Generation of Animals · Book II

Aristotle · a new plain-English translation from the original language

1| For it would seem worth being content even to attain this kind of knowledge, rather than lying there dead and non-existent. Animals differ from things that merely live only in respect of perception. But since it is necessary also to live, if a thing is an animal, then whenever it becomes necessary to complete the function of the living thing, it pairs and mates and comes to be, as it were, like a plant, just as we said. The testaceans among animals, being between animals and plants, as belonging to both kinds, perform the function of neither: for as a plant it has neither the female nor the male and does not generate into another, but as an animal it does not bear fruit from itself as plants do, but is composed and generated from some earthy and moist composition. But about the generation of these we must speak later.

2| That the female and the male are principles of generation has been said before, and what their power is and the account of their substance. But as to why it comes about, and why the one is female and the other male — so far as this happens by necessity and from the first mover and from what kind of matter, we must try, as we proceed, to state the account; but so far as it is for the sake of the better and by the cause that is for the sake of something, its principle comes from above. For since some existing things are eternal and divine while others admit of both being and not being, and the fine and the divine is always, in accordance with its own nature, a cause of the better among things that admit of being otherwise, while what is not eternal admits of both being and of partaking of the worse and the better, and since soul is better than body, and the ensouled is better than the unensouled because of soul, and being is better than not being, and living than not living — for these causes there is generation of animals. For since the nature of such a kind cannot be eternal, in whatever way this is possible, in that way what comes to be is eternal.

3| In number, then, this is impossible (for the substance of existing things lies in the particular, and if the particular were of this kind, it would be eternal); but in species it is possible. Hence there is always a genus of human beings and of animals and of plants. And since the female and the male are the principle of these, the female and the male would exist among beings for the sake of generation. And since the cause that first moves, in which the account resides along with the form of the matter, is better and more divine in its nature, it is also better for the superior to be separated from the inferior. For this reason, in whatever cases it is possible, and to whatever extent it is possible, the male is separated from the female. For the principle of motion, which is present as male in the things that come to be, is better and more divine; matter is what is female. The male comes together and mates with the female for the work of generation, for this work is common to both. Now insofar as they partake of the female and the male, they live — which is why plants too partake of life — but insofar as they have perception, they belong to the genus of animals. And in almost all of these, among the animals capable of locomotion, the female and the male are separated, for the reasons stated.

4| And of these, some, as was said, emit seed in mating, while others do not. The cause of this is that the more honorable and more self-sufficient in nature are such as to have partaken of size. And this does not come about without vital heat, for the larger must necessarily be moved by a greater power, and heat is capable of causing motion. For this reason, speaking generally, the blooded animals are larger than the bloodless, and the animals capable of locomotion are larger than the stationary ones; and these emit seed on account of their heat and their size. Concerning male and female, then, and the cause for which each of them exists, this has been said. Of animals, some bring to completion and send forth outside something like themselves, namely all those that bear live young into visible form; others produce something unarticulated and not yet possessing its own shape. Of these latter, the blooded ones lay eggs, the bloodless produce larvae. An egg differs from a larva: an egg is that from part of which the thing that comes to be is generated, while the remainder is nourishment for the thing coming to be; a larva is that from the whole of which the whole of the thing that comes to be is generated. Of those that bring to completion an offspring visibly like themselves and that bear live young, some bear live young directly within themselves, such as man and horse and ox, and among sea creatures the dolphin and the other such animals; others first lay eggs within themselves and only then in this way bear live young outside, such as the animals called cartilaginous fish.

5| Of the egg-laying animals, some emit a complete egg, such as birds and all the four-footed egg-layers and all the footless ones, such as lizards and tortoises and most of the kind of serpents (for the eggs of these, once they come out, no longer undergo growth); others emit an incomplete egg, such as fish and the crustaceans and the animals called cephalopods; for the eggs of these grow after they come out. All animals that bear live young or lay eggs are blooded, and all blooded animals either bear live young or lay eggs, except those that are entirely infertile. Of the bloodless animals, the insects produce larvae, namely all those that come to be from mating or that themselves mate. For there are some such insects that come to be spontaneously, yet are female and male, and something is produced from their mating, though what is produced is incomplete; the cause of this has been stated before elsewhere. Now there occurs much overlapping among the kinds. For not all two-footed animals bear live young (birds lay eggs), nor do all lay eggs (man bears live young); nor do all four-footed animals lay eggs (horse and ox and countless others bear live young), nor do all bear live young (lizards and crocodiles and many others lay eggs).

6| Nor does it make a difference whether they have feet or not: for footless animals bear live young, such as vipers and the cartilaginous fish, while others lay eggs, such as the kind of fish and the other serpents; and among those that have feet, many both lay eggs and bear live young, such as the four-footed animals mentioned. And among these same live-bearers, some have feet, such as man, and some are footless, such as the whale and the dolphin. So it is not possible to divide them in this way, nor is any of the organs of locomotion the cause of this difference; rather, the more perfect in nature among animals bear live young, namely those that partake of a purer principle; for nothing bears live young within itself unless it receives breath and respires. More perfect are those that are hotter in nature and moister and not earthy. The boundary of natural heat is the lung, in those animals that are blooded; for in general those that have a lung are hotter than those that do not, and among these very ones, those that have a lung that is not spongy nor compact nor sparse in blood, but full of blood and soft, are hotter still. And just as the animal is complete while the larva and the egg are incomplete, so the complete is naturally generated from the more complete. Those that are hotter because they have a lung, but drier in nature, or those that are colder but moister, some lay a complete egg, while others, having laid eggs, bear live young within themselves.

7| For birds and scaly animals achieve completion because of heat, but lay eggs because of dryness; the selachians, however, are less hot than these but more moist, so that they share in both: for they both lay eggs and bear live young within themselves — they lay eggs because they are cold, and bear live young because they are moist, for the moist is life-giving, while the dry is furthest removed from what has soul. Since they are neither feathered nor scaly nor covered with scales — signs rather of a dry and earthy nature — they produce a soft egg; for just as the earthy element does not rise to the surface in the animal itself, neither does it in the egg. And because of this they lay their eggs into themselves; for if the egg went outside it would be destroyed, having no protective covering. But the colder and drier animals lay eggs, yet the egg is incomplete, and it is hard-shelled, because, being earthy, they produce an incomplete egg, so that it may be preserved by having the protection of a shell-like covering. Now fish, being scaly, and the soft-shelled crustaceans, being earthy, produce hard-shelled eggs. But the cephalopods, just as they themselves are viscous in the nature of their body, so they preserve their eggs, incomplete as they produce them, by producing a great deal of viscous matter around the embryo. And all the insects produce grubs.

8| All insects are bloodless, and this is why, though they produce grubs, they do so outside the body. But not all bloodless animals simply produce grubs; for insects and those that bring forth an incomplete egg overlap with one another — for instance the scaly fish, the soft-shelled crustaceans, and the cephalopods. For the eggs of these are grub-like, since they receive their growth outside; while in the case of the insects, the grubs, as they advance, become egg-like — in what manner we shall determine later. One must understand how well and in due order nature renders generation. For the more complete and hotter animals render their offspring complete in quality (in respect of quantity none of the animals do so completely, since all, once born, undergo growth), and these accordingly generate living animals within themselves right away. The second class does not generate complete offspring within themselves right away, for they bear live young only after first laying eggs within themselves, and it is outside that they bear the young alive; others do not generate a complete animal but generate an egg, and this egg is complete. But those whose nature is still colder than these produce an egg, yet not a complete egg — it is completed outside — as with the class of scaly fish, the soft-shelled crustaceans, and the cephalopods.

9| The fifth kind, and the coldest, does not even lay eggs from itself, but even this sort of thing happens to it outside, as has been said. For insects first bring forth grubs; and as the grub advances it becomes egg-like (for what is called the chrysalis has the capacity of an egg); then from this an animal is generated, reaching the completion of its generation in the third change. Now some animals are not generated from seed, as was said before as well; but all the blooded animals that come to be from copulation are generated from seed, the male emitting semen into the female, and when this has entered, the animals are constituted and take on their proper form — some within the animals themselves, namely those that bear live young, others in eggs and seeds and other such secretions. Concerning these there is a greater difficulty: how ever does the plant, or any animal whatever, come to be from the seed? For necessarily what comes to be must come to be both from something and by something and as something. Now as to that from which it is, the matter — which some animals have first within themselves, taking it from the female, namely those that are not born alive but produce grubs or lay eggs, while others take it from the female for a long time by suckling, as do those that are born alive not only outside but also within.

