by Aristotle..............................................................................................................................................1

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Book II...............................................................................................................................................................21

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Book IV ..............................................................................................................................................................55

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Book V...............................................................................................................................................................71

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WE have now discussed the other parts of animals, both generally and with reference to the peculiarities of

each kind, explaining how each part exists on account of such a cause, and I mean by this the final cause.

There are four causes underlying everything: first, the final cause, that for the sake of which a thing exists;

secondly, the formal cause, the definition of its essence (and these two we may regard pretty much as one and

the same); thirdly, the material; and fourthly, the moving principle or efficient cause.

We have then already discussed the other three causes, for the definition and the final cause are the same, and

the material of animals is their parts of the whole animal the nonhomogeneous parts, of these again the

homogeneous, and of these last the socalled elements of all matter. It remains to speak of those parts which

contribute to the generation of animals and of which nothing definite has yet been said, and to explain what is

the moving or efficient cause. To inquire into this last and to inquire into the generation of each animal is in a

way the same thing; and, therefore, my plan has united them together, arranging the discussion of these parts

Now some animals come into being from the union of male and female, i.e. all those kinds of animal which

possess the two sexes. This is not the case with all of them; though in the sanguinea with few exceptions the

creature, when its growth is complete, is either male or female, and though some bloodless animals have

sexes so that they generate offspring of the same kind, yet other bloodless animals generate indeed, but not

offspring of the same kind; such are all that come into being not from a union of the sexes, but from decaying

earth and excrements. To speak generally, if we take all animals which change their locality, some by

swimming, others by flying, others by walking, we find in these the two sexes, not only in the sanguinea but

also in some of the bloodless animals; and this applies in the case of the latter sometimes to the whole class,

as the cephalopoda and crustacea, but in the class of insects only to the majority. Of these, all which are

produced by union of animals of the same kind generate also after their kind, but all which are not produced

by animals, but from decaying matter, generate indeed, but produce another kind, and the offspring is neither

male nor female; such are some of the insects. This is what might have been expected, for if those animals

which are not produced by parents had themselves united and produced others, then their offspring must have

been either like or unlike to themselves. If like, then their parents ought to have come into being in the same

way; this is only a reasonable postulate to make, for it is plainly the case with other animals. If unlike, and yet

able to copulate, then there would have come into being again from them another kind of creature and again

another from these, and this would have gone on to infinity. But Nature flies from the infinite, for the infinite

But all those creatures which do not move, as the testacea and animals that live by clinging to something else,

inasmuch as their nature resembles that of plants, have no sex any more than plants have, but as applied to

them the word is only used in virtue of a similarity and analogy. For there is a slight distinction of this sort,

since even in plants we find in the same kind some trees which bear fruit and others which, while bearing

none themselves, yet contribute to the ripening of the fruits of those which do, as in the case of the figtree

The same holds good also in plants, some coming into being from seed and others, as it were, by the

spontaneous action of Nature, arising either from decomposition of the earth or of some parts in other plants,

for some are not formed by themselves separately but are produced upon other trees, as the mistletoe. Plants,

Of the generation of animals we must speak as various questions arise in order in the case of each, and we

must connect our account with what has been said. For, as we said above, the male and female principles may

be put down first and foremost as origins of generation, the former as containing the efficient cause of

generation, the latter the material of it. The most conclusive proof of this is drawn from considering how and

whence comes the semen; for there is no doubt that it is out of this that those creatures are formed which are

produced in the ordinary course of Nature; but we must observe carefully the way in which this semen

actually comes into being from the male and female. For it is just because the semen is secreted from the two

sexes, the secretion taking place in them and from them, that they are first principles of generation. For by a

male animal we mean that which generates in another, and by a female that which generates in itself;

wherefore men apply these terms to the macrocosm also, naming Earth mother as being female, but

addressing Heaven and the Sun and other like entities as fathers, as causing generation.

Male and female differ in their essence by each having a separate ability or faculty, and anatomically by

certain parts; essentially the male is that which is able to generate in another, as said above; the female is that

which is able to generate in itself and out of which comes into being the offspring previously existing in the

parent. And since they are differentiated by an ability or faculty and by their function, and since instruments

or organs are needed for all functioning, and since the bodily parts are the instruments or organs to serve the

faculties, it follows that certain parts must exist for union of parents and production of offspring. And these

must differ from each other, so that consequently the male will differ from the female. (For even though we

speak of the animal as a whole as male or female, yet really it is not male or female in virtue of the whole of

itself, but only in virtue of a certain faculty and a certain part just as with the part used for sight or

Now as a matter of fact such parts are in the female the socalled uterus, in the male the testes and the penis,

in all the sanguinea; for some of them have testes and others the corresponding passages. There are

corresponding differences of male and female in all the bloodless animals also which have this division into

opposite sexes. But if in the sanguinea it is the parts concerned in copulation that differ primarily in their

forms, we must observe that a small change in a first principle is often attended by changes in other things

depending on it. This is plain in the case of castrated animals, for, though only the generative part is disabled,

yet pretty well the whole form of the animal changes in consequence so much that it seems to be female or

not far short of it, and thus it is clear than an animal is not male or female in virtue of an isolated part or an

isolated faculty. Clearly, then, the distinction of sex is a first principle; at any rate, when that which

distinguishes male and female suffers change, many other changes accompany it, as would be the case if a

The sanguinea are not all alike as regards testes and uterus. Taking the former first, we find that some of them

have not testes at all, as the classes of fish and of serpents, but only two spermatic ducts. Others have testes

indeed, but internally by the loin in the region of the kidneys, and from each of these a duct, as in the case of

those animals which have no testes at all, these ducts unite also as with those animals; this applies (among

animals breathing air and having a lung) to all birds and oviparous quadrupeds. For all these have their testes

internal near the loin, and two ducts from these in the same way as serpents; I mean the lizards and tortoises

and all the scaly reptiles. But all the vivipara have their testes in front; some of them inside at the end of the

abdomen, as the dolphin, not with ducts but with a penis projecting externally from them; others outside,

either pendent as in man or towards the fundament as in swine. They have been discriminated more

The uterus is always double, just as the testes are always two in the male. It is situated either near the

pudendum (as in women, and all those animals which bring forth alive not only externally but also internally,

and all fish that lay eggs externally) or up towards the hypozoma (as in all birds and in viviparous fishes).

The uterus is also double in the crustacea and the cephalopoda, for the membranes which include their

socalled eggs are of the nature of a uterus. It is particularly hard to distinguish in the case of the poulps, so

that it seems to be single, but the reason of this is that the bulk of the body is everywhere similar.

It is double also in the larger insects; in the smaller the question is uncertain owing to the small size of the

With regard to the difference of the spermatic organs in males, if we are to investigate the causes of their

existence, we must first grasp the final cause of the testes. Now if Nature makes everything either because it

is necessary or because it is better so, this part also must be for one of these two reasons. But that it is not

necessary for generation is plain; else had it been possessed by all creatures that generate, but as it is neither

serpents have testes nor have fish; for they have been seen uniting and with their ducts full of milt. It remains

then that it must be because it is somehow better so. Now it is true that the business of most animals is, you

may say, nothing else than to produce young, as the business of a plant is to produce seed and fruit. But still

as, in the case of nutriment, animals with straight intestines are more violent in their desire for food, so those

which have not testes but only ducts, or which have them indeed but internally, are all quicker in

accomplishing copulation. But those which are to be more temperate in the one case have not straight

intestines, and in the other have their ducts twisted to prevent their desire being too violent and hasty. It is for

this that the testes are contrived; for they make the movement of the spermatic secretion steadier, preserving

the folding back of the passages in the vivipara, as horses and the like, and in man. (For details see the

Enquiries about Animals.) For the testes are no part of the ducts but are only attached to them, as women

fasten stones to the loom when weaving; if they are removed the ducts are drawn up internally, so that

castrated animals are unable to generate; if they were not drawn up they would be able, and before now a bull

mounting immediately after castration has caused conception in the cow because the ducts had not yet been

drawn up. In birds and oviparous quadrupeds the testes receive the spermatic secretion, so that its expulsion is

slower than in fishes. This is clear in the case of birds, for their testes are much enlarged at the time of

copulation, and all those which pair at one season of the year have them so small when this is past that they

are almost indiscernible, but during the season they are very large. When the testes are internal the act of

copulation is quicker than when they are external, for even in the latter case the semen is not emitted before

Besides, quadrupeds have the organ of copulation, since it is possible for them to have it, but for birds and the

footless animals it is not possible, because the former have their legs under the middle of the abdomen and

the latter have no legs at all; now the penis depends from that region and is situated there. (Wherefore also the

legs are strained in intercourse, both the penis and the legs being sinewy.) So that, since it is not possible for

them to have this organ, they must necessarily either have no testes also, or at any rate not have them there, as

Further, with those animals at any rate that have external testes, the semen is collected together before

emission, and emission is due to the penis being heated by its movement; it is not ready for emission at

All the vivipira have their testes in front, internally or externally, except the hedgehog; he alone has them

near the loin. This is for the same reason as with birds, because their union must be quick, for the hedgehog

does not, like the other quadrupeds, mount upon the back of the female, but they conjugate standing upright

So much for the reasons why those animals have testes which have them, and why they are sometimes

All those animals which have no testes are deficient in this part, as has been said, not because it is better to be

so but simply because of necessity, and secondly because it is necessary that their copulation should be

speedy. Such is the nature of fish and serpents. Fish copulate throwing themselves alongside of the females

and separating again quickly. For as men and all such creatures must hold their breath before emitting the

semen, so fish at such times must cease taking in the seawater, and then they perish easily. Therefore they

must not mature the semen during copulation, as viviparous landanimals do, but they have it all matured

together before the time, so as not to be maturing it while in contact but to emit it ready matured. So they

have no testes, and the ducts are straight and simple. There is a small part similar to this connected with the

testes in the system of quadrupeds, for part of the reflected duct is sanguineous and part is not; the fluid is

already semen when it is received by and passes through this latter part, so that once it has arrived there it is

soon emitted in these quadrupeds also. Now in fishes the whole passage resembles the last section of the

Serpents copulate twining round one another, and, as said above, have neither testes nor penis, the latter

because they have no legs, the former because of their length, but they have ducts like for on account of their

extreme length the seminal fluid would take too long in its passage and be cooled if it were further delayed by

testes. (This happens also if the penis is large; such men are less fertile than when it is smaller because the

semen, if cold, is not generative, and that which is carried too far is cooled.) So much for the reason why

some animals have testes and others not. Serpents intertwine because of their inaptitude to cast themselves

alongside of one another. For they are too long to unite closely with so small a part and have no organs of

attachment, so they make use of the suppleness of their bodies, intertwining. Wherefore also they seem to be

slower in copulation than fish, not only on account of the length of the ducts but also of this elaborate

It is not easy to state the facts about the uterus in female animals, for there are many points of difference. The

vivipara are not alike in this part; women and all the vivipara with feet have the uterus low down by the

pudendum, but the cartilaginous viviparous fish have it higher up near the hypozoma. In the ovipara, again, it

is low in fish (as in women and the viviparous quadrupeds), high in birds and all oviparous quadrupeds. Yet

even these differences are on a principle. To begin with the ovipara, they differ in the manner of laying their

eggs, for some produce them imperfect, as fishes whose eggs increase and are finally developed outside of

them. The reason is that they produce many young, and this is their function as it is with plants. If then they

perfected the egg in themselves they must needs be few in number, but as it is, they have so many that each

uterus seems to be an egg, at any rate in the small fishes. For these are the most productive, just as with the

other animals and plants whose nature is analogous to theirs, for the increase of size turns with them to seed.

But the eggs of birds and the quadrupedal ovipara are perfect when produced. In order that these may be

preserved they must have a hard covering (for their envelope is soft so long as they are increasing in size),

and the shell is made by heat squeezing out the moisture for the earthy material; consequently the place must

be hot in which this is to happen. But the part about the hypozoma is hot, as is shown by that being the part

which concocts the food. If then the eggs must be within the uterus, then the uterus must be near the

hypozoma in those creatures which produce their eggs in a perfect form. Similarly it must be low down in

those which produce them imperfect, for it is profitable that it should be so. And it is more natural for the

uterus to be low down than high up, when Nature has no other business in hand to hinder it; for its end is low

down, and where is the end, there is the function, and the uterus itself is naturally where the function is.

We find differences in the vivipara also as compared with one another. Some produce their young alive, not

only externally, but also internally, as men, horses, dogs, and all those which have hair, and among aquatic

But the cartilaginous fish and the vipers produce their young alive externally, but first produce eggs

internally. The egg is perfect, for so only can an animal be generated from an egg, and nothing comes from an

imperfect one. It is because they are of a cold nature, not hot as some assert, that they do not lay their eggs

At least they certainly produce their eggs in a soft envelope, the reason being that they have but little heat and

so their nature does not complete the process of drying the eggshell. Because, then, they are cold they

produce softshelled eggs, and because the eggs are soft they do not produce them externally; for that would

The process is for the most part the same as in birds, for the egg descends and the young is hatched from it

near the vagina, where the young is produced in those animals which are viviparous from the beginning.

Therefore in such animals the uterus is dissimilar to that of both the vivipara and ovipara, because they

participate in both classes; for it is at once near the hypozoma and also stretching along downwards in all the

cartilaginous fishes. But the facts about this and the other kinds of uterus must be gathered from inspection of

the drawings of dissections and from the Enquiries. Thus, because they are oviparous, laying perfect eggs,

they have the uterus placed high, but, as being viviparous, low, participating in both classes.

Animals that are viviparous from the beginning all have it low, Nature here having no other business to

interfere with her, and their production having no double character. Besides this, it is impossible for animals

to be produced alive near the hypozoma, for the foetus must needs be heavy and move, and that region in the

mother is vital and would not be able to bear the weight and the movement. Thirdly, parturition would be

difficult because of the length of the passage to be traversed; even as it is there is difficulty with women if

they draw up the uterus in parturition by yawning or anything of the kind, and even when empty it causes a

feeling of suffocation if moved upwards. For if a uterus is to hold a living animal it must be stronger than in

ovipara, and therefore in all the vivipara it is fleshy, whereas when the uterus is near the hypozoma it is

membranous. And this is clear also in the case of the animals which produce young by the mixed method, for

their eggs are high up and sideways, but the living young are produced in the lower part of the uterus.

So much for the reason why differences are found in the uterus of various animals, and generally why it is

Why is the uterus always internal, but the testes sometimes internal, sometimes external? The reason for the

uterus always being internal is that in this is contained the egg or foetus, which needs guarding, shelter, and

maturation by concoction, while the outer surface of the body is easily injured and cold. The testes vary in

position because they also need shelter and a covering to preserve them and to mature the semen; for it would

be impossible for them, if chilled and stiffened, to be drawn up and discharge it. Therefore, whenever the

testes are visible, they have a cuticular covering known as the scrotum. If the nature of the skin is opposed to

this, being too hard to be adapted for enclosing them or for being soft like a true 'skin', as with the scaly

integument of fish and reptiles, then the testes must needs be internal. Therefore they are so in dolphins and

all the cetacea which have them, and in the oviparous quadrupeds among the scaly animals. The skin of birds

also is hard so that it will not conform to the size of anything and enclose it neatly. (This is another reason

with all these animals for their testes being internal besides those previously mentioned as arising necessarily

from the details of copulation.) For the same reason they are internal in the elephant and hedgehog, for the

[The position of the uterus differs in animals viviparous within themselves and those externally oviparous,

and in the latter class again it differs in those which have the uterus low and those which have it near the

hypozoma, as in fishes compared with birds and oviparous quadrupeds. And it is different again in those

which produce young in both ways, being oviparous internally and viviparous externally. For those which are

viviparous both internally and externally have the uterus placed on the abdomen, as men, cattle, dogs, and the

like, since it is expedient for the safety and growth of the foetus that no weight should be upon the uterus.]

The passages also are different through which the solid and liquid excreta pass out in all the vivipara.

Wherefore both males and females in this class all have a part whereby the urine is voided, and this serves

also for the issue of the semen in males, of the offspring in females. This passage is situated above and in

front of the passage of the solid excreta. The passage is the same as that of the solid nutriment in all those

animals that have no penis, in all the ovipara, even those of them that have a bladder, as the tortoises. For it is

for the sake of generation, not for the evacuation of the urine, that the passages are double; but because the

semen is naturally liquid, the liquid excretion also shares the same passage. This is clear from the fact that all

animals produce semen, but all do not void liquid excrement. Now the spermatic passages of the male must

be fixed and must not wander, and the same applies to the uterus of the female, and this fixing must take

place at either the front or the back of the body. To take the uterus first, it is in the front of the body in

vivipara because of the foetus, but at the loin and the back in ovipara. All animals which are internally

oviparous and externally viviparous are in an intermediate condition because they participate in both classes,

being at once oviparous and viviparous. For the upper part of the uterus, where the eggs are produced, is

under the hypozoma by the loin and the back, but as it advances is low at the abdomen; for it is in that part

that the animal is viviparous. In these also the passage for solid excrement and for copulation is the same, for

The same applies to the passages in the male, whether they have testes or no, as to the uterus of the ovipara.

For in all of them, not only in the ovipara, the ducts adhere to the back and the region of the spine. For they

must not wander but be settled, and that is the character of the region of the back, which gives continuity and

stability. Now in those which have internal testes, the ducts are fixed from the first, and they are fixed in like

manner if the testes are external; then they meet together towards the region of the penis.

The like applies to the ducts in the dolphins, but they have their testes hidden under the abdominal cavity.

We have now discussed the situation of the parts contributing to generation, and the causes thereof.

The bloodless animals do not agree either with the sanguinea or with each other in the fashion of the parts

contributing to generation. There are four classes still left to deal with, first the crustacea, secondly the

cephalopoda, thirdly the insects, and fourthly the testacea. We cannot be certain about all of them, but that

The crustacea copulate like the retromingent quadrupeds, fitting their tails to one another, the one supine and

the other prone. For the flaps attached to the sides of the tail being long prevent them from uniting with the

belly against the back. The males have fine spermatic ducts, the females a membranous uterus alongside the

The cephalopoda entwine together at the mouth, pushing against one another and enfolding their arms. This

attitude is necessary, because Nature has bent backwards the end of the intestine and brought it round near the

mouth, as has been said before in the treatise on the parts of animals. The female has a part corresponding to

the uterus, plainly to be seen in each of these animals, for it contains an egg which is at first indivisible to the

eye but afterwards splits up into many; each of these eggs is imperfect when deposited, as with the oviparous

fishes. In the cephalopoda (as also in the crustacea) the same passage serves to void the excrement and leads

to the part like a uterus, for the male discharges the seminal fluid through this passage. And it is on the lower

surface of the body, where the mantle is open and the seawater enters the cavity. Hence the union of the

male with the female takes place at this point, for it is necessary, if the male discharges either semen or a part

of himself or any other force, that he should unite with her at the uterine passage. But the insertion, in the

case of the poulps, of the arm of the male into the funnel of the female, by which arm the fishermen say the

male copulates with her, is only for the sake of attachment, and it is not an organ useful for generation, for it

is outside the passage in the male and indeed outside the body of the male altogether.

Sometimes also cephalopoda unite by the male mounting on the back of the female, but whether for

Some insects copulate and the offspring are produced from animals of the same name, just as with the

sanguinea; such are the locusts, cicadae, spiders, wasps, and ants. Others unite indeed and generate; but the

result is not a creature of the same kind, but only a scolex, and these insects do not come into being from

animals but from putrefying matter, liquid or solid; such are fleas, flies, and cantharides. Others again are

neither produced from animals nor unite with each other; such are gnats, 'conopes', and many similar kinds.

In most of those which unite the female is larger than the male. The males do not appear to have spermatic

passages. In most cases the male does not insert any part into the female, but the female from below upwards

into the male; this has been observed in many cases (as also that the male mounts the female), the opposite in

few cases; but observations are not yet comprehensive enough to enable us to make a distinction of classes.

And generally it is the rule with most of the oviparous fish and oviparous quadrupeds that the female is larger

than the because this is expedient in view of the increase of bulk in conception by reason of the eggs. In the

female the part analogous to the uterus is cleft and extends along the intestine, as with the other animals; in

this are produced the results of conception. This is clear in locusts and all other large insects whose nature it

Such is the character of the generative organs in animals which were not spoken of before. It remains now to

speak of the homogeneous parts concerned, the seminal fluid and milk. We will take the former first, and

Some animals manifestly emit semen, as all the sanguinea, but whether the insects and cephalopoda do so is

uncertain. Therefore this is a question to be considered, whether all males do so, or not all; and if not all, why

some do and some not; and whether the female also contributes any semen or not; and, if not semen, whether

she does not contribute anything else either, or whether she contributes something else which is not semen.

We must also inquire what those animals which emit semen contribute by means of it to generation, and

generally what is the nature of semen, and of the socalled catamenia in all animals which discharge this

Now it is thought that all animals are generated out of semen, and that the semen comes from the parents.

Wherefore it is part of the same inquiry to ask whether both male and female produce it or only one of them,

and to ask whether it comes from the whole of the body or not from the whole; for if the latter is true it is

reasonable to suppose that it does not come from both parents either. Accordingly, since some say that it

The proofs from which it can be argued that the semen comes from each and every part of the body may be

reduced to four. First, the intensity of the pleasure of coition; for the same state of feeling is more pleasant if

multiplied, and that which affects all the parts is multiplied as compared with that which affects only one or a

few. Secondly, the alleged fact that mutilations are inherited, for they argue that since the parent is deficient

in this part the semen does not come from thence, and the result is that the corresponding part is not formed

in the offspring. Thirdly, the resemblances to the parents, for the young are born like them part for part as

well as in the whole body; if then the coming of the semen from the whole body is cause of the resemblance

of the whole, so the parts would be like because it comes from each of the parts. Fourthly, it would seem to

be reasonable to say that as there is some first thing from which the whole arises, so it is also with each of the

parts, and therefore if semen or seed is cause of the whole so each of the parts would have a seed peculiar to

itself. And these opinions are plausibly supported by such evidence as that children are born with a likeness

to their parents, not in congenital but also in acquired characteristics; for before now, when the parents have

had scars, the children have been born with a mark in the form of the scar in the same place, and there was a

case at Chalcedon where the father had a brand on his arm and the letter was marked on the child, only

confused and not clearly articulated. That is pretty much the evidence on which some believe that the semen

On examining the question, however, the opposite appears more likely, for it is not hard to refute the above

arguments and the view involves impossibilities. First, then, the resemblance of children to parents is no

proof that the semen comes from the whole body, because the resemblance is found also in voice, nails, hair,

and way of moving, from which nothing comes. And men generate before they yet have certain characters,

such as a beard or grey hair. Further, children are like their more remote ancestors from whom nothing has

come, for the resemblances recur at an interval of many generations, as in the case of the woman in Elis who

had intercourse with the Aethiop; her daughter was not an Aethiop but the son of that daughter was. The

same thing applies also to plants, for it is clear that if this theory were true the seed would come from all parts

of plants also; but often a plant does not possess one part, and another part may be removed, and a third

grows afterwards. Besides, the seed does not come from the pericarp, and yet this also comes into being with

We may also ask whether the semen comes from each of the homogeneous parts only, such as flesh and bone

and sinew, or also from the heterogeneous, such as face and hands. For if from the former only, we object

that resemblance exists rather in the heterogeneous parts, such as face and hands and feet; if then it is not

because of the semen coming from all parts that children resemble their parents in these, what is there to stop

the homogeneous parts also from being like for some other reason than this? If the semen comes from the

heterogeneous alone, then it does not come from all parts; but it is more fitting that it should come from the

homogeneous parts, for they are prior to the heterogeneous which are composed of them; and as children are

born like their parents in face and hands, so they are, necessarily, in flesh and nails. If the semen comes from

both, what would be the manner of generation? For the heteroeneous parts are composed of the homogneous,

so that to come from the former would be to come from the latter and from their composition. To make this

clearer by an illustration, take a written name; if anything came from the whole of it, it would be from each of

the syllables, and if from these, from the letters and their composition. So that if really flesh and bones are

composed of fire and the like elements, the semen would come rather from the elements than anything else,

for how can it come from their composition? Yet without this composition there would be no resemblance. If

again something creates this composition later, it would be this that would be the cause of the resemblance,

Further, if the parts of the future animal are separated in the semen, how do they live? and if they are

And what about the generative parts? For that which comes from the male is not similar to what comes from

Again, if the semen comes from all parts of both parents alike, the result is two animals, for the offspring will

have all the parts of both. Wherefore Empedocles seems to say what agrees pretty well with this view (if we

are to adopt it), to a certain extent at any rate, but to be wrong if we think otherwise. What he says agrees

with it when he declares that there is a sort of tally in the male and female, and that the whole offspring does

not come from either, 'but sundered is the fashion of limbs, some in man's...' For why does not the female

generate from herself if the semen comes from all parts alike and she has a receptacle ready in the uterus?

But, it seems, either it does not come from all the parts, or if it does it is in the way Empedocles says, not the

same parts coming from each parent, which is why they need intercourse with each other.

Yet this also is impossible, just as much as it is impossible for the parts when full grown to survive and have

life in them when torn apart, as Empedocles accounts for the creation of animals; in the time of his 'Reign of

Love', says he, 'many heads sprang up without necks,' and later on these isolated parts combined into animals.

Now that this is impossible is plain, for neither would the separate parts be able to survive without having any

soul or life in them, nor if they were living things, so to say, could several of them combine so as to become

one animal again. Yet those who say that semen comes from the whole of the body really have to talk in that

way, and as it happened then in the earth during the 'Reign of Love', so it happens according to them in the

body. Now it is impossible that the parts should be united together when they come into being and should

come from different parts of the parent, meeting together in one place. Then how can the upper and lower,

right and left, front and back parts have been 'sundered'? All these points are unintelligible. Further, some

parts are distinguished by possessing a faculty, others by being in certain states or conditions; the

heterogeneous, as tongue and hand, by the faculty of doing something, the homogeneous by hardness and

softness and the other similar states. Blood, then, will not be blood, nor flesh flesh, in any and every state. It

is clear, then, that that which comes from any part, as blood from blood or flesh from flesh, will not be

identical with that part. But if it is something different from which the blood of the offspring comes, the

coming of the semen from all the parts will not be the cause of the resemblance, as is held by the supporters

of this theory. For if blood is formed from something which is not blood, it is enough that the semen come

from one part only, for why should not all the other parts of the offspring as well as blood be formed from

one part of the parent? Indeed, this theory seems to be the same as that of Anaxagoras, that none of the

homogeneous parts come into being, except that these theorists assume, in the case of the generation of

Then, again, how will these parts that came from all the body of the parent be increased or grow? It is true

that Anaxagoras plausibly says that particles of flesh out of the food are added to the flesh. But if we do not

say this (while saying that semen comes from all parts of the body), how will the foetus become greater by

the addition of something else if that which is added remain unchanged? But if that which is added can

change, then why not say that the semen from the very first is of such a kind that blood and flesh can be made

out of it, instead of saying that it itself is blood and flesh? Nor is there any other alternative, for surely we

cannot say that it is increased later by a process of mixing, as wine when water is poured into it. For in that

case each element of the mixture would be itself at first while still unmixed, but the fact rather is that flesh

and bone and each of the other parts is such later. And to say that some part of the semen is sinew and bone is

Besides all this there is a difficulty if the sex is determined in conception (as Empedocles says: 'it is shed in

clean vessels; some wax female, if they fall in with cold'). Anyhow, it is plain that both men and women

change not only from infertile to fertile, but also from bearing female to bearing male offspring, which looks

as if the cause does not lie in the semen coming from all the parent or not, but in the mutual proportion or

disproportion of that comes from the woman and the man, or in something of this kind. It is clear, then, if we

are to put this down as being so, that the female sex is not determined by the semen coming from any

particular part, and consequently neither is the special sexual part so determined (if really the same semen can

become either male or female child, which shows that the sexual part does not exist in the semen). Why, then,

should we assert this of this part any more than of others? For if semen does not come from this part, the

Again, some creatures come into being neither from parents of the same kind nor from parents of a different

kind, as flies and the various kinds of what are called fleas; from these are produced animals indeed, but not

in this case of similar nature but a kind of scolex. It is plain in this case that the young of a different kind are

not produced by semen coming from all parts of the parent, for they would then resemble them, if indeed

Further even among animals some produce many young from a single coition (and something like this is

universal among plants, for it is plain that they bear all the fruit of a whole season from a single movement).

