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  that is why they change in colour along with it, for they become

  white, black, and all sorts of colours according to that of the

  skin. But the teeth do nothing of the sort, for they are made out of

  the bones in all animals that have both bones and teeth. Of all the

  bones they alone go on growing through life, as is plain with the

  teeth which grow out of the straight line so as no longer to touch

  each other. The reason for their growth, as a final cause, is their

  function, for they would soon be worn down if there were not some

  means of saving them; even as it is they are altogether worn down in

  old age in some animals which eat much and have not large teeth, their

  growth not being in proportion to their detrition. And so Nature has

  contrived well to meet the case in this also, for she causes the

  failure of the teeth to synchronize with old age and death. If life

  lasted for a thousand or ten thousand years the original teeth must

  have been very large indeed, and many sets of them must have been

  produced, for even if they had grown continuously they would still

  have been worn smooth and become useless for their work. The final

  cause of their growth has been now stated, but besides this as a

  matter of fact the growth of the teeth is not the same as that of

  the other bones. The latter all come into being in the first formation

  of the embryo and none of them later, but the teeth do so later.

  Therefore it is possible for them to grow again after the first set

  falls out, for though they touch the bones they are not connate with

  them. They are formed, however, out of the nutriment distributed to

  the bones, and so have the same nature, even when the bones have their

  own number complete.

  Other animals are born in possession of teeth or their analogue

  (unless in cases contrary to Nature), because when they are set

  free from the parent they are more perfect than man; but man (also

  unless in cases contrary to Nature) is born without them.

  The reason will be stated later why some teeth are formed and fall

  out but others do not fall out.

  It is because such parts are formed from a residue that man is the

  most naked in body of all animals and has the smallest nails in

  proportion to his size; he has the least amount of earthy residue, but

  that part of the blood which is not concocted is the residue, and

  the earthy part in the bodies of all animals is the least concocted.

  We have now stated how each of the parts is formed and what is the

  cause of their generation.

  7

  In viviparous animals, as said before, the embryo gets its growth

  through the umbilical cord. For since the nutritive power of the soul,

  as well as the others, is present in animals, it straightway sends off

  this cord like a root to the uterus. The cord consists of

  blood-vessels in a sheath, more numerous in the larger animals as

  cattle and the like, one in the smallest, two in those of intermediate

  size. Through this cord the embryo receives its nourishment in the

  form of blood, for the uterus is the termination of many

  blood-vessels. All animals with no front teeth in the upper jaw, and

  all those which have them in both jaws and whose uterus has not one

  great blood-vessel running through it but many close together instead-

  all these have in the uterus the so-called cotyledons (with which the

  umbilical cord connects and is closely united; for the vessels which

  pass through the cord run backwards and forwards between embryo and

  uterus and split up into smaller vessels all over the uterus; where

  they terminate, there are found the cotyledons). Their convexity is

  turned towards the uterus, the concavity towards the embryo. Between

  uterus and embryo are the chorion and the membranes. As the embryo

  grows and approaches perfection the cotyledons become smaller and

  finally disappear when it is perfected. For Nature sends the

  sanguineous nutriment for the embryo into this part of the uterus as

  she sends milk into the breasts, and because the cotyledons are

  gradually aggregated from many into a few the body of the cotyledon

  becomes like an eruption or inflammation. So long as the embryo is

  comparatively small, being unable to receive much nutriment, they

  are plain and large, but when it has increased in size they fall in

  together.

  But most of the animals which have front teeth in both jaws and no

  horns have no cotyledons in the uterus, but the umbilical cord runs to

  meet one blood-vessel, which is large and extends throughout the

  uterus. Of such animals some produce one young at a time, some more

  than one, but the same description applies to both these classes.

  (This should be studied with the aid of the examples drawn in the

  Anatomy and the Enquiries.) For the young, if numerous, are

  attached each to its umbilical cord, and this to the blood-vessel of

  the mother; they are arranged next to one another along the stream

  of the blood-vessel as along a canal; and each embryo is enclosed in

  its membranes and chorion.

  Those who say that children are nourished in the uterus by sucking

  some lump of flesh or other are mistaken. If so, the same would have

  been the case with other animals, but as it is we do not find this

  (and this can easily be observed by dissection). Secondly, all

  embryos alike, whether of creatures that fly or swim or walk, are

  surrounded by fine membranes separating them from the uterus and

  from the fluids which are formed in it; but neither in these

  themselves is there anything of the kind, nor is it possible for the

  embryo to take nourishment by means of any of them. Thirdly, it is

  plain that all creatures developed in eggs grow when separated from

  the uterus.