10| Either something outside produces it, or else something is present in the semen and seed; and this is either some part of the soul, or the soul itself, or something that would possess soul. That any of the internal organs, or any of the other parts, should produce something from outside would seem irrational; for it is impossible for a thing to move without touching, and impossible for a thing to be affected without something moving it. Therefore something is present within the embryo itself, either as a part of it or as something separated from it. That it should be something else, separated, is irrational. For once the animal has been generated, does this thing perish or does it remain? But nothing of this kind appears to be present that is not a part of the whole, whether of the plant or of the animal. But indeed it is also absurd that it should perish, whether it made all the parts or only some of them; for what will make the rest? For if it made the heart and then perished, and the heart made another part, by the same reasoning either all must perish or all must remain. It is preserved, then. It is therefore a part of the thing itself, which is present in the semen from the very start. And if indeed there is nothing belonging to the soul that does not belong to some part of the body, then some part would immediately be ensouled.

11| How then do the other parts come to be? Either all the parts come to be at once — heart, lung, liver, eye, and each of the rest — or in succession, as in the poems called the poems of Orpheus; for there he says the animal comes to be in a manner like the plaiting of a net. That they do not come to be at once is clear even to perception: for some of the parts appear already present, others not. And that it is not on account of smallness that they fail to appear is clear; for the lung, though greater in size than the heart, appears later than the heart in the original generation. Since, then, one part is prior and another posterior, does the one make the other, and is this because it is next to it, or rather does this come to be after that? I mean, for example, that it is not the heart, once it has come to be, that makes the liver, and the liver in turn some other part, but rather this comes after that, just as a man comes after a child, but not by the child's agency. The reason for this is that, in things that come to be by nature or by craft, what is potentially comes to be by the agency of what is actually; so that the form and the shape would need to be present in that prior thing — for example, the form of the liver would have to be in the heart.

12| But this account is otherwise absurd and fanciful as well. But moreover, that some part of the animal or plant, already generated, should be present at once in the seed — whether capable of making the rest or not — is impossible, if everything comes to be from seed and semen. For it is clear that it would have come to be by the agency of whatever made the seed, if indeed it is present in it from the start. But the seed must come to be first, and this is the work of the parent. None of the parts, therefore, can possibly already be present in it. The agent that makes the parts, then, does not have them within itself. But neither is it outside; and one or the other must be the case. One must try, then, to resolve these difficulties; for perhaps one of the statements made is not straightforward — for example, how it is that a thing cannot come to be by the agency of something outside. For in one sense this is possible, in another it is not. Now to say 'the semen' or 'that from which the semen comes' makes no difference, insofar as the semen has within itself the motion which that agent set going. And it is possible for this to move that, and that to move something else, and for the process to be like the automatic puppets among marvels. For the parts, even while at rest, possess a certain capacity in themselves; and when something external first sets one of them in motion, immediately the next part comes to be in actuality.

13| Just as in automatic devices, in a certain way that thing moves it without touching anything now, though it did touch something before; likewise too whatever the semen comes from, or whatever made the semen, has touched something but no longer touches it — in a certain way it is the movement present within it, just as house-building produces the house. That there is something which produces the thing generated, then, but not in the way that a particular "this" does, nor is it present within as something already completed from the start, is clear. But how each thing comes to be must be grasped starting from this point, taking as our first principle that whatever comes to be by nature or by craft comes to be, by something actually existing, out of something potentially of that kind. Now the semen is of this sort, and has movement and a starting-principle of this kind, so that when the movement ceases each of the parts comes to be, and comes to be ensouled. For there is no face that does not have soul, nor flesh; but once destroyed, they will be called "face" and "flesh" only equivocally, just as if they had come to be made of stone or of wood. The uniform parts and the organic parts come to be at the same time; and just as we would not say that fire alone made an axe or any other tool, so neither does it make a foot or a hand. In the same way it does not make flesh either.

14| For flesh too has some function of its own. Hardness and softness, stickiness and brittleness, and whatever other affections belong to the ensouled parts — heat and cold could produce these, but they can no longer produce the account by which this part is already flesh and that part bone; rather it is the movement coming from the begetter, which is the thing in actuality that the matter it comes from is potentially — just as also in things that come to be by craft. For heat and cold make the iron hard or soft, but what makes it a sword is the movement of the tools, which has the account belonging to the craft. For craft is a starting-principle and form of the thing that comes to be, but in something else; whereas the movement of nature is in the thing itself, coming from another nature which has the form in actuality. Does the semen have soul, or not? The same account applies to the parts as well: for there will be no soul in anything else except that of which it is the soul, nor will a part exist without sharing in soul, except equivocally, like the eye of a dead man. It is clear, then, that the semen both has soul and is soul potentially. And a thing can be nearer to or further from itself in potentiality, as the sleeping geometer is further from actuality than the one who is awake, and this one further than the one who is actually studying.

15| Of this generation, then, no part is the cause, but rather the first thing that moved it, from outside. For nothing generates itself; but once it has come to be, it already increases itself. That is why some one thing comes to be first, and not everything at once. And this must necessarily be the first thing to come to be which has the starting-principle of growth; for whether it is a plant or an animal, this — the nutritive faculty — belongs alike to all of them. And this is what is generative of another thing like itself; for this is the function of everything that is by nature complete, both animal and plant. And it is necessary for this reason: that when something comes to be, it must grow. Now what it begot was something of the same name — for instance a man begets a man — but it grows through itself. So it is itself, being something, that causes the growth. If, then, this is some one thing and is first, this must be the first thing to come to be. So if the heart comes to be first in some animals, and in those not having a heart, the part analogous to it comes to be first, then the starting-principle would be from the heart in those that have one, and from the analogous part in the others. What, then, is the cause as starting-principle of the generation of each thing, the first mover and craftsman, has been stated, in answer to the difficulties raised earlier.

16| But concerning the nature of the semen one might raise a difficulty. For the semen comes out of the animal thick and white, but on cooling it becomes liquid like water, and the color of water. This would seem strange: for water does not thicken by heat, yet the semen comes out from inside thick, out of what is hot, and on cooling becomes liquid. And yet watery things do congeal by cold; but the semen does not congeal when set out in the open in frosty weather — it becomes more liquid instead, as though thickened by its opposite. But then it is not reasonable that it should be thickened by heat either. For things that have more earth in them condense and thicken when boiled, as milk does too. So it ought to become solid on cooling. But in fact nothing solid comes of it — the whole thing is like water. This, then, is the difficulty. For if it is water, water does not appear to be thickened by heat, and yet the semen comes out thick and hot, out of a hot body; but if it is out of earth, or a mixture of earth and water, it ought not to become entirely liquid and like water. Or else we have not distinguished all the relevant facts. For it is not only moisture composed of water and earthy matter that thickens, but also that composed of water and pneuma — for example foam too becomes thicker and white, and the smaller and less visible the bubbles are, the whiter and more compact the mass appears.

17| Oil is affected in the same way: it thickens when pneuma mixes into it; that is why what is being whitened also becomes thicker, since the watery element in it is being separated out by the heat that is generating pneuma. And litharge, mixed with water or with oil, produces a great bulk out of a small quantity, and something solid out of liquid, and something white out of black. The cause is that pneuma is mixed in, which produces the bulk and shows through as whiteness, just as in foam and in snow — for snow too is a kind of foam. And water itself, when mixed with oil, becomes thick and white; for by the rubbing, pneuma is enclosed within it, and oil itself contains much pneuma; for the fatty element is neither of earth nor of water, but of pneuma. That is why it floats on water: for the air within it, as though in a vessel, carries it upward and makes it float, and is the cause of its lightness. And in cold weather and frosts oil thickens, but does not freeze solid; for it does not freeze on account of its heat (for air is hot and does not freeze), but because it condenses and is compacted, oil becomes thicker as though by the cold.