And yet how would this be possible if the semen were secreted from all the body? For from a single coition

and a single segregation of the semen scattered throughout the body must needs follow only a single

secretion. Nor is it possible for it to be separated in the uterus, for this would no longer be a mere separation

Again, the cuttings from a plant bear seed; clearly, therefore, even before they were cut from the parent plant,

they bore their fruit from their own mass alone, and the seed did not come from all the plant.

But the greatest proof of all is derived from observations we have sufficiently established on insects. For, if

not in all, at least in most of these, the female in the act of copulation inserts a part of herself into the male.

This, as we said before, is the way they copulate, for the females manifestly insert this from below into the

males above, not in all cases, but in most of those observed. Hence it seems clear that, when the males do

emit semen, then also the cause of the generation is not its coming from all the body, but something else

which must be investigated hereafter. For even if it were true that it comes from all the body, as they say,

they ought not to claim that it comes from all parts of it, but only from the creative part from the workman,

so to say, not the material he works in. Instead of that, they talk as if one were to say that the semen comes

from the shoes, for, generally speaking, if a son is like his father, the shoes he wears are like his father's

As to the vehemence of pleasure in sexual intercourse, it is not because the semen comes from all the body,

but because there is a strong friction (wherefore if this intercourse is often repeated the pleasure is diminished

in the persons concerned). Moreover, the pleasure is at the end of the act, but it ought, on the theory, to be in

each of the parts, and not at the same time, but sooner in some and later in others.

If mutilated young are born of mutilated parents, it is for the same reason as that for which they are like them.

And the young of mutilated parents are not always mutilated, just as they are not always like their parents; the

cause of this must be inquired into later, for this problem is the same as that.

Again, if the female does not produce semen, it is reasonable to suppose it does not come from all the body of

the male either. Conversely, if it does not come from all the male it is not unreasonable to suppose that it does

not come from the female, but that the female is cause of the generation in some other way. Into this we must

next inquire, since it is plain that the semen is not secreted from all the parts.

In this investigation and those which follow from it, the first thing to do is to understand what semen is, for

then it will be easier to inquire into its operations and the phenomena connected with it. Now the object of

semen is to be of such a nature that from it as their origin come into being those things which are naturally

formed, not because there is any agent which makes them from it as simply because this is the semen. Now

we speak of one thing coming from another in many senses; it is one thing when we say that night comes

from day or a man becomes man from boy, meaning that A follows B; it is another if we say that a statue is

made from bronze and a bed from wood, and so on in all the other cases where we say that the thing made is

made from a material, meaning that the whole is formed from something preexisting which is only put into

shape. In a third sense a man becomes unmusical from being musical, sick from being well, and generally in

this sense contraries arise from contraries. Fourthly, as in the 'climax' of Epicharmus; thus from slander

comes railing and from this fighting, and all these are from something in the sense that it is the efficient

cause. In this last class sometimes the efficient cause is in the things themselves, as in the last mentioned (for

the slander is a part of the whole trouble), and sometimes external, as the art is external to the work of art or

the torch to the burning house. Now the offspring comes from the semen, and it is plainly in one of the two

following senses that it does so either the semen is the material from which it is made, or it is the first

efficient cause. For assuredly it is not in the sense of A being after B, as the voyage comes from, i.e. after, the

Panathenaea; nor yet as contraries come from contraries, for then one of the two contraries ceases to be, and a

third substance must exist as an immediate underlying basis from which the new thing comes into being. We

must discover then, in which of the two other classes the semen is to be put, whether it is to be regarded as

matter, and therefore acted upon by something else, or as a form, and therefore acting upon something else,

or as both at once. For perhaps at the same time we shall see clearly also how all the products of semen come

into being from contraries, since coming into being from contraries is also a natural process, for some animals

do so, i.e. from male and female, others from only one parent, as is the case with plants and all those animals

in which male and female are not separately differentiated. Now that which comes from the generating parent

is called the seminal fluid, being that which first has in it a principle of generation, in the case of all animals

whose nature it is to unite; semen is that which has in it the principles from both united parents, as the first

mixture which arises from the union of male and female, be it a foetus or an ovum, for these already have in

them that which comes from both. (Semen, or seed, and grain differ only in the one being earlier and the

other later, grain in that it comes from something else, i.e. the seed, and seed in that something else, the grain,

We must again take up the question what the primary nature of what is called semen is. Needs must

everything which we find in the body either be (1) one of the natural parts, whether homogeneous or

heterogeneous, or (2) an unnatural part such as a growth, or (3) a secretion or excretion, or (4) wasteproduct,

or (5) nutriment. (By secretion or excretion I mean the residue of the nutriment, by wasteproduct that which

Now that semen cannot be a part of the body is plain, for it is homogeneous, and from the homogeneous

nothing is composed, e.g. from only sinew or only flesh; nor is it separated as are all the other parts. But

neither is it contrary to Nature nor a defect, for it exists in all alike, and the development of the young animal

It remains, then, that it must be either a wasteproduct or a secretion or excretion. Now the ancients seem to

think that it is a wasteproduct, for when they say that it comes from all the body by reason of the heat of the

movement of the body in copulation, they imply that it is a kind of wasteproduct. But these are contrary to

Nature, and from such arises nothing according to Nature. So then it must be a secretion or excretion.

But, to go further into it, every secretion or excretion is either of useless or useful nutriment; by 'useless' I

mean that from which nothing further is contributed to natural growth, but which is particularly mischievous

to the body if too much of it is consumed; by 'useful' I mean the opposite. Now it is evident that it cannot be

of the former character, for such is most abundant in persons of the worst condition of body through age or

sickness; semen, on the contrary, is least abundant in them for either they have none at all or it is not fertile,

Semen, then, is part of a useful secretion. But the most useful is the last and that from which finally is formed

each of the parts of the body. For secretions are either earlier or later; of the nutriment in the first stage the

secretion is phlegm and the like, for phlegm also is a secretion of the useful nutriment, an indication of this

being that if it is mixed with pure nutriment it is nourishing, and that it is used up in cases of illness. The final

secretion is the smallest in proportion to the quantity of nutriment. But we must reflect that the daily

nutriment by which animals and plants grow is but small, for if a very little be added continually to the same

So we must say the opposite of what the ancients said. For whereas they said that semen is that which comes

from all the body, we shall say it is that whose nature is to go to all of it, and what they thought a

wasteproduct seems rather to be a secretion. For it is more reasonable to suppose that the last extract of the

nutriment which goes to all parts resembles that which is left over from it, just as part of a painter's colour is

often left over resembling that which he has used up. Wasteproducts, on the contrary, are always due to

A further proof that it is not a wasteproduct, but rather a secretion, is the fact that the large animals have few

young, the small many. For the large must have more waste and less secretion, since the great size of the

body causes most of the nutriment to be used up, so that the residue or secretion is small.

Again, no place has been set apart by Nature for wasteproducts but they flow wherever they can find an

easy passage in the body, but a place has been set apart for all the natural secretions; thus the lower intestine

serves for the excretion of the solid nutriment, the bladder for that of the liquid; for the useful part of the

nutriment we have the upper intestine, for the spermatic secretions the uterus and pudenda and breasts, for it

And the resulting phenomena are evidence that semen is what we have said, and these result because such is

the nature of the secretion. For the exhaustion consequent on the loss of even a very little of the semen is

conspicuous because the body is deprived of the ultimate gain drawn from the nutriment. With some few

persons, it is true, during a short time in the flower of their youth the loss of it, if it be excessive in quantity,

is an alleviation (just as in the case of the nutriment in its first stage, if too much have been taken, since

getting rid of this also makes the body more comfortable), and so it may be also when other secretions come

away with it, for in that case it is not only semen that is lost but also other influences come away mingled

with it, and these are morbid. Wherefore, with some men at least, that which comes from them proves

sometimes incapable of procreation because the seminal element in it is so small. But still in most men and as

a general rule the result of intercourse is exhaustion and weakness rather than relief, for the reason given.

Moreover, semen does not exist in them either in childhood or in old age or in sickness in the last case

because of weakness, in old age because they do not sufficiently concoct their food, and in childhood because

they are growing and so all the nutriment is used up too soon, for in about five years, in the case of human

beings at any rate, the body seems to gain half the height that is gained in all the rest of life.

In many animals and plants we find a difference in this connexion not only between kinds as compared with

kinds, but also between similar individuals of the same kind as compared with each other, e.g. man with man

or vine with vine. Some have much semen, others little, others again none at all, not through weakness but

the contrary, at any rate in some cases. This is because the nutriment is used up to form the body, as with

some human beings, who, being in good condition and developing much flesh or getting rather too fat,

produce less semen and are less desirous of intercourse. Like this is what happens with those vines which

'play the goat', that is, luxuriate wantonly through too much nutrition, for hegoats when fat are less inclined

to mount the female; for which reason they thin them before breeding from them, and say that the vines 'play

the goat', so calling it from the condition of the goats. And fat people, women as well as men, appear to be

less fertile than others from the fact that the secretion when in process of concoction turns to fat with those

who are too wellnourished. For fat also is a healthy secretion due to good living.

In some cases no semen is produced at all, as by the willow and poplar. This condition is due to each of the

two causes, weakness and strength; the former prevents concoction of the nutriment, the latter causes it to be

all consumed, as said above. In like manner other animals produce much semen through weakness as well as

through strength, when a great quantity of a useless secretion is mixed with it; this sometimes results in actual

disease when a passage is not found to carry off the impurity, and though some recover of this, others actually

die of it. For corrupt humours collect here as in the urine, which also has been known to cause disease.

[Further the same passage serves for urine and semen; and whatever animals have both kinds of excrement,

that of liquid and that of solid nutriment, discharge the semen by the same passage as the liquid excrement

(for it is a secretion of a liquid, since the nutriment of all animals is rather liquid than solid), but those which

have no liquid excrement discharge it at the passage of the solid residua. Moreover, wasteproducts are

always morbid, but the removal of the secretion is useful; now the discharge of the semen participates in both

characteristics because it takes up some of the nonuseful nutriment. But if it were a wasteproduct it would

From what has been said, it is clear that semen is a secretion of useful nutriment, and that in its last stage,

After this we must distinguish of what sort of nutriment it is a secretion, and must discuss the catamenia

which occur in certain of the vivipara. For thus we shall make it clear (1) whether the female also produces

semen like the male and the foetus is a single mixture of two semens, or whether no semen is secreted by the

female, and, (2) if not, whether she contributes nothing else either to generation but only provides a

receptacle, or whether she does contribute something, and, if so, how and in what manner she does so.

We have previously stated that the final nutriment is the blood in the sanguinea and the analogous fluid in the

other animals. Since the semen is also a secretion of the nutriment, and that in its final stage, it follows that it

will be either (1) blood or that which is analogous to blood, or (2) something formed from this. But since it is

from the blood, when concocted and somehow divided up, that each part of the body is made, and since the

semen if properly concocted is quite of a different character from the blood when it is separated from it, but if

not properly concocted has been known in some cases to issue in a bloody condition if one forces oneself too

often to coition, therefore it is plain that semen will be a secretion of the nutriment when reduced to blood,

being that which is finally distributed to the parts of the body. And this is the reason why it has so great

power, for the loss of the pure and healthy blood is an exhausting thing; for this reason also it is natural that

the offspring should resemble the parents, for that which goes to all the parts of the body resembles that

which is left over. So that the semen which is to form the hand or the face or the whole animal is already the

hand or face or whole animal undifferentiated, and what each of them is actually such is the semen

potentially, either in virtue of its own mass or because it has a certain power in itself. I mention these

alternatives here because we have not yet made it clear from the distincti;1H the vivipara. For thus we shall

make it clear (1) ons drawn hitherto whether it is the matter of the semen that is the cause of generation, or

whether it has in it some faculty and efficient cause thereof, for the hand also or any other bodily part is not

hand or other part in a true sense if it be without soul or some other power, but is only called by the same

On this subject, then, so much may be laid down. But since it is necessary (1) that the weaker animal also

should have a secretion greater in quantity and less concocted, and (2) that being of such a nature it should be

a mass of sanguineous liquid, and (3) since that which Nature endows with a smaller portion of heat is

weaker, and (4) since it has already been stated that such is the character of the female putting all these

considerations together we see that the sanguineous matter discharged by the female is also a secretion. And

It is plain, then, that the catamenia are a secretion, and that they are analogous in females to the semen in

males. The circumstances connected with them are evidence that this view is correct. For the semen begins to

appear in males and to be emitted at the same time of life that the catamenia begin to flow in females, and

that they change their voice and their breasts begin to develop. So, too, in the decline of life the generative

The following signs also indicate that this discharge in females is a secretion. Generally speaking women

suffer neither from haemorrhoids nor bleeding at the nose nor anything else of the sort except when the

catamenia are ceasing, and if anything of the kind occurs the flow is interfered with because the discharge is

Further, the bloodvessels of women stand out less than those of men, and women are rounder and smoother

because the secretion which in men goes to these vessels is drained away with the catamenia. We must

suppose, too, that the same cause accounts for the fact that the bulk of the body is smaller in females than in

males among the vivipara, since this is the only class in which the catamenia are discharged from the body.

And in this class the fact is clearest in women, for the discharge is greater in women than in the other

animals. Wherefore her pallor and the absence of prominent bloodvessels is most conspicuous, and the

Now since this is what corresponds in the female to the semen in the male, and since it is not possible that

two such discharges should be found together, it is plain that the female does not contribute semen to the

generation of the offspring. For if she had semen she would not have the catamenia; but, as it is, because she

It has been stated then that the catamenia are a secretion as the semen is, and confirmation of this view may

be drawn from some of the phenomena of animals. For fat creatures produce less semen than lean ones, as

observed before. The reason is that fat also, like semen, is a secretion, is in fact concocted blood, only not

concocted in the same way as the semen. Thus, if the secretion is consumed to form fat the semen is naturally

deficient. And so among the bloodless animals the cephalopoda and crustacea are in best condition about the

time of producing eggs, for, because they are bloodless and no fat is formed in them, that which is analogous

And a proof that the female does not emit similar semen to the male, and that the offspring is not formed by a

mixture of both, as some say, is that often the female conceives without the sensation of pleasure in

intercourse, and if again the pleasure is experience by her no less than by the male and the two sexes reach

their goal together, yet often no conception takes place unless the liquid of the socalled catamenia is present

in a right proportion. Hence the female does not produce young if the catamenia are absent altogether, nor

often when, they being present, the efflux still continues; but she does so after the purgation. For in the one

case she has not the nutriment or material from which the foetus can be framed by the power coming from the

male and inherent in the semen, and in the other it is washed away with the catamenia because of their

abundance. But when after their occurrence the greater part has been evacuated, the remainder is formed into

a foetus. Cases of conception when the catamenia do not occur at all, or of conception during their discharge

instead of after it, are due to the fact that in the former instance there is only so much liquid to begin with as

remains behind after the discharge in fertile women, and no greater quantity is secreted so as to come away

from the body, while in the latter instance the mouth of the uterus closes after the discharge. When, therefore,

the quantity already expelled from the body is great but the discharge still continues, only not on such a scale

as to wash away the semen, then it is that conception accompanies coition. Nor is it at all strange that the

catamenia should still continue after conception (for even after it they recur to some extent, but are scanty and

do not last during all the period of gestation; this, however, is a morbid phenomenon, wherefore it is found

only in a few cases and then seldom, whereas it is that which happens as a regular thing that is according to

It is clear then that the female contributes the material for generation, and that this is in the substance of the

Some think that the female contributes semen in coition because the pleasure she experiences is sometimes

similar to that of the male, and also is attended by a liquid discharge. But this discharge is not seminal; it is

merely proper to the part concerned in each case, for there is a discharge from the uterus which occurs in

some women but not in others. It is found in those who are fairskinned and of a feminine type generally, but

not in those who are dark and of a masculine appearance. The amount of this discharge, when it occurs, is

sometimes on a different scale from the emission of semen and far exceeds it. Moreover, different kinds of

food cause a great difference in the quantity of such discharges; for instance some pungentlyflavoured foods

cause them to be conspicuously increased. And as to the pleasure which accompanies coition it is due to

emission not only of semen, but also of a spiritus, the coming together of which precedes the emission. This

is plain in the case of boys who are not yet able to emit semen, but are near the proper age, and of men who

are impotent, for all these are capable of pleasure by attrition. And those who have been injured in the

generative organs sometimes suffer from diarrhoea because the secretion, which they are not able to concoct

and turn into semen, is diverted into the intestine. Now a boy is like a woman in form, and the woman is as it

were an impotent male, for it is through a certain incapacity that the female is female, being incapable of

concocting the nutriment in its last stage into semen (and this is either blood or that which is analogous to it

in animals which are bloodless owing to the coldness of their nature). As then diarrhoea is caused in the

bowels by the insufficient concoction of the blood, so are caused in the bloodvessels all discharges of blood,

including that of the catamenia, for this also is such a discharge, only it is natural whereas the others are

Thus it is clear that it is reasonable to suppose that generation comes from this. For the catamenia are semen

not in a pure state but in need of working up, as in the formation of fruits the nutriment is present, when it is

not yet sifted thoroughly, but needs working up to purify it. Thus the catamenia cause generation mixture

with the semen, as this impure nutriment in plants is nutritious when mixed with pure nutriment.

And a sign that the female does not emit semen is the fact that the pleasure of intercourse is caused by touch

in the same region of the female as of the male; and yet is it not from thence that this flow proceeds. Further,

it is not all females that have it at all, but only the sanguinea, and not all even of these, but only those whose

uterus is not near the hypozoma and which do not lay eggs; it is not found in the animals which have no

blood but only the analogous fluid (for what is blood in the former is represented by another fluid in the

latter). The reason why neither the latter nor those sanguinea mentioned (i.e. those whose uterus is low and

which do not lay eggs) have this effluxion is the dryness of their bodies; this allows but little matter to be

secreted, only enough for generation but not enough to be discharged from the body. All animals that are

viviparous without producing eggs first (such are man and all quadrupeds which bend their hindlegs

outwards, for all these are viviparous without producing eggs) all these have the catamenia, unless they are

defective in development as the mule, only the efflux is not abundant as in women. Details of the facts in

The catamenia are more abundant in women than in the other animals, and men emit the most semen in

proportion to their size. The reason is that the composition of their bodies is liquid and hot compared to

others, for more matter must be secreted in such a case. Further, man has no such parts in his body as those to

which the superfluous matter is diverted in the other animals; for he has no great quantity of hair in

There is evidence that the semen is in the catamenia, for, as said before, this secretion appears in the male at

the same time of life as the catamenia in the female; this indicates that the parts destined to receive each of

these secretions are differentiated at the same time in both sexes; and as the neighboring parts in both become

swollen the hair of puberty springs forth in both alike. As the parts in question are on the point of

differentiating they are distended by the spiritus; this is clearer in males in the testes, but appears also about

the breasts; in females it is more marked in the breasts, for it is when they have risen two fingers' breadth that

Now, in all living things in which the male and female are not separated the semen (or seed) is a sort of

embryo; by embryo I mean the first mixture of male and female; hence, from one semen comes one bodys

for example, one stalk of wheat from one grain, as one animal from one egg (for twin eggs are really two

eggs). But in whatever kinds the sexes are distinguished, in these many animals may come from one emission

of semen, showing that the semen differs in its nature in plants and animals. A proof of this is that animals

which can bear more than one young one at a time do so in consequence of only one coition. Whereby, too, it

is plain that the semen does not come from the whole of the body; for neither would on to their size. The

reason is that the composition of thei2J the different parts of the semen already be separated as soon as

discharged from the same part, nor could they be separated in the uterus if they had once entered it all

together; but what does happen is just what one would expect, since what the male contributes to generation

is the form and the efficient cause, while the female contributes the material. In fact, as in the coagulation of

milk, the milk being the material, the figjuice or rennet is that which contains the curdling principle, so acts

the secretion of the male, being divided into parts in the female. Why it is sometimes divided into more or

fewer parts, and sometimes not divided at all, will be the subject of another discussion. But because it does

not differ in kind at any rate this does not matter, but what does matter is only that each part should

correspond to the material, being neither too little to concoct it and fix it into form, nor too much so as to dry

it up; it then generates a number of offspring. But from this first formative semen, if it remains one, and is not

That, then, the female does not contribute semen to generation, but does contribute something, and that this is

the matter of the catamenia, or that which is analogous to it in bloodless animals, is clear from what has been

said, and also from a general and abstract survey of the question. For there must needs be that which

generates and that from which it generates; even if these be one, still they must be distinct in form and their

essence must be different; and in those animals that have these powers separate in two sexes the body and

nature of the active and the passive sex must also differ. If, then, the male stands for the effective and active,

and the female, considered as female, for the passive, it follows that what the female would contribute to the

semen of the male would not be semen but material for the semen to work upon. This is just what we find to

be the case, for the catamenia have in their nature an affinity to the primitive matter.

So much for the discussion of this question. At the same time the answer to the next question we have to

investigate is clear from these considerations, I mean how it is that the male contributes to generation and

how it is that the semen from the male is the cause of the offspring. Does it exist in the body of the embryo as

a part of it from the first, mingling with the material which comes from the female? Or does the semen

communicate nothing to the material body of the embryo but only to the power and movement in it? For this

power is that which acts and makes, while that which is made and receives the form is the residue of the

secretion in the female. Now the latter alternative appears to be the right one both a priori and in view of the

facts. For, if we consider the question on general grounds, we find that, whenever one thing is made from two

of which one is active and the other passive, the active agent does not exist in that which is made; and, still

more generally, the same applies when one thing moves and another is moved; the moving thing does not

exist in that which is moved. But the female, as female, is passive, and the male, as male, is active, and the

principle of the movement comes from him. Therefore, if we take the highest genera under which they each

fall, the one being active and motive and the other passive and moved, that one thing which is produced

comes from them only in the sense in which a bed comes into being from the carpenter and the wood, or in

which a ball comes into being from the wax and the form. It is plain then that it is not necessary that anything

at all should come away from the male, and if anything does come away it does not follow that this gives rise

to the embryo as being in the embryo, but only as that which imparts the motion and as the form; so the

This a priori argument is confirmed by the facts. For it is for this reason that some males which unite with the

female do not, it appears, insert any part of themselves into the female, but on the contrary the female inserts

a part of herself into the male; this occurs in some insects. For the effect produced by the semen in the female

(in the case of those animals whose males do insert a part) is produced in the case of these insects by the heat

and power in the male animal itself when the female inserts that part of herself which receives the secretion.

And therefore such animals remain united a long time, and when they are separated the young are produced

quickly. For the union lasts until that which is analogous to the semen has done its work, and when they

separate the female produces the embryo quickly; for the young is imperfect inasmuch as all such creatures

What occurs in birds and oviparous fishes is the greatest proof that neither does the semen come from all

parts of the male nor does he emit anything of such a nature as to exist within that which is generated, as part

of the material embryo, but that he only makes a living creature by the power which resides in the semen (as

we said in the case of those insects whose females insert a part of themselves into the male). For if a henbird

is in process of producing windeggs and is then trodden by the cock before the egg has begun to whiten and

while it is all still yellow, then they become fertile instead of being windeggs. And if while it is still yellow

she be trodden by another cock, the whole brood of chicks turn out like the second cock. Hence some of those

who are anxious to rear fine birds act thus; they change the cocks for the first and second treading, not as if

they thought that the semen is mingled with the egg or exists in it, or that it comes from all parts of the cock;

for if it did it would have come from both cocks, so that the chick would have all its parts doubled. But it is

by its force that the semen of the male gives a certain quality to the material and the nutriment in the female,

for the second semen added to the first can produce this effect by heat and concoction, as the egg acquires

The same conclusion is to be drawn from the generation of oviparous fishes. When the female has laid her

eggs, the male spinkles the milt over them, and those eggs are fertilized which it reaches, but not the others;

this shows that the male does not contribute anything to the quantity but only to the quality of the embryo.

From what has been said it is plain that the semen does not come from the whole of the body of the male in

those animals which emit it, and that the contribution of the female to the generative product is not the same

as that of the male, but the male contributes the principle of movement and the female the material. This is

why the female does not produce offspring by herself, for she needs a principle, i.e. something to begin the

movement in the embryo and to define the form it is to assume. Yet in some animals, as birds, the nature of

the female unassisted can generate to a certain extent, for they do form something, only it is incomplete; I

For the same reason the development of the embryo takes place in the female; neither the male himself nor

the female emits semen into the male, but the female receives within herself the share contributed by both,

because in the female is the material from which is made the resulting product. Not only must the mass of

material exist there from which the embryo is formed in the first instance, but further material must

constantly be added that it may increase in size. Therefore the birth must take place in the female. For the

carpenter must keep in close connexion with his timber and the potter with his clay, and generally all

workmanship and the ultimate movement imparted to matter must be connected with the material concerned,

From these considerations we may also gather how it is that the male contributes to generation. The male

does not emit semen at all in some animals, and where he does this is no part of the resulting embryo; just so

no material part comes from the carpenter to the material, i.e. the wood in which he works, nor does any part

of the carpenter's art exist within what he makes, but the shape and the form are imparted from him to the

material by means of the motion he sets up. It is his hands that move his tools, his tools that move the

material; it is his knowledge of his art, and his soul, in which is the form, that moves his hands or any other

part of him with a motion of some definite kind, a motion varying with the varying nature of the object made.

In like manner, in the male of those animals which emit semen Nature uses the semen as a tool and as

possessing motion in actuality, just as tools are used in the products of any art, for in them lies in a certain

sense the motion of the art. Such, then, is the way in which these males contribute to generation. But when

the male does not emit semen, but the female inserts some part of herself into the male, this is parallel to a

case in which a man should carry the material to the workman. For by reason of weakness in such males

Nature is not able to do anything by any secondary means, but the movements imparted to the material are

scarcely strong enough when Nature herself watches over them. Thus here she resembles a modeller in clay

rather than a carpenter, for she does not touch the work she is forming by means of tools, but, as it were, with

In all animals which can move about, the sexes are separated, one individual being male and one female,

though both are the same in species, as with man and horse. But in plants these powers are mingled, female

not being separated from male. Wherefore they generate out of themselves, and do not emit semen but

produce an embryo, what is called the seed. Empedocles puts this well in the line: 'and thus the tall trees

oviposit; first olives...' For as the egg is an embryo, a certain part of it giving rise to the animal and the rest

being nutriment, so also from a part of the seed springs the growing plant, and the rest is nutriment for the

In a certain sense the same thing happens also in those animals which have the sexes separate. For when there

is need for them to generate the sexes are no longer separated any more than in plants, their nature desiring

that they shall become one; and this is plain to view when they copulate and are united, that one animal is

It is the nature of those creatures which do not emit semen to remain united a long time until the male

element has formed the embryo, as with those insects which copulate. The others so remain only until the

male has discharged from the parts of himself introduced something which will form the embryo in a longer

time, as among the sanguinea. For the former remain paired some part of a day, while the semen forms the

And animals seem literally to be like divided plants, as though one should separate and divide them, when

In all this Nature acts like an intelligent workman. For to the essence of plants belongs no other function or

business than the production of seed; since, then, this is brought about by the union of male and female,

Nature has mixed these and set them together in plants, so that the sexes are not divided in them. Plants,

however, have been investigated elsewhere. But the function of the animal is not only to generate (which is

common to all living things), but they all of them participate also in a kind of knowledge, some more and

some less, and some very little indeed. For they have senseperception, and this is a kind of knowledge. (If

we consider the value of this we find that it is of great importance compared with the class of lifeless objects,

but of little compared with the use of the intellect. For against the latter the mere participation in touch and

taste seems to be practically nothing, but beside absolute insensibility it seems most excellent; for it would

seem a treasure to gain even this kind of knowledge rather than to lie in a state of death and nonexistence.)

