous within

  themselves and those externally oviparous, and in the latter class

  again it differs in those which have the uterus low and those which

  have it near the hypozoma, as in fishes compared with birds and

  oviparous quadrupeds. And it is different again in those which produce

  young in both ways, being oviparous internally and viviparous

  externally. For those which are viviparous both internally and

  externally have the uterus placed on the abdomen, as men, cattle,

  dogs, and the like, since it is expedient for the safety and growth of

  the foetus that no weight should be upon the uterus.]

  13

  The passages also are different through which the solid and liquid

  excreta pass out in all the vivipara. Wherefore both males and females

  in this class all have a part whereby the urine is voided, and this

  serves also for the issue of the semen in males, of the offspring in

  females. This passage is situated above and in front of the passage of

  the solid excreta. The passage is the same as that of the solid

  nutriment in all those animals that have no penis, in all the ovipara,

  even those of them that have a bladder, as the tortoises. For it is

  for the sake of generation, not for the evacuation of the urine,

  that the passages are double; but because the semen is naturally

  liquid, the liquid excretion also shares the same passage. This is

  clear from the fact that all animals produce semen, but all do not

  void liquid excrement. Now the spermatic passages of the male must

  be fixed and must not wander, and the same applies to the uterus of

  the female, and this fixing must take place at either the front or the

  back of the body. To take the uterus first, it is in the front of

  the body in vivipara because of the foetus, but at the loin and the

  back in ovipara. All animals which are internally oviparous and

  externally viviparous are in an intermediate condition because they

  participate in both classes, being at once oviparous and viviparous.

  For the u
pper part of the uterus, where the eggs are produced, is

  under the hypozoma by the loin and the back, but as it advances is low

  at the abdomen; for it is in that part that the animal is

  viviparous. In these also the passage for solid excrement and for

  copulation is the same, for none of these, as has been said already,

  has a separate pudendum.

  The same applies to the passages in the male, whether they have

  testes or no, as to the uterus of the ovipara. For in all of them, not

  only in the ovipara, the ducts adhere to the back and the region of

  the spine. For they must not wander but be settled, and that is the

  character of the region of the back, which gives continuity and

  stability. Now in those which have internal testes, the ducts are

  fixed from the first, and they are fixed in like manner if the

  testes are external; then they meet together towards the region of the

  penis.

  The like applies to the ducts in the dolphins, but they have their

  testes hidden under the abdominal cavity.

  We have now discussed the situation of the parts contributing to

  generation, and the causes thereof.

  14

  The bloodless animals do not agree either with the sanguinea or with

  each other in the fashion of the parts contributing to generation.

  There are four classes still left to deal with, first the crustacea,

  secondly the cephalopoda, thirdly the insects, and fourthly the

  testacea. We cannot be certain about all of them, but that most of

  them copulate is plain; in what manner they unite must be stated

  later.

  The crustacea copulate like the retromingent quadrupeds, fitting

  their tails to one another, the one supine and the other prone. For

  the flaps attached to the sides of the tail being long prevent them

  from uniting with the belly against the back. The males have fine

  spermatic ducts, the females a membranous uterus alongside the

  intestine, cloven on each side, in which the egg is produced.