18| For these same causes the semen too comes out from inside compact and white, containing much hot pneuma because of the internal heat; but once it has come out, when the heat breathes off and the air is cooled, it becomes liquid and dark; for what is left is the water, and whatever small amount of earthy matter there is, as in phlegm, and in the semen as it dries out. The semen, then, is a combination of pneuma and water, and pneuma is hot air; that is why it is liquid in nature, because it comes from water. As for Ctesias of Cnidus — what he has said about the semen of elephants is plainly false. For he says that on drying it hardens so much that it becomes like amber. But this does not happen; rather it is necessary that one animal's semen be more earthy than another's, and especially so in those that have much earthy matter in the bulk of their body. But it is thick and white because pneuma is mixed into it. For indeed the semen of all animals is white; Herodotus does not speak the truth when he says that the semen of the Ethiopians is black, as though it were necessary that everything belonging to those black in color should be black — and this though he could see that their teeth too are white. The cause of the whiteness of the semen is that the seed is a kind of foam, and foam is white, especially the kind composed of the fewest parts and of parts so small that each bubble is invisible — which is what happens also with water and oil when they are mixed and rubbed together, as was said before.

19| It seems that the ancients too were not unaware that the nature of seed is frothy; at any rate they named the presiding god of intercourse from this capacity. So the cause of the stated difficulty has been given, and it is clear that seed does not solidify for this reason: air does not solidify. Next after this it is worth raising the question, and stating, whether none of the fluid the male emits into the female is a part of the resulting embryo that enters it — where does its bodily substance go, if it works by the capacity present within it? And we must determine whether what forms in the female takes on something from what entered it, or nothing at all; and concerning the soul in virtue of which a thing is called an animal (a thing is an animal in virtue of the perceptive part of the soul), whether it is present in the seed and in the embryo or not, and where it comes from. For one could not posit the embryo as soulless, deprived of life in every way; for the seeds and embryos of animals are no less alive than those of plants, and are capable of generation up to a point. That they have the nutritive soul, then, is clear (why it is necessary to grasp this first is clear from what has been determined about the soul elsewhere); and as they progress, they also have the perceptive soul, in virtue of which a thing is an animal. For a thing does not become an animal and a human being at the same time, nor an animal and a horse, and likewise with the other animals; for the end comes later, and what is proper to each thing is the end of its coming-to-be.

20| That is why, concerning intellect too — when and how those things that share in this principle come to partake of it, and from where — there is the greatest difficulty, and one must be eager to grasp it as far as possible and to whatever extent is attainable. Now as for the nutritive soul, it is clear that the seeds and the embryos, once separated off, must be said to have it in potentiality but not in actuality, until, like the embryos that have become separate, they draw nutriment and perform the function of this kind of soul. For at first all such things seem to live the life of a plant, and correspondingly it is clear that the same must be said about the perceptive soul and the soul capable of intellect. For all of them must be possessed in potentiality before actuality. And it is necessary either that none of them exist beforehand and all come to be present, or that all pre-exist, or that some do and some do not — and that they come to be present either in the matter without having entered in the seed of the male, or having come there from that source, while in the male either all come to be present from outside, or none, or some do and some do not. That it is not possible for all to pre-exist, then, is clear from the following. For whatever principles have a bodily activity, it is clear that these cannot exist without a body — for instance, walking without feet;

21| so it is also impossible for them to enter from outside. For they cannot enter by themselves, being inseparable, nor can they enter within a body; for the seed is a residue of nutriment undergoing change. It remains, then, that intellect alone enters from outside and alone is divine; for bodily activity has nothing in common with its activity. Now the capacity of every soul seems to have shared in a body other than and more divine than the so-called elements; and just as souls differ from one another in worth and lack of worth, so too this kind of nature differs. For in the seed of all there is present that which makes seeds capable of generation, the so-called hot. But this is not fire nor any such power, but the pneuma enclosed within the seed and within the frothy substance, and the nature within the pneuma, being analogous to the element of the stars. That is why fire generates no animal, and nothing appears to form when things are subjected to fire, whether in moist things or in dry things; whereas the heat of the sun and that of animals — not only the heat that acts through the seed, but even if some other residue of their nature happens to be present, this too has a vital principle. That the heat in animals, then, is neither fire nor derives its origin from fire, is clear from considerations of this sort;

22| and the body of the seminal fluid, in which the seed of the soul's principle is carried along together with it, is in part separable from body — in those cases where the divine is enclosed within it (such is the so-called intellect) — and in part inseparable; this, the body of the seminal fluid, dissolves and turns to pneuma, having a moist and watery nature. For this reason one should not seek for it always to go back outside, nor for it to be any part of the form that has been formed, any more than one should seek this of the fig-juice that curdles milk; for this too changes, and is no part of the resulting masses. Concerning the soul, then, how the embryo and the seed have it and how they do not have it, has been determined: for they have it in potentiality, but do not have it in actuality. And since the seed is a residue and moves with the same motion by which the body grows, as the ultimate nutriment is being distributed, when it arrives at the uterus it forms and moves the residue of the female with the same motion with which it itself happens to be moved. For that too is a residue, and has all the parts in potentiality, but none in actuality — for such things too have parts in potentiality, in respect of which the female differs from the male. For just as from maimed parents there sometimes come maimed offspring and sometimes not, so too from a female sometimes comes a female, and sometimes not, but a male.

23| For the female is, as it were, a maimed male, and the menstrual fluid is seed, though not pure; for it lacks one thing only, the principle of soul. And for this reason, in those animals in which wind-eggs occur, the egg that forms has the parts of both, but does not have the principle, and therefore does not become ensouled; for it is the seed of the male that brings this. But when the residue of the female partakes of such a principle, it becomes an embryo. In moist things that are bodily and are heated, a surrounding forms, just as in boiled liquids as they cool a dry crust forms round them. And what holds all bodies together is the viscous element; and this, as things advance and grow larger, takes on the nature of sinew, which holds together the parts of animals — being sinew in some, and the analogous thing in others. And of the same form are skin and vein and membrane and every such kind; for these differ by the more and the less, and in general by excess and deficiency.

24| Of animals, those that have a less complete nature, when the embryo has become complete but the animal not yet complete, send it forth outside; the causes have been stated earlier. It is complete, however, precisely when one of the embryos is male and the other female, in those cases where this difference exists among the things generated; for some generate neither female nor male — those that do not themselves come to be from female and male, nor from animals mating. And concerning the generation of these we shall speak later. Animals that bear their young alive within themselves, being the complete kind of animal, keep the developing animal attached and growing within themselves until the point at which they produce the animal and send it forth outside. Those that bear their young alive outside but at first lay eggs within themselves — when they have produced the complete egg, in some of these the egg is released, as in those that lay eggs outside, and the animal comes to be from the egg within the female; in others, when the nutriment from the egg has been used up, the animal is completed from the uterus, and for this reason the egg is not released from the uterus. This difference is found in the cartilaginous fish, concerning which we must speak later, on their own account.

25| But now we must begin first from what is primary. The complete animals are primary, and such are the ones that bear live young, and among these man is first. Now the secretion of semen occurs in all of them just as with any other residue. For each thing travels to its own proper place, with no force applied by the breath, nor any other such cause compelling it, as some say — claiming that the genitals draw it in like cupping-glasses, the residue being forced along together with the breath, as though this residue of wet or dry nutriment could travel somewhere else if it were not forced, on the grounds that people expel these residues' outlets together with the breath they have accumulated — and this, they say, holds in common for everything that needs to be moved: for it is by holding the breath that strength arises, since even without this forcing the residues are excreted, even in people who are asleep, if the places happen to be relaxed and full of residue. It would be a similar claim if someone said that in plants too the seeds — the text is corrupt at this point — and the fruit are moved by breath on each occasion. But the cause of this, as has been said, is that all things have parts receptive of their residues — for the useless ones, such as the dry and the wet, and, for blood, the so-called veins. In females, then, around the region of the womb, since the two veins split off from above — the great vein and the aorta — many fine veins terminate in the womb; and when these become overfilled from the nutriment, and nature is unable to concoct it because of coldness, it is excreted through the finest veins into the womb, the veins being unable, because of their narrowness, to receive the excess in quantity, and the condition that results is like a hemorrhage.