Now it is by senseperception that an animal differs from those organisms which have only life. But since, if

it is a living animal, it must also live; therefore, when it is necessary for it to accomplish the function of that

which has life, it unites and copulates, becoming like a plant, as we said before.

Testaceous animals, being intermediate between animals and plants, perform the function of neither class as

belonging to both. As plants they have no sexes, and one does not generate in another; as animals they do not

bear fruit from themselves like plants; but they are formed and generated from a liquid and earthy concretion.

THAT the male and the female are the principles of generation has been previously stated, as also what is

their power and their essence. But why is it that one thing becomes and is male, another female? It is the

business of our discussion as it proceeds to try and point out (1) that the sexes arise from Necessity and the

first efficient cause, (2) from what sort of material they are formed. That (3) they exist because it is better and

on account of the final cause, takes us back to a principle still further remote.

Now (1) some existing things are eternal and divine whilst others admit of both existence and nonexistence.

But (2) that which is noble and divine is always, in virtue of its own nature, the cause of the better in such

things as admit of being better or worse, and what is not eternal does admit of existence and nonexistence,

and can partake in the better and the worse. And (3) soul is better than body, and living, having soul, is

thereby better than the lifeless which has none, and being is better than not being, living than not living.

These, then, are the reasons of the generation of animals. For since it is impossible that such a class of things

as animals should be of an eternal nature, therefore that which comes into being is eternal in the only way

possible. Now it is impossible for it to be eternal as an individual (though of course the real essence of things

is in the individual) were it such it would be eternal but it is possible for it as a species. This is why there is

always a class of men and animals and plants. But since the male and female essences are the first principles

of these, they will exist in the existing individuals for the sake of generation. Again, as the first efficient or

moving cause, to which belong the definition and the form, is better and more divine in its nature than the

material on which it works, it is better that the superior principle should be separated from the inferior.

Therefore, wherever it is possible and so far as it is possible, the male is separated from the female. For the

first principle of the movement, or efficient cause, whereby that which comes into being is male, is better and

more divine than the material whereby it is female. The male, however, comes together and mingles with the

A thing lives, then, in virtue of participating in the male and female principles, wherefore even plants have

some kind of life; but the class of animals exists in virtue of senseperception. The sexes are divided in

nearly all of these that can move about, for the reasons already stated, and some of them, as said before, emit

semen in copulation, others not. The reason of this is that the higher animals are more independent in their

nature, so that they have greater size, and this cannot exist without vital heat; for the greater body requires

more force to move it, and heat is a motive force. Therefore, taking a general view, we may say that

sanguinea are of greater size than bloodless animals, and those which move about than those which remain

fixed. And these are just the animals which emit semen on account of their heat and size.

So much for the cause of the existence of the two sexes. Some animals bring to perfection and produce into

the world a creature like themselves, as all those which bring their young into the world alive; others produce

something undeveloped which has not yet acquired its own form; in this latter division the sanguinea lay

eggs, the bloodless animals either lay an egg or give birth to a scolex. The difference between egg and scolex

is this: an egg is that from a part of which the young comes into being, the rest being nutriment for it; but the

whole of a scolex is developed into the whole of the young animal. Of the vivipara, which bring into the

world an animal like themselves, some are internally viviparous (as men, horses, cattle, and of marine

animals dolphins and the other cetacea); others first lay eggs within themselves, and only after this are

externally viviparous (as the cartilaginous fishes). Among the ovipara some produce the egg in a perfect

condition (as birds and all oviparous quadrupeds and footless animals, e.g. lizards and tortoises and most

snakes; for the eggs of all these do not increase when once laid). The eggs of others are imperfect; such are

those of fishes, crustaceans, and cephalopods, for their eggs increase after being produced.

All the vivipara are sanguineous, and the sanguinea are either viviparous or oviparous, except those which are

altogether infertile. Among bloodless animals the insects produce a scolex, alike those that are generated by

copulation and those that copulate themselves though not so generated. For there are some insects of this sort,

which though they come into being by spontaneous generation are yet male and female; from their union

something is produced, only it is imperfect; the reason of this has been previously stated.

These classes admit of much crossdivision. Not all bipeds are viviparous (for birds are oviparous), nor are

they all oviparous (for man is viviparous), nor are all quadrupeds oviparous (for horses, cattle, and countless

others are viviparous), nor are they all viviparous (for lizards, crocodiles, and many others lay eggs). Nor

does the presence or absence of feet make the difference between them, for not only are some footless

animals viviparous, as vipers and the cartilaginous fishes, while others are oviparous, as the other fishes and

serpents, but also among those which have feet many are oviparous and many viviparous, as the quadrupeds

above mentioned. And some which have feet, as man, and some which have not, as the whale and dolphin,

are internally viviparous. By this character then it is not possible to divide them, nor is any of the locomotive

organs the cause of this difference, but it is those animals which are more perfect in their nature and

participate in a purer element which are viviparous, for nothing is internally viviparous unless it receive and

breathe out air. But the more perfect are those which are hotter in their nature and have more moisture and are

not earthy in their composition. And the measure of natural heat is the lung when it has blood in it, for

generally those animals which have a lung are hotter than those which have not, and in the former class again

those whose lung is not spongy nor solid nor containing only a little blood, but soft and full of blood. And as

the animal is perfect but the egg and the scolex are imperfect, so the perfect is naturally produced from the

more perfect. If animals are hotter as shown by their possessing a lung but drier in their nature, or are colder

but have more moisture, then they either lay a perfect egg or are viviparous after laying an egg within

themselves. For birds and scaly reptiles because of their heat produce a perfect egg, but because of their

dryness it is only an egg; the cartilaginous fishes have less heat than these but more moisture, so that they are

intermediate, for they are both oviparous and viviparous within themselves, the former because they are cold,

the latter because of their moisture; for moisture is vivifying, whereas dryness is furthest removed from what

has life. Since they have neither feathers nor scales such as either reptiles or other fishes have, all which are

signs rather of a dry and earthy nature, the egg they produce is soft; for the earthy matter does not come to the

surface in their eggs any more than in themselves. This is why they lay eggs in themselves, for if the egg

Animals that are cold and rather dry than moist also lay eggs, but the egg is imperfect; at the same time,

because they are of an earthy nature and the egg they produce is imperfect, therefore it has a hard integument

that it may be preserved by the protection of the shelllike covering. Hence fishes, because they are scaly,

and crustacea, because they are of an earthy nature, lay eggs with a hard integument.

The cephalopods, having themselves bodies of a sticky nature, preserve in the same way the imperfect eggs

they lay, for they deposit a quantity of sticky material about the embryo. All insects produce a scolex. Now

all the insects are bloodless, wherefore all creatures that produce a scolex from themselves are so. But we

cannot say simply that all bloodless animals produce a scolex, for the classes overlap one another, (1) the

insects, (2) the animals that produce a scolex, (3) those that lay their egg imperfect, as the scaly fishes, the

crustacea, and the cephalopoda. I say that these form a gradation, for the eggs of these latter resemble a

scolex, in that they increase after oviposition, and the scolex of insects again as it develops resembles an egg;

We must observe how rightly Nature orders generation in regular gradation. The more perfect and hotter

animals produce their young perfect in respect of quality (in respect of quantity this is so with no animal, for

the young always increase in size after birth), and these generate living animals within themselves from the

first. The second class do not generate perfect animals within themselves from the first (for they are only

viviparous after first laying eggs), but still they are externally viviparous. The third class do not produce a

perfect animal, but an egg, and this egg is perfect. Those whose nature is still colder than these produce an

egg, but an imperfect one, which is perfected outside the body, as the class of scaly fishes, the crustacea, and

the cephalopods. The fifth and coldest class does not even lay an egg from itself; but so far as the young ever

attain to this condition at all, it is outside the body of the parent, as has been said already. For insects produce

a scolex first; the scolex after developing becomes egglike (for the socalled chrysalis or pupa is equivalent

to an egg); then from this it is that a perfect animal comes into being, reaching the end of its development in

Some animals then, as said before, do not come into being from semen, but all the sanguinea do so which are

generated by copulation, the male emitting semen into the female when this has entered into her the young

are formed and assume their peculiar character, some within the animals themselves when they are

There is a considerable difficulty in understanding how the plant is formed out of the seed or any animal out

of the semen. Everything that comes into being or is made must (1) be made out of something, (2) be made

by the agency of something, and (3) must become something. Now that out of which it is made is the

material; this some animals have in its first form within themselves, taking it from the female parent, as all

those which are not born alive but produced as a scolex or an egg; others receive it from the mother for a long

time by sucking, as the young of all those which are not only externally but also internally viviparous. Such,

then, is the material out of which things come into being, but we now are inquiring not out of what the parts

of an animal are made, but by what agency. Either it is something external which makes them, or else

something existing in the seminal fluid and the semen; and this must either be soul or a part of soul, or

Now it would appear irrational to suppose that any of either the internal organs or the other parts is made by

something external, since one thing cannot set up a motion in another without touching it, nor can a thing be

affected in any way by another if it does not set up a motion in it. Something then of the sort we require exists

in the embryo itself, being either a part of it or separate from it. To suppose that it should be something else

separate from it is irrational. For after the animal has been produced does this something perish or does it

remain in it? But nothing of the kind appears to be in it, nothing which is not a part of the whole plant or

animal. Yet, on the other hand, it is absurd to say that it perishes after making either all the parts or only some

of them. If it makes some of the parts and then perishes, what is to make the rest of them? Suppose this

something makes the heart and then perishes, and the heart makes another organ, by the same argument either

all the parts must perish or all must remain. Therefore it is preserved and does not perish. Therefore it is a

part of the embryo itself which exists in the semen from the beginning; and if indeed there is no part of the

soul which does not exist in some part of the body, it would also be a part containing soul in it from the

How, then, does it make the other parts? Either all the parts, as heart, lung, liver, eye, and all the rest, come

into being together or in succession, as is said in the verse ascribed to Orpheus, for there he says that an

animal comes into being in the same way as the knitting of a net. That the former is not the fact is plain even

to the senses, for some of the parts are clearly visible as already existing in the embryo while others are not;

that it is not because of their being too small that they are not visible is clear, for the lung is of greater size

than the heart, and yet appears later than the heart in the original development. Since, then, one is earlier and

another later, does the one make the other, and does the later part exist on account of the part which is next to

it, or rather does the one come into being only after the other? I mean, for instance, that it is not the fact that

the heart, having come into being first, then makes the liver, and the liver again another organ, but that the

liver only comes into being after the heart, and not by the agency of the heart, as a man becomes a man after

being a boy, not by his agency. An explanation of this is that, in all the productions of Nature or of art, what

already exists potentially is brought into being only by what exists actually; therefore if one organ formed

another the form and the character of the later organ would have to exist in the earlier, e.g. the form of the

Yet again, if the whole animal or plant is formed from semen or seed, it is impossible that any part of it

should exist ready made in the semen or seed, whether that part be able to make the other parts or no. For it is

plain that, if it exists in it from the first, it was made by that which made the semen. But semen must be made

first, and that is the function of the generating parent. So, then, it is not possible that any part should exist in

But neither can this agent be external, and yet it must needs be one or other of the two. We must try, then, to

solve this difficulty, for perhaps some one of the statements made cannot be made without qualification, e.g.

the statement that the parts cannot be made by what is external to the semen. For if in a certain sense they

cannot, yet in another sense they can. (Now it makes no difference whether we say 'the semen' or 'that from

which the semen comes', in so far as the semen has in itself the movement initiated by the other.) It is

possible, then, that A should move B, and B move C; that, in fact, the case should be the same as with the

automatic machines shown as curiosities. For the parts of such machines while at rest have a sort of

potentiality of motion in them, and when any external force puts the first of them in motion, immediately the

next is moved in actuality. As, then, in these automatic machines the external force moves the parts in a

certain sense (not by touching any part at the moment, but by having touched one previously), in like manner

also that from which the semen comes, or in other words that which made the semen, sets up the movement

in the embryo and makes the parts of it by having first touched something though not continuing to touch it.

In a way it is the innate motion that does this, as the act of building builds the house. Plainly, then, while

there is something which makes the parts, this does not exist as a definite object, nor does it exist in the

But how is each part formed? We must answer this by starting in the first instance from the principle that, in

all products of Nature or art, a thing is made by something actually existing out of that which is potentially

such as the finished product. Now the semen is of such a nature, and has in it such a principle of motion, that

when the motion is ceasing each of the parts comes into being, and that as a part having life or soul. For there

is no such thing as face or flesh without life or soul in it; it is only equivocally that they will be called face or

flesh if the life has gone out of them, just as if they had been made of stone or wood. And the homogeneous

parts and the organic come into being together. And just as we should not say that an axe or other instrument

or organ was made by the fire alone, so neither shall we say that foot or hand were made by heat alone. The

same applies also to flesh, for this too has a function. While, then, we may allow that hardness and softness,

stickiness and brittleness, and whatever other qualities are found in the parts that have life and soul, may be

caused by mere heat and cold, yet, when we come to the principle in virtue of which flesh is flesh and bone is

bone, that is no longer so; what makes them is the movement set up by the male parent, who is in actuality

what that out of which the offspring is made is in potentiality. This is what we find in the products of art; heat

and cold may make the iron soft and hard, but what makes a sword is the movement of the tools employed,

this movement containing the principle of the art. For the art is the startingpoint and form of the product;

only it exists in something else, whereas the movement of Nature exists in the product itself, issuing from

Has the semen soul, or not? The same argument applies here as in the question concerning the parts. As no

part, if it participate not in soul, will be a part except in an equivocal sense (as the eye of a dead man is still

called an 'eye'), so no soul will exist in anything except that of which it is soul; it is plain therefore that semen

But a thing existing potentially may be nearer or further from its realization in actuality, as e.g. a

mathematician when asleep is further from his realization in actuality as engaged in mathematics than when

he is awake, and when awake again but not studying mathematics he is further removed than when he is so

studying. Accordingly it is not any part that is the cause of the soul's coming into being, but it is the first

moving cause from outside. (For nothing generates itself, though when it has come into being it

thenceforward increases itself.) Hence it is that only one part comes into being first and not all of them

together. But that must first come into being which has a principle of increase (for this nutritive power exists

in all alike, whether animals or plants, and this is the same as the power that enables an animal or plant to

generate another like itself, that being the function of them all if naturally perfect). And this is necessary for

the reason that whenever a living thing is produced it must grow. It is produced, then, by something else of

the same name, as e.g. man is produced by man, but it is increased by means of itself. There is, then,

something which increases it. If this is a single part, this must come into being first. Therefore if the heart is

first made in some animals, and what is analogous to the heart in the others which have no heart, it is from

We have thus discussed the difficulties previously raised on the question what is the efficient cause of

The next question to be mooted concerns the nature of semen. For whereas when it issues from the animal it

is thick and white, yet on cooling it becomes liquid as water, and its colour is that of water. This would

appear strange, for water is not thickened by heat; yet semen is thick when it issues from within the animal's

body which is hot, and becomes liquid on cooling. Again, watery fluids freeze, but semen, if exposed in frosts

to the open air, does not freeze but liquefies, as if it was thickened by the opposite of cold. Yet it is

unreasonable, again, to suppose that it is thickened by heat. For it is only substances having a predominance

of earth in their composition that coagulate and thicken on boiling, e.g. milk. It ought then to solidify on

cooling, but as a matter of fact it does not become solid in any part but the whole of it goes like water.

This then is the difficulty. If it is water, water evidently does not thicken through heat, whereas the semen is

thick and both it and the body whence it issues are hot. If it is made of earth or a mixture of earth and water, it

Perhaps, however, we have not discriminated all the possibilities. It is not only the liquids composed of water

and earthy matter that thicken, but also those composed of water and air; foam, for instance, becomes thicker

and white, and the smaller and less visible the bubbles in it, the whiter and firmer does the mass appear. The

same thing happens also with oil; on mixing with air it thickens, wherefore that which is whitening becomes

thicker, the watery part in it being separated off by the heat and turning to air. And if oxide of lead is mixed

with water or even with oil, the mass increases greatly and changes from liquid and dark to firm and white,

the reason being that air is mixed in with it which increases the mass and makes the white shine through, as in

foam and snow (for snow is foam). And water itself on mingling with oil becomes thick and white, because

air is entangled in it by the act of pounding them together, and oil itself has much air in it (for shininess is a

property of air, not of earth or water). This too is why it floats on the surface of the water, for the air

contained in it as in a vessel bears it up and makes it float, being the cause of its lightness. So too oil is

thickened without freezing in cold weather and frosts; it does not freeze because of its heat (for the air is hot

and will not freeze), but because the air is forced together and compressed, as..., by the cold, the oil becomes

thicker. These are the reasons why semen is firm and white when it issues from within the animal; it has a

quantity of hot air in it because of the internal heat; afterwards, when the heat has evaporated and the air has

cooled, it turns liquid and dark; for the water, and any small quantity of earthy matter there may be, remain in

Semen, then, is a compound of spirit (pneuma) and water, and the former is hot air (aerh); hence semen is

liquid in its nature because it is made of water. What Ctesias the Cnidian has asserted of the semen of

elephants is manifestly untrue; he says that it hardens so much in drying that it becomes like amber. But this

does not happen, though it is true that one semen must be more earthy than another, and especially so with

animals that have much earthy matter in them because of the bulk of their bodies. And it is thick and white

because it is mixed with spirit, for it is also an invariable rule that it is white, and Herodotus does not report

the truth when he says that the semen of the Aethiopians is black, as if everything must needs be black in

those who have a black skin, and that too when he saw their teeth were white. The reason of the whiteness of

semen is that it is a foam, and foam is white, especially that which is composed of the smallest parts, small in

the sense that each bubble is invisible, which is what happens when water and oil are mixed and shaken

together, as said before. (Even the ancients seem to have noticed that semen is of the nature of foam; at least

This then is the explanation of the problem proposed, and it is plain too that this is why semen does not

The next question to raise and to answer is this. If, in the case of those animals which emit semen into the

female, that which enters makes no part of the resulting embryo, where is the material part of it diverted if (as

we have seen) it acts by means of the power residing in it? It is not only necessary to decide whether what is

forming in the female receives anything material, or not, from that which has entered her, but also concerning

the soul in virtue of which an animal is so called (and this is in virtue of the sensitive part of the soul) does

this exist originally in the semen and in the unfertilized embryo or not, and if it does whence does it come?

For nobody would put down the unfertilized embryo as soulless or in every sense bereft of life (since both the

semen and the embryo of an animal have every bit as much life as a plant), and it is productive up to a certain

point. That then they possess the nutritive soul is plain (and plain is it from the discussions elsewhere about

soul why this soul must be acquired first). As they develop they also acquire the sensitive soul in virtue of

which an animal is an animal. For e.g. an animal does not become at the same time an animal and a man or a

horse or any other particular animal. For the end is developed last, and the peculiar character of the species is

the end of the generation in each individual. Hence arises a question of the greatest difficulty, which we must

strive to solve to the best of our ability and as far as possible. When and how and whence is a share in reason

acquired by those animals that participate in this principle? It is plain that the semen and the unfertilized

embryo, while still separate from each other, must be assumed to have the nutritive soul potentially, but not

actually, except that (like those unfertilized embryos that are separated from the mother) it absorbs

nourishment and performs the function of the nutritive soul. For at first all such embryos seem to live the life

of a plant. And it is clear that we must be guided by this in speaking of the sensitive and the rational soul. For

all three kinds of soul, not only the nutritive, must be possessed potentially before they are possessed in

actuality. And it is necessary either (1) that they should all come into being in the embryo without existing

previously outside it, or (2) that they should all exist previously, or (3), that some should so exist and others

not. Again, it is necessary that they should either (1) come into being in the material supplied by the female

without entering with the semen of the male, or (2) come from the male and be imparted to the material in the

female. If the latter, then either all of them, or none, or some must come into being in the male from outside.

Now that it is impossible for them all to preexist is clear from this consideration. Plainly those principles

whose activity is bodily cannot exist without a body, e.g. walking cannot exist without feet. For the same

reason also they cannot enter from outside. For neither is it possible for them to enter by themselves, being

inseparable from a body, nor yet in a body, for the semen is only a secretion of the nutriment in process of

change. It remains, then, for the reason alone so to enter and alone to be divine, for no bodily activity has any

Now it is true that the faculty of all kinds of soul seems to have a connexion with a matter different from and

more divine than the socalled elements; but as one soul differs from another in honour and dishonour, so

differs also the nature of the corresponding matter. All have in their semen that which causes it to be

productive; I mean what is called vital heat. This is not fire nor any such force, but it is the spiritus included

in the semen and the foamlike, and the natural principle in the spiritus, being analogous to the element of

the stars. Hence, whereas fire generates no animal and we do not find any living thing forming in either solids

or liquids under the influence of fire, the heat of the sun and that of animals does generate them. Not only is

this true of the heat that works through the semen, but whatever other residuum of the animal nature there

may be, this also has still a vital principle in it. From such considerations it is clear that the heat in animals

Let us return to the material of the semen, in and with which comes away from the male the spiritus

conveying the principle of soul. Of this principle there are two kinds; the one is not connected with matter,

and belongs to those animals in which is included something divine (to wit, what is called the reason), while

the other is inseparable from matter. This material of the semen dissolves and evaporates because it has a

liquid and watery nature. Therefore we ought not to expect it always to come out again from the female or to

form any part of the embryo that has taken shape from it; the case resembles that of the figjuice which

curdles milk, for this too changes without becoming any part of the curdling masses.

It has been settled, then, in what sense the embryo and the semen have soul, and in what sense they have not;

Now semen is a secretion and is moved with the same movement as that in virtue of which the body increases

(this increase being due to subdivision of the nutriment in its last stage). When it has entered the uterus it puts

into form the corresponding secretion of the female and moves it with the same movement wherewith it is

moved itself. For the female's contribution also is a secretion, and has all the arts in it potentially though none

of them actually; it has in it potentially even those parts which differentiate the female from the male, for just

as the young of mutilated parents are sometimes born mutilated and sometimes not, so also the young born of

a female are sometimes female and sometimes male instead. For the female is, as it were, a mutilated male,

and the catamenia are semen, only not pure; for there is only one thing they have not in them, the principle of

soul. For this reason, whenever a windegg is produced by any animal, the egg so forming has in it the parts

of both sexes potentially, but has not the principle in question, so that it does not develop into a living

creature, for this is introduced by the semen of the male. When such a principle has ben imparted to the

Liquid but corporeal substances become surrounded by some kind of covering on heating, like the solid scum

which forms on boiled foods when cooling. All bodies are held together by the glutinous; this quality, as the

embryo develops and increases in size, is acquired by the sinewy substance, which holds together the parts of

animals, being actual sinew in some and its analogue in others. To the same class belong also skin,

bloodvessels, membranes, and the like, for these differ in being more or less glutinous and generally in

In those animals whose nature is comparatively imperfect, when a perfect embryo (which, however, is not yet

a perfect animal) has been formed, it is cast out from the mother, for reasons previously stated. An embryo is

then complete when it is either male or female, in the case of those animals who possess this distinction, for

some (i.e. all those which are not themselves produced from a male or female parent nor from a union of the

two) produce an offspring which is neither male nor female. Of the generation of these we shall speak later.

The perfect animals, those internally viviparous, keep the developing embryo within themselves and in close

A third class is externally viviparous but first internally oviparous; they develop the egg into a perfect

condition, and then in some cases the egg is set free as with creatures externally oviparous, and the animal is

produced from the egg within the mother's body; in other cases, when the nutriment from the egg is

consumed, development is completed by connection with the uterus, and therefore the egg is not set free from

the uterus. This character marks the cartilaginous fish, of which we must speak later by themselves.

Here we must make our first start from the first class; these are the perfect or viviparous animals, and of these

the first is man. Now the secretion of the semen takes place in all of them just as does that of any other

residual matter. For each is conveyed to its proper place without any force from the breath or compulsion of

any other cause, as some assert, saying that the generative parts attract the semen like cuppingglasses, aided

by the force of the breath, as if it were possible for either this secretion or the residue of the solid and liquid

nutriment to go anywhere else than they do without the exertion of such a force. Their reason is that the

discharge of both is attended by holding the breath, but this is a common feature of all cases when it is

necessary to move anything, because strength arises through holding the breath. Why, even without this force

the secretions or excretions are discharged in sleep if the parts concerned are full of them and are relaxed.

