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black; thus the change is not a violent one nor, so to say, contrary

  to Nature; at least, is it not a change into another nature. But in

  animals producing many young not only do the same phenomena occur, but

  also the numerous embryos hinder one another from becoming perfect and

  interfere with the generative motions imparted by the semen.

  A difficulty may be raised concerning (1) the production of many

  young and the multiplication of the parts in a single young one, and

  (2) the production of few young or only one and the deficiency of

  the parts. Sometimes animals are born with too many toes, sometimes

  with one alone, and so on with the other parts, for they may be

  multiplied or they may be absent. Again, they may have the

  generative parts doubled, the one being male, the other female; this

  is known in men and especially in goats. For what are called

  'tragaenae' are such because they have both male and female generative

  parts; there is a case also of a goat being born with a horn upon

  its leg. Changes and deficiencies are found also in the internal

  parts, animals either not possessing some at all, or possessing them

  in a rudimentary condition, or too numerous or in the wrong place.

  No animal, indeed, has ever been born without a heart, but they are

  born without a spleen or with two spleens or with one kidney; there is

  no case again of total absence of the liver, but there are cases of

  its being incomplete. And all these phenomena have been seen in

  animals perfect and alive. Animals also which naturally have a

  gall-bladder are found without one; others are found to have more than

  one. Cases are known, too, of the organs changing places, the liver

  being on the left, the spleen on the right. These phenomena have

  been observed, as stated above, in animals whose growth is

  perfected; at the time of birth great confusion of every kind has been

  found. Those deficiency which only depart a little from Nature

  commonly live; not so those which depart further, when the unnatural

  condition is in the parts which are sovereign over life.

  The question then about all these cases is this. Are we to suppose

  that a single cause is responsible for the production of a single

  young one and for the deficiency of the parts, and another but still a

  single cause for the production of many young and the multiplication

  of parts, or not?

  In the first place it seems only reasonable to wonder why some

  animals produce many young, others only one. For it is the largest

  animals that produce one, e.g. the elephant, camel, horse, and the

  other solid-hoofed ungulates; of these some are larger than all

  other animals, while the others are of a remarkable size. But the dog,

  the wolf, and practically all the fissipeds, produce many, even the

  small members of the class, as the mouse family. The cloven-footed

  animals again produce few, except the pig, which belongs to those that

  produce many. This certainly seems surprising, for we should expect

  the large animals to be able to generate more young and to secrete

  more semen. But precisely what we wonder at is the reason for not

  wondering; it is just because of their size that they do not produce

  many young, for the nutriment is expended in such animals upon

  increasing the body. But in the smaller animals Nature takes away from

  the size and adds the excess so gained to the seminal secretion.

  Moreover, more semen must needs be used in generation by the larger

  animal, and little by the smaller. Therefore many small ones may be

  produced together, but it is hard for many large ones to be so, and to

  those intermediate in size Nature has assigned the intermediate

  number. We have formerly given the reason why some animals are

  large, some smaller, and some between the two, and speaking generally,

  with regard to the number of young produced, the solid-hoofed

  produce one, the cloven-footed few, the many-toed many. (The reason

  of this is that, generally speaking, their sizes correspond to this

  difference.) It is not so, however, in all cases; for it is the

  largeness and smallness of the body that is cause of few or many young

  being born, not the fact that the kind of animal has one, two, or many

  toes. A proof of this is that the elephant is the largest of animals

  and yet is many-toed, and the camel, the next largest, is

  cloven-footed. And not only in animals that walk but also in those

  that fly or swim the large ones produce few, the small many, for the

  same reason. In like manner also it is not the largest plants that

  bear most fruit.