26| Now the period is not fixed with precision for women, but it tends to occur, reasonably enough, as the month wanes; for the bodies of animals become colder whenever the surrounding air also happens to become so, and the conjunctions of the months are cold because of the moon's failing light, which is why the conjunctions of the months turn out to be more wintry than the mid-points. So then, once the residue has changed into blood, the menses tend to occur according to the period stated, but when it has not been concocted, something is always secreted little by little; and this is why the white discharge occurs in females while they are still small children. Now when both these secretions of residue occur in moderation, they preserve the body, inasmuch as a purging occurs of the residues that are causes of disease for the body; but when they fail to occur, or occur in excess, this does harm; for it produces either diseases or a wasting away of the body, and this is why the white discharge, when it occurs continuously and in excess, takes growth away from children. This residue, then, occurs in females of necessity, on account of the causes stated; for since nature is unable to concoct it, residue must arise not only from the useless nutriment, but also in the veins, and, increasing in quantity, it must overflow through the finest veins.

27| But for the sake of the better and of the end, nature makes use of this residue by directing it toward this place, for the sake of generation, so that another thing of the same sort as the parent may come to be; for it already exists potentially as the same sort of thing as the body of which it is a secretion. In all females, then, residue must necessarily arise — more in the blooded ones, and among these, most of all in the human female; and in the others too some such mass must necessarily collect in the region of the womb. The cause of why it is more in the blooded animals, and most of all among these in humans, has been stated earlier; but as for why such a residue exists in all females, but not in all males, the reason is that some males do not emit seed at all, but rather, just as the animals mentioned earlier fashion, by the motion in the seed, what is formed out of the matter present in the female, so too these seedless males, by the motion within themselves, in this part from which the semen is secreted in others, do the same thing and form the offspring. This is the region around the diaphragm, in all that have one; for the heart, or what is analogous to it, is the starting point of nature, while what lies below is an addition, and exists for its sake. The cause, then, of why not all males have a generative residue, but all females do, is that the animal is an ensouled body..

28| The female always supplies the matter, the male what fashions it; for this is the capacity we say each of them has, and this is what it is to be female and to be male, respectively. So the female must necessarily supply body and bulk, but the male need not; for it is not necessary that either the organs or the agent itself be present in the things that come to be. The body is from the female, but the soul is from the male; for the soul is the substance of a particular body. And this is why, whenever a female and male of kinds that are not the same but not too far apart mate (and those mate whose periods of time are equal and whose gestations are close, and whose body sizes are not far apart), the first offspring comes to be, by way of resemblance, common to both, as with the offspring produced from a fox and a dog, and from a partridge and a domestic fowl; but as time goes on, and generation after generation comes from these, the outcome in the end conforms to the female in form, just as foreign seeds conform to the region they are sown in. For it is this — the female, or the region — that supplies the matter and the body to the seeds. And this is why, in females, the receptive part is not a mere duct, but the womb has an internal cavity;

29| whereas in males that emit seed there are ducts, and these are bloodless, and each of the residues both is in its own proper place and becomes a residue at one and the same time; before that, nothing is a residue, unless by great force and contrary to nature. The cause, then, on account of which the generative residues are secreted in animals, has been stated. But when the semen comes from the male, in those animals that emit seed, it forms the purest part of the female's residue into offspring. For the greater part even of the menstrual fluid is useless and watery, just as the wateriest part of the male's seed is also useless — including that of a single emission taken as a whole; and in most cases the earlier portion is less fertile than the later; for it has less vital heat because it is unconcocted, whereas the concocted part has thickness and is more fully embodied. In those females — whether women or the females of other animals — in whom no emission occurs outward, because there is not much useless residue present in a secretion of this sort, what is present internally is only as much as is left over in animals that do emit it outward, and this the male's power, present in the semen secreted, forms into offspring, or else it does so once the part analogous to the womb has come to the male, as appears to happen in some insects.

30| That the moisture which arises together with pleasure in females contributes nothing to the embryo has been stated earlier. This would seem most supported by the fact that, just as with males, so too with women there occurs at night what people call having an erotic dream. But this is no evidence at all; for it also happens to young males who are on the verge of maturity but emit nothing yet, or who do emit but infertilely. So then, without the male's emission during intercourse it is impossible to conceive, and it is likewise impossible without the female's residue, whether it has come forth outward or is present internally in sufficient amount. Yet women do conceive from such intercourse even when the pleasure that customarily occurs in females does not happen, if the region happens to be aroused and the womb has descended nearby. But for the most part it happens the other way, because the mouth of the womb has not closed up when the secretion occurs, the secretion accompanied by which pleasure customarily arises both in males and in women; and when things are in this state, the male's semen also has an easier passage. The release of semen does not occur internally, as some suppose (for the mouth of the womb is narrow), but toward the front, at the very place where the female emits the moisture that arises in some of them — it is there that the male also emits his seed, if one draws it out to check.

31| So sometimes the seed stays in this condition, but sometimes, if the womb happens to be in due proportion and warm because of the purging, it draws the seed in. Here is a sign of this: fluids applied to it beforehand come away dry. Further, in whatever animals have their wombs next to the diaphragm, as with birds and the fish that bear their young alive, it is impossible for the seed not to be drawn in there, rather than simply being released and arriving on its own. The place draws the seed to itself because of the heat present in it. And the excretion and accumulation of the menses kindles heat in this part, so that, just as cupping-glasses among instruments, when rinsed out with something hot, draw the water into themselves as their mouth turns inward, so too the womb draws in the seed. This, then, is the way the drawing-in comes about; but as some say, it does not happen in any way at all by means of organic parts fitted for intercourse. And the opposite in fact results for those who say that the woman too emits seed: for wombs that are emitting outward turn out, on their account, to have to draw back inward again, if the seed is to mix with that of the male. But for it to happen in this roundabout way is superfluous, and nature does nothing superfluous. Now when the secretion in the womb belonging to the female is composed by the seed of the male, which acts in a way similar to rennet upon milk—

32| for rennet, too, is milk possessing vital heat, which brings what is alike together into one and makes it curdle, and this is what the seed does in relation to the substance of the menses; for milk and the menses have the same nature. As the bodily matter comes together, the fluid is separated out, and as the earthy parts dry, membranes form around it in a circle, both of necessity and for the sake of something: for it is necessary that the outer parts dry out both as they are heated and as they are cooled, and the animal must not be in fluid but set apart from it. Of these coverings, some are called membranes and others chorions, differing in degree; and both are present alike in animals that produce eggs and those that bear their young alive. Once the embryo has already been formed, it behaves in a way similar to things that are sown as seed. For in seeds too there is a first principle present in the seed itself; and once this has been separated out, though it was present potentially before, from it are sent forth both the shoot and the root. It is by the root that the plant takes its nourishment, for the plant needs growth. So too in the embryo, though in a way all the parts are present in it potentially, it is above all the principle that is present, well ahead of the rest. That is why the heart is the first thing separated out in actuality.

33| And this is clear not only from perception (for that is in fact what happens), but also from reasoning. For once the thing has been separated off from both parents, what has come to be must govern itself, like a child sent off to live away from its father. So it must have a principle, from which, later on, the ordering of the body for animals also comes about. For if this principle were ever going to come from outside and be present only later, one could raise not only the difficulty of when, but also the point that, whenever each of the parts is being separated off, this principle must of necessity be present first, since it is from it that both the growth and the motion of the other parts come. That is why those who say, as Democritus does, that the outer parts of animals are distinguished first and the inner parts later, are not speaking correctly, as though animals were like wooden or stone figures. For such things have no principle at all, whereas animals all have one, and have it within. That is why the heart is seen to be marked off first in all blooded animals; for it is the principle both of the uniform parts and of the non-uniform parts. Indeed, this is properly to be recognized as the principle of the animal and of the whole system, from the moment it needs nourishment; for what already exists, after all, grows. And the final form of nourishment for an animal is blood, or its analogue.