One might as well say that it is by the breath that the seeds of plants are always segregated to the places

where they are wont to bear fruit. No, the real cause, as has been stated already, is that there are special parts

for receiving all the secretions, alike the useless (as the residues of the liquid and solid nutriment), and the

To consider now the region of the uterus in the female the two bloodvessels, the great vessel and the aorta,

divide higher up, and many fine vessels from them terminate in the uterus. These become overfilled from

the nourishment they convey, nor is the female nature able to concoct it, because it is colder than man's; so

the blood is excreted through very fine vessels into the uterus, these being unable on account of their

narrowness to receive the excessive quantity, and the result is a sort of haemorrhage. The period is not

accurately defined in women, but tends to return during the waning of the moon. This we should expect, for

the bodies of animals are colder when the environment happens to become so, and the time of change from

one month to another is cold because of the absence of the moon, whence also it results that this time is

stormier than the middle of the month. When then the residue of the nourishment has changed into blood, the

catamenia tend to occur at the abovementioned period, but when it is not concocted a little matter at a time

is always coming away, and this is why 'whites' appear in females while still small, in fact mere children. If

both these discharges of the secretions are moderate, the body remains in good health, for they act as a

purification of the secretions which are the causes of a morbid state of body; if they do not occur at all or if

they are excessive, they are injurious, either causing illness or pulling down the patient; hence whites, if

continuous and excessive, prevent girls from growing. This secretion then is necessarily discharged by

females for the reasons given; for, the female nature being unable to concoct the nourishment thoroughly,

there must not only be left a residue of the useless nutriment, but also there must be a residue in the

bloodvessels, and this filling the channels of the finest vessels must overflow. Then Nature, aiming at the

best end, uses it up in this place for the sake of generation, that another creature may come into being of the

same kind as the former was going to be, for the menstrual blood is already potentially such as the body from

In all females, then, there must necessarily be such a secretion, more indeed in those that have blood and of

these most of all in man, but in the others also some matter must be collected in the uterine region. The

reason why there is more in those that have blood and most in man has been already given, but why, if all

females have such a secretion, have not all males one to correspond? For some of them do not emit semen

but, just as those which do emit it fashion by the movement in the semen the mass forming from the material

supplied by the female, so do the animals in question bring the same to pass and exert the same formative

power by the movement within themselves in that part from whence the semen is secreted. This is the region

about the diaphragm in all those animals which have one, for the heart or its analogue is the first principle of

a natural body, while the lower part is a mere addition for the sake of it. Now the reason why it is not all

males that have a generative secretion, while all females do, is that the animal is a body with Soul or life; the

female always provides the material, the male that which fashions it, for this is the power that we say they

each possess, and this is what is meant by calling them male and female. Thus while it is necessary for the

female to provide a body and a material mass, it is not necessary for the male, because it is not within the

work of art or the embryo that the tools or the maker must exist. While the body is from the female, it is the

soul that is from the male, for the soul is the reality of a particular body. For this reason if animals of a

different kind are crossed (and this is possible when the periods of gestation are equal and conception takes

place nearly at the same season and there is no great difference in the of the animals), the first cross has a

common resemblance to both parents, as the hybrid between fox and dog, partridge and domestic fowl, but as

time goes on and one generation springs from another, the final result resembles the female in form, just as

foreign seeds produce plants varying in accordance with the country in which they are sown. For it is the soil

that gives to the seeds the material and the body of the plant. And hence the part of the female which receives

the semen is not a mere passage, but the uterus has a considerable width, whereas the males that emit semen

Each of the secretions becomes such at the moment when it is in its proper place; before that there is nothing

We have thus stated the reason for which the generative secretions are formed in animals. But when the

semen from the male (in those animals which emit semen) has entered, it puts into form the purest part of the

female secretion (for the greater part of the catamenia also is useless and fluid, as is the most fluid part of the

male secretion, i.e. in a single emission, the earlier discharge being in most cases apt to be infertile rather than

the later, having less vital heat through want of concoction, whereas that which is concocted is thick and of a

If there is no external discharge, either in women or other animals, on account of there not being much

useless and superfluous matter in the secretion, then the quantity forming within the female altogether is as

much as what is retained within those animals which have an external discharge; this is put into form by the

power of the male residing in the semen secreted by him, or, as is clearly seen to happen in some insects, by

It has been previously stated that the discharge accompanying sexual pleasure in the female contributes

nothing to the embryo. The chief argument for the opposite view is that what are called bad dreams occur by

night with women as with men; but this is no proof, for the same thing happens to young men also who do

not yet emit semen, and to those who do emit semen but whose semen is infertile.

It is impossible to conceive without the emission of the male in union and without the secretion of the

corresponding female material, whether it be discharged externally or whether there is only enough within the

body. Women conceive, however, without experiencing the pleasure usual in such intercourse, if the part

chance to be in heat and the uterus to have descended. But generally speaking the opposite is the case,

because the os uteri is not closed when the discharge takes place which is usually accompanied by pleasure in

women as well as men, and when this is so there is a readier way for the semen of the male to be drawn into

The actual discharge does not take place within the uterus as some think, the os uteri being too narrow, but it

is in the region in front of this, where the female discharges the moisture found in some cases, that the male

emits the semen. Sometimes it remains in this place; at other times, if the uterus chance to be conveniently

placed and hot on account of the purgation of the catamenia, it draws it within itself. A proof of this is that

pessaries, though wet when applied, are removed dry. Moreover, in all those animals which have the uterus

near the hypozoma, as birds and viviparous fishes, it is impossible that the semen should be so discharged as

to enter it; it must be drawn into it. This region, on account of the heat which is in it, attracts the semen. The

discharge and collection of the catamenia also excite heat in this part. Hence it acts like coneshaped vessels

which, when they have been washed out with hot water, their mouth being turned downwards, draw water

into themselves. And this is the way things are drawn up, but some say that nothing of the kind happens with

the organic parts concerned in copulation. Precisely the opposite is the case of those who say the woman

emits semen as well as the man, for if she emits it outside the uterus this must then draw it back again into

itself if it is to be mixed with the semen of the male. But this is a superfluous proceeding, and Nature does

When the material secreted by the female in the uterus has been fixed by the semen of the male (this acts in

the same way as rennet acts upon milk, for rennet is a kind of milk containing vital heat, which brings into

one mass and fixes the similar material, and the relation of the semen to the catamenia is the same, milk and

the catamenia being of the same nature) when, I say, the more solid part comes together, the liquid is

separated off from it, and as the earthy parts solidify membranes form all round it; this is both a necessary

result and for a final cause, the former because the surface of a mass must solidify on heating as well as on

cooling, the latter because the foetus must not be in a liquid but be separated from it. Some of these are called

membranes and others choria, the difference being one of more or less, and they exist in ovipara and vivipara

When the embryo is once formed, it acts like the seeds of plants. For seeds also contain the first principle of

growth in themselves, and when this (which previously exists in them only potentially) has been

differentiated, the shoot and the root are sent off from it, and it is by the root that the plant gets nourishment;

for it needs growth. So also in the embryo all the parts exist potentially in a way at the same time, but the first

principle is furthest on the road to realization. Therefore the heart is first differentiated in actuality. This is

clear not only to the senses (for it is so) but also on theoretical grounds. For whenever the young animal has

been separated from both parents it must be able to manage itself, like a son who has set up house away from

his father. Hence it must have a first principle from which comes the ordering of the body at a later stage also,

for if it is to come in from outside at later period to dwell in it, not only may the question be asked at what

time it is to do so, but also we may object that, when each of the parts is separating from the rest, it is

necessary that this principle should exist first from which comes growth and movement to the other parts.

(Wherefore all who say, as did Democritus, that the external parts of animals are first differentiated and the

internal later, are much mistaken; it is as if they were talking of animals of stone or wood. For such as these

have no principle of growth at all, but all animals have, and have it within themselves.) Therefore it is that the

heart appears first distinctly marked off in all the sanguinea, for this is the first principle or origin of both

homogeneous and heterogeneous parts, since from the moment that the animal or organism needs

nourishment, from that moment does this deserve to be called its principle or origin. For the animal grows,

and the nutriment, in its final stage, of an animal is the blood or its analogue, and of this the bloodvessels

are the receptacle, wherefore the heart is the principle or origin of these also. (This is clear from the Enquiries

Since the embryo is already potentially an animal but an imperfect one, it must obtain its nourishment from

elsewhere; accordingly it makes use of the uterus and the mother, as a plant does of the earth, to get

nourishment, until it is perfected to the point of being now an animal potentially locomotive. So Nature has

first designed the two bloodvessels from the heart, and from these smaller vessels branch off to the uterus.

These are what is called the umbilicus, for this is a bloodvessel, consisting of one or more vessels in

different animals. Round these is a skinlike integument, because the weakness of the vessels needs

protection and shelter. The vessels join on to the uterus like the roots of plants, and through them the embryo

receives its nourishment. This is why the animal remains in the uterus, not, as Democritus says, that the parts

of the embryo may be moulded in conformity with those of the mother. This is plain in the ovipara, for they

Here a difficulty may be raised. If the blood is the nourishment, and if the heart, which first comes into being,

already contains blood, and the nourishment comes from outside, whence did the first nourishment enter?

Perhaps it is not true that all of it comes from outside just as in the seeds of plants there is something of this

nature, the substance which at first appears milky, so also in the material of the animal embryo the

The embryo, then, grows by means of the umbilicus in the same way as a plant by its roots, or as animals

themselves when separated from the nutriment within the mother, of which we must speak later at the time

appropriate for discussing them. But the parts are not differentiated, as some suppose, because like is

naturally carried to like. Besides many other difficulties involved in this theory, it results from it that the

homogeneous parts ought to come into being each one separate from the rest, as bones and sinews by

themselves, and flesh by itself, if one should accept this cause. The real cause why each of them comes into

being is that the secretion of the female is potentially such as the animal is naturally, and all the parts are

potentially present in it, but none actually. It is also because when the active and the passive come in contact

with each other in that way in which the one is active and the other passive (I mean in the right manner, in the

right place, and at the right time), straightway the one acts and the other is acted upon. The female, then,

provides matter, the male the principle of motion. And as the products of art are made by means of the tools

of the artist, or to put it more truly by means of their movement, and this is the activity of the art, and the art

is the form of what is made in something else, so is it with the power of the nutritive soul. As later on in the

case of mature animals and plants this soul causes growth from the nutriment, using heat and cold as its tools

(for in these is the movement of the soul), and each thing comes into being in accordance with a certain

formula, so also from the beginning does it form the product of nature. For the material by which this latter

grows is the same as that from which it is constituted at first; consequently also the power which acts upon it

is identical with that which originally generated it; if then this acting power is the nutritive soul, this is also

the generative soul, and this is the nature of every organism, existing in all animals and plants. [But the other

parts of the soul exist in some animals, not in others.] In plants, then, the female is not separated from the

male, but in those animals in which it is separated the male needs the female besides.

And yet the question may be raised why it is that, if indeed the female possesses the same soul and if it is the

secretion of the female which is the material of the embryo, she needs the male besides instead of generating

entirely from herself. The reason is that the animal differs from the plant by having senseperception; if the

sensitive soul is not present, either actually or potentially, and either with or without qualification, it is

impossible for face, hand, flesh, or any other part to exist; it will be no better than a corpse or part of a corpse.

If then, when the sexes are separated, it is the male that has the power of making the sensitive soul, it is

impossible for the female to generate an animal from itself alone, for the process in question was seen to

involve the male quality. Certainly that there is a good deal in the difficulty stated is plain in the case of the

birds that lay windeggs, showing that the female can generate up to a certain point unaided. But this still

involves a difficulty; in what way are we to say that their eggs live? It neither possible that they should live in

the same way as fertile eggs (for then they would produce a chick actually alive), nor yet can they be called

eggs only in the sense in which an egg of wood or stone is so called, for the fact that these eggs go bad shows

that they previously participate in some way in life. It is plain, then, that they have some soul potentially.

What sort of soul will this be? It must be the lowest surely, and this is the nutritive, for this exists in all

animals and plants alike. Why then does it not perfect the parts and the animal? Because they must have a

sensitive soul, for the parts of animals are not like those of a plant. And so the female animal needs the help

of the male, for in these animals we are speaking of the male is separate. This is exactly what we find, for the

windeggs become fertile if the male tread the female in a certain space of time. About the cause of these

If there is any kind of animal which is female and has no male separate from it, it is possible that this may

generate a young one from itself without copulation. No instance of this worthy of credit has been observed

up to the present at any rate, but one case in the class of fishes makes us hesitate. No male of the socalled

erythrinus has ever yet been seen, but females, and specimens full of roe, have been seen. Of this, however,

we have as yet no proof worthy of credit. Again, some members of the class of fishes are neither male nor

female, as eels and a kind of mullets found in stagnant waters. But whenever the sexes are separate the female

cannot generate perfectly by herself alone, for then the male would exist in vain, and Nature makes nothing in

vain. Hence in such animals the male always perfects the work of generation, for he imparts the sensitive

soul, either by means of the semen or without it. Now the parts of the embryo already exist potentially in the

material, and so when once the principle of movement has been imparted to them they develop in a chain one

after another, as the wheels are moved one by another in the automatic machines. When some of the natural

philosophers say that like is brought to like, this must be understood, not in the sense that the parts are moved

as changing place, but that they stay where they are and the movement is a change of quality (such as

softness, hardness, colour, and the other differences of the homogeneous parts); thus they become in actuality

what they previously were in potentiality. And what comes into being first is the first principle; this is the

heart in the sanguinea and its analogue in the rest, as has been often said already. This is plain not only to the

senses (that it is first to come into being), but also in view of its end; for life fails in the heart last of all, and it

happens in all cases that what comes into being last fails first, and the first last, Nature running a double

course, so to say, and turning back to the point from whence she started. For the process of becoming is from

the nonexistent to the existent, and that of perishing is back again from the existent to the nonexistent.

After this, as said already, the internal parts come into being before the external. The greater become visible

before the less, even if some of them do not come into being before them. First the parts above the hypozoma

are differentiated and are superior in size; the part below is both smaller and less differentiated. This happens

in all animals in which exists the distinction of upper and lower, except in the insects; the growth of those

that produce a scolex is towards the upper part, for this is smaller in the beginning. The cephalopoda are the

only locomotive animals in which the distinction of upper and lower does not exist.

What has been said applies to plants also, that the upper portion is earlier in development than the lower, for

The agency by which the parts of animals are differentiated is air, not however that of the mother nor yet of

the embryo itself, as some of the physicists say. This is manifest in birds, fishes, and insects. For some of

these are separated from the mother and produced from an egg, within which the differentiation takes place;

other animals do not breathe at all, but are produced as a scolex or an egg; those which do breathe and whose

parts are differentiated within the mother's uterus yet do not breathe until the lung is perfected, and the lung

and the preceding parts are differentiated before they breathe. Moreover, all polydactylous quadrupeds, as

dog, lion, wolf, fox, jackal, produce their young blind, and the eyelids do not separate till after birth.

Manifestly the same holds also in all the other parts; as the qualitative, so also the quantitative differentia

comes into being, preexisting potentially but being actualized later by the same causes by which the

qualitative distinction is produced, and so the eyelids become two instead of one. Of course air must be

present, because heat and moisture are present, the former acting and the latter being acted upon.

Some of the ancient naturephilosolphers made an attempt to state which part comes into being after which,

but were not sufficiently acquainted with the facts. It is with the parts as with other things; one naturally

exists prior to another. But the word 'prior' is used in more senses than one. For there is a difference between

the end or final cause and that which exists for the sake of it; the latter is prior in order of development, the

former is prior in reality. Again, that which exists for the sake of the end admits of division into two classes,

(1) the origin of the movement, (2) that which is used by the end; I mean, for instance, (1) that which can

generate, (2) that which serves as an instrument to what is generated, for the one of these, that which makes,

must exist first, as the teacher before the learner, and the other later, as the pipes are later than he who learns

to play upon them, for it is superfluous that men who do not know how to play should have pipes. Thus there

are three things: first, the end, by which we mean that for the sake of which something else exists; secondly,

the principle of movement and of generation, existing for the sake of the end (for that which can make and

generate, considered simply as such, exists only in relation to what is made and generated); thirdly, the

useful, that is to say what the end uses. Accordingly, there must first exist some part in which is the principle

of movement (I say a part because this is from the first one part of the end and the most important part too);

next after this the whole and the end; thirdly and lastly, the organic parts serving these for certain uses. Hence

if there is anything of this sort which must exist in animals, containing the principle and end of all their

nature, this must be the first to come into being first, that is, considered as the moving power, but

simultaneous with the whole embryo if considered as a part of the end. Therefore all the organic parts whose

nature is to bring others into being must always themselves exist before them, for they are for the sake of

something else, as the beginning for the sake of the end; all those parts which are for the sake of something

else but are not of the nature of beginnings must come into being later. So it is not easy to distinguish which

of the parts are prior, those which are for the sake of another or that for the sake of which are the former. For

the parts which cause the movement, being prior to the end in order of development, come in to cause

confusion, and it is not easy to distinguish these as compared with the organic parts. And yet it is in

accordance with this method that we must inquire what comes into being after what; for the end is later than

some parts and earlier than others. And for this reason that part which contains the first principle comes into

being first, next to this the upper half of the body. This is why the parts about the head, and particularly the

eyes, appear largest in the embryo at an early stage, while the parts below the umbilicus, as the legs, are

small; for the lower parts are for the sake of the upper, and are neither parts of the end nor able to form it.

But they do not say well nor do they assign a necessary cause who say simply that 'it always happens so', and

imagine that this is a first principle in these cases. Thus Democritus of Abdera says that 'there is no beginning

of the infinite; now the cause is a beginning, and the eternal is infinite; in consequence, to ask the cause of

anything of this kind is to seek for a beginning of the infinite'. Yet according to this argument, which forbids

us to seek the cause, there will be no proof of any eternal truth whatever; but we see that there is a proof of

many such, whether by 'eternal' we mean what always happens or what exists eternally; it is an eternal truth

that the angles of a triangle are always equal to two right angles, or that the diagonal of a square is

incommensurable with the side, and nevertheless a cause and a proof can be given for these truths. While,

then, it is well said that we must not take on us to seek a beginning (or first principle) of all things, yet this is

not well said of all things whatever that always are or always happen, but only of those which really are first

principles of the eternal things; for it is by another method, not by proof, that we acquire knowledge of the

first principle. Now in that which is immovable and unchanging the first principle is simply the essence of the

thing, but when we come to those things which come into being the principles are more than one, varying in

kind and not all of the same kind; one of this number is the principle of movement, and therefore in all the

sanguinea the heart is formed first, as was said at the beginning, and in the other animals that which is

From the heart the bloodvessels extend throughout the body as in the anatomical diagrams which are

represented on the wall, for the parts lie round these because they are formed out of them. The homogeneous

parts are formed by heat and cold, for some are put together and solidified by the one and some by the other.

The difference between these has already been discussed elsewhere, and it has been stated what kinds of

things are soluble by liquid and fire, and what are not soluble by liquid and cannot be melted by fire. The

nutriment then oozes through the bloodvessels and the passages in each of the parts, like water in unbaked

pottery, and thus is formed the flesh or its analogues, being solidified by cold, which is why it is also

dissolved by fire. But all the particles given off which are too earthy, having but little moisture and heat, cool

as the moisture evaporates along with the heat; so they become hard and earthy in character, as nails, horns,

hoofs, and beaks, and therefore they are softened by fire but none of them is melted by it, while some of

them, as eggshells, are soluble in liquids. The sinews and bones are formed by the internal heat as the

moisture dries, and hence the bones are insoluble by fire like pottery, for like it they have been as it were

baked in an oven by the heat in the process of development. But it is not anything whatever that is made into

flesh or bone by the heat, but only something naturally fitted for the purpose; nor is it made in any place or

time whatever, but only in a place and time naturally so fitted. For neither will that which exists potentially

be made except by that moving agent which possesses the actuality, nor will that which possesses the

actuality make anything whatever; the carpenter would not make a box except out of wood, nor will a box be

made out of the wood without the carpenter. The heat exists in the seminal secretion, and the movement and

activity in it is sufficient in kind and in quantity to correspond to each of the parts. In so far as there is any

deficiency or excess, the resulting product is in worse condition or physically defective, in like manner as in

the case of external substances which are thickened by boiling that they may be more palatable or for any

other purpose. But in the latter case it is we who apply the heat in due measure for the motion required; in the

former it is the nature of the male parent that gives it, or with animals spontaneously generated it is the

movement and heat imparted by the right season of the year that it is the cause.

Cooling, again, is mere deprivation of heat. Nature makes use of both; they have of necessity the power of

bringing about different results, but in the development of the embryo we find that the one cools and the other

heats for some definite purpose, and so each of the parts is formed; thus it is in one sense by necessity, in

another for a final cause, that they make the flesh soft, the sinews solid and elastic, the bones solid and brittle.

The skin, again, is formed by the drying of the flesh, like the scum upon boiled substances; it is so formed not

only because it is on the outside, but also because what is glutinous, being unable to evaporate, remains on

the surface. While in other animals the glutinous is dry, for which reason the covering of the invertebrates is

testaceous or crustaceous, in the vertebrates it is rather of the nature of fat. In all of these which are not of too

earthy a nature the fat is collected under the covering of the skin, a fact which points to the skin being formed

out of such a glutinous substance, for fat is somewhat glutinous. As we said, all these things must be

understood to be formed in one sense of necessity, but in another sense not of necessity but for a final cause.

The upper half of the body, then, is first marked out in the order of development; as time goes on the lower

also reaches its full size in the sanguinea. All the parts are first marked out in their outlines and acquire later

on their colour and softness or hardness, exactly as if Nature were a painter producing a work of art, for

painters, too, first sketch in the animal with lines and only after that put in the colours.

Because the source of the sensations is in the heart, therefore this is the part first formed in the whole animal,

and because of the heat of this organ the cold forms the brain, where the bloodvessels terminate above,

corresponding to the heat of the heart. Hence the parts about the head begin to form next in order after the

heart, and surpass the other parts in size, for the brain is from the first large and fluid.

There is a difficulty about what happens with the eyes of animals. Though from the beginning they appear

very large in all creatures, whether they walk or swim or fly, yet they are the last of the parts to be formed

completely, for in the intervening time they collapse. The reason is this. The senseorgan of the eyes is set

upon certain passages, as are the other senseorgans. Whereas those of touch and taste are simply the body

itself or some part of the body of animals, those of smell and hearing are passages connecting with the

external air and full themselves of innate spiritus; these passages end at the small bloodvessels about the

brain which run thither from the heart. But the eye is the only senseorgan that has a bodily constitution

peculiar to itself. It is fluid and cold, and does not exist from the first in the place which it occupies later in

the same way as the other parts do, for they exist potentially to begin with and actually come into being later,

but the eye is the purest part of the liquidity about the brain drained off through the passages which are

visible running from them to the membrane round the brain. A proof of this is that, apart from the brain, there

is no other part in the head that is cold and fluid except the eye. Of necessity therefore this region is large at

first but falls in later. For the same thing happens with the brain; at first it is liquid and large, but in course of

evaporation and concoction it becomes more solid and falls in; this applies both to the brain and the eyes. The

head is very large at first, on account of the brain, and the eyes appear large because of the liquid in them.

They are the last organs to reach completion because the brain is formed with difficulty; for it is at a late

period that it gets rid of its coldness and fluidity; this applies to all animals possessing a brain, but especially

to man. For this reason the 'bregma' is the last of the bones to be formed; even after birth this bone is still soft

in children. The cause of this being so with men more than with other animals is the fact that their brain is the

most fluid and largest. This again is because the heat in man's heart is purest. His intellect shows how well he

is tempered, for man is the wisest of animals. And children for a long time have no control over their heads

on account of the heaviness of the brain; and the same applies to the parts which it is necessary to move, for it

is late that the principle of motion gets control over the upper parts, and last of all over those whose motion is

not connected directly with it, as that of the legs is not. Now the eyelid is such a part. But since Nature makes

nothing superfluous nor in vain, it is clear also that she makes nothing too late or too soon, for if she did the

result would be either in vain or superfluous. Hence it is necessary that the eyelids should be separated at the

same time as the heart is able to move them. So then the eyes of animals are perfected late because of the

amount of concoction required by the brain, and last of all the parts because the motion must be very strong

before it can affect parts so far from the first principle of motion and so cold. And it is plain that such is the

nature of the eyelids, for if the head is affected by never so little heaviness through sleepiness or drunkenness

or anything else of the kind, we cannot raise the eyelids though their own weight is so small. So much for the

question how the eyes come into being, and why and for what cause they are the last to be fully developed.

Each of the other parts is formed out of the nutriment, those most honourable and participating in the

sovereign principle from the nutriment which is first and purest and fully concocted, those which are only

necessary for the sake of the former parts from the inferior nutriment and the residues left over from the

other. For Nature, like a good householder, is not in the habit of throwing away anything from which it is

possible to make anything useful. Now in a household the best part of the food that comes in is set apart for

the free men, the inferior and the residue of the best for the slaves, and the worst is given to the animals that

live with them. Just as the intellect acts thus in the outside world with a view to the growth of the persons

concerned, so in the case of the embryo itself does Nature form from the purest material the flesh and the

body of the other senseorgans, and from the residues thereof bones, sinews, hair, and also nails and hoofs

and the like; hence these are last to assume their form, for they have to wait till the time when Nature has

The bones, then, are made in the first conformation of the parts from the seminal secretion or residue. As the

animal grows the bones grow from the natural nourishment, being the same as that of the sovereign parts, but

of this they only take up the superfluous residues. For everywhere the nutriment may be divided into two

kinds, the first and the second; the former is 'nutritious', being that which gives its essence both to the whole

and to the parts; the latter is concerned with growth, being that which causes quantitative increase. But these

must be distinguished more fully later on. The sinews are formed in the same way as the bones and out of the

same materials, the Seminal and nutritious residue. Nails, hair, hoofs, horns, beaks, the spurs of cocks, and

any other similar parts, are on the contrary formed from the nutriment which is taken later and only

concerned with growth, in other words that which is derived from the mother, or from the outer world after

birth. For this reason the bones on the one hand only grow up to a certain point (for there is a limit of size in

all animals, and therefore also of the growth of the bones; if these had been always able to grow, all animals

that have bone or its analogue would grow as long as they lived, for these set the limit of size to animals.

What is the reason of their not always increasing in size must be stated later.) Hair, on the contrary, and

growths akin to hair go on growing as long as they exist at all, and increase yet more in diseases and when the

body is getting old and wasting, because more residual matter is left over, as owing to old age and disease

less is expended on the important parts, though when the residual matter also fails through age the hair fails

with it. But the contrary is the case with the bones, for they waste away along with the body and the other

parts. Hair actually goes on growing after death; it does not, however, begin growing then.

About the teeth a difficulty may be raised. They have actually the same nature 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 have both bones and

teeth. Of all the bones they alone go on growing through life, as is plain with the teeth which grow out of the

straight line so as no longer to touch each other. The reason for their growth, as a final cause, is their

function, for they would soon be worn down if there were not some means of saving them; even as it is they

are altogether worn down in old age in some animals which eat much and have not large teeth, their growth

not being in proportion to their detrition. And so Nature has contrived well to meet the case in this also, for

she causes the failure of the teeth to synchronize with old age and death. If life lasted for a thousand or ten

thousand years the original teeth must have been very large indeed, and many sets of them must have been

produced, for even if they had grown continuously they would still have been worn smooth and become

useless for their work. The final cause of their growth has been now stated, but besides this as a matter of fact

the growth of the teeth is not the same as that of the other bones. The latter all come into being in the first

formation of the embryo and none of them later, but the teeth do so later. Therefore it is possible for them to

grow again after the first set falls out, for though they touch the bones they are not connate with them. They

are formed, however, out of the nutriment distributed to the bones, and so have the same nature, even when

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

The reason will be stated later why some teeth are formed and fall out but others do not fall out.

It is because such parts are formed from a residue that man is the most naked in body of all animals and has

the smallest nails in proportion to his size; he has the least amount of earthy residue, but that part of the blood

which is not concocted is the residue, and the earthy part in the bodies of all animals is the least concocted.

We have now stated how each of the parts is formed and what is the cause of their generation.

In viviparous animals, as said before, the embryo gets its growth through the umbilical cord. For since the

nutritive power of the soul, as well as the others, is present in animals, it straightway sends off this cord like a

root to the uterus. The cord consists of bloodvessels 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 bloodvessels. All

animals with no front teeth in the upper jaw, and all those which have them in both jaws and whose uterus

has not one great bloodvessel running through it but many close together instead all these have in the

uterus the socalled cotyledons (with which the umbilical cord connects and is closely united; for the vessels

which pass through the cord run backwards and forwards between embryo and uterus and split up into

smaller vessels all over the uterus; where they terminate, there are found the cotyledons). Their convexity is

turned towards the uterus, the concavity towards the embryo. Between uterus and embryo are the chorion and

the membranes. As the embryo grows and approaches perfection the cotyledons become smaller and finally

disappear when it is perfected. For Nature sends the sanguineous nutriment for the embryo into this part of

the uterus as she sends milk into the breasts, and because the cotyledons are gradually aggregated from many

into a few the body of the cotyledon becomes like an eruption or inflammation. So long as the embryo is

comparatively small, being unable to receive much nutriment, they are plain and large, but when it has

But most of the animals which have front teeth in both jaws and no horns have no cotyledons in the uterus,

but the umbilical cord runs to meet one bloodvessel, which is large and extends throughout the uterus. Of

such animals some produce one young at a time, some more than one, but the same description applies to

both these classes. (This should be studied with the aid of the examples drawn in the Anatomy and the

Enquiries.) For the young, if numerous, are attached each to its umbilical cord, and this to the bloodvessel of

the mother; they are arranged next to one another along the stream of the bloodvessel as along a canal; and

Those who say that children are nourished in the uterus by sucking some lump of flesh or other are mistaken.