  We have explained then why some animals naturally produce many

  young, some but few, and some only one; in the difficulty now stated

  we may rather be surprised with reason at those which produce many,

  since such animals are often seen to conceive from a single

  copulation. Whether the semen of the male contributes to the

  material of the embryo by itself becoming a part of it and mixing with

  the semen of the female, or whether, as we say, it does not act in

  this way but brings together and fashions the material within the

  female and the generative secretion as the fig-juice does the liquid

  substance of milk, what is the reason why it does not form a single

  animal of considerable size? For certainly in the parallel case the

  fig-juice is not separated if it has to curdle a large quantity of

  milk, but the more the milk and the more the fig-juice put into it, so

  much the greater is the curdled mass. Now it is no use to say that the

  several regions of the uterus attract the semen and therefore more

  young than one are formed, because the regions are many and the

  cotyledons are more than one. For two embryos are often formed in

  the same region of the uterus, and they may be seen lying in a row

  in animals that produce many, when the uterus is filled with the

  embryos. (This is plain from the dissections.) Rather the truth is

  this. As animals complete their growth there are certain limits to

  their size, both upwards and downwards, beyond which they cannot go,

  but it is in the space between these limits that they exceed or fall

  short of one another in size, and it is within these limits that one

  man (or any other animal) is larger or smaller than another. So also

  the generative material from which each animal is formed is not

  without a quantitative limit in both directions, nor can it be

  formed from any quantity you please. Whenever then an animal, for

  the cause assigned, discharges more of the female secretion than is

  needed for beginning the existence of a single animal, it is not

  possible that only one should be formed out of all this, but a

  number limited by the appropriate size in each case; nor will the

  semen of the male, or the power residing in the semen, form anything

  either more or less than what is according to Nature. In like

  manner, if the male emits more semen than is necessary, or more powers

  in different parts of the semen as it is divided, however much it is

  it will not make anything greater; on the contrary it will dry up

  the ma
terial of the female and destroy it. So fire also does not

  continue to make water hotter in proportion as it is itself increased,

  but there is a fixed limit to the heat of which water is capable; if

  that is once reached and the fire is then increased, the water no

  longer gets hotter but rather evaporates and at last disappears and is

  dried up. Now since it appears that the secretion of the female and

  that from the male need to stand in some proportionate relation to one

  another (I mean in animals of which the male emits semen), what

  happens in those that produce many young is this: from the very

  first the semen emitted by the male has power, being divided, to

  form several embryos, and the material contributed by the female is so

  much that several can be formed out of it. (The parallel of

  curdling milk, which we spoke of before, is no longer in point here,

  for what is formed by the heat of the semen is not only of a certain

  quantity but also of a certain quality, whereas with fig-juice and

  rennet quantity alone is concerned.) This then is just the reason why

  in such animals the embryos formed are numerous and do not all unite

  into one whole; it is because an embryo is not formed out of any

  quantity you please, but whether there is too much or too little, in

  either case there will be no result, for there is a limit set alike to

  the power of the heat which acts on the material and to the material

  so acted upon.

  On the same principle many embryos are not formed, though the

  secretion is much, in the large animals which produce only one young

  one, for in them also both the material and that which works upon it

  are of a certain quantity. So then they do not secrete such material

  in too great quantity for the reason previously stated, and what

  they do secrete is naturally just enough for one embryo alone to be

  formed from it. If ever too much is secreted, then twins are born.

  Hence such cases seem to be more portentous, because they are contrary

  to the general and customary rule.

  Man belongs to all three classes, for he produces one only and

  sometimes many or few, though naturally he almost always produces one.

  Because of the moisture and heat of his body he may produce many [for

  semen is naturally fluid and hot], but because of his size he

  produces few or one. On account of this it results that in man alone

  among animals the period of gestation is irregular; whereas the period

  is fixed in the rest, there are several periods in man, for children

  are born at seven months and at ten months and at the times between,

  for even those of eight months do live though less often than the

  rest. The reason may be gathered from what has just been said, and the

  question has been discussed in the Problems. Let this explanation

  suffice for these points.

  The cause why the parts may be multiplied contrary to Nature is

  the same as the cause of the birth of twins. For the reason exists

  already in the embryo, whenever it aggregates more material at any

  point of itself than is required by the nature of the part. The result

  is then that either one of its parts is larger than the others, as a

  finger or hand or foot or any of the other extremities or limbs; or

  again if the embryo is cleft there may come into being more than one

  such part, as eddies do in rivers; as the water in these is carried

  along with a certain motion, if it dash against anything two systems

  or eddies come into being out of one, each retaining the same

  motion; the same thing happens also with the embryos. The abnormal

  parts generally are attached near those they resemble, but sometimes

  at a distance because of the movement- taking place in the embryo, and

  especially because of the excess of material returning to that place

  whence it was taken away while retaining the form of that part

  whence it arose as a superfluity.

