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
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