If so, the same would have been the case with other animals, but as it is we do not find this (and this can

easily be observed by dissection). Secondly, all embryos alike, whether of creatures that fly or swim or walk,

are surrounded by fine membranes separating them from the uterus and from the fluids which are formed in

it; but neither in these themselves is there anything of the kind, nor is it possible for the embryo to take

nourishment by means of any of them. Thirdly, it is plain that all creatures developed in eggs grow when

Natural intercourse takes place between animals of the same kind. However, those also unite whose nature is

near akin and whose form is not very different, if their size is much the same and if the periods of gestation

are equal. In other animals such cases are rare, but they occur with dogs and foxes and wolves; the Indian

dogs also spring from the union of a dog with some wild doglike animal. A similar thing has been seen to

take place in those birds that are amative, as partridges and hens. Among birds of prey hawks of different

form are thought to unite, and the same applies to some other birds. Nothing worth mentioning has been

observed in the inhabitants of the sea, but the socalled 'rhinobates' especially is thought to spring from the

union of the 'rhini' and 'batus'. And the proverb about Libya, that 'Libya is always producing something new',

is said to have originated from animals of different species uniting with one another in that country, for it is

said that because of the want of water all meet at the few places where springs are to be found, and that even

Of the animals that arise from such union all except mules are found to copulate again with each other and to

be able to produce young of both sexes, but mules alone are sterile, for they do not generate by union with

one another or with other animals. The problem why any individual, whether male or female, is sterile is a

general one, for some men and women are sterile, and so are other animals in their several kinds, as horses

and sheep. But this kind, of mules, is universally so. The causes of sterility in other animals are several. Both

men and women are sterile from birth when the parts useful for union are imperfect, so that men never grow a

beard but remain like eunuchs, and women do not attain puberty; the same thing may befall others as their

years advance, sometimes on account of the body being too well nourished (for men who are in too good

condition and women who are too fat the seminal secretion is taken up into the body, and the former have no

semen, the latter no catamenia); at other times by reason of sickness men emit the semen in a cold and liquid

state, and the discharges of women are bad and full of morbid secretions. Often, too, in both sexes this state is

caused by injuries in the parts and regions contributory to copulation. Some such cases are curable, others

incurable, but the subjects especially remain sterile if anything of the sort has happened in the first formation

of the parts in the embryo, for then are produced women of a masculine and men of a feminine appearance,

and in the former the catamenia do not occur, in the latter the semen is thin and cold. Hence it is with good

reason that the semen of men is tested in water to find out if it is infertile, for that which is thin and cold is

quickly spread out on the surface, but the fertile sinks to the bottom, for that which is well concocted is hot

indeed, but that which is firm and thick is well concocted. They test women by pessaries to see if the smells

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

see if they colour the saliva. If these results do not follow it is a sign that the passages of the body, through

which the catamenia are secreted, are clogged and closed. For the region about the eyes is, of all the head,

that most nearly connected with the generative secretions; a proof of this is that it alone is visibly changed in

sexual intercourse, and those who indulge too much in this are seen to have their eyes sunken in. The reason

is that the nature of the semen is similar to that of the brain, for the material of it is watery (the heat being

acquired later). And the seminal purgations are from the region of the diaphragm, for the first principle of

nature is there, so that the movements from the pudenda are communicated to the chest, and the smells from

In men, then, and in other kinds, as said before, such deficiency occurs sporadically, but the whole of the

mule kind is sterile. The reason has not been rightly given by Empedocles and Democritus, of whom the

former expresses himself obscurely, the latter more intelligibly. For they offer their demonstration in the case

of all these animals alike which unite against their affinities. Democritus says that the genital passages of

mules are spoilt in the mother's uterus because the animals from the first are not produced from parents of the

same kind. But we find that though this is so with other animals they are none the less able to generate; yet, if

this were the reason, all others that unite in this manner ought to be barren. Empedocles assigns as his reason

that the mixture of the 'seeds' becomes dense, each of the two seminal fluids out of which it is made being

soft, for the hollows in each fit into the densities of the other, and in such cases a hard substance is formed

out of soft ones, like bronze mingled with tin. Now he does not give the correct reason in the case of bronze

and tin (we have spoken of them in the Problems) nor, to take general ground, does he take his principles

from the intelligible. How do the 'hollows' and 'solids' fit into one another to make the mixing, e.g. in the case

of wine and water? This saying is quite beyond us; for how we are to understand the 'hollows' of the wine and

water is too far beyond our perception. Again, when, as a matter of fact, horse is born of horse, ass of ass, and

mule of horse and ass in two ways according as the parents are stallion and sheass or jackass and mare, why

in the last case does there result something so 'dense' that the offspring is sterile, whereas the offspring of

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

female horse is soft. But both sexes of the horse cross with both sexes of the ass, and the offspring of both

crosses are barren, according to Empedocles, because from both is produced something 'dense', the 'seeds'

being 'soft'. If so, the offspring of stallion and mare ought also to be sterile. If one of them alone united with

the ass, it might be said that the cause of the mule's being unable to generate was the unlikeness of that one to

the generative fluid of the ass; but, as it is, whatever be the character of that generative fluid with which it

unites in the ass, such it is also in the animal of its own kind. Then, again, the argument is intended to apply

to both male and female mules alike, but the male does generate at seven years of age, it is said; it is the

female alone that is entirely sterile, and even she is so only because she does not complete the development

Perhaps an abstract proof might appear to be more plausible than those already given; I call it abstract

because the more general it is the further is it removed from the special principles involved. It runs somewhat

as follows. From male and female of the same species there are born in course of nature male and female of

the same species as the parents, e.g. male and female puppies from male and female dog. From parents of

different species is born a young one different in species; thus if a dog is different from a lion, the offspring

of male dog and lioness or of lion and bitch will be different from both parents. If this is so, then since (1)

mules are produced of both sexes and are not different in species from one another, and (2) a mule is born of

horse and ass and these are different in species from mules, it is impossible that anything should be produced

from mules. For (1) another kind cannot be, because the product of male and female of the same species is

also of the same species, and (2) a mule cannot be, because that is the product of horse and ass which are

different in form, [and it was laid down that from parents different in form is born a different animal]. Now

this theory is too general and empty. For all theories not based on the special principles involved are empty;

they only appear to be connected with the facts without being so really. As geometrical arguments must start

from 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 of different species are

fertile with one another, as was said before. So we must not inquire into questions of natural science in this

fashion any more than any other questions; we shall be more likely to find the reason by considering the facts

peculiar to the two kinds concerned, horse and ass. In the first place, each of them, if mated with its own

kind, bears only one young one; secondly, the females are not always able to conceive from the male

(wherefore breeders put the horse to the mare again at intervals). Indeed, both the mare is deficient in

catamenia, discharging less than any other quadruped, and the sheass does not admit the impregnation, but

ejects the semen with her urine, wherefore men follow flogging her after intercourse. Again the ass is an

animal of cold nature, and so is not wont to be produced in wintry regions because it cannot bear cold, as in

Scythia and the neighbouring country and among the Celts beyond Iberia, for this country also is cold. For

this cause they do not put the jackasses to the females at the equinox, as they do with horses, but about the

summer solstice, in order that the assfoals may be born in a warm season, for the mothers bear at the same

season as that in which they are impregnated, the period of gestation in both horse and ass being one year.

The animal, then, being, as has been said of such a cold nature, its semen also must be cold. A proof of this is

that if a horse mount a female already impregnated by an ass he does not destroy the impregnation of the ass,

but if the ass be the second to mount her he does destroy that of the horse because of the coldness of his own

semen. When, therefore, they unite with each other, the generative elements are preserved by the heat of the

one of them, that contributed by the horse being the hotter; for in the ass both the semen of the male and the

material contributed by the female are cold, and those of the horse, in both sexes, are hotter. Now when either

hot is added to cold or cold to hot so as to mix, the 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

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 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 very near generating a sterile animal when mated with its own species. Thus when

the difficulty of a cross contrary to nature is added, (when too even in the other case when united with their

own species they with difficulty produce a single young one), 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 female mules grow large because the matter which is secreted in other animals

to form the catamenia is diverted to growth. But since the period of gestation in such animals is a year, the

mule must not only conceive, if she is to be fertile, but must also nourish the embryo till birth, and this is

impossible if there are no catamenia. But there are 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 smell to the pudenda of the

females, as do the other solidhoofed ungulates, but only to the evacuation 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

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 dwarf 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 socalled 'metachoera' in swine, for a 'metachoerum' also

is a pig injured 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 injured during gestation, and resemble ginni and metachoera.

WE have now spoken about the sterility of mules, and about those 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 into the female. But among the ovipara (1) birds

produce a perfect hardshelled egg, unless it be injured by disease, and the eggs of birds are all

twocoloured. (2) The cartilaginous fishes, as has been often said already, are oviparous internally but

produce the young alive, the egg changing previously from one part of the uterus to another; and their egg is

softshelled and of one colour. One of this class alone does not produce the young from the egg within itself,

the socalled 'frog'; the reason of which must be stated later. (3) All other oviparous fishes produce an egg of

one colour, but this is imperfect, for its growth is completed outside the mother's body by the same cause as

Concerning the uterus of these classes of animals, what differences there are among them and for what

reasons, has been stated previously. For in some of the viviparous creatures it is high up near the hypozoma,

in others low down by the pudenda; the former in the cartilaginous fishes, the latter in animals both internally

and externally viviparous, such as man and horse and the rest; in the ovipara it is sometimes low, as in the

Some embryos are formed in birds spontaneously, which are called windeggs and 'zephyria' by some; these

occur in birds which are not given to flight nor rapine but which produce many young, for these birds have

much residual matter, whereas in the birds of prey all such secretion is diverted to the wings and

wingfeathers, while the body is small and dry and hot. (The secretion corresponding in henbirds to

catamenia, and the semen of the cock, are residues.) Since then both the wings and the semen are made from

residual matter, nature cannot afford to spend much upon both. And for this same reason the birds of prey are

neither given to treading much nor to laying many eggs, as are the heavy birds and those flying birds whose

bodies are bulky, as the pigeon and so forth. For such residual matter is secreted largely in the heavy birds not

given to flying, such as fowls, partridges, and so on, wherefore their males tread often and their females

produce much material. Of such birds some lay many eggs at a time and some lay often; for instance, the

fowl, the partridge, and the Libyan ostrich lay many eggs, while the pigeon family do not lay many but lay

often. For these are between the birds of prey and the heavy ones; they are flyers like the former, but have

bulky bodies like the latter; hence, because they are flyers and the residue is diverted that. way, they lay few

eggs, but they lay often because of their having bulky bodies and their stomachs being hot and very active in

concoction, and because moreover they can easily procure their food, whereas the birds of prey do so with

Small birds also tread often and are very fertile, as are sometimes small plants, for what causes bodily growth

in others turn in them to a seminal residuum. Hence the Adrianic fowls lay most eggs, for because of the

smallness of their bodies the nutriment is used up in producing young. And other birds are more fertile than

gamefowl, for their bodies are more fluid and bulkier, whereas those of gamefowl are leaner and drier,

since a passionate spirit is found rather in such bodies as the latter. Moreover the thinness and weakness of

the legs contribute to making the former class of birds naturally inclined to tread and to be fertile, as we find

also in the human species; for the nourishment which otherwise goes to the legs is turned in such into a

seminal secretion, what Nature takes from the one place being added at the other. Birds of prey, on the

contrary, have a strong walk and their legs are thick owing to their habits, so that for all these reasons they

neither tread nor lay much. The kestrel is the most fertile; for this is nearly the only bird of prey which drinks,

and its moisture, both innate and acquired, along with its heat is favourable to generative products. Even this

The cuckoo, though not a bird of prey, lays few eggs, because it is of a cold nature, as is shown by the

cowardice of the bird, whereas a generative animal should be hot and moist. That it is cowardly is plain, for it

The pigeon family are in the habit of laying two for the most part, for they neither lay one (no bird does

except the cuckoo, and even that sometimes lays two) nor yet many, but they frequently produce two, or three

It is plain from the facts that with the birds that lay many eggs the nutriment is diverted to the semen. For

most trees, if they bear too much fruit, wither away after the crop when nutriment is not reserved for

themselves, and this seems to be what happens to annuals, as leguminous plants, corn, and the like. For they

consume all their nutriment to make seed, their kind being prolific. And some fowls after laying too much, so

as even to lay two eggs in a day, have died after this. For both the birds the plants become exhausted, and this

condition is an excess of secretion of residual matter. A similar condition is the cause of the later sterility of

the lioness, for at the first birth she produces five or six, then in the next year four, and again three cubs, then

the next number down to one, then none at all, showing that the residue is being used up and the generative

We have now stated in which birds windeggs are found, and also what sort of birds lay many eggs or few,

and for what reasons. And windeggs, as said before, come into being because while it is the material for

generation that exists in the female of all animals, birds have no discharge of catamenia like viviparous

sanguinea (for they occur in all these latter, more in some, less in others, and in some only enough in quantity

just to mark the class). The same applies to fish as to birds, and so in them as in birds is found an embryonic

formation without impregnation, but it is less obvious because their nature is colder. The secretion

corresponding to the catamenia of vivipara is formed in birds at the appropriate season for the discharge of

superfluous matter, and, because the region near the hypozoma is hot, it is perfected so far as size is

concerned, but in birds and fishes alike it is imperfect for generation without the seminal fluid of the male;

the cause of this has been previously given. Windeggs are not formed in the flying birds, for the same

reason as prevents their laying many eggs; for the residual matter in birds of prey is small, and they need the

male to give an impulse for the discharge of it. The windeggs are produced in greater numbers than the

impregnated but smaller in size for one and the same reason; they are smaller in size because they are

imperfect, and because they are smaller in size they are more in number. They are less pleasant for food

because they are less concocted, for in all foods the concocted is more agreeable. It has been sufficiently

observed, then, that neither birds' nor fishes' eggs are perfected for generation without the males. As for

embryos being formed in fish also (though in a less degree) without the males, the fact has been observed

especially in river fish, for some are seen to have eggs from the first, as has been written in the Enquiries

concerning them. And generally speaking in the case of birds even the impregnated eggs are not wont for the

most part to attain their full growth unless the hen be trodden continually. The reason of this is that just as

with women intercourse with men draws down the secretion of the catamenia (for the uterus being heated

attracts the moisture and the passages are opened), so this happens also with birds; the residual matter

corresponding to the catamenia advances a little at a time, and is not discharged externally, because its

amount is small and the uterus is high up by the hypozoma, but trickles together into the uterus itself. For as

the embryo of the vivipara grows by means of the umbilical cord, so the egg grows through this matter

flowing to it through the uterus. For when once the hens have been trodden, they all continue to have eggs

almost without intermission, though very small ones. Hence some are wont to speak of windeggs as not

coming into being independently but as mere relics from a previous impregnation. But this is a false view, for

sufficient observations have been made of their arising without impregnation in chickens and goslings. Also

the female partridges which are taken out to act as decoys, whether they have ever been impregnated or not,

immediately on smelling the male and hearing his call, become filled with eggs in the latter case and lay them

in the former. The reason why this happens is the same as in men and quadrupeds, for if their bodies chance

to be in rut they emit semen at the mere sight of the female or at a slight touch. And such birds are of a

lascivious and fertile nature, so that the impulse they need is but small when they are in this excited

condition, and the secreting activity takes place quickly in them, windeggs forming in the unimpregnated

and the eggs in those which have been impregnated growing and reaching perfection swiftly.

Among creatures that lay eggs externally birds produce their egg perfect, fish imperfect, but the eggs of the

latter complete their growth outside as has been said before. The reason is that the fish kind is very fertile;

now it is impossible for many eggs to reach completion within the mother and therefore they lay them

outside. They are quickly discharged, for the uterus of externally oviparous fishes is near the generative

passage. While the eggs of birds are twocoloured, those of all fish are onecoloured. The cause of the

double colour may be seen from considering the power of each of the two parts, the white and the yolk. For

the matter of the egg is secreted from the blood [No bloodless animal lays eggs,] and that the blood is the

material of the body has been often said already. The one part, then, of the egg is nearer the form of the

animal coming into being, that is the hot part; the more earthy part gives the substance of the body and is

further removed. Hence in all twocoloured eggs the animal receives the first principle of generation from

the white (for the vital principle is in that which is hot), but the nutriment from the yolk. Now in animals of a

hotter nature the part from which the first principle arises is separated off from the part from which comes the

nutriment, the one being white and the other yellow, and the white and pure is always more than the yellow

and earthy; but in the moister and less hot the yolk is more in quantity and more fluid. This is what we find in

lake birds, for they are of a moister nature and are colder than the land birds, so that the socalled 'lecithus' or

yolk in the eggs of such birds is large and less yellow because the white is less separated off from it. But

when we come to the ovipara which are both of a cold nature and also moister (such is the fish kind) we find

the white not separated at all because of the small size of the eggs and the quantity of the cold and earthy

matter; therefore all fish eggs are of one colour, and white compared with yellow, yellow compared with

white. Even the windeggs of birds have this distinction of colour, for they contain that out of which will

come each of the two parts, alike that whence arises the principle of life and that whence comes the

nutriment; only both these are imperfect and need the influence of the male in addition; for windeggs

become fertile if impregnated by the male within a certain period. The difference in colour, however, is not

due to any difference of sex, as if the white came from the male, the yolk from the female; both on the

contrary come from the female, but the one is cold, the other hot. In all cases then where the hot part is

considerable it is separated off, but where it is little it cannot be so; hence the eggs of such animals, as has

been said, are of one colour. The semen of the male only puts them into form; and therefore at first the egg in

birds appears white and small, but as it advances it is all yellow as more of the sanguineous material is

continually mixed with it; finally as the hot part is separated the white takes up a position all round it and

equally distributed on all sides, as when a liquid boils; for the white is naturally liquid and contains in itself

the vital heat; therefore it is separated off all round, but the yellow and earthy part is inside. And if we

enclose many eggs together in a bladder or something of the kind and boil them over a fire so as not to make

the movement of the heat quicker than the separation of the white and yolk in the eggs, then the same process

takes place in the whole mass of the eggs as in a single egg, all the yellow part coming into the middle and

The principle of the male is separated off in eggs at the point where the egg is attached to the uterus, and the

reason why the shape of twocoloured eggs is unsymmetrical, and not perfectly round but sharper at one end,

is that the part of the white in which is contained this principle must differ from the rest. Therefore the egg is

harder at this point than below, for it is necessary to shelter and protect this principle. And this is why the

sharp end of the egg comes out of the hen later than the blunt end; for the part attached to the uterus comes

out later, and the egg is attached at the point where is the said principle, and the principle is in the sharp end.

The same is the case also in the seeds of plants; the principle of the seed is attached sometimes to the twig,

sometimes to the husk, sometimes to the pericarp. This is plain in the leguminous plants, for where the two

cotyledons of beans and of similar seeds are united, there is the seed attached to the parent plant, and there is

A difficulty may be raised about the growth of the egg; how is it derived from the uterus? For if animals

derive their nutriment through the umbilical cord, through what do eggs derive it? They do not, like a scolex,

acquire their growth by their own means. If there is anything by which they are attached to the uterus, what

becomes of this when the egg is perfected? It does not come out with the egg as the cord does with animals;

for when its egg is perfected the shell forms all round it. This problem is rightly raised, but it is not observed

that the shell is at first only a soft membrane, and that it is only after the egg is perfected that it becomes hard

and brittle; this is so nicely adjusted that it is still soft when it comes out (for otherwise it would cause pain in

laying), but no sooner has it come out than it is fixed hard by cooling, the moisture quickly evaporating

because there is but little of it, and the earthy part remaining. Now at first a certain part of this membrane at

the sharp end of eggs resembles an umbilical cord, and projects like a pipe from them while they are still

small. It is plainly visible in small aborted eggs, for if the bird be drenched with water or suddenly chilled in

any other way and cast out the egg too soon, it appears still sanguineous and with a small tail like an

umbilical cord running through it. As the egg becomes larger this is more twisted round and becomes smaller,

and when the egg is perfected this end is the sharp end. Under this is the inner membrane which separates the

white and the yolk from this. When the egg is perfected, the whole of it is set free, and naturally the umbilical

The egg is discharged in the opposite way from the young of vivipara; the latter are born headfirst, the part

where is the first principle leading, but the egg is discharged as it were feet first; the reason of this being what

has been stated, that the egg is attached to the uterus at the point where is the first principle.

The young bird is produced out of the egg by the mother's incubating and aiding the concoction, the creature

developing out of part of the egg, and receiving growth and completion from the remaining part. For Nature

not only places the material of the creature in the egg but also the nourishment sufficient for its growth; for

since the mother bird cannot perfect her young within herself she produces the nourishment in the egg along

with it. Whereas the nourishment, what is called milk, is produced for the young of vivipara in another part,

in the breasts, Nature does this for birds in the egg. The opposite, however, is the case to what people think

and what is asserted by Alcmaeon of Crotona. For it is not the white that is the milk, but the yolk, for it is this

that is the nourishment of the chick, whereas they think it is the white because of the similarity of colour.

The chick then, as has been said, comes into being by the incubation of the mother; yet if the temperature of

the season is favourable, or if the place in which the eggs happen to lie is warm, the eggs are sufficiently

concocted without incubation, both those of birds and those of oviparous quadrupeds. For these all lay their

eggs upon the ground, where they are concocted by the heat in the earth. Such oviparous quadrupeds as do

visit their eggs and incubate do so rather for the sake of protecting them than of incubation.

The eggs of these quadrupeds are formed in the same way as those of birds, for they are hardshelled and

twocoloured, and they are formed near the hypozoma as are those of birds, and in all other respects

resemble them both internally and externally, so that the inquiry into their causes is the same for all. But

whereas the eggs of quadrupeds are hatched out by the mere heat of the weather owing to their strength, those

of birds are more exposed to destruction and need the motherbird. Nature seems to wish to implant in

animals a special sense of care for their young: in the inferior animals this lasts only to the moment of giving

birth to the incompletely developed animal; in others it continues till they are perfect; in all that are more

intelligent, during the bringing up of the young also. In those which have the greatest portion in intelligence

we find familiarity and love shown also towards the young when perfected, as with men and some

quadrupeds; with birds we find it till they have produced and brought up their young, and therefore if the

hens do not incubate after laying they get into worse condition, as if deprived of something natural to them.

The young is perfected within the egg more quickly in sunshiny weather, the season aiding in the work, for

concoction is a kind of heat. For the earth aids in the concoction by its heat, and the brooding hen does the

same, for she applies the heat that is within her. And it is in the hot season, as we should expect, that the eggs

are more apt to be spoilt and the socalled 'uria' or rotten eggs are produced; for just as wines turn sour in the

heats from the sediment rising (for this is the cause of their being spoilt), so is it with the yolk in eggs, for the

sediment and yolk are the earthy part in each case, wherefore the wine becomes turbid when the sediment

mixes with it, and the like applies to the eggs that are spoiling because of the yolk. It is natural then that such

should be the case with the birds that lay many eggs, for it is not easy to give the fitting amount of heat to all,

but (while some have too little) others have too much and this makes them turbid, as it were by putrefaction.

But this happens none the less with the birds of prey though they lay few eggs, for often one of the two

becomes rotten, and the third practically always, for being of a hot nature they make the moisture in the eggs

to overboil so to say. For the nature of the white is opposed to that of the yolk; the yolk congeals in frosts but

liquefies on heating, and therefore it liquefies on concoction in the earth or by reason of incubation, and

becoming liquid serves as nutriment for the developing chick. If exposed to heat and roasted it does not

become hard, because though earthy in nature it is only so in the same way as wax is; accordingly on heating

too much the eggs become watery and rotten, [if they be not from a liquid residue]. The white on the contrary

is not congealed by frost but rather liquefies (the reason of which has been stated before), but on exposure to

heat becomes solid. Therefore being concocted in the development of the chick it is thickened. For it is from

this that the young is formed (whereas the yolk turns to nutriment) and it is from this that the parts derive

their growth as they are formed one after another. This is why the white and the yolk are separated by

membranes, as being different in nature. The precise details of the relation of the parts to one another both at

the beginning of generation and as the animals are forming, and also the details of the membranes and

umbilical cords, must be learnt from what has been written in the Enquiries; for the present investigation it is

sufficient to understand this much clearly, that, when the heart has been first formed and the great

bloodvessel has been marked off from it, two umbilical cords run from the vessel, the one to the membrane

which encloses the yolk, the other to the membrane resembling a chorion which surrounds the whole embryo;

this latter runs round on the inside of the membrane of the shell. Through the one of these the embryo

receives the nutriment from the yolk, and the yolk becomes larger, for it becomes more liquid by heating.

This is because the nourishment, being of a material character in its first form, must become liquid before it

can be absorbed, just as it is with plants, and at first this embryo, whether in an egg or in the mother's uterus,

lives the life of a plant, for it receives its first growth and nourishment by being attached to something else.

The second umbilical cord runs to the surrounding chorion. For we must understand that, in the case of

animals developed in eggs, the chick has the same relation to the yolk as the embryo of the vivipara has to the

mother so long as it is within the mother (for since the nourishment of the embryo of the ovipara is not

completed within the mother, the embryo takes part of it away from her). So also the relation of the chick to

the outermost membrane, the sanguineous one, is like that of the mammalian embryo to the uterus. At the

same time the eggshell surrounds both the yolk and the membrane analogous to the uterus, just as if it

should be put round both the embryo itself and the whole of the mother, in the vivipara. This is so because

the embryo must be in the uterus and attached to the mother. Now in the vivipara the uterus is within the

mother, but in the ovipara it is the other way about, as if one should say that the mother was in the uterus, for

that which comes from the mother, the nutriment, is the yolk. The reason is that the process of nourishment is

As the creature grows the umbilicus running the chorion collapses first, because it is here that the young is to

come out; what is left of the yolk, and the umbilical cord running to the yolk, collapse later. For the young

must have nourishment as soon as it is hatched; it is not nursed by the mother and cannot immediately

procure its nourishment for itself; therefore the yolk enters within it along with its umbilicus and the flesh

This then is the manner in which animals produced from perfect eggs are hatched in all those, whether birds

or quadrupeds, which lay the egg with a hard shell. These details are plainer in the larger creatures; in the

The class of fishes is also oviparous. Those among them which have the uterus low down lay an imperfect

egg for the reason previously given,' but the socalled 'selache' or cartilaginous fishes produce a perfect egg

within themselves but are externally viviparous except one which they call the 'frog'; this alone lays a perfect

egg externally. The reason is the nature of its body, for its head is many times as large as the rest of the body

and is spiny and very rough. This is also why it does not receive its young again within itself nor produce

them alive to begin with, for as the size and roughness of the head prevents their entering so it would prevent

their exit. And while the egg of the cartilaginous fishes is softshelled (for they cannot harden and dry its

circumference, being colder than birds), the egg of the frogfish alone is solid and firm to protect it outside,

but those of the rest are of a moist and soft nature, for they are sheltered within and by the body of the

The young are produced from the egg in the same way both with those externally perfected (the frogfishes)

and those internally, and the process in these eggs is partly similar to, partly different from that in birds' eggs.

In the first place they have not the second umbilicus which runs to the chorion under the surrounding shell.

The reason of this is that they have not the surrounding shell, for it is no use to them since the mother shelters

them, and the shell is a protection to the eggs against external injury between laying and hatching out.