34| The vessel for this is the veins; and this is why the heart is the principle of these too. This is clear from observations and from dissections. Now since the embryo is already an animal potentially, though not yet complete, it must take its nourishment from elsewhere; and so it makes use of the womb, which holds it as earth holds a plant, for taking nourishment, until it is brought to completion so as to be already an animal potentially capable of movement. That is why nature first sketched out the two veins from the heart; and from these, small veins are attached, reaching to the womb, by what is called the navel. For the navel is a vein—one in some animals, more than one in others. Around these is a skin-like sheath [the so-called navel] because the weakness of the veins requires safeguarding and covering. The veins, like roots, connect to the womb, and through them the embryo takes its nourishment. This, in fact, is why the animal remains in the womb—not, as Democritus says, so that its parts may be molded to match the parts of the one that holds it. For this is plain in the case of egg-laying animals: their parts undergo differentiation within the eggs, separated from the womb. But one might raise this difficulty: if blood is nourishment, and the heart is the first part to come to be as a blooded thing, and blood is nourishment, and nourishment comes from outside, then where did the first nourishment come in from?

35| Or rather, it is not true that all nourishment comes from outside; instead, right from the start, just as in the seeds of plants there is present a certain milk-like substance that appears first, so too in the material of animals the residue of the initial composition is nourishment. The growth of the embryo, then, comes about through the navel in the same way that it does through the roots for plants, and also for the animals themselves once they have been released from the nourishment within them—matters that must be discussed later, at the proper points in the argument. The differentiation of the parts does not come about, as some suppose, because like is naturally borne toward like (for besides the many other difficulties this account faces, it would follow, if one accepted this cause, that each of the uniform parts comes to be separately—bones by themselves, sinews by themselves, and flesh by itself); rather, it is because the residue belonging to the female is potentially such as the animal is by nature, and the parts are present in it potentially, though none in actuality, that each of them comes to be through this cause; and because, when the active and the passive come into contact in the way in which the one is active and the other passive (and by "the way" I mean the how, the where, and the when), at once the one acts and the other is acted upon.

36| The female, then, provides the matter, and the male provides the principle of motion. Now just as the things produced by a craft come about through instruments—or, more truly speaking, through their motion, and this motion is the actuality of the craft, while the craft is the form of the things that come to be in something else—so too the capacity of the nutritive soul, just as later, in the animals and plants themselves, it produces growth out of nourishment, using heat and cold as its instruments, so to speak (for the motion belonging to the nutritive soul is present in these, and each thing comes to be by a certain proportion), so too from the very beginning it puts together what comes to be by nature. For it is the same matter by which the thing grows and out of which it is first composed, so that the productive capacity too is the same as the one operating from the beginning—though this one is greater. If, then, this is the nutritive soul, it is also the generative soul; and this is the nature belonging to each thing, present both in plants and in all animals. But the other parts of the soul are present in some animals and not in others. In plants, then, the female is not separated from the male—

37| In animals where the sexes are separate, the female needs the male. Yet one might raise a difficulty: if the female has the same soul, and the matter is the residue that belongs to the female, why does she need the male at all, rather than generating offspring by herself out of herself? The reason is that an animal differs from a plant in having perception; and it is impossible for there to be a face or a hand or flesh or any other part without a perceptive soul present in it, either in actuality or in potentiality, and either in some respect or without qualification; otherwise it will be like a corpse, or a part of a corpse. If, then, the male is what produces a soul of this kind, then wherever the female and the male are separate, it is impossible for the female to generate an animal by itself out of itself; for what we have just said was precisely what it is to be male. That the stated difficulty does have some point is clear from birds that lay wind-eggs, since the female is able to generate up to a certain point on her own. But this too presents a difficulty: how are we to say that their eggs are alive? For they cannot be alive in the way fertile eggs are — since then an actually ensouled thing would come from them — nor in the way wood or stone is; for there is a certain decay that befalls even these eggs, as things that in some way previously shared in life.

38| It is clear, then, that they possess some soul potentially. What kind, then, is this? It must be the lowest kind. This is the nutritive soul, for it belongs alike to all animals and plants. Why then does it not bring the parts, and the animal, to completion? Because these need to have a perceptive soul; for the parts of animals are not like the parts of a plant. Hence the female needs partnership with the male, since in these cases the male is kept separate. And this is exactly what happens: wind-eggs become fertile if the male mounts the female at the right time. But the cause of this will be determined later. If, however, there is some genus that is female but has no separate male, it is possible for this to generate an animal out of itself. This has not yet been observed with any confidence up to now, but it does make one hesitate in the genus of fishes; for of the so-called red mullets, no male has ever yet been observed, only females full of spawn. But we do not yet have trustworthy experience of these matters; and there are also fish within the genus of fishes that are neither female nor male, such as eels and a certain kind of grey mullet found around marshy rivers.

39| In all cases where the female and the male are kept separate, it is impossible for the female by itself to generate to completion; for then the male would exist in vain, and nature does nothing in vain. This is why in such cases the male always brings the process of generation to completion. For it is the male that produces the perceptive soul, either through itself or through the seed. Since the parts are already present potentially in the matter, when a principle of motion arises, what follows next falls into sequence, just as in automated marvels. And what some of the natural philosophers mean when they speak of things "moving toward what is like them" should be understood not as the parts changing place as they move, but as remaining in place and being altered in softness and hardness and colors and the other differentiae proper to the uniform parts, becoming in actuality what had previously existed only in potentiality. And the principle comes to be first. This is the heart in blooded animals, and in the others the analogous part, as has often been said. This is clear not only from observation, in that it comes to be first, but also with regard to the end; for life departs from this point last, and this holds true in every case — as if nature were running a double course and winding back to the starting point from which it set out.

40| For coming-to-be is a passage from not-being into being, and destruction is a passage back from being into not-being. After the principle, as has been said, the internal parts come to be before the external ones. And the parts that have magnitude appear before the smaller ones — though in some cases they do not actually come to be earlier, they only appear to. The parts above the midriff are the first to be articulated, and they differ in size, while the lower part is both smaller and less clearly defined. This happens in all animals in which upper and lower are distinguished, except in insects; and among these, in the grub-born kind the growth proceeds upward, since the upper part is smaller to begin with. Both upper and lower remain undifferentiated only in the case of the soft-bodied creatures among those that move about. The same thing that has been said holds also for plants: the upper growth is prior in coming-to-be to what lies below, for seeds send out roots before they send out shoots. The parts of animals are marked off by pneuma — not, however, by the pneuma of the parent that generates them, nor by their own, as some of the natural philosophers claim. This is clear from the case of birds, fish, and insects. For once separated from the parent that generated them, they come to be from an egg, within which they receive their articulation; and some animals do not breathe at all, yet are grub-born or egg-born.

41| Those that do breathe, and receive their articulation within the womb, do not breathe until the lung has reached completion; and this organ, along with the parts in front of it, is articulated before breathing begins. Further, all the many-toed quadrupeds — for example dog, lion, wolf, fox, jackal — bear their young blind, and the eyelid opens only later, after birth. So it is clear that in the same way, in all other cases too, just as quality comes to be, so quantity too comes to be, existing beforehand potentially and only afterward in actuality, under the same causes by which quality is also determined, and two come to be out of one. Pneuma must necessarily be present, because there is moisture and heat, the one acting, the other being acted upon. Some of the ancient natural philosophers attempted to say which of the parts comes to be after which, without being very well acquainted with what actually happens. For among the parts, as among other things, one is by nature prior to another. But "prior" already has several senses: that for the sake of which, and that which exists for the sake of this, differ from one another, and one of them is prior to the other in coming-to-be, the other in substance. And that for the sake of which also has two differentiae: the one is that from which the motion comes, the other is that which the end makes use of.

