by Aristotle
About the teeth a difficulty may be raised. They have actually the same nature [20] as the bones, and are formed out of the bones, but nails, hair, horns, and the like are formed out of the skin, and 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 [25] 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 [30] 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, for they are worn away faster than they grow. 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 would have had to be very large indeed, and many sets of them would have had to have been produced, for even if they had grown [745b1] 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 nature 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 [5] 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 naturally connected to 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.
[10] 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 [15] 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 what is not concocted is the residue, and the earthy part in the bodies of all animals is the least concocted. We have now stated [20] 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 [25] 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 non-ambidentates and all ambidentates [30] 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 and split up 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 [746a1] sends the sanguineous nutriment for the embryo into this part of the uterus as it were into 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 [5] 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 shrink.
But most of the animals which are stunted and ambidentate have no cotyledons in the uterus, but the umbilical cord runs to meet one blood-vessel, which [10] 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 Anatomies and the Histories.) For the young are attached each to its umbilical [15] cord, and this to the blood-vessel; 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 [20] 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 [25] the kind, nor is it possible for the embryo to take nourishment by means of any of them. And it is plain that all creatures developed in eggs grow when separated from the uterus. Thus those, e.g. Democritus, who put forward this view are mistaken.
Copulation takes place naturally between animals of the same kind. However, [30] 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 and jackals; the Indian dogs also spring from the union of a dog with some wild dog-like [746b1] animal. A similar thing has been seen to take place in those birds that are salacious, 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 [5] observed in the inhabitants of the sea, but the so-called ‘rhinobates’ especially is thought to spring from the union of the rhinè and the 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 [10] 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 [15] 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 [20] several kinds, as horses and sheep. But this kind, that 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 [25] may befall others as their years advance, sometimes on account of the body being too well nourished (for in men who are in too good condition and women who are too fat the seminal residue is taken up into the body, and the former have no semen, the latter no menstrual discharge); at other times by reason of sickness men emit the [30] semen in a cold and liquid state, and the discharges of women are bad and full of morbid residues. Often, too, in both sexes this state is caused by deformities 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 [747a1] women of a masculine and men of a feminine appearance, and in the former the menstrual discharge does 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 [5] 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 permeate from below upwards to the breath from the mouth, and by colours [10] 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 residue is secreted, are clogged and closed. For the region about the eyes is, of all the head, the most [15] seminal part; 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 discharges are from the region of the diaphragm, for the first principle of nature is there, so that the [20] movements from the pudenda are communicated to the chest, and the smells from the chest are all 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 [25] rightly given by Empedocles and Democritus, of whom the former expresses himself obscurely, the latter more intelligibly. For they offer a single demonstration for all animals 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 [30] 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 for their sterility, all others that unite in this manner ought to be sterile. 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 [747b1] 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 [5] general ground, does he take his principles from the intelligible. For 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 [10] fact, horse is born of horse, ass of ass, and mule of horse and ass (it does not matter which is the male and which the female), 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 [15] 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 sterile, 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 [20] 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 demonstration is intended to apply to both male and female mules alike, but the male alone does generate at seven years of age, it is said, whereas the female is entirely sterile, and [25] 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 appropriate principles. It runs somewhat as follows. From male [30] 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 [748a1] bitch will be different from both parents. If this is so, then since mules are produced of both sexes and are not different in species from one another, and 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 another kind cannot be, because the [5] product of male and female of the same species is also of the same species, and 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 appropriate principles are empty; they only appear to be connected with the facts [10] without being so really. As geometrical arguments must start from geometrical 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 produced from different species are fertile, 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 [15] 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 (that is why breeders put the horse to the [20] mare again at intervals). Indeed, the mare is deficient in menstrual flow, discharging less than any other quadruped, and the she-ass does not admit the impregnation, but ejects the semen with her urine, which is why men follow flogging her after intercourse. Again the ass is an animal of cold nature, and so is not wont to [25] 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 [30] 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 [748b1] 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 and the material are cold, and those of the horse are hotter. Now when either hot is added to cold or cold to hot so as to mix, the [5] result is that the embryo itself arising from these is preserved and thus these animals are fertile when crossed with one another, but the animal produced by them is no longer fertile but unable to produce perfect offspring.
And in general each of these animals naturally tends towards sterility. The ass has all the disadvantages already mentioned, and if it should not begin to generate [10] after the first shedding of teeth, it no longer generates at all; so near is the constitution of the ass to being sterile. The horse is much the same; it tends naturally towards sterility, and to make it entirely so it is only necessary that its generative secretion should become colder; now this is what happens to it when mixed with the corresponding secretion of the ass. The ass in like manner comes [15] very near generating a sterile animal when mated with its own species. Thus when the difficulty of a cross contrary to nature is added to the difficulty they have in producing a single young one when united with their own species, the result of the cross, being still more sterile and contrary to nature, will need nothing further to make it sterile, but will be so of necessity.
We find also that the bodies of mules grow large because the matter which is [20] secreted in other animals to form the menstrual flow is diverted to growth. But since the period of gestation in such animals is a
year, the mule must not only conceive but must also nourish the embryo till birth, and this is impossible if there is no menstrual discharge. But there is none in the mule; the useless part of the nutriment is discharged with the excretion from the bladder—this is why male mules do not [25] sniff at the pudenda of the females, as do the other solid-hoofed animals, but only at the excretion itself—and the rest of the nutriment is used up to increase the size of the body. Hence it is sometimes possible for the female to conceive, as has been known to happen before now, but it is impossible for her to complete the process of [30] nourishing the embryo and bringing it to birth.
The male, again, may sometimes generate, both because the male sex is naturally hotter than the female and because it does not contribute any material substance to the mixture. The result in such cases is a ginnus, that is to say, a [749a1] deformed mule; for ginni are produced also from the crossing of horse and ass when the embryo is diseased in the uterus. The ginnus is in fact like the so-called metachoera in swine, for a metachoerum also is a pig deformed in the uterus; this may happen to any pig. The origin of human dwarfs is similar, for these also have their parts and their whole development deformed during gestation, and resemble [5] ginni and metachoera.
BOOK III
1 · We have now spoken about the sterility of mules, and about those [10] animals which are viviparous both externally and within themselves. The generation of the oviparous sanguinea is to a certain extent similar to that of the animals that walk, and all may be embraced in the same general statement; but in other respects there are differences in them both as compared with each other and with those that walk. All alike are generated from sexual union, the male emitting semen [15] into the female. But among the ovipara birds produce a perfect hard-shelled egg, unless it be injured by disease, and the eggs of birds are all two-coloured. The Selachian fishes, as has been often said already, are oviparous internally but [20] produce the young alive, the egg changing previously from one part of the uterus to another; and their egg is soft-shelled and of one colour. One of this class alone does not produce the young from the egg within itself, the so-called fishing-frog; the reason of which must be stated later. All other oviparous fishes produce an egg of [25] one colour, but this is imperfect, for its growth is completed outside the mother’s body by the same cause as are those eggs which are perfected within.