Secondly, the process in these also begins on the surface of the egg but not where it is attached to the uterus,

as in birds, for the chick is developed from the sharp end and that is where the egg was attached. The reason

is that the egg of birds is separated from the uterus before it is perfected, but in most though not all

cartilaginous fishes the egg is still attached to the uterus when perfect. While the young develops upon the

surface the egg is consumed by it just as in birds and the other animals detached from the uterus, and at last

the umbilicus of the now perfect fish is left attached to the uterus. The like is the case with all those whose

eggs are detached from the uterus, for in some of them the egg is so detached when it is perfect.

The question may be asked why the development of birds and cartilaginous fishes differs in this respect. The

reason is that in birds the white and yolk are separate, but fish eggs are onecoloured, the corresponding

matter being completely mixed, so that there is nothing to stop the first principle being at the opposite end,

for the egg is of the same nature both at the point of attachment and at the opposite end, and it is easy to draw

the nourishment from the uterus by passages running from this principle. This is plain in the eggs which are

not detached, for in some of the cartilaginous fish the egg is not detached from the uterus, but is still

connected with it as it comes downwards with a view to the production of the young alive; in these the young

fish when perfected is still connected by the umbilicus to the uterus when the egg has been consumed. From

this it is clear that previously also, while the egg was still round the young, the passages ran to the uterus.

In these respects and for the reasons given the development of cartilaginous fishes differs from that of birds,

but otherwise it takes place in the same way. For they have the one umbilicus in like manner as that of birds

connecting with the yolk, only in these fishes it connects with the whole egg (for it is not divided into white

and yolk but all onecoloured), and get their nourishment from this, and as it is being consumed the flesh in

Such is the process of development in those fish that produce a perfect egg within themselves but are

Most of the other fish are externally oviparous, all laying an imperfect egg except the frogfish; the reason of

this exception has been previously stated, and the reason also why the others lay imperfect eggs. In these also

the development from the egg runs on the same lines as that of the cartilaginous and internally oviparous

fishes, except that the growth is quick and from small beginnings and the outside of the egg is harder. The

growth of the egg is like that of a scolex, for those animals which produce a scolex give birth to a small thing

at first and this grows by itself and not through any attachment to the parent. The reason is similar to that of

the growth of yeast, for yeast also grows great from a small beginning as the more solid part liquefies and the

liquid is aerated. This is effected in animals by the nature of the vital heat, in yeasts by the heat of the juice

commingled with them. The eggs then grow of necessity through this cause (for they have in them

superfluous yeasty matter), but also for the sake of a final cause, for it is impossible for them to attain their

whole growth in the uterus because these animals have so many eggs. Therefore are they very small when set

free and grow quickly, small because the uterus is narrow for the multitude of the eggs, and growing quickly

that the race may not perish, as it would if much of the time required for the whole development were spent

in this growth; even as it is most of those laid are destroyed before hatching. Hence the class of fish is

prolific, for Nature makes up for the destruction by numbers. Some fish actually burst because of the size of

the eggs, as the fish called 'belone', for its eggs are large instead of numerous, what Nature has taken away in

A proof that these fish also are oviparous is the fact that even viviparous fish, such as the cartilaginous, are

first internally oviparous, for hence it is plain that the whole class of fishes is oviparous. Where, however,

both sexes exist and the eggs are produced in consequence of impregnation, the eggs do not arrive at

completion unless the male sprinkle his milt upon them. Some erroneously assert that all fish are female

except in the cartilaginous fishes, for they think that the females of fish differ from what are supposed to be

males only in the same way as in those plants where the one bears fruit but the other is fruitless, as olive and

oleaster, fig and caprifig. They think the like applies to fish except the cartilaginous, for they do not dispute

the sexes in these. And yet there is no difference in the males of cartilaginous fishes and those belonging to

the oviparous class in respect of the organs for the milt, and it is manifest that semen can be squeezed out of

males of both classes at the right season. The female also has a uterus. But if the whole class were females

and some of them unproductive (as with mules in the class of bushytailed animals), then not only should

those which lay eggs have a uterus but also the others, only the uterus of the latter should be different from

that of the former. But, as it is, some of them have organs for milt and others have a uterus, and this

distinction obtains in all except two, the erythrinus and the channa, some of them having the milt organs,

others a uterus. The difficulty which drives some thinkers to this conclusion is easily solved if we look at the

facts. They say quite correctly that no animal which copulates produces many young, for of all those that

generate from themselves perfect animals or perfect eggs none is prolific on the same scale as the oviparous

fishes, for the number of eggs in these is enormous. But they had overlooked the fact that fisheggs differ

from those of birds in one circumstance. Birds and all oviparous quadrupeds, and any of the cartilaginous fish

that are oviparous, produce a perfect egg, and it does not increase outside of them, whereas the eggs of fish

are imperfect and do so complete their growth. Moreover the same thing applies to cephalopods also and

crustacea, yet these animals are actually seen copulating, for their union lasts a long time, and it is plain in

these cases that the one is male and the other has a uterus. Finally, it would be strange if this distinction did

not exist in the whole class, just as male and female in all the vivipara. The cause of the ignorance of those

who make this statement is that the differences in the copulation and generation of various animals are of all

kinds and not obvious, and so, speculating on a small induction, they think the same must hold good in all

So also those who assert that conception in female fishes is caused by their swallowing the semen of the male

have not observed certain points when they say this. For the males have their milt and the females their eggs

at about the same time of year, and the nearer the female is to laying the more abundant and the more liquid is

the milt formed in the male. And just as the increase of the milt in the male and of the roe in the female takes

place at the same time, so is it also with their emission, for neither do the females lay all their eggs together,

but gradually, nor do the males emit all the milt at once. All these facts are in accordance with reason. For

just as the class of birds in some cases has eggs without impregnation, but few and seldom, impregnation

being generally required, so we find the same thing, though to a less degree, in fish. But in both classes these

spontaneous eggs are infertile unless the male, in those kinds where the male exists, shed his fluid upon them.

Now in birds this must take place while the eggs are still within the mother, because they are perfect when

discharged, but in fish, because the eggs are imperfect and complete their growth outside the mother in all

cases, those outside are preserved by the sprinkling of the milt over them, even if they come into being by

impregnation, and here it is that the milt of the males is used up. Therefore it comes down the ducts and

diminishes in quantity at the same time as this happens to the eggs of the females, for the males always attend

them, shedding their milt upon the eggs as they are laid. Thus then they are male and female, and all of them

copulate (unless in any kind the distinction of sex does not exist), and without the semen of the male no such

What helps in the deception is also the fact that the union of such fishes is brief, so that it is not observed

even by many of the fishermen, for none of them ever watches anything of the sort for the sake of knowledge.

Nevertheless their copulation has been seen, for fish [when the tail part does not prevent it] copulate like the

dolphins by throwing themselves alongside of one another. But the dolphins take longer to get free again,

whereas such fishes do so quickly. Hence, not seeing this, but seeing the swallowing of the milt and the eggs,

even the fishermen repeat the same simple tale, so much noised abroad, as Herodotus the storyteller, as if fish

were conceived by the mother's swallowing the milt, not considering that this is impossible. For the passage

which enters by way of the mouth runs to the intestines, not to the uterus, and what goes into the intestines

must be turned into nutriment, for it is concocted; the uterus, however, is plainly full of eggs, and from

A similar story is told also of the generation of birds. For there are some who say that the raven and the ibis

unite at the mouth, and among quadrupeds that the weasel brings forth its young by the mouth; so say

Anaxagoras and some of the other physicists, speaking too superficially and without consideration.

Concerning the birds, they are deceived by a false reasoning, because the copulation of ravens is seldom seen,

but they are often seen uniting with one another with their beaks, as do all the birds of the raven family; this

is plain with domesticated jackdaws. Birds of the pigeon kind do the same, but, because they also plainly

copulate, therefore they have not had the same legend told of them. But the raven family is not amorous, for

they are birds that produce few young, though this bird also has been seen copulating before now. It is a

strange thing, however, that these theorists do not ask themselves how the semen enters the uterus through the

intestine, which always concocts whatever comes into it, as the nutriment; and these birds have a uterus like

others, and eggs are found them near the hypozoma. And the weasel has a uterus in like manner to the other

quadrupeds; by what passage is the embryo to get from it to the mouth? But this opinion has arisen because

the young of the weasel are very small like those of the other fissipeds, of which we shall speak later, and

Much deceived also are those who make a foolish statement about the trochus and the hyena. Many say that

the hyena, and Herodorus the Heracleot says that the trochus, has two pudenda, those of the male and of the

female, and that the trochus impregnates itself but the hyena mounts and is mounted in alternate years. This is

untrue, for the hyena has been seen to have only one pudendum, there being no lack of opportunity for

observation in some districts, but hyenas have under the tail a line like the pudendum of the female. Both

male and female have such a mark, but the males are taken more frequently; this casual observation has given

Touching the generation of fish, the question may be raised, why it is that in the cartilaginous fish neither the

females are seen discharging their eggs nor the males their milt, whereas in the nonviviparous fishes this is

seen in both sexes. The reason is that the whole cartilaginous class do not produce much semen, and further

the females have their uterus near hypozoma. For the males and females of the one class of fish differ from

the males and females of the other class in like manner, for the cartilaginous are less productive of semen.

But in the oviparous fish, as the females lay their eggs on account of their number, so do the males shed their

milt on account of its abundance. For they have more milt than just what is required for copulation, as Nature

prefers to expend the milt in helping to perfect the eggs, when the female has deposited them, rather than in

forming them at first. For as has been said both further back and in our recent discussions, the eggs of birds

are perfected internally but those of fish externally. The latter, indeed, resemble in a way those animals which

produce a scolex, for the product discharged by them is still more imperfect than a fish's egg. It is the male

that brings about the perfection of the egg both of birds and of fishes, only in the former internally, as they

are perfected internally, and in the latter externally, because the egg is imperfect when deposited; but the

In birds the windeggs become fertile, and those previously impregnated by one kind of cock change their

nature to that of the later cock. And if the eggs be behindhand in growth, then, if the same cock treads the hen

again after leaving off treading for a time, he causes them to increase quickly, not, however, at any period

whatever of their development, but if the treading take place before the egg changes so far that the white

begins to separate from the yolk. But in the eggs of fishes no such limit of time has been laid down, but the

males shed their milt quickly upon them to preserve them. The reason is that these eggs are not

twocoloured, and hence there is no such limit of time fixed with them as with those of birds. This fact is

what we should expect, for by the time that the white and yolk are separated off from one another, the birds

egg already contains the principle that comes from the male parent.... for the male contributes to this.

Windeggs, then, participate in generation so far as is possible for them. That they should be perfected into

an animal is impossible, for an animal requires senseperception; but the nutritive faculty of the soul is

possessed by females as well as males, and indeed by all living things, as has been often said, wherefore the

egg itself is perfect only as the embryo of a plant, but imperfect as that of an animal. If, then, there had been

no male sex in the class of birds, the egg would have been produced as it is in some fishes, if indeed there is

any kind of fish of such a nature as to generate without a male; but it has been said of them before that this

has not yet been satisfactorily observed. But as it is both sexes exist in all birds, so that, considered as a plant,

the egg is perfect, but in so far as it is not a plant it is not perfect, nor does anything else result from it; for

neither has it come into being simply like a real plant nor from copulation like an animal. Eggs, however,

produced from copulation but already separated into white and yolk take after the first cock; for they already

contain both principles, which is why they do not change again after the second impregnation.

The young are produced in the same way also by the cephalopoda, e.g. sepias and the like, and by the

crustacea, e.g. carabi and their kindred, for these also lay eggs in consequence of copulation, and the male has

often been seen uniting with the female. Therefore those who say that all fish are female and lay eggs without

copulation are plainly speaking unscientifically from this point of view also. For it is a wonderful thing to

suppose that the former animals lay eggs in consequence of copulation and that fish do not; if again they were

unaware of this, it is a sign of ignorance. The union of all these creatures lasts a considerable time, as in

insects, and naturally so, for they are bloodless and therefore of a cold nature.

In the sepias and calamaries or squids the eggs appear to be two, because the uterus is divided and appears

double, but that of the poulps appears to be single. The reason is that the shape of the uterus in the poulp is

round in form and spherical, the cleavage being obscure when it is filled with eggs. The uterus of the carabi is

also bifid. All these animals also lay an imperfect egg for the same reason as fishes. In the carabi and their

like the females produce their eggs so as to keep them attached to themselves, which is why the sideflaps of

the females are larger than those of the males, to protect the eggs; the cephalopoda lay them away from

themselves. The males of the cephalopoda sprinkle their milt over the females, as the male fish do over the

eggs, and it becomes a sticky and glutinous mass, but in the carabi and their like nothing of the sort has been

seen or can be naturally expected, for the egg is under the female and is hardshelled. Both these eggs and

The sepia while developing is attached to the egg by its front part, for here alone is it possible, because this

animal alone has its front and back pointing in the same direction. For the position and attitude of the young

We have now spoken of the generation of other animals, those that walk, fly, and swim; it remains to speak of

insects and testacea according to the plan laid down. Let us begin with the insects. It was observed previously

that some of these are generated by copulation, others spontaneously, and besides this that they produce a

scolex, and why this is so. For pretty much all creatures seem in a certain way to produce a scolex first, since

the most imperfect embryo is of such a nature; and in all animals, even the viviparous and those that lay a

perfect egg, the first embryo grows in size while still undifferentiated into parts; now such is the nature of the

scolex. After this stage some of the ovipara produce the egg in a perfect condition, others in an imperfect, but

it is perfected outside as has been often stated of fish. With animals internally viviparous the embryo

becomes egglike in a certain sense after its original formation, for the liquid is contained in a fine

membrane, just as if we should take away the shell of the egg, wherefore they call the abortion of an embryo

Those insects which generate at all generate a scolex, and those which come into being spontaneously and not

from copulation do so at first from a formation this nature. I say that the former generate a scolex, for we

must put down caterpillars also and the product of spiders as a sort of scolex. And yet some even of these and

many of the others may be thought to resemble eggs because of their round shape, but we must not judge by

shapes nor yet by softness and hardness (for what is produced by some is hard), but by the fact that the whole

of them is changed into the body of the creature and the animal is not developed from a part of them. All

these products that are of the nature of a scolex, after progressing and acquiring their full size, become a sort

of egg, for the husk about them hardens and they are motionless during this period. This is plain in the scolex

of bees and wasps and in caterpillars. The reason of this is that their nature, because of its imperfection,

oviposits as it were before the right time, as if the scolex, while still growing in size, were a soft egg. Similar

to this is also what happens with all other insects which come into being without copulation in wool and other

such materials and in water. For all of them after the scolex stage become immovable and their integument

dries round them, and after this the latter bursts and there comes forth as from an egg an animal perfected in

Another point is quite natural, which may wondered at by many. Caterpillars at first take nourishment, but

after this stage do so no longer, but what is called by some the chrysalis is motionless. The same applies to

the scolex of wasps and bees, but after this comes into being the socalled nymph.... and have nothing of the

kind. For an egg is also of such a nature that when it has reached perfection it grows no more in size, but at

first it grows and receives nourishment until it is differentiated and becomes a perfect egg. Sometimes the

scolex contains in itself the material from which it is nourished and obtains such an addition to its size, e.g. in

bees and wasps; sometimes it gets its nourishment from outside itself, as caterpillars and some others.

It has thus been stated why such animals go through a double development and for what reason they become

immovable again after moving. And some of them come into being by copulation, like birds and vivipara and

There is much difficulty about the generation of bees. If it is really true that in the case of some fishes there is

such a method of generation that they produce eggs without copulation, this may well happen also with bees,

to judge from appearances. For they must (1) either bring the young brood from elsewhere, as some say, and

if so the young must either be spontaneously generated or produced by some other animal, or (2) they must

generate them themselves, or (3) they must bring some and generate others, for this also is maintained by

some, who say that they bring the young of the drones only. Again, if they generate them it must be either

with or without copulation; if the former, then either (1) each kind must generate its own kind, or (2) some

one kind must generate the others, or (3) one kind must unite with another for the purpose (I mean for

instance (1) that bees may be generated from the union of bees, drones from that of drones, and kings from

that of kings, or (2) that all the others may be generated from one, as from what are called kings and leaders,

or (3) from the union of drones and bees, for some say that the former are male, the latter female, while

others say that the bees are male and the drones female). But all these views are impossible if we reason first

upon the facts peculiar to bees and secondly upon those which apply more generally to other animals also.

For if they do not generate the young but bring them from elsewhere, then bees ought to come into being

also, if the bees did not carry them off, in the places from which the old bees carry the germs. For why, if new

bees come into existence when the germs are transported, should they not do so if the germs are left there?

They ought to do so just as much, whether the germs are spontaneously generated in the flowers or whether

some animal generates them. And if the germs were of some other animal, then that animal ought to be

produced from them instead of bees. Again, that they should collect honey is reasonable, for it is their food,

but it is strange that they should collect the young if they are neither their own offspring nor food. With what

object should they do so? for all animals that trouble themselves about the young labour for what appears to

But, again, it is also unreasonable to suppose that the bees are female and the drones male, for Nature does

not give weapons for fighting to any female, and while the drones are stingless all the bees have a sting. Nor

is the opposite view reasonable, that the bees are male and the drones female, for no males are in the habit of

working for their offspring, but as it is the bees do this. And generally, since the brood of the drones is found

coming into being among them even if there is no mature drone present, but that of the bees is not so found

without the presence of the kings (which is why some say that the young of the drones alone is brought in

from outside), it is plain that they are not produced from copulation, either (1) of bee with bee or drone with

drone or (2) of bees with drones. (That they should import the brood of the drones alone is impossible for the

reasons already given, and besides it is unreasonable that a similar state of things should not prevail with all

the three kinds if it prevails with one.) Then, again, it is also impossible that the bees themselves should be

some of them male and some female, for in all kinds of animals the two sexes differ. Besides they would in

that case generate their own kind, but as it is their brood is not found to come into being if the leaders are not

among them, as men say. And an argument against both theories, that the young are generated by union of the

bees with one another or with the drones, separately or with one another, is this: none of them has ever yet

been seen copulating, whereas this would have often happened if the sexes had existed in them. It remains

then, if they are generated by copulation at all, that the kings shall unite to generate them. But the drones are

found to come into being even if no leaders are present, and it is not possible that the bees should either

import their brood or themselves generate them by copulation. It remains then, as appears to be the case in

certain fishes, that the bees should generate the drones without copulation, being indeed female in respect of

generative power, but containing in themselves both sexes as plants do. Hence also they have the instrument

of offence, for we ought not to call that female in which the male sex is not separated. But if this is found to

be the case with drones, if they come into being without copulation, then as it is necessary that the same

account should be given of the bees and the kings and that they also should be generated without copulation.

Now if the brood of the bees had been found to come into being among them without the presence of the

kings, it would necessarily follow that the bees also are produced from bees themselves without copulation,

but as it is, since those occupied with the tendance of these creatures deny this, it remains that the kings must

As bees are a peculiar and extraordinary kind of animal so also their generation appears to be peculiar. That

bees should generate without copulation is a thing which may be paralleled in other animals, but that what

they generate should not be of the same kind is peculiar to them, for the erythrinus generates an erythrinus

and the channa a channa. The reason is that bees themselves are not generated like flies and similar creatures,

but from a kind different indeed but akin to them, for they are produced from the leaders. Hence in a sort of

way their generation is analogous. For the leaders resemble the drones in size and the bees in possessing a

sting; so the bees are like them in this respect, and the drones are like them in size. For there must needs be

some overlapping unless the same kind is always to be produced from each; but this is impossible, for at that

rate the whole class would consist of leaders. The bees, then, are assimilated to them their power of

generation, the drones in size; if the latter had had a sting also they would have been leaders, but as it is this

much of the difficulty has been solved, for the leaders are like both kinds at once, like the bees in possessing

But the leaders also must be generated from something. Since it is neither from the bees nor from the drones,

it must be from their own kind. The grubs of the kings are produced last and are not many in number.

Thus what happens is this: the leaders generate their own kind but also another kind, that of the bees; the bees

again generate another kind, the drones, but do not also generate their own kind, but this has been denied

them. And since what is according to Nature is always in due order, therefore it is necessary that it should be

denied to the drones even to generate another kind than themselves. This is just what we find happening, for

though the drones are themselves generated, they generate nothing else, but the process reaches its limit in the

third stage. And so beautifully is this arranged by Nature that the three kinds always continue in existence and

Another fact is also natural, that in fine seasons much honey is collected and many drones are produced but in

rainy reasons a large brood of ordinary bees. For the wet causes more residual matter to be formed in the

bodies of the leaders, the fine weather in that of the bees, for being smaller in size they need the fine weather

more than the kings do. It is right also that the kings, being as it were made with a view to producing young,

should remain within, freed from the labour of procuring necessaries, and also that they should be of a

considerable size, their bodies being, as it were, constituted with a view to bearing young, and that the drones

should be idle as having no weapon to fight for the food and because of the slowness of their bodies. But the

bees are intermediate in size between the two other kinds, for this is useful for their work, and they are

workers as having to support not only their young but also their fathers. And it agrees with our views that the

bees attend upon their kings because they are their offspring (for if nothing of the sort had been the case the

facts about their leadership would be unreasonable), and that, while they suffer the kings to do no work as

being their parents, they punish the drones as their children, for it is nobler to punish one's children and those

who have no work to perform. The fact that the leaders, being few, generate the bees in large numbers seems

to be similar to what obtains in the generation of lions, which at first produce five, afterwards a smaller

number each time at last one and thereafter none. So the leaders at first produce a number of workers,

afterwards a few of their own kind; thus the brood of the latter is smaller in number than that of the former,

but where Nature has taken away from them in number she has made it up again in size.

Such appears to be the truth about the generation of bees, judging from theory and from what are believed to

be the facts about them; the facts, however, have not yet been sufficiently grasped; if ever they are, then

credit must be given rather to observation than to theories, and to theories only if what they affirm agrees

A further indication that bees are produced without copulation is the fact that the brood appears small in the

cells of the comb, whereas, whenever insects are generated by copulation, the parents remain united for a

long time but produce quickly something of the nature of a scolex and of a considerable size.

Concerning the generation of animals akin to them, as hornets and wasps, the facts in all cases are similar to a

certain extent, but are devoid of the extraordinary features which characterize bees; this we should expect, for

they have nothing divine about them as the bees have. For the socalled 'mothers' generate the young and

mould the first part of the combs, but they generate by copulation with one another, for their union has often

been observed. As for all the differences of each of these kind from one another and from bees, they must be

Having spoken of the generation of all insects, we must now speak of the testacea. Here also the facts of

generation are partly like and partly unlike those in the other classes. And this is what might be expected. For

compared with animals they resemble plants, compared with plants they resemble animals, so that in a sense

they appear to come into being from semen, but in another sense not so, and in one way they are

spontaneously generated but in another from their own kind, or some of them in the latter way, others in the

former. Because their nature answers to that of plants, therefore few or no kinds of testacea come into being

on land, e.g. the snails and any others, few as they are, that resemble them; but in the sea and similar waters

there are many of all kinds of forms. But the class of plants has but few and one may say practically no

representatives in the sea and such places, all such growing on the land. For plants and testacea are

analogous; and in proportion as liquid has more quickening power than solid, water than earth, so much does

the nature of testacea differ from that of plants, since the object of testacea is to be in such a relation to water

as plants are to earth, as if plants were, so to say, landoysters, oysters waterplants.

For such a reason also the testacea in the water vary more in form than those on the land. For the nature of

liquid is more plastic than that of earth and yet not much less material, and this is especially true of the

inhabitants of the sea, for fresh water, though sweet and nutritious, is cold and less material. Wherefore

animals having no blood and not of a hot nature are not produced in lakes nor in the fresher among brackish

waters, but only exceptionally, but it is in estuaries and at the mouths of rivers that they come into being, as

testacea and cephalopoda and crustacea, all these being bloodless and of a cold nature. For they seek at the

same time the warmth of the sun and food; now the sea is not only water but much more material than fresh

water and hot in its nature; it has a share in all the parts of the universe, water and air and earth, so that it also

has a share in all living things which are produced in connexion with each of these elements. Plants may be

assigned to land, the aquatic animals to water, the land animals to air, but variations of quantity and distance

make a great and wonderful difference. The fourth class must not be sought in these regions, though there

certainly ought to be some animal corresponding to the element of fire, for this is counted in as the fourth of

the elementary bodies. But the form which fire assumes never appears to be peculiar to it, but it always exists

in some other of the elements, for that which is ignited appears to be either air or smoke or earth. Such a kind

of animal must be sought in the moon, for this appears to participate in the element removed in the third

degree from earth. The discussion of these things however belongs to another subject.

To return to testacea, some of them are formed spontaneously, some emit a sort of generative substance from

themselves, but these also often come into being from a spontaneous formation. To understand this we must

grasp the different methods of generation in plants; some of these are produced from seed, some from slips,

planted out, some by budding off alongside, as the class of onions. In the last way produced mussels, for

smaller ones are always growing off alongside the original, but the whelks, the purplefish, and those which

are said to 'spawn' emit masses of a liquid slime as if originated by something of a seminal nature. We must

not, however, consider that anything of the sort is real semen, but that these creatures participate in the

resemblance to plants in the manner stated above. Hence when once one such creature has been produced,

then is produced a number of them. For all these creatures are liable to be even spontaneously generated, and

so to be formed still more plentifully in proportion if some are already existing. For it is natural that each

should have some superfluous residue attached to it from the original, and from this buds off each of the

creatures growing alongside of it. Again, since the nutriment and its residue possess a like power, it is likely

that the product of those testacea which 'spawn' should resemble the original formation, and so it is natural

All those which do not bud off or 'spawn' are spontaneously generated. Now all things formed in this way,

whether in earth or water, manifestly come into being in connexion with putrefaction and an admixture of

rainwater. For as the sweet is separated off into the matter which is forming, the residue of the mixture takes

such a form. Nothing comes into being by putrefying, but by concocting; putrefaction and the thing putrefied

is only a residue of that which is concocted. For nothing comes into being out of the whole of anything, any

more than in the products of art; if it did art would have nothing to do, but as it is in the one case art removes

the useless material, in the other Nature does so. Animals and plants come into being in earth and in liquid

because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full

of soul. Therefore living things form quickly whenever this air and vital heat are enclosed in anything. When

they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble. Whether what

is forming is to be more or less honourable in kind depends on the embracing of the psychical principle; this

again depends on the medium in which the generation takes place and the material which is included. Now in

the sea the earthy matter is present in large quantities, and consequently the testaceous animals are formed

from a concretion of this kind, the earthy matter hardening round them and solidifying in the same manner as

bones and horns (for these cannot be melted by fire), and the matter (or body) which contains the life being

The class of snails is the only class of such creatures that has been seen uniting, but it has never yet been

sufficiently observed whether their generation is the result of the union or not.