42| I mean, for example, the generative part and the instrumental part, in relation to the thing generated; for of these, the productive one must exist first — just as the one who teaches exists before the one who learns, but the flutes come after the one learning to play the flute, since it would be pointless for flutes to exist for people who do not know how to play them. There being three things — first, the end, which we call that for the sake of which; second, the things that exist for the sake of this, namely the principle of motion and generation (for the productive and generative, insofar as they are such, stand in relation to the thing produced and generated); and third, the useful thing, that which the end makes use of — of necessity there must first exist some part in which the principle of motion resides (for this is itself immediately a part of the end, one part and the most authoritative one), then after this the whole, that is, the end itself, and third and last the instrumental parts that serve these for certain uses. So if there is something of this kind, which must exist in animals, possessing within itself the principle and end of the whole nature, this must of necessity come to be first — first insofar as it is a moving principle, and along with the whole insofar as it is a part of the end. So that, of the instrumental parts, those that are generative in nature must always exist earlier, since they exist for the sake of something else, as the principle does, while those that are not of this kind, but exist for the sake of something else, come later.

43| That is why it is not easy to distinguish which of the parts are prior — those that exist for the sake of something else, or that for whose sake they exist. For the parts that produce motion intrude, being prior in generation to the end, and it is not easy to distinguish the motion-producing parts from the organic ones. And yet by this method one must inquire what comes into being after what: for the end comes later in some cases, and earlier in others. And for this reason the part that has the principle comes into being first, and next after it the upper cavity. That is why the parts around the head and the eyes appear largest at the start in embryos, while the parts below the navel, such as the limbs, are small: for the lower parts exist for the sake of the upper, and are neither parts of the end nor productive of it. Nor do those speak well about the necessity involved in the 'why', who say that this is simply how it always happens, and think that this is itself a principle in things — as Democritus of Abdera does, on the ground that there is no principle of what is eternal and infinite, but the 'why' is a principle, and the eternal is infinite, so that to ask the 'why' about any such thing is, he says, to seek a principle of the infinite.

44| And yet on this reasoning, by which they think one ought not to seek the 'why', there will be no demonstration of anything eternal; but demonstration is evidently found for many things, some of which are always coming to be and others which always are — since a triangle's having its angles equal to two right angles is always so, and the incommensurability of the diagonal with the side is eternal, and yet there is some cause of these things and a demonstration of them. Now it is right to say that one should not seek a principle of everything, but it is not right to say this of everything that always is and everything that always comes to be; rather it holds only of whatever happen to be principles of eternal things — for a principle is known in a different way, and not by demonstration. A principle, in the case of unchanging things, is what the thing is; but in the case of things that come to be there are already several principles, and in a different way, and not all the same way: one of them, one in number, is that from which the motion comes. That is why all blooded animals have a heart first, as was said at the start; and in the other animals, whatever is analogous to the heart comes into being first. And from the heart the veins are stretched out, just as those who sketch ground-plans do on walls; for the parts are arranged around these veins, inasmuch as they come into being out of them, and their generation is out of the uniform parts by cooling and heating.

45| For things are formed and set solid, some by cold, others by heat. The difference between these has been discussed earlier, elsewhere — which things are dissolved by moisture and by fire, and which are not dissolved by moisture and not melted by fire. Now through the veins and the passages within each part, the nourishment oozes through, just as water does in unfired earthenware jars, and flesh comes into being, or whatever is analogous to it, being formed by cold — which is why it is also dissolved by fire. But whatever of the growing parts are excessively earthy, having little moisture and heat, these, as they cool and the moisture evaporates along with the heat, become hard and earthy in form — such as claws, horns, hooves, and beaks. That is why they are softened by fire but not melted at all, though some are softened by moisture, such as the shells of eggs. By the heat within, the sinews and the bones come into being, as the moisture is dried out. That is why bones too are not dissolved by fire, like pottery: for they have been baked, as it were in a kiln, by the heat present at their generation. And this heat does not make just any chance thing into flesh or bone, nor in any chance place, but makes what is naturally suited to become so, in the place where it is naturally suited, and at the time when it is naturally suited.

46| For neither will what is potentially be brought about by a mover that does not itself have the actuality, nor will what has the actuality make something out of just any chance material — just as a carpenter could not make a chest except out of wood, nor will a chest come to be out of the wood without him. The heat is present in the seminal residue, having motion and actuality of just such an amount and kind as is proportioned to each of the parts. And to the degree that it falls short or exceeds this, it produces a worse result, or the thing that comes to be is deformed — in a way similar to things put together outside the body by boiling, for the enjoyment of food or for some other piece of work. But there, we ourselves supply the proportioning of the heat for the process; here, in generation, nature — the nature of the begetter — supplies it. In things that come to be spontaneously, the cause is the motion and heat of the season. Cooling is a privation of heat. Nature makes use of both, since it has, of necessity, the power to make this thing so and that thing so; yet in things that come to be for the sake of something, it happens that one process cools and another heats, and that each of the parts comes to be as it does — the flesh soft, made so partly of necessity and partly for the sake of something, the sinew dry and tensile, and the bone dry and brittle.

47| The skin comes into being as the flesh dries out, just as with boiled liquids there forms what is called the 'scum'. Its generation happens not only because it is the outermost part, but also because the viscous element rises to the surface, being unable to evaporate. Now in the other animals the viscous element is dry (which is why the shells of testaceans and crustaceans form the outer covering of the bloodless animals), but in the blooded animals the viscous element is more oily. And of these, whatever do not have a very earthy nature, the fatty matter collects beneath the covering of the skin, since the skin itself comes into being out of just such viscosity — for what is oily has a certain viscosity of its own. All these things, as we have said, must be described as coming to be partly of necessity and partly not of necessity but for the sake of something. First, then, the upper cavity is marked off in the process of generation, while the lower part, as time goes on, takes on its growth in the blooded animals. And all the parts are first distinguished by their outlines, and only later take on their colors and their softness and hardness — exactly as though they were being fashioned by nature acting as a painter; for painters too, having first sketched the outline in lines, then fill in the animal with colors.

48| Now because the principle of the senses, and of the whole animal, is in the heart, the heart comes into being first; and because of its heat — at the point where the veins terminate above — the cold forms the brain, as the counterpart to the heat around the heart. That is why the parts around the head take on their generation continuously after the heart, and differ from the other parts in size; for the brain is copious and moist from the start. But what happens with the eyes of animals presents a difficulty. For they appear largest of all the parts from the start, in land animals, swimming animals, and flying animals alike, yet they are the last of the parts to reach completion — for in the intervening time they shrink. The cause is that the sense-organ of the eyes is, like the other sense-organs, situated at passages; but the organ of touch and of taste is immediately either the body itself or some part of the body of the animals, while smell and hearing are passages connecting to the outside air, full of connate breath, and terminating at the little veins around the brain that stretch from the heart; but the eye alone among the sense-organs has a body that is its own.

49| It is moist and cold, and does not pre-exist in that place the way the other parts do — potentially at first, then coming to be in actuality later. Rather, the purest part is separated off from the moisture around the brain through the passages, which are visibly carrying it from the brain to the membrane around the brain. Here is evidence of this: no other part in the head besides the brain is moist and cold, and the eye is cold and moist. So of necessity the place first takes on size, and later contracts. For the same thing happens with the brain itself: at first it is moist and abundant, but as it exhales and is concocted it becomes more solid and contracts — the brain (and its coverings) and the size of the eyes alike. At the start, because of the brain the head is largest, and because of the moisture in the eyes the eyes appear large. They reach completion last, because the brain too coalesces only with difficulty; for it is late in giving up its coldness and moistness, in all animals, but most of all in humans. This is also why the crown of the skull's bones is the last to form.

50| For even once the embryos have already been born into the world, this bone of infants is still soft. The cause of this happening most in the case of humans is that they have the moistest brain, and the most abundant, of all animals. The cause of that, in turn, is that the heat in their heart is the purest. Their intelligence shows this good blending: for man is the most intelligent of animals. Children likewise lack control of their heads for a long time, because of the weight around the brain. And the same holds for the parts that need to be moved: for the principle of motion only late, and last of all, gains mastery over the parts above, in the case of parts whose motion is not directly bound up with it the way the limbs' motion is. The eyelid is such a part. Since nature does nothing superfluous or in vain, clearly it makes this neither too late nor too early; otherwise what came to be would be in vain or superfluous. So it is necessary that the eyelids separate and become able to move at one and the same time. It is late, then, because of the great amount of concoction that goes on around the brain, that the eyes reach completion in animals; and it is last of all, because moving parts so far from the principle, and so thoroughly cooled, requires a motion that has gained a very strong mastery.