  Natural intercourse takes place between animals of the same kind.

  However, those also unite whose nature is near akin and whose form

  is not very different, if their size is much the same and if the

  periods of gestation are equal. In other animals such cases are

  rare, but they occur with dogs and foxes and wolves; the Indian dogs

  also spring from the union of a dog with some wild dog-like animal.

  A similar thing has been seen to take place in those birds that are

  amative, as partridges and hens. Among birds of prey hawks of

  different form are thought to unite, and the same applies to some

  other birds. Nothing worth mentioning has been observed in the

  inhabitants of the sea, but the so-called 'rhinobates' especially is

  thought to spring from the union of the 'rhini' and 'batus'. And the

  proverb about Libya, that 'Libya is always producing something new',

  is said to have originated from animals of different species uniting

  with one another in that country, for it is said that because of the

  want of water all meet at the few places where springs are to be

  found, and that even different kinds unite in consequence.

  Of the animals that arise from such union all except mules are found

  to copulate again with each other and to be able to produce young of

  both sexes, but mules alone are sterile, for they
do not generate by

  union with one another or with other animals. The problem why any

  individual, whether male or female, is sterile is a general one, for

  some men and women are sterile, and so are other animals in their

  several kinds, as horses and sheep. But this kind, of mules, is

  universally so. The causes of sterility in other animals are

  several. Both men and women are sterile from birth when the parts

  useful for union are imperfect, so that men never grow a beard but

  remain like eunuchs, and women do not attain puberty; the same thing

  may befall others as their years advance, sometimes on account of

  the body being too well nourished (for men who are in too good

  condition and women who are too fat the seminal secretion is taken

  up into the body, and the former have no semen, the latter no

  catamenia); at other times by reason of sickness men emit the semen

  in a cold and liquid state, and the discharges of women are bad and

  full of morbid secretions. Often, too, in both sexes this state is

  caused by injuries in the parts and regions contributory to

  copulation. Some such cases are curable, others incurable, but the

  subjects especially remain sterile if anything of the sort has

  happened in the first formation of the parts in the embryo, for then

  are produced women of a masculine and men of a feminine appearance,

  and in the former the catamenia do not occur, in the latter the

  semen is thin and cold. Hence it is with good reason that the semen of

  men is tested in water to find out if it is infertile, for that

  which is thin and cold is quickly spread out on the surface, but the

  fertile sinks to the bottom, for that which is well concocted is hot

  indeed, but that which is firm and thick is well concocted. They

  test women by pessaries to see if the smells thereof permeate from

  below upwards to the breath from the mouth and by colours smeared upon

  the eyes to see if they colour the saliva. If these results do not

  follow it is a sign that the passages of the body, through which the

  catamenia are secreted, are clogged and closed. For the region about

  the eyes is, of all the head, that most nearly connected with the

  generative secretions; a proof of this is that it alone is visibly

  changed in sexual intercourse, and those who indulge too much in

  this are seen to have their eyes sunken in. The reason is that the

  nature of the semen is similar to that of the brain, for the

  material of it is watery (the heat being acquired later). And the

  seminal purgations are from the region of the diaphragm, for the first

  principle of nature is there, so that the movements from the pudenda

  are communicated to the chest, and the smells from the chest are

  perceived through the respiration.

  8

  In men, then, and in other kinds, as said before, such deficiency

  occurs sporadically, but the whole of the mule kind is sterile. The

  reason has not been rightly given by Empedocles and Democritus, of

  whom the former expresses himself obscurely, the latter more

  intelligibly. For they offer their demonstration in the case of all

  these animals alike which unite against their affinities. Democritus

  says that the genital passages of mules are spoilt in the mother's

  uterus because the animals from the first are not produced from

  parents of the same kind. But we find that though this is so with

  other animals they are none the less able to generate; yet, if this

  were the reason, all others that unite in this manner ought to be

  barren. Empedocles assigns as his reason that the mixture of the

  'seeds' becomes dense, each of the two seminal fluids out of which

  it is made being soft, for the hollows in each fit into the

  densities of the other, and in such cases a hard substance is formed

  out of soft ones, like bronze mingled with tin. Now he does not give

  the correct reason in the case of bronze and tin- (we have spoken of

  them in the Problems)- nor, to take general ground, does he take his

  principles from the intelligible. How do the 'hollows' and 'solids'