It may be asked, if we wish to follow the right line of investigation, what it is in such animals the formation

of which corresponds to the material principle. For in the females this is a residual secretion of the animal,

potentially such as that from which it came, by imparting motion to which the principle derived from the

male perfects the animal. But here what must be said to correspond to this, and whence comes or what is the

moving principle which corresponds to the male? We must understand that even in animals which generate it

is from the incoming nourishment that the heat in the animal makes the residue, the beginning of the

conception, by secretion and concoction. The like is the case also in plants, except that in these (and also in

some animals) there is no further need of the male principle, because they have it mingled with the female

principle within themselves, whereas the residual secretion in most animals does need it. The nourishment

again of some is earth and water, of others the more complicated combinations of these, so that what the heat

in animals produces from their nutriment, this does the heat of the warm season in the environment put

together and combine by concoction out of the seawater on the earth. And the portion of the psychical

principle which is either included along with it or separated off in the air makes an embryo and puts motion

into it. Now in plants which are spontaneously generated the method of formation is uniform; they arise from

a part of something, and while some of it is the startingpoint of the plant, some is the first nourishment of

the young shoots.... Other animals are produced in the form of a scolex, not only those bloodless animals

which are not generated from parents but even some sanguinea, as a kind of mullet and some other river

fishes and also the eel kind. For all of these, though they have but little blood by nature, are nevertheless

sanguinea, and have a heart with blood in it as the origin of the parts; and the socalled 'entrails of earth', in

Hence one might suppose, in connexion with the origin of men and quadrupeds, that, if ever they were really

'earthborn' as some say, they came into being in one of two ways; that either it was by the formation of a

scolex at first or else it was out of eggs. For either they must have had in themselves the nutriment for growth

(and such a conception is a scolex) or they must have got it from elsewhere, and that either from the mother

or from part of the conception. If then the former is impossible (I mean that nourishment should flow to them

from the earth as it does in animals from the mother), then they must have got it from some part of the

It is plain then that, if there really was any such beginning of the generation of all animals, it is reasonable to

suppose to have been one of these two, scolex or egg. But it is less reasonable to suppose that it was from

eggs, for we do not see such generation occurring with any animal, but we do see the other both in the

sanguinea above mentioned and in the bloodless animals. Such are some of the insects and such are the

testacea which we are discussing; for they do not develop out of a part of something (as do animals from

eggs), and they grow like a scolex. For the scolex grows towards the upper part and the first principle, since

in the lower part is the nourishment for the upper. And this resembles the development of animals from eggs,

except that these latter consume the whole egg, whereas in the scolex, when the upper part has grown by

taking up into itself part of the substance in the lower part, the lower part is then differentiated out of the rest.

The reason is that in later life also the nourishment is absorbed by all animals in the part below the

That the scolex grows in this way is plain in the case of bees and the like, for at first the lower part is large in

them and the upper is smaller. The details of growth in the testacea are similar. This is plain in the whorls of

the turbinata, for always as the animal grows the whorls become larger towards the front and what is called

We have now pretty well described the manner of the development of these and the other spontaneously

generated animals. That all the testacea are formed spontaneously is clear from such facts as these. They

come into being on the side of boats when the frothy mud putrefies. In many places where previously nothing

of the kind existed, the socalled limnostrea, a kind of oyster, have come into being when the spot turned

muddy through want of water; thus when a naval armament cast anchor at Rhodes a number of clay vessels

were thrown out into the sea, and after some time, when mud had collected round them, oysters used to be

found in them. Here is another proof that such animals do not emit any generative substance from

themselves; when certain Chians carried some live oysters over from Pyrrha in Lesbos and placed them in

narrow straits of the sea where tides clash, they became no more numerous as time passed, but increased

greatly in size. The socalled eggs contribute to generation but are only a condition, like fat in the sanguinea,

and therefore the oysters are savoury at these periods. A proof that this substance is not really eggs is the fact

that such 'eggs' are always found in some testacea, as in pinnae, whelks, and purplefish; only they are

sometimes larger and sometimes smaller; in others as pectens, mussels, and the socalled limnostrea, they are

not always present but only in the spring; as the season advances they dwindle and at last disappear

altogether; the reason being that the spring is favourable to their being in good condition. In others again, as

the ascidians, nothing of the sort is visible. (The details concerning these last, and the places in which they

WE have thus spoken of the generation of animals both generally and separately in all the different classes.

But, since male and female are distinct in the most perfect of them, and since we say that the sexes are first

principles of all living things whether animals or plants, only in some of them the sexes are separated and in

others not, therefore we must speak first of the origin of the sexes in the latter. For while the animal is still

It is disputed, however, whether the embryo is male or female, as the case may be, even before the distinction

is plain to our senses, and further whether it is thus differentiated within the mother or even earlier. It is said

by some, as by Anaxagoras and other of the physicists, that this antithesis exists from the beginning in the

germs or seeds; for the germ, they say, comes from the male while the female only provides the place in

which it is to be developed, and the male is from the right, the female from the left testis, and so also that the

male embryo is in the right of the uterus, the female in the left. Others, as Empedocles, say that the

differentiation takes place in the uterus; for he says that if the uterus is hot or cold what enters it becomes

male or female, the cause of the heat or cold being the flow of the catamenia, according as it is colder or

hotter, more 'antique' or more 'recent'. Democritus of Abdera also says that the differentiation of sex takes

place within the mother; that however it is not because of heat and cold that one embryo becomes female and

another male, but that it depends on the question which parent it is whose semen prevails, not the whole of

the semen, but that which has come from the part by which male and female differ from one another. This is a

better theory, for certainly Empedocles has made a rather lighthearted assumption in thinking that the

difference between them is due only to cold and heat, when he saw that there was a great difference in the

whole of the sexual parts, the difference in fact between the male pudenda and the uterus. For suppose two

animals already moulded in embryo, the one having all the parts of the female, the other those of the male;

suppose them then to be put into the uterus as into an oven, the former when the oven is hot, the latter when it

is cold; then on the view of Empedocles that which has no uterus will be female and that which has will be

male. But this is impossible. Thus the theory of Democritus would be the better of the two, at least as far as

this goes, for he seeks for the origin of this difference and tries to set it forth; whether he does so well or not

Again, if heat and cold were the cause of the difference of the parts, this ought to have been stated by those

who maintain the view of Empedocles; for to explain the origin of male and female is practically the same

thing as to explain this, which is the manifest difference between them. And it is no small matter, starting

from temperature as a principle, to collect the cause of the origin of these parts, as if it were a necessary

consequence for this part which they call the uterus to be formed in the embryo under the influence of cold

but not under that of heat. The same applies also to the parts which serve for intercourse, since these also

Moreover male and female twins are often found together in the same part of the uterus; this we have

observed sufficiently by dissection in all the vivipara, both land animals and fish. Now if Empedocles had not

seen this it was only natural for him to fall into error in assigning this cause of his; but if he had seen it it is

strange that he should still think the heat or cold of the uterus to be the cause, since on his theory both these

twins would have become either male or female, but as it is we do not see this to be the fact.

Again he says that the parts of the embryo are 'sundered', some being in the male and some in the female

parent, which is why they desire intercourse with one another. If so it is necessary that the sexual parts like

the rest should be separated from one another, already existing as masses of a certain size, and that they

should come into being in the embryo on account of uniting with one another, not on account of cooling or

heating of the semen. But perhaps it would take too long to discuss thoroughly such a cause as this which is

stated by Empedocles, for its whole character seems to be fanciful. If, however, the facts about semen are

such as we have actually stated, if it does not come from the whole of the body of the male parent and if the

secretion of the male does not give any material at all to the embryo, then we must make a stand against both

Empedocles and Democritus and any one else who argues on the same lines. For then it is not possible that

the body of the embryo should exist 'sundered', part in the female parent and part in the male, as Empedocles

says in the words: 'But the nature of the limbs hath been sundered, part in the man's...'; nor yet that a whole

embryo is drawn off from each parent and the combination of the two becomes male or female according as

And, to take a more general view, though it is better to say that the one part makes the embryo female by

prevailing through some superiority than to assign nothing but heat as the cause without any reflection, yet,

as the form of the pudendum also varies along with the uterus from that of the father, we need an explanation

of the fact that both these parts go along with each other. If it is because they are near each other, then each of

the other parts also ought to go with them, for one of the prevailing parts is always near another part where

the struggle is not yet decided; thus the offspring would be not only female or male but also like its mother or

Besides, it is absurd to suppose that these parts should come into being as something isolated, without the

body as a whole having changed along with them. Take first and foremost the bloodvessels, round which

the whole mass of the flesh lies as round a framework. It is not reasonable that these should become of a

certain quality because of the uterus, but rather that the uterus should do so on account of them. For though it

is true that each is a receptacle of blood of some kind, still the system of the vessels is prior to the other; the

moving principle must needs always be prior to that which it moves, and it is because it is itself of a certain

quality that it is the cause of the development. The difference, then, of these parts as compared with each

other in the two sexes is only a concomitant result; not this but something else must be held to be the first

principle and the cause of the development of an embryo as male or female; this is so even if no semen is

secreted by either male or female, but the embryo is formed in any way you please.

The same argument as that with which we meet Empedocles and Democritus will serve against those who say

that the male comes from the right and the female from the left. If the male contributes no material to the

embryo, there can be nothing in this view. If, as they say, he does contribute something of the sort, we must

confront them in the same way as we did the theory of Empedocles, which accounts for the difference

between male and female by the heat and cold of the uterus. They make the same mistake as he does, when

they account for the difference by their 'right and left', though they see that the sexes differ actually by the

whole of the sexual parts; for what reason then is the body of the uterus to exist in those embryos which come

from the left and not in those from the right? For if an embryo have come from the left but has not acquired

this part, it will be a female without a uterus, and so too there is nothing to stop another from being a male

with a uterus! Besides as has been said before, a female embryo has been observed in the right part of the

uterus, a male in the left, or again both at once in the same part, and this not only once but several times.

Some again, persuaded of the truth of a view resembling that of these philosophers, say that if a man

copulates with the right or left testis tied up the result is male or female offspring respectively; so at least

Leophanes asserted. And some say that the same happens in the case of those who have one or other testis

excised, not speaking truth but vaticinating what will happen from probabilities and jumping at the

conclusion that it is so before seeing that it proves to be so. Moreover, they know not that these parts of

animals contribute nothing to the production of one sex rather than the other; a proof of this is that many

animals in which the distinction of sex exists, and which produce both male and female offspring,

nevertheless have no testes, as the footless animals; I mean the classes of fish and of serpents.

To suppose, then, either that heat and cold are the causes of male and female, or that the different sexes come

from the right and left, is not altogether unreasonable in itself; for the right of the body is hotter than the left,

and the concocted semen is hotter than the unconcocted; again, the thickened is concocted, and the more

thickened is more fertile. Yet to put it in this way is to seek for the cause from too remote a startingpoint;

We have, then, previously spoken elsewhere of both the body as a whole and its parts, explaining what each

part is and for what reason it exists. But (1) the male and female are distinguished by a certain capacity and

incapacity. (For the male is that which can concoct the blood into semen and which can form and secrete and

discharge a semen carrying with it the principle of form by 'principle' I do not mean a material principle out

of which comes into being an offspring resembling the parent, but I mean the first moving cause, whether it

have power to act as such in the thing itself or in something else but the female is that which receives

semen, indeed, but cannot form it for itself or secrete or discharge it.) And (2) all concoction works by means

of heat. Therefore the males of animals must needs be hotter than the females. For it is by reason of cold and

incapacity that the female is more abundant in blood in certain parts of her anatomy, and this abundance is an

evidence of the exact opposite of what some suppose, thinking that the female is hotter than the male for this

reason, i.e. the discharge of the catamenia. It is true that blood is hot, and that which has more of it is hotter

than that which has less. But they assume that this discharge occurs through excess of blood and of heat, as if

it could be taken for granted that all blood is equally blood if only it be liquid and sanguineous in colour, and

as if it might not become less in quantity but purer in quality in those who assimilate nourishment properly.

In fact they look upon this residual discharge in the same light as that of the intestines, when they think that a

greater amount of it is a sign of a hotter nature, whereas the truth is just the opposite. For consider the

production of fruit; the nutriment in its first stage is abundant, but the useful product derived from it is small,

indeed the final result is nothing at all compared to the quantity in the first stage. So is it with the body; the

various parts receive and work up the nutriment, from the whole of which the final result is quite small. This

is blood in some animals, in some its analogue. Now since (1) the one sex is able and the other is unable to

reduce the residual secretion to a pure form, and (2) every capacity or power in an organism has a certain

corresponding organ, whether the faculty produces the desired results in a lower degree or in a higher degree,

and the two sexes correspond in this manner (the terms 'able' and 'unable' being used in more senses than

one) therefore it is necessary that both female and male should have organs. Accordingly the one has the

Again, Nature gives both the faculty and the organ to each individual at the same time, for it is better so.

Hence each region comes into being along with the secretions and the faculties, as e.g. the faculty of sight is

not perfected without the eye, nor the eye without the faculty of sight; and so too the intestine and bladder

come into being along with the faculty of forming the excreta. And since that from which an organ comes

into being and that by which it is increased are the same (i.e. the nutriment), each of the parts will be made

out of such a material and such residual matter as it is able to receive. In the second place, again, it is formed,

as we say, in a certain sense, out of its opposite. Thirdly, we must understand besides this that, if it is true that

when a thing perishes it becomes the opposite of what it was, it is necessary also that what is not under the

sway of that which made it must change into its opposite. After these premisses it will perhaps be now clearer

for what reason one embryo becomes female and another male. For when the first principle does not bear

sway and cannot concoct the nourishment through lack of heat nor bring it into its proper form, but is

defeated in this respect, then must needs the material which it works on change into its opposite. Now the

female is opposite to the male, and that in so far as the one is female and the other male. And since it differs

in its faculty, its organ also is different, so that the embryo changes into this state. And as one part of

firstrate importance changes, the whole system of the animal differs greatly in form along with it. This may

be seen in the case of eunuchs, who, though mutilated in one part alone, depart so much from their original

appearance and approximate closely to the female form. The reason of this is that some of the parts are

principles, and when a principle is moved or affected needs must many of the parts that go along with it

If then (1) the male quality or essence is a principle and a cause, and (2) the male is such in virtue of a certain

capacity and the female is such in virtue of an incapacity, and (3) the essence or definition of the capacity and

of the incapacity is ability or inability to concoct the nourishment in its ultimate stage, this being called blood

in the sanguinea and the analogue of blood in the other animals, and (4) the cause of this capacity is in the

first principle and in the part which contains the principle of natural heat therefore a heart must be formed in

the sanguinea (and the resulting animal will be either male or female), and in the other kinds which possess

This, then, is the first principle and cause of male and female, and this is the part of the body in which it

resides. But the animal becomes definitely female or male by the time when it possesses also the parts by

which the female differs from the male, for it is not in virtue of any part you please that it is male or female,

To recapitulate, we say that the semen, which is the foundation of the embryo, is the ultimate secretion of the

nutriment. By ultimate I mean that which is carried to every part of the body, and this is also the reason why

the offspring is like the parent. For it makes no difference whether we say that the semen comes from all the

parts or goes to all of them, but the latter is the better. But the semen of the male differs from the

corresponding secretion of the female in that it contains a principle within itself of such a kind as to set up

movements also in the embryo and to concoct thoroughly the ultimate nourishment, whereas the secretion of

the female contains material alone. If, then, the male element prevails it draws the female element into itself,

but if it is prevailed over it changes into the opposite or is destroyed. But the female is opposite to the male,

and is female because of its inability to concoct and of the coldness of the sanguineous nutriment. And Nature

assigns to each of the secretions the part fitted to receive it. But the semen is a secretion, and this in the hotter

animals with blood, i.e. the males, is moderate in quantity, wherefore the recipient parts of this secretion in

males are only passages. But the females, owing to inability to concoct, have a great quantity of blood, for it

cannot be worked up into semen. Therefore they must also have a part to receive this, and this part must be

unlike the passages of the male and of a considerable size. This is why the uterus is of such a nature, this

We have thus stated for what reason the one becomes female and the other male. Observed facts confirm

what we have said. For more females are produced by the young and by those verging on old age than by

those in the prime of life; in the former the vital heat is not yet perfect, in the latter it is failing. And those of a

moister and more feminine state of body are more wont to beget females, and a liquid semen causes this more

than a thicker; now all these characteristics come of deficiency in natural heat.

Again, more males are born if copulation takes place when north than when south winds are blowing. For in

the latter case the animals produce more secretion, and too much secretion is harder to concoct; hence the

Also the fact that the catamenia occur in the course of nature rather when the month is waning is due to the

same causes. For this time of the month is colder and moister because of the waning and failure of the moon;

as the sun makes winter and summer in the year as a whole, so does the moon in the month. This is not due to

the turning of the moon, but it grows warmer as the light increases and colder as it wanes.

The shepherds also say that it not only makes a difference in the production of males and females if

copulation takes place during northern or southerly winds, but even if the animals while copulating look

towards the south or north; so small a thing will sometimes turn the scale and cause cold or heat, and these

The male and female, then, are distinguished generally, as compared with one another in connexion with the

production of male and female offspring, for the causes stated. However, they also need a certain

correspondence with one another to produce at all, for all things that come into being as products of art or of

Nature exist in virtue of a certain ratio. Now if the hot preponderates too much it dries up the liquid; if it is

very deficient it does not solidify it; for the artistic or natural product we need the due mean between the

extremes. Otherwise it will be as in cooking; too much fire burns the meat, too little does not cook it, and in

either case the process is a failure. So also there is need of due proportion in the mixture of the male and

female elements. And for this cause it often happens to many of both sexes that they do not generate with one

another, but if divorced and remarried to others do generate; and these oppositions show themselves

sometimes in youth, sometimes in advanced age, alike as concerns fertility or infertility, and as concerns

One country also differs from another in these respects, and one water from another, for the same reasons.

For the nourishment and the medical condition of the body are of such or such a kind because of the

tempering of the surrounding air and of the food entering the body, especially the water; for men consume

more of this than of anything else, and this enters as nourishment into all food, even solids. Hence hard

The same causes must be held responsible for the following groups of facts. (1) Some children resemble their

parents, while others do not; some being like the father and others like the mother, both in the body as a

whole and in each part, male and female offspring resembling father and mother respectively rather than the

other way about. (2) They resemble their parents more than remoter ancestors, and resemble those ancestors

more than any chance individual. (3) Some, though resembling none of their relations, yet do at any rate

resemble a human being, but others are not even like a human being but a monstrosity. For even he who does

not resemble his parents is already in a certain sense a monstrosity; for in these cases Nature has in a way

departed from the type. The first departure indeed is that the offspring should become female instead of male;

this, however, is a natural necessity. (For the class of animals divided into sexes must be preserved, and as it

is possible for the male sometimes not to prevail over the female in the mixture of the two elements, either

through youth or age or some other such cause, it is necessary that animals should produce female young).

And the monstrosity, though not necessary in regard of a final cause and an end, yet is necessary accidentally.

As for the origin of it, we must look at it in this way. If the generative secretion in the catamenia is properly

concocted, the movement imparted by the male will make the form of the embryo in the likeness of itself.

(Whether we say that it is the semen or this movement that makes each of the parts grow, makes no

difference; nor again whether we say that it 'makes them grow' or 'forms them from the beginning', for the

formula of the movement is the same in either case.) Thus if this movement prevail, it will make the embryo

male and not female, like the father and not like the mother; if it prevail not, the embryo is deficient in that

faculty in which it has not prevailed. By 'each faculty' I mean this. That which generates is not only male but

also a particular male, e.g. Coriscus or Socrates, and it is not only Coriscus but also a man. In this way some

of the characteristics of the father are more near to him, others more remote from him considered simply as a

parent and not in reference to his accidental qualities (as for instance if the parent is a scholar or the

neighbour of some particular person). Now the peculiar and individual has always more force in generation

than the more general and wider characteristics. Coriscus is both a man and an animal, but his manhood is

nearer to his individual existence than is his animalhood. In generation both the individual and the class are

operative, but the individual is the more so of the two, for this is the only true existence. And the offspring is

produced indeed of a certain quality, but also as an individual, and this latter is the true existence. Therefore it

is from the forces of all such existences that the efficient movements come which exist in the semen;

potentially from remoter ancestors but in a higher degree and more nearly from the individual (and by the

individual I mean e.g. Coriscus or Socrates). Now since everything changes not into anything haphazard but

into its opposite, therefore also that which is not prevailed over in generation must change and become the

opposite, in respect of that particular force in which the paternal and efficient or moving element has not

prevailed. If then it has not prevailed in so far as it is male, the offspring becomes female; if in so far as it is

Coriscus or Socrates, the offspring does not resemble the father but the mother. For as 'father' and 'mother' are

opposed as general terms, so also the individual father is opposed to the individual mother. The like applies

also to the forces that come next in order, for the offspring always changes rather into the likeness of the

nearer ancestor than the more remote, both in the paternal and in the maternal line.

Some of the movements exist in the semen actually, others potentially; actually, those of the father and the

general type, as man and animal; potentially those of the female and the remoter ancestors. Thus the male and

efficient principle, if it lose its own nature, changes to its opposites, but the movements which form the

embryo change into those nearly connected with them; for instance, if the movement of the male parent be

resolved, it changes by a very slight difference into that of his father, and in the next instance into that of his

grandfather; and in this way not only in the male but also in the female line the movement of the female

parent changes into that of her mother, and, if not into this, then into that of her grandmother; and similarly

Naturally then it is most likely that the characteristics of 'male' and of the individual father will go together,

whether they prevail or are prevailed over. For the difference between them is small so that there is no

difficulty in both concurring, for Socrates is an individual man with certain characters. Hence for the most

part the male offspring resemble the father, and the female the mother. For in the latter case the loss of both

characters takes place at once, and the change is into the two opposites; now is opposed to male, and the

But if the movement coming from the male principle prevails while that coming from the individual Socrates

does not, or vice versa, then the result is that male children are produced resembling the mother and female

If again the movements be resolved, if the male character remain but the movement coming from the

individual Socrates be resolved into that of the father of Socrates, the result will be a male child resembling

its grandfather or some other of its more remote ancestors in the male line on the same principle. If the male

principle be prevailed over, the child will be female and resembling most probably its mother, but, if the

movement coming from the mother also be resolved, it will resemble its mother's mother or the resemblance

will be to some other of its more remote ancestors in the female line on the same principle.

The same applies also to the separate parts, for often some of these take after the father, and others after the

mother, and yet others after some of the remoter ancestors. For, as has been often said already, some of the

movements which form the parts exist in the semen actually and others potentially. We must grasp certain

fundamental general principles, not only that just mentioned (that some of the movements exist potentially

and others actually), but also two others, that if a character be prevailed over it changes into its opposite, and,

if it be resolved, is resolved into the movement next allied to it if less, into that which is near, if more, into

that which is further removed. Finally, the movements are so confused together that there is no resemblance

to any of the family or kindred, but the only character that remains is that common to the race, i.e. it is a

human being. The reason of this is that this is closely knit up with the individual characteristics; 'human

being' is the general term, while Socrates, the father, and the mother, whoever she may be, are individuals.

The reason why the movements are resolved is this. The agent is itself acted upon by that on which it acts;

thus that which cuts is blunted by that which is cut by it, that which heats is cooled by that which is heated by

it, and in general the moving or efficient cause (except in the case of the first cause of all) does itself receive

some motion in return; e.g. what pushes is itself in a way pushed again and what crushes is itself crushed

again. Sometimes it is altogether more acted upon than is the thing on which it acts, so that what is heating or

cooling something else is itself cooled or heated; sometimes having produced no effect, sometimes less than

it has itself received. (This question has been treated in the special discussion of action and reaction, where it

is laid down in what classes of things action and reaction exist.) Now that which is acted on escapes and is

not mastered by the semen, either through deficiency of power in the concocting and moving agent or

because what should be concocted and formed into distinct parts is too cold and in too great quantity. Thus

the moving agent, mastering it in one part but not in another, makes the embryo in formation to be multiform,

as happens with athletes because they eat so much. For owing to the quantity of their food their nature is not

able to master it all, so as to increase and arrange their form symmetrically; therefore their limbs develop

irregularly, sometimes indeed almost so much that no one of them resembles what it was before. Similar to

this is also the disease known as satyrism, in which the face appears like that of a satyr owing to a quantity of

We have thus discussed the cause of all these phenomena, (1) female and male offspring are produced, (2)

why some are similar to their parents, female to female and male to male, and others the other way about,

females being similar to the father and males to the mother, and in general why some are like their ancestors

while others are like none of them, and all this as concerns both the body as a whole and each of the parts

separately. Different accounts, however, have been given of these phenomena by some of the

naturephilosophers; I mean why children are like or unlike their parents. They give two versions of the

reason. Some say that the child is more like that parent of the two from whom comes more semen, this

applying equally both to the body as a whole and to the separate parts, on the assumption that semen comes

from each part of both parents; if an equal part comes from each, then, they say, the child is like neither. But

if this is false, if semen does not come off from the whole body of the parents, it is clear that the reason

assigned cannot be the cause of likeness and unlikeness. Moreover, they are hard put to it to explain how it is

that a female child can be like the father and a male like the mother. For (1) those who assign the same cause

of sex as Empedocles or Democritus say what is on other grounds impossible, and (2) those who say that it is

determined by the greater or smaller amount of semen coming the male or female parent, and that this is why

one child is male and another female, cannot show how the female is to resemble the father and the male the

mother, for it is impossible that more should come from both at once. Again, for what reason is a child

generally like its ancestors, even the more remote? None of the semen has come from them at any rate.

But those who account for the similarity in the manner which remains to be discussed, explain this point

better, as well as the others. For there are some who say that the semen, though one, is as it were a common

mixture (panspermia) of many elements; just as, if one should mix many juices in one liquid and then take

some from it, it would be possible to take, not an equal quantity always from each juice, but sometimes more

of one and sometimes more of another, sometimes some of one and none at all of another, so they say it is

with the generative fluid, which is a mixture of many elements, for the offspring resembles that parent from

which it has derived most. Though this theory is obscure and in many ways fictitious, it aims at what is better

expressed by saying that what is called 'panspermia' exists potentially, not actually; it cannot exist actually,

but it can do so potentially. Also, if we assign only one sort of cause, it is not easy to explain all the

phenomena, (1) the distinction of sex, (2) why the female is often like the father and the male like the mother,

and again (3) the resemblance to remoter ancestors, and further (4) the reason why the offspring is sometimes

unlike any of these but still a human being, but sometimes, (5) proceeding further on these lines, appears

finally to be not even a human being but only some kind of animal, what is called a monstrosity.

For, following what has been said, it remains to give the reason for such monsters. If the movements imparted

by the semen are resolved and the material contributed by the mother is not controlled by them, at last there

remains the most general substratum, that is to say the animal. Then people say that the child has the head of

a ram or a bull, and so on with other animals, as that a calf has the head of a child or a sheep that of an ox. All

these monsters result from the causes stated above, but they are none of the things they are said to be; there is

only some similarity, such as may arise even where there is no defect of growth. Hence often jesters compare

some one who is not beautiful to a 'goat breathing fire', or again to a 'ram butting', and a certain

physiognomist reduced all faces to those of two or three animals, and his arguments often prevailed on

That, however, it is impossible for such a monstrosity to come into existence I mean one animal in another

is shown by the great difference in the period of gestation between man, sheep, dog, and ox, it being

This is the description of some of the monsters talked about; others are such because certain parts of their

The account of the cause of monstrosities is very close and similar in a way to that of the cause of animals

Democritus said that monstrosities arose because two emissions of seminal fluid met together, the one

succeeding the other at an interval of time; that the later entering into the uterus reinforced the earlier so that

the parts of the embryo grow together and get confused with one another. But in birds, he says, since

copulation takes place quickly, both the eggs and their colour always cross one another. But if it is the fact, as

it manifestly is, that several young are produced from one emission of semen and a single act of intercourse,

it is better not to desert the short road to go a long way about, for in such cases it is absolutely necessary that

this should occur when the semen is not separated but all enters the female at once.