  In certain cases we find a double set of generative organs [one

  male and the other female]. When such duplication occurs the one is

  always functional but not the other, because it is always

  insufficiently supplied with nourishment as being contrary to

  Nature; it is attached like a growth (for such growths also receive

  nourishment though they are a later development than the body proper

  and contrary to Nature.) If the formative power prevails, both are

  similar; if it is altogether vanquished, both are similar; but if it

  prevail here and be vanquished there, then the one is female and the

  other male. (For whether we consider the reason why the whole

  animal is male or female, or why the parts are so, makes no

  difference.)

  When we meet with deficiency in such parts, e.g. an extremity or one

  of the other members, we must assume the same cause as when the embryo

  is altogether aborted (abortion of embryos happens frequently).

  Outgrowths differ from the production of many young in the manner

  stated before; monsters differ from these in that most of them are due

  to embryos growing together. Some however are also of the following

  kind, when the monstrosity affects greater and more sovereign parts,

  as for instance some monsters have two spleens or more than two

  kidneys. Further, the parts may migrate, the movements which form

  the embryo being diverted and the material changing its place. We must

  decide whether the monstrous animal is one or is composed of several

  grown together by considering the vital principle; thus, if the

  heart is a part of such a kind then that which has one heart will be

  one animal, the multiplied parts being mere outgrowths, but those

  which have more than one heart will be two animals grown together

  through their embryos having been confused.

  It also often happens even in many animals that do not seem to be

  defective and whose growth is now complete, that some of their

  passages may have grown together or others may have been diverted from

  the normal course. Thus in some women before now the os uteri has

  remained closed, so that when the time for the catamenia has arrived

  pain has attacked them, till either the passage has burst open of

  its own accord or the physicians have removed the impediment; some

  such cases have ended in death if the rupture has been made too

  violently or if it has been impossible to make it at all. In some boys

  on the other hand the end of the penis has not coincided with the

  end of the passage where the urine is voided, but the passage has

  ended below, so that they crouch sitting to void it, and if the testes

  are drawn up they appear from a distance to have both male and

  female generative organs. The passage of the solid food also has

  been closed before now in sheep and some other animals; there was a

  cow in Perinthus which passed fine matter, as if it were sifted,

  through the bladder, and when the anus was cut open it quickly

  closed up again nor could they succeed in keeping it open.

  We have now spoken of the production of few and many young, and of

  the outgrowth of superfluous parts or of their deficiency, and also of

  monstrosities.


  5

  Superfoetation does not occur at all in some animals but does in

  others; of the former some are able to bring the later formed embryo

  to birth, while others can only do so sometimes. The reason why it

  does not occur in some is that they produce only one young one, for it

  is not found in solid-hoofed animals and those larger than these, as

  owing to their size the secretion of the female is all used up for the

  one embryo. For all these have large bodies, and when an animal is

  large its foetus is large in proportion, e.g. the foetus of the

  elephant is as big as a calf. But superfoetation occurs in those which

  produce many young because the production of more than one at a

  birth is itself a sort of superfoetation, one being added to

  another. Of these all that are large, as man, bring to birth the later

  embryo, if the second impregnation takes place soon after the first,

  for such an event has been observed before now. The reason is that

  given above, for even in a single act of intercourse the semen

  discharged is more than enough for one embryo, and this being

  divided causes more than one child to be born, the one of which is

  later than the other. But when the embryo has already grown to some

  size and it so happens that copulation occurs again, superfoetation

  sometimes takes place, but rarely, since the uterus generally closes

  in women during the period of gestation. If this ever happens (for

  this also has occurred) the mother cannot bring the second embryo

  to perfection, but it is cast out in a state like what are called

  abortions. For just as, in those animals that bear only one, all the

  secretion of the female is converted to the first formed embryo

  because of its size, so it is here also; the only difference is that

  in the former case this happens at once, in the latter when the foetus

  has attained to some size, for then they are in the same state as

  those that bear only one. In like manner, since man naturally would

  produce many young, and since the size of the uterus and the

  quantity of the female secretion are both greater than is necessary

  for one embryo, only not so much so as to bring to birth a second,