51| The eyelids show that they have this nature: for if any heaviness at all occurs around the head, whether from sleep or drink or something else of that kind, we are unable to lift the eyelids, small as the weight they carry is. Concerning the eyes, then, it has been said how and why they come to be, and for what cause they are the last to receive their articulation. Each of the other parts comes to be from the nourishment: the most honored parts, which share in the most authoritative principle, come from the concocted, purest, and first nourishment; the necessary parts, which exist for the sake of these, come from the inferior nourishment and from the residues and excess-products. For nature, like a good household manager, is accustomed to throw nothing away from which something useful can be made. In households, of the food that is produced, the best is set aside for the free members, the worse — and its surplus — for the servants, and the worst is given even to the animals raised alongside them. So just as, outside the body, the intellect governs growth this way, so too, within the things that are coming to be, nature composes flesh and the bodies of the other sense-organs out of the purest matter, but bones and sinews and hair, and further nails and hooves and everything of that kind, out of the residues.

52| That is why these come together last, once nature's residue is already being produced. The nature of the bones comes to be in the first formation of the parts, out of the seminal residue, and as the animal grows it takes its growth from the natural nourishment — from the very same nourishment the principal parts grow from, but from the leftover and residual portion of it. For in everything there is a first and a second, and of nourishment one part is nutritive and the other growth-producing: the nutritive is what supplies being to the whole and to the parts, the growth-producing is what brings about the increase in size — matters that must be determined more fully later. The sinews are formed in the same way as the bones, and from the same sources, from the seminal residue and the nutritive residue. But nails, hair, hooves, horns, beaks, the spurs of birds, and any other part of that kind, come from acquired nourishment, of the growth-producing sort — both what is acquired from the female and what comes from outside afterward. This is why the bones take on growth only up to a certain point: for there is a limit of size set for all animals, and hence also a limit to the growth of their bones.

53| For if these went on growing forever, then the animals that have the analogous part to bone would also keep growing for as long as they lived; for it is these that set the limit of size for animals. The cause why they do not always keep growing must be stated later. Hair, and what is akin to it, grows for as long as it exists, and grows more during illnesses and as bodies age and waste away, because more residue is left over — since less is being spent on the principal parts on account of old age and illness. Indeed, when this supply too fails because of advanced age, the hair fails along with it. Bones are the opposite: they waste away together with the body and its parts. Hair grows even in the dead, though it does not begin growing from nothing in them. About teeth one might raise a puzzle. For they have the same nature as bones, and come to be from the bones, whereas nails, hair, horns, and the like come from the skin — which is why they change color along with the skin, becoming white or black or every sort of color to match the skin's color. Teeth do not change at all in this way; for they come from the bones, in every animal that has both teeth and bones.

54| They alone, unlike the other bones, go on growing throughout life; this is clear in the case of teeth that grow aslant and wear against one another's edge. The cause of their growth, so far as the 'for the sake of which' goes, lies in their function: for they would quickly be worn away if there were no ongoing replenishment, since even now, in some animals as they age — those that eat a great deal but do not have large teeth — the teeth are worn away entirely, because they are ground down at a greater rate than their growth can keep up with. This too, then, is something nature has well contrived to meet what happens: it arranges for the teeth's failing to coincide with old age and the end of life. And if a lifetime lasted ten thousand years, or a thousand, the original teeth would have to become enormous and to grow in afresh many times over; for even if they had continuous growth, they would still become useless for their work once worn smooth. The purpose for which they receive their growth has now been stated. But it turns out that teeth do not have quite the same nature as the other bones either: the others all come to be in the first formation and none of them later, whereas teeth come later. That is also why, once they have fallen out, they can grow in again — for they are attached to the bones, but not grown together with them. Yet they do come to be from the nourishment that is distributed into the bones, and so they share the bones' nature, and they come to be at the point when the bones already have their full number.

55| So the other animals are born already having their teeth, or the part analogous to teeth, unless something contrary to nature occurs — because they are released from generation in a more finished state than man is. Man, unless something contrary to nature happens, is born without them. The cause why some teeth come in and later fall out, while others do not fall out, will be stated later. And because such parts come from residue, this is why man is the most hairless in body of all animals, and has the smallest nails in proportion to his size: for he has the least earthy residue, and residue is what is unconcocted, and the earthy element in bodies is, of all things, the least concocted.

56| We have said, then, how each of the parts is put together, and what the cause of generation is. The embryos of animals that bear their young alive get their growth, as was said earlier, through the attachment of the umbilical cord. For since the nutritive capacity of the soul is present in animals, the embryo at once sends out the umbilical cord like a root into the uterus. The umbilical cord contains veins within its sheath — more of them in the larger animals, such as the ox and the like, two in the middle-sized ones, and one in the smallest. Through this it takes its blood-nourishment, for the uterus is the terminus of many veins. Now all the animals that do not have teeth in both jaws, and those animals with teeth in both jaws whose uterus does not have one large vein running through it but instead many close-packed veins in place of one, have in their uteruses what are called cotyledons, which are convex toward the uterus and hollow toward the embryo. Between the uterus and the embryo are the chorion and the membranes. As the embryo grows and reaches completion the cotyledons become smaller, and finally, once it is complete, they disappear. For nature sets out the uterus's blood-nourishment for the embryos at these points, as if at breasts, and because it collects there little by little out of many veins, the body of the cotyledon comes to be something like an efflorescence or an inflammation.

57| So then, as long as the embryo is small and unable to take in much nourishment, the cotyledons are plainly visible and indeed larger, but once it has grown they collapse. Most of the docked animals, even those with teeth in both jaws, do not have a cotyledon in their uteruses; instead the umbilical cord runs into a single vein, and this vein is stretched through the uterus with considerable size. Since some of these animals bear one young at a time and others bear many, the several embryos are related to it in the same way as the single one is. One must observe these things both from the specimens seen in dissections and from what has been written in the Researches on Animals. For animals grow naturally out of the umbilical cord, and the umbilical cord out of the vein, one connected to the next, as though the vein were flowing past like a channel; and around each of the embryos are both the membranes and the chorion. Those who say that infants are nourished in the uterus by sucking on some little piece of flesh are not speaking correctly. For the same thing would happen in the other animals too, but in fact it is not observed to (this is easy to see by dissection); and around all embryos alike — the winged, the swimming, and those that walk on land — thin membranes enclose them in the same way, separating them from the uterus and from the fluids that form there, in which there is nothing of that kind, nor is it possible to draw nourishment through any of these.

58| It is clear that all egg-laying animals get their growth once separated and outside the womb. Mating occurs among animals, by nature, between those of the same kind; but it also occurs between those whose nature is close, though not identical in species, provided their sizes are close and the periods of their gestation are equal. Such matings are rare among most animals, but they do occur among dogs, foxes, and wolves; and the Indian dogs are born from some dog-like wild beast mated with a dog. This has also been observed among birds that mate readily, for instance among partridges and hens; and among the hook-clawed birds the hawks that differ in species are thought to mate with one another; and it happens in the same way among certain other birds too. Among sea creatures nothing of note has been observed, though the creatures called rhinobatoi are thought above all to arise from the mating of the rhine and the batos. There is also the proverbial saying about Libya, that Libya is always breeding something new, said because of the mating there of animals not of the same stock; for on account of the scarcity of water, all the animals converge on the few places that have springs, and so even those not of the same kind mate together.