  fit into one another to make the mixing, e.g. in the case of wine

  and water? This saying is quite beyond us; for how we are to

  understand the 'hollows' of the wine and water is too far beyond our

  perception. Again, when, as a matter of fact, horse is born of

  horse, ass of ass, and mule of horse and ass in two ways according

  as the parents are stallion and she-ass or jackass and mare, why in

  the last case does there result something so 'dense' that the

  offspring is sterile, whereas the offspring of male and female

  horse, male and female ass, is not sterile? And yet the generative

  fluid of the male and female horse is soft. But both sexes of the

  horse cross with both sexes of the ass, and the offspring of both

  crosses are barren, according to Empedocles, because from both is

  produced something 'dense', the 'seeds' being 'soft'. If so, the

  offspring of stallion and mare ought also to be sterile. If one of

  them alone united with the ass, it might be said that the cause of the

  mule's being unable to generate was the unlikeness of that one to

  the generative fluid of the ass; but, as it is, whatever be the

  character of that generative fluid with which it unites in the ass,

  such it is also in the animal of its own kind. Then, again, the

  argument is intended to apply to both male and female mules alike, but

  the male does generate at seven years of age, it is said; it is the

  female alone that is entirely sterile, and even she is so only because

  she does not complete the development of the embryo, for a female mule

  has been known to conceive.

  Perhaps an abstract proof might appear to be more plausible than

  those already given; I call it abstract because the more general it is

  the further is it removed from the special principles involved. It

  runs somewhat as follows. From male and female of the same species

  there are born in course of nature male and female of the same species

  as the parents, e.g. male and female puppies from male and female dog.

  From parents of different species is born a young one different in

  species; thus if a dog is different from a lion, the offspring of male

  dog and lioness or of lion and bitch will be different from both

  parents. If this is so, then since (1) mules are produced of both

  sexes and are not different in species from one another, and (2) a

  mule is born of horse and ass and these are different in species

  from mules, it is impossible that anything should be produced from

  mules. For (1) another kind cannot be, because the product of male and

  female of the same species is also of the same species, and (2) a mule

  cannot be, because that is the product of horse and ass which are

  different in form, [and it was laid down that from parents

  different in form is born a different animal]. Now this theory is too

  general and empty. For all theories not based on the special

  principles involved are empty; they only appear to be connected with

  the facts without being so really. As geometrical arguments must start

  from geome
trical principles, so it is with the others; that which is

  empty may seem to be something, but is really nothing. Now the basis

  of this particular theory is not true, for many animals of different

  species are fertile with one another, as was said before. So we must

  not inquire into questions of natural science in this fashion any more

  than any other questions; we shall be more likely to find the reason

  by considering the facts peculiar to the two kinds concerned, horse

  and ass. In the first place, each of them, if mated with its own kind,

  bears only one young one; secondly, the females are not always able to

  conceive from the male (wherefore breeders put the horse to the

  mare again at intervals). Indeed, both the mare is deficient in

  catamenia, discharging less than any other quadruped, and the

  she-ass does not admit the impregnation, but ejects the semen with her

  urine, wherefore men follow flogging her after intercourse. Again

  the ass is an animal of cold nature, and so is not wont to be produced

  in wintry regions because it cannot bear cold, as in Scythia and the

  neighbouring country and among the Celts beyond Iberia, for this

  country also is cold. For this cause they do not put the jackasses

  to the females at the equinox, as they do with horses, but about the

  summer solstice, in order that the ass-foals may be born in a warm

  season, for the mothers bear at the same season as that in which

  they are impregnated, the period of gestation in both horse and ass

  being one year. The animal, then, being, as has been said of such a

  cold nature, its semen also must be cold. A proof of this is that if a

  horse mount a female already impregnated by an ass he does not destroy

  the impregnation of the ass, but if the ass be the second to mount her

  he does destroy that of the horse because of the coldness of his own

  semen. When, therefore, they unite with each other, the generative

  elements are preserved by the heat of the one of them, that

  contributed by the horse being the hotter; for in the ass both the

  semen of the male and the material contributed by the female are cold,

  and those of the horse, in both sexes, are hotter. Now when either hot