59| Now the other offspring produced from such mixed matings are observed, in turn, to mate and breed with one another again, and to be capable of producing both female and male young; but mules alone among such offspring are sterile, for they breed neither from one another nor by mating with other animals. The problem, stated generally, is this: for what cause is a thing sterile, or male, or female? For there are sterile women and sterile men, and among the other animals too, within each kind — horses and sheep, for instance. But the case of the mule is different: the whole kind is sterile. The causes of sterility in the other cases are several. For sometimes, from birth, when the parts useful for mating are maimed, both women and men become sterile, so that the women do not reach puberty and the men do not grow a beard, but remain permanently in a eunuch-like state. In others the same thing happens as their age advances, sometimes on account of the good nourishment of their bodies (for as the women grow fatter and the men grow more vigorous in body, the seminal residue is used up on the body itself, so that the women stop having their monthly discharge and the men stop producing seed), and sometimes because of disease: some men emit fluid that is watery and cold, while in women the menstrual discharges are poor and full of morbid residues; and for many men and many women this condition also comes about through defects in the parts and the places useful for intercourse.

60| Some of these conditions are curable and some incurable, and those that arise this way at the body's original formation remain sterile most persistently of all; for there come to be mannish women and womanish men, and the women do not have their monthly discharge, while the men's seed is thin and cold. This is why, reasonably enough, men's seed is tested for sterility by the trial in water: for what is thin and cold quickly disperses on the surface, while what is fertile sinks to the bottom; for what has been concocted is hot, and what is compact and has thickness to it has been concocted. Women are tested both by pessaries, to see whether the odors pass through to the breath and travel out through the mouth from below upward, and by pigments smeared into the eyes, to see whether they color the saliva in the mouth. For when these things fail to occur, it shows that the body has the passages through which the residue is secreted confused together and grown shut; and the region around the eyes is the most seed-charged part of those around the head. This is shown by the fact that in sexual intercourse it alone visibly changes its appearance, and in those who indulge often in sexual pleasures the eyes visibly give way. The cause is that the nature of seed is similar to that of the brain.

61| for its matter is watery, and its heat is acquired. And the seminal discharges come from the diaphragm, for that is where the origin of this nature lies, so that the motions reach the chest from the joints; and the odors from the chest produce sensation by way of the breathing. Now among human beings and the other kinds, as has been said before, this sort of defect occurs only in particular individuals, but the kind of the mules is sterile as a whole. As for the cause, the accounts given by Empedocles and Democritus — the one speaking obscurely, Democritus more intelligibly — are not correctly stated. For they both give the same demonstration for all cases alike of animals mating outside their own kind. Democritus says that the passages in the uteruses of mules are ruined because the origin of the animals does not come from creatures of the same kind. But it happens with other animals that this same thing holds true of them, and yet they breed no less; and yet, if this really were the cause, the other animals that mate in this way ought to be sterile too. Empedocles attributes it to the mixture of the seeds becoming dense, since the seed of each parent is soft; for the hollow parts of each fit together with the dense parts of the other, and out of such soft materials something hard results, just as when bronze is mixed with tin — and he is not correct either about the case of bronze and tin (this has been discussed in the Problems concerning them) or, in general, since he does not base his principles on anything well established.

62| For how do the hollow and the solid parts, fitting together with one another, produce mixture — for example, of wine and water? This statement is beyond us; for how one is supposed to take the hollow parts of the wine and of the water is altogether contrary to perception. Further, since it happens that from horses a horse is born, and from donkeys a donkey, and from a horse and a donkey a mule — either sex of the horse being paired with either sex of the donkey — why is it that from these a mixture is produced so dense that the offspring is sterile, while from a male and female horse, or from a male and female donkey, no sterile offspring results? And yet the seed of the male horse is soft, and so is that of the female. Both the female horse and the male mate with the donkey — with both the male donkey and the female donkey. And the reason sterile offspring result from both combinations, as he says, is that from both a single thing comes to be, the seeds being soft. So the offspring of a male and female horse ought to be sterile as well, on this account. For if only one of the two mated with the donkey, it would be possible to say that the other was the cause of the offspring's not being generated as similar, comparing it to the seed of the donkey; but as it is, the seed with which the horse mixes when mating with the donkey is of the very same kind as that with which it mixes when mating with its own kind.

63| Further, the demonstration has been stated as applying equally to both, to the female and to the male; but, as they say, only the male mule breeds, and only once it is seven years old — while the female is entirely sterile, or rather sterile in the sense that she does not carry to term, since a mule has actually been known to conceive. Perhaps, though, a more plausible demonstration than those stated would seem to be the dialectical one. I call it dialectical for this reason: the more universal an argument is, the further it stands from the principles proper to the subject. It runs something like this. If it is natural that from a male and female of the same species there comes to be, whether male or female, an offspring of the same species as its parents — for example, from a male and female dog, a dog, male or female — while from parents differing in species there comes to be something differing in species from both, for example, if a dog is different in species from a lion, then from a male dog and a female lion there comes something different from both, and from a male lion and a female dog something different from both — then, since a mule, male and female, is produced, and these two do not differ in species from each other, and the mule is produced from a horse and a donkey, and horses and donkeys differ in species from mules, it is impossible for offspring to be produced from mules. For it is not possible to produce another kind, since from a male and female of the same species only a thing the same in species results.

64| But the mule is produced from a horse and a donkey, which differ in species, and it was laid down that from things differing in species a different animal results; this argument, then, is far too universal, and empty. For arguments not drawn from the principles proper to the subject are empty — they merely seem to belong to the facts, without actually doing so. For arguments drawn from geometrical principles are geometrical, and likewise in the other cases; and what is empty seems to be something, but is nothing. Nor is the premise true, since many offspring produced from parents not of the same species turn out fertile, as was said earlier. In this way, then, one should not conduct inquiry either about the other cases or about natural things generally; rather, by examining the attributes belonging to the kind horse and the kind donkey, one could better grasp the cause — namely, first, that each of them, among animals of its own kind, bears a single offspring at a time; and next, that the females do not always conceive from the males, which is why breeders let horses cover the mares at intervals, since the mares cannot bear continuously. Now the mare is not subject to a monthly discharge, but emits the least of all the quadrupeds; the she-donkey, on the other hand, does not readily accept mating, but urinates out the seed, which is why handlers follow along whipping her.

65| Further, the donkey is a cold animal, and for that reason it does not thrive in wintry regions, since it is by nature ill-suited to cold — for example among the Scythians and the country bordering theirs, nor among the Celts beyond Iberia, for that country too is cold. For this same reason breeders let the males cover the she-donkeys not at the equinox, as with horses, but around the summer solstice, so that the foals may be born in the warm season; for birth occurs in the same season in which mating took place, since both the mare and the she-donkey carry for a year. And since, as has been said, the donkey is cold by nature, its seed too must necessarily be cold. Here is a sign of this: for this reason, if a stallion mounts a mare already covered by a donkey, it does not spoil the donkey's mating; but if the donkey mounts after the horse, it spoils the horse's mating, on account of the coldness of its seed. So then, when the two are mixed with each other, the offspring is preserved because of the heat of the one; for what is secreted from the horse is hotter — the donkey's matter and seed are both cold, while the horse's are hotter. And when hot is mixed onto cold, or cold onto hot, the result is that the conceptus produced from these is itself preserved, and that these parents are fertile with one another; but what is produced from these offspring is no longer fertile, but sterile as regards producing complete offspring.

66| In general, each of the two, horse and donkey, is by nature well-disposed toward sterility. For the donkey, besides the other traits mentioned, has this further trait: if it does not begin to breed right after its first mating season, it never breeds at all thereafter — so closely does the body of donkeys verge on being barren. The horse is likewise: it too is well-disposed toward sterility, and it falls short of being sterile only by as much as is needed to make its offspring colder — and this happens when its seed is mixed with the donkey's secretion. The donkey too, in the same way, comes close to sterility even in mating within its own kind, so that when the unnatural union is added to this — given that, mating with its own kind, it could barely produce a single offspring — the offspring produced from the unnatural union will be still more sterile, and being unnatural it will need nothing further to make it sterile, but will of necessity be sterile. It also happens that the bodies of mules grow large, because the secretion that would otherwise go to the menstrual discharge is turned instead toward growth. And since the gestation of such animals lasts a year, the female mule must not only conceive but also nourish the young to term; but this is impossible without the menstrual discharges occurring. In female mules this discharge does not occur; instead, the useless residue is excreted along with the residue from the bladder (which is why male mules, unlike the other solid-hoofed animals, do not smell the genitals of the females in heat, but only this residue itself), while the rest is turned toward growth and size.

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