If, then, we must attribute the cause to the semen of the male, this will be the way we shall have to state it,

but we must rather by all means suppose that the cause lies in the material contributed by the female and in

the embryo as it is forming. Hence also such monstrosities appear very rarely in animals producing only one

young one, more frequently in those producing many, most of all in birds and among birds in the common

fowl. For this bird produces many young, not only because it lays often like the pigeon family, but also

because it has many embryos at once and copulates all the year round. Therefore it produces many double

eggs, for the embryos grow together because they are near one another, as often happens with many fruits. In

such double eggs, when the yolks are separated by the membrane, two separate chickens are produced with

nothing abnormal about them; when the yolks are continuous, with no division between them, the chickens

produced are monstrous, having one body and head but four legs and four wings; this is because the upper

parts are formed earlier from the white, their nourishment being drawn from the yolk, whereas the lower part

A snake has also been observed with two heads for the same reason, this class also being oviparous and

producing many young. Monstrosities, however, are rarer among them owing to the shape of the uterus, for

Nothing of the kind occurs with bees and wasps, because their brood is in separate cells. But in the fowl the

opposite is the case, whereby it is plain that we must hold the cause of such phenomena to lie in the material.

So, too, monstrosities are commoner in other animals if they produce many young. Hence they are less

common in man, for he produces for the most part only one young one and that perfect; even in man

monstrosities occur more often in regions where the women give birth to more than one at a time, as in

Egypt. And they are commoner in sheep and goats, since they produce more young. Still more does this apply

to the fissipeds, for such animals produce many young and imperfect, as the dog, the young of these creatures

being generally blind. Why this happens and why they produce many young must be stated later, but in them

Nature has made an advance towards the production of monstrosities in that what they generate, being

imperfect, is so far unlike the parent; now monstrosities also belong to the class of things unlike the parent.

Therefore this accident also often invades animals of such a nature. So, too, it is in these that the socalled

'metachoera' are most frequent, and the condition of these also is in a way monstrous, since both deficiency

and excess are monstrous. For the monstrosity belongs to the class of things contrary to Nature, not any and

every kind of Nature, but Nature in her usual operations; nothing can happen contrary to Nature considered as

eternal and necessary, but we speak of things being contrary to her in those cases where things generally

happen in a certain way but may also happen in another way. In fact, even in the case of monstrosities,

whenever things occur contrary indeed to the established order but still always in a certain way and not at

random, the result seems to be less of a monstrosity because even that which is contrary to Nature is in a

certain sense according to Nature, whenever, that is, the formal nature has not mastered the material nature.

Therefore they do not call such things monstrosities any more than in the other cases where a phenomenon

occurs habitually, as in fruits; for instance, there is a vine which some call 'capneos'; if this bear black grapes

they do not judge it a monstrosity because it is in the habit of doing this very often. The reason is that it is in

its nature intermediate between white and black; thus the change is not a violent one nor, so to say, contrary

to Nature; at least, is it not a change into another nature. But in animals producing many young not only do

the same phenomena occur, but also the numerous embryos hinder one another from becoming perfect and

A difficulty may be raised concerning (1) the production of many young and the multiplication of the parts in

a single young one, and (2) the production of few young or only one and the deficiency of the parts.

Sometimes animals are born with too many toes, sometimes with one alone, and so on with the other parts,

for they may be multiplied or they may be absent. Again, they may have the generative parts doubled, the one

being male, the other female; this is known in men and especially in goats. For what are called 'tragaenae' are

such because they have both male and female generative parts; there is a case also of a goat being born with a

horn upon its leg. Changes and deficiencies are found also in the internal parts, animals either not possessing

some at all, or possessing them in a rudimentary condition, or too numerous or in the wrong place. No

animal, indeed, has ever been born without a heart, but they are born without a spleen or with two spleens or

with one kidney; there is no case again of total absence of the liver, but there are cases of its being

incomplete. And all these phenomena have been seen in animals perfect and alive. Animals also which

naturally have a gallbladder are found without one; others are found to have more than one. Cases are

known, too, of the organs changing places, the liver being on the left, the spleen on the right. These

phenomena have been observed, as stated above, in animals whose growth is perfected; at the time of birth

great confusion of every kind has been found. Those deficiency which only depart a little from Nature

commonly live; not so those which depart further, when the unnatural condition is in the parts which are

The question then about all these cases is this. Are we to suppose that a single cause is responsible for the

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

In the first place it seems only reasonable to wonder why some animals produce many young, others only

one. For it is the largest animals that produce one, e.g. the elephant, camel, horse, and the other solidhoofed

ungulates; of these some are larger than all other animals, while the others are of a remarkable size. But the

dog, the wolf, and practically all the fissipeds, produce many, even the small members of the class, as the

mouse family. The clovenfooted animals again produce few, except the pig, which belongs to those that

produce many. This certainly seems surprising, for we should expect the large animals to be able to generate

more young and to secrete more semen. But precisely what we wonder at is the reason for not wondering; it is

just because of their size that they do not produce many young, for the nutriment is expended in such animals

upon increasing the body. But in the smaller animals Nature takes away from the size and adds the excess so

gained to the seminal secretion. Moreover, more semen must needs be used in generation by the larger

animal, and little by the smaller. Therefore many small ones may be produced together, but it is hard for

many large ones to be so, and to those intermediate in size Nature has assigned the intermediate number. We

have formerly given the reason why some animals are large, some smaller, and some between the two, and

speaking generally, with regard to the number of young produced, the solidhoofed produce one, the

clovenfooted few, the manytoed many. (The reason of this is that, generally speaking, their sizes

correspond to this difference.) It is not so, however, in all cases; for it is the largeness and smallness of the

body that is cause of few or many young being born, not the fact that the kind of animal has one, two, or

many toes. A proof of this is that the elephant is the largest of animals and yet is manytoed, and the camel,

the next largest, is clovenfooted. And not only in animals that walk but also in those that fly or swim the

large ones produce few, the small many, for the same reason. In like manner also it is not the largest plants

We have explained then why some animals naturally produce many young, some but few, and some only one;

in the difficulty now stated we may rather be surprised with reason at those which produce many, since such

animals are often seen to conceive from a single copulation. Whether the semen of the male contributes to the

material of the embryo by itself becoming a part of it and mixing with the semen of the female, or whether, as

we say, it does not act in this way but brings together and fashions the material within the female and the

generative secretion as the figjuice does the liquid substance of milk, what is the reason why it does not

form a single animal of considerable size? For certainly in the parallel case the figjuice is not separated if it

has to curdle a large quantity of milk, but the more the milk and the more the figjuice put into it, so much

the greater is the curdled mass. Now it is no use to say that the several regions of the uterus attract the semen

and therefore more young than one are formed, because the regions are many and the cotyledons are more

than one. For two embryos are often formed in the same region of the uterus, and they may be seen lying in a

row in animals that produce many, when the uterus is filled with the embryos. (This is plain from the

dissections.) Rather the truth is this. As animals complete their growth there are certain limits to their size,

both upwards and downwards, beyond which they cannot go, but it is in the space between these limits that

they exceed or fall short of one another in size, and it is within these limits that one man (or any other animal)

is larger or smaller than another. So also the generative material from which each animal is formed is not

without a quantitative limit in both directions, nor can it be formed from any quantity you please. Whenever

then an animal, for the cause assigned, discharges more of the female secretion than is needed for beginning

the existence of a single animal, it is not possible that only one should be formed out of all this, but a number

limited by the appropriate size in each case; nor will the semen of the male, or the power residing in the

semen, form anything either more or less than what is according to Nature. In like manner, if the male emits

more semen than is necessary, or more powers in different parts of the semen as it is divided, however much

it is it will not make anything greater; on the contrary it will dry up the material of the female and destroy it.

So fire also does not continue to make water hotter in proportion as it is itself increased, but there is a fixed

limit to the heat of which water is capable; if that is once reached and the fire is then increased, the water no

longer gets hotter but rather evaporates and at last disappears and is dried up. Now since it appears that the

secretion of the female and that from the male need to stand in some proportionate relation to one another (I

mean in animals of which the male emits semen), what happens in those that produce many young is this:

from the very first the semen emitted by the male has power, being divided, to form several embryos, and the

material contributed by the female is so much that several can be formed out of it. (The parallel of curdling

milk, which we spoke of before, is no longer in point here, for what is formed by the heat of the semen is not

only of a certain quantity but also of a certain quality, whereas with figjuice and rennet quantity alone is

concerned.) This then is just the reason why in such animals the embryos formed are numerous and do not all

unite into one whole; it is because an embryo is not formed out of any quantity you please, but whether there

is too much or too little, in either case there will be no result, for there is a limit set alike to the power of the

On the same principle many embryos are not formed, though the secretion is much, in the large animals

which produce only one young one, for in them also both the material and that which works upon it are of a

certain quantity. So then they do not secrete such material in too great quantity for the reason previously

stated, and what they do secrete is naturally just enough for one embryo alone to be formed from it. If ever

too much is secreted, then twins are born. Hence such cases seem to be more portentous, because they are

Man belongs to all three classes, for he produces one only and sometimes many or few, though naturally he

almost always produces one. Because of the moisture and heat of his body he may produce many [for semen

is naturally fluid and hot], but because of his size he produces few or one. On account of this it results that in

man alone among animals the period of gestation is irregular; whereas the period is fixed in the rest, there are

several periods in man, for children are born at seven months and at ten months and at the times between, for

even those of eight months do live though less often than the rest. The reason may be gathered from what has

just been said, and the question has been discussed in the Problems. Let this explanation suffice for these

The cause why the parts may be multiplied contrary to Nature is the same as the cause of the birth of twins.

For the reason exists already in the embryo, whenever it aggregates more material at any point of itself than is

required by the nature of the part. The result is then that either one of its parts is larger than the others, as a

finger or hand or foot or any of the other extremities or limbs; or again if the embryo is cleft there may come

into being more than one such part, as eddies do in rivers; as the water in these is carried along with a certain

motion, if it dash against anything two systems or eddies come into being out of one, each retaining the same

motion; the same thing happens also with the embryos. The abnormal parts generally are attached near those

they resemble, but sometimes at a distance because of the movement taking place in the embryo, and

especially because of the excess of material returning to that place whence it was taken away while retaining

In certain cases we find a double set of generative organs [one male and the other female]. When such

duplication occurs the one is always functional but not the other, because it is always insufficiently supplied

with nourishment as being contrary to Nature; it is attached like a growth (for such growths also receive

nourishment though they are a later development than the body proper and contrary to Nature.) If the

formative power prevails, both are similar; if it is altogether vanquished, both are similar; but if it prevail here

and be vanquished there, then the one is female and the other male. (For whether we consider the reason why

the whole animal is male or female, or why the parts are so, makes no difference.)

When we meet with deficiency in such parts, e.g. an extremity or one of the other members, we must assume

the same cause as when the embryo is altogether aborted (abortion of embryos happens frequently).

Outgrowths differ from the production of many young in the manner stated before; monsters differ from these

in that most of them are due to embryos growing together. Some however are also of the following kind,

when the monstrosity affects greater and more sovereign parts, as for instance some monsters have two

spleens or more than two kidneys. Further, the parts may migrate, the movements which form the embryo

being diverted and the material changing its place. We must decide whether the monstrous animal is one or is

composed of several grown together by considering the vital principle; thus, if the heart is a part of such a

kind then that which has one heart will be one animal, the multiplied parts being mere outgrowths, but those

which have more than one heart will be two animals grown together through their embryos having been

It also often happens even in many animals that do not seem to be defective and whose growth is now

complete, that some of their passages may have grown together or others may have been diverted from the

normal course. Thus in some women before now the os uteri has remained closed, so that when the time for

the catamenia has arrived pain has attacked them, till either the passage has burst open of its own accord or

the physicians have removed the impediment; some such cases have ended in death if the rupture has been

made too violently or if it has been impossible to make it at all. In some boys on the other hand the end of the

penis has not coincided with the end of the passage where the urine is voided, but the passage has ended

below, so that they crouch sitting to void it, and if the testes are drawn up they appear from a distance to have

both male and female generative organs. The passage of the solid food also has been closed before now in

sheep and some other animals; there was a cow in Perinthus which passed fine matter, as if it were sifted,

through the bladder, and when the anus was cut open it quickly closed up again nor could they succeed in

We have now spoken of the production of few and many young, and of the outgrowth of superfluous parts or

Superfoetation does not occur at all in some animals but does in others; of the former some are able to bring

the later formed embryo to birth, while others can only do so sometimes. The reason why it does not occur in

some is that they produce only one young one, for it is not found in solidhoofed animals and those larger

than these, as owing to their size the secretion of the female is all used up for the one embryo. For all these

have large bodies, and when an animal is large its foetus is large in proportion, e.g. the foetus of the elephant

is as big as a calf. But superfoetation occurs in those which produce many young because the production of

more than one at a birth is itself a sort of superfoetation, one being added to another. Of these all that are

large, as man, bring to birth the later embryo, if the second impregnation takes place soon after the first, for

such an event has been observed before now. The reason is that given above, for even in a single act of

intercourse the semen discharged is more than enough for one embryo, and this being divided causes more

than one child to be born, the one of which is later than the other. But when the embryo has already grown to

some size and it so happens that copulation occurs again, superfoetation sometimes takes place, but rarely,

since the uterus generally closes in women during the period of gestation. If this ever happens (for this also

has occurred) the mother cannot bring the second embryo to perfection, but it is cast out in a state like what

are called abortions. For just as, in those animals that bear only one, all the secretion of the female is

converted to the first formed embryo because of its size, so it is here also; the only difference is that in the

former case this happens at once, in the latter when the foetus has attained to some size, for then they are in

the same state as those that bear only one. In like manner, since man naturally would produce many young,

and since the size of the uterus and the quantity of the female secretion are both greater than is necessary for

one embryo, only not so much so as to bring to birth a second, therefore women and mares are the only

animals which admit the male during gestation, the former for the reason stated, and mares both because of

the barrenness of their nature and because their uterus is of superfluous size, too large for one but too small to

allow a second embryo to be brought to perfection by superfoetation. And the mare is naturally inclined to

sexual intercourse because she is in the same case as the barren among women; these latter are barren

because they have no monthly discharge (which corresponds to the act of intercourse in males) and mares

have exceedingly little. And in all the vivipara the barren females are so inclined, because they resemble the

males when the semen has collected in the testes but is not being got rid of. For the discharge of the

catamenia is in females a sort of emission of semen, they being unconcocted semen as has been said before.

Hence it is that those women also who are incontinent in regard to such intercourse cease from their passion

for it when they have borne many children, for, the seminal secretion being then drained off, they no longer

desire this intercourse. And among birds the hens are less disposed that way than the cocks, because the

uterus of the henbird is up near the hypozoma; but with the cockbirds it is the other way, for their testes are

drawn up within them, so that, if any kind of such birds has much semen naturally, it is always in need of this

intercourse. In females then it encourages copulation to have the uterus low down, but in males to have the

It has been now stated why superfoetation is not found in some animals at all, why it is found in others which

sometimes bring the later embryos to birth and sometimes not, and why some such animals are inclined to

Some of those animals in which superfoetation occurs can bring the embryos to birth even if a long time

elapses between the two impregnations, if their kind is spermatic, if their body is not of a large size, and if

they bear many young. For because they bear many their uterus is spacious, because they are spermatic the

generative discharge is copious, and because the body is not large but the discharge is excessive and in

greater measure than is required for the nourishment wanted for the embryo, therefore they can not only form

animals but also bring them to birth later on. Further, the uterus in such animals does not close up during

gestation because there is a quantity of the residual discharge left over. This has happened before now even in

women, for in some of them the discharge continues during all the time of pregnancy. In women, however,

this is contrary to Nature, so that the embryo suffers, but in such animals it is according to Nature, for their

body is so formed from the beginning, as with hares. For superfoetation occurs in these animals, since they

are not large and they bear many young (for they have many toes and the manytoed animals bear many),

and they are spermatic. This is shown by their hairiness, for the quantity of their hair is excessive, these

animals alone having hair under the feet and within the jaws. Now hairiness is a sign of abundance of residual

matter, wherefore among men also the hairy are given to sexual intercourse and have much semen rather than

the smooth. In the hare it often happens that some of the embryos are imperfect while others of its young are

Some of the vivipara produce their young imperfect, others perfect; the onehoofed and clovenfooted

perfect, most of the manytoed imperfect. The reason of this is that the onehoofed produce one young one,

and the clovenfooted either one or two generally speaking; now it is easy to bring the few to perfection. All

the manytoed animals that bear their young imperfect give birth to many. Hence, though they are able to

nourish the embryos while newly formed, their bodies are unable to complete the process when the embryos

have grown and acquired some size. So they produce them imperfect, like those animals which generate a

scolex, for some of them when born are scarcely brought into form at all, as the fox, bear, and lion, and some

of the rest in like manner; and nearly all of them are blind, as not only the animals mentioned but also the

dog, wolf, and jackal. The pig alone produces both many and perfect young, and thus here alone we find any

overlapping; it produces many as do the manytoed animals, but is clovenfooted or solidhoofed (for there

certainly are solidhoofed swine). They bear, then, many young because the nutriment which would

otherwise go to increase their size is diverted to the generative secretion (for considered as a solidhoofed

animal the pig is not a large one), and also it is more often clovenhoofed, striving as it were with the nature

of the solidhoofed animals. For this reason it produces sometimes only one, sometimes two, but generally

many, and brings them to perfection before birth because of the good condition of its body, being like a rich

The young of some birds also are hatched imperfect, that is to say blind; this applies to all small birds which

lay many eggs, as crows and rooks, jays, sparrows, swallows, and to all those which lay few eggs without

producing abundant nourishment along with the young, as ringdoves, turtledoves, and pigeons. Hence if

the eyes of swallows while still young be put out they recover their sight again, for the birds are still

developing, not yet developed, when the injury is inflicted, so that the eyes grow and sprout afresh. And in

general the production of young before they are perfect is owing to inability to continue nourishing them, and

they are born imperfect because they are born too soon. This is plain also with sevenmonths children, for

since they are not perfected it often happens that even the passages, e.g. of the ears and nostrils, are not yet

opened in some of them at birth, but only open later as they are growing, and many such infants survive.

In man males are more often born defective than females, but in the other animals this is not the case. The

reason is that in man the male is much superior to the female in natural heat, and so the male foetus moves

about more than the female, and on account of moving is more liable to injury, for what is young is easily

injured since it is weak. For this same reason also the female foetus is not perfected equally with the male in

man (but they are so in the other animals, for in them the female is not later in developing than the male). For

while within the mother the female takes longer in developing, but after birth everything is perfected more

quickly in females than in males; I mean, for instance, puberty, the prime of life, and old age. For females are

weaker and colder in nature, and we must look upon the female character as being a sort of natural

deficiency. Accordingly while it is within the mother it develops slowly because of its coldness (for

development is concoction, and it is heat that concocts, and what is hotter is easily concocted); but after birth

it quickly arrives at maturity and old age on account of its weakness, for all inferior things come sooner to

their perfection or end, and as this is true of works of art so it is of what is formed by Nature. For the reason

just given also twins are less likely to survive in man if one be male and one female, but this is not at all so in

the other animals; for in man it is contrary to Nature that they should run an equal course, as their

development does not take place in equal periods, but the male must needs be too late or the female too early;

in the other animals, however, it is not contrary to Nature. A difference is also found between man and the

other animals in respect of gestation, for animals are in better bodily condition most of the time, whereas in

most women gestation is attended with discomfort. Their way of life is partly responsible for this, for being

sedentary they are full of more residual matter; among nations where the women live a laborious life

gestation is not equally conspicuous and those who are accustomed to work bear children easily both there

and elsewhere; for work consumes the residual matter, but those who are sedentary have a great deal of it in

them because not only is there no monthly discharge during pregnancy but also they do no work; therefore

their travail is painful. But work exercises them so that they can hold their breath, upon which depends the

ease or difficulty of childbirth. These circumstances then, as we have said, contribute to cause the difference

between women and the other animals in this state, but the most important thing is this: in some animals the

discharge corresponding to the catamenia is but small, and in some not visible at all, but in women it is

greater than in any other animal, so that when this discharge ceases owing to pregnancy they are troubled (for

if they are not pregnant they are afflicted with ailments whenever the catamenia do not occur); and they are

more troubled as a rule at the beginning of pregnancy, for the embryo is able indeed to stop the catamenia but

is too small at first to consume any quantity of the secretion; later on it takes up some of it and so alleviates

the mother. In the other animals, on the contrary, the residual matter is but small and so corresponds with the

growth of the foetus, and as the secretions which hinder nourishment are being consumed by the foetus the

mother is in better bodily condition than usual. The same holds good also with aquatic animals and birds. If it

ever happens that the body of the mother is no longer in good condition when the foetus is now becoming

large, the reason is that its growth needs more nourishment than the residual matter supplies. (In some few

women it happens that the body is in a better state during pregnancy; these are women in whose body the

residual matter is small so that it is all used up along with the nourishment that goes to the foetus.)

We must also speak of what is known as mola uteri, which occurs rarely in women but still is found

sometimes during pregnancy. For they produce what is called a mola; it has happened before now to a

woman, after she had had intercourse with her husband and supposed she had conceived, that at first the size

of her belly increased and everything else happened accordingly, but yet when the time for birth came on, she

neither bore a child nor was her size reduced, but she continued thus for three or four years until dysentery

came on, endangering her life, and she produced a lump of flesh which is called mola. Moreover this

condition may continue till old age and death. Such masses when expelled from the body become so hard that

they can hardly be cut through even by iron. Concerning the cause of this phenomenon we have spoken in the

Problems; the same thing happens to the embryo in the womb as to meats half cooked in roasting, and it is

not due to heat, as some say, but rather to the weakness of the maternal heat. (For their nature seems to be

incapable, and unable to perfect or to put the last touches to the process of generation. Hence it is that the

mola remains in them till old age or at any rate for a long time, for in its nature it is neither perfect nor

altogether a foreign body.) It is want of concoction that is the reason of its hardness, as with halfcooked

A difficulty is raised as to why this does not occur in other animals, unless indeed it does occur and has

entirely escaped observation. We must suppose the reason to be that woman alone among animals is subject

to troubles of the uterus, and alone has a superfluous amount of catamenia and is unable to concoct them;

when, then, the embryo has been formed of a liquid hard to concoct, then comes the socalled mola into

being, and this happens naturally in women alone or at any rate more than in other animals.

Milk is formed in the females of all internally viviparous animals, becoming useful for the time of birth. For

Nature has made it for the sake of the nourishment of animals after birth, so that it may neither fail at this

time at all nor yet be at all superfluous; this is just what we find happening, unless anything chance contrary

to Nature. In the other animals the period of gestation does not vary, and so the milk is concocted in time to

suit this moment, but in man, since there are several times of birth, it must be ready at the first of these; hence

in women the milk is useless before the seventh month and only then becomes useful. That it is only

concocted at the last stages is what we should expect to happen also as being due to a necessary cause. For at

first such residual matter when secreted is used up for the development of the embryo; now the nutritious part

in all things is the sweetest and the most concocted, and thus when all such elements are removed what

remains must become of necessity bitter and illflavoured. As the embryo is perfecting, the residual matter

left over increases in quantity because the part consumed by the embryo is less; it is also sweeter since the

easily concocted part is less drawn away from it. For it is no longer expended on moulding the embryo but

only on slightly increasing its growth, it being now fixed because it has reached perfection (for in a sense

there is a perfection even of an embryo). Therefore it comes forth from the mother and changes its mode of

development, as now possessing what belongs to it; and no longer takes that which does not belong to it; and

The milk collects in the upper part of the body and the breasts because of the original plan of the organism.

For the part above the hypozoma is the sovereign part of the animal, while that below is concerned with

nourishment and residual matter, in order that all animals which move about may contain within themselves

nourishment enough to make them independent when they move from one place to another. From this upper

part also is produced the generative secretion for the reason mentioned in the opening of our discussion. But

both the secretion of the male and the catamenia of the female are of a sanguineous nature, and the first

principle of this blood and of the bloodvessels is the heart, and the heart is in this part of the body.

Therefore it is here that the change of such a secretion must first become plain. This is why the voice changes

in both sexes when they begin to bear seed (for the first principle of the voice resides there, and is itself

changed when its moving cause changes). At the same time the parts about the breasts are raised visibly even

in males but still more in females, for the region of the breasts becomes empty and spongy in them because so

much material is drained away below. This is so not only in women but also in those animals which have the

This change in the voice and the parts about the mammae is plain even in other creatures to those who have

experience of each kind of animal, but is most remarkable in man. The reason is that in man the production of

secretion is greatest in both sexes in proportion to their size as compared with other animals; I mean that of

the catamenia in women and the emission of semen in men. When, therefore, the embryo no longer takes up

the secretion in question but yet prevents its being discharged from the mother, it is necessary that the

residual matter should collect in all those empty parts which are set upon the same passages. And such is the

position of the mammae in each kind of animals for both causes; it is so both for the sake of what is best and

It is here, then, that the nourishment in animals is now formed and becomes thoroughly concocted. As for the

cause of concoction, we may take that already given, or we may take the opposite, for it is a reasonable view

also that the embryo being larger takes more nourishment, so that less is left over about this time, and the less

That milk has the same nature as the secretion from which each animal is formed is plain, and has been stated

previously. For the material which nourishes is the same as that from which Nature forms the animal in

generation. Now this is the sanguineous liquid in the sanguinea, and milk is blood concocted (not corrupted;

Empedocles either mistook the fact or made a bad metaphor when he composed the line: 'On the tenth day of

the eighth month the milk comes into being, a white pus', for putrefaction and concoction are opposite things,

and pus is a kind of putrefaction but milk is concocted). While women are suckling children the catamenia do

not occur according to Nature, nor do they conceive; if they do conceive, the milk dries up. This is because

the nature of the milk and of the catamenia is the same, and Nature cannot be so productive as to supply both

at once; if the secretion is diverted in the one direction it must needs cease in the other, unless some violence

is done contrary to the general rule. But this is as much as to say that it is contrary to Nature, for in all cases

where it is not impossible for things to be otherwise than they generally are but where they may so happen,

The time also at which the young animal is born has been well arranged. For when the nourishment coming

through the umbilical cord is no longer sufficient for the foetus because of its size, then at the same time the

milk becomes useful for the nourishment of the newlyborn animal, and the bloodvessels round which the

socalled umbilical cord lies as a coat collapse as the nourishment is no longer passing through it; for these

The natural birth of all animals is headforemost, because the parts above the umbilical cord are larger than

those below. The body then, being suspended from the cord as in a balance, inclines towards the heavy end,

The period of gestation is, as a matter of fact, determined generally in each animal in proportion to the length

of its life. This we should expect, for it is reasonable that the development of the longlived animals should

take a longer time. Yet this is not the cause of it, but the periods only correspond accidentally for the most

part; for though the larger and more perfect sanguinea do live a long time, yet the larger are not all

longerlived. Man lives a longer time than any animal of which we have any credible experience except the

elephant, and yet the human kind is smaller than that of the bushytailed animals and many others. The real

cause of long life in any animal is its being tempered in a manner resembling the environing air, along with

certain other circumstances of its nature, of which we will speak later; but the cause of the time of gestation is

the size of the offspring. For it is not easy for large masses to arrive at their perfection in a small time,

whether they be animals or, one may say, anything else whatever. That is why horses and animals akin to

them, though living a shorter time than man, yet carry their young longer; for the time in the former is a year,

but in the latter ten months at the outside. For the same reason also the time is long in elephants; they carry

We find, as we might expect, that in all animals the time of gestation and development and the length of life

aims at being measured by naturally complete periods. By a natural period I mean, e.g. a day and night, a

month, a year, and the greater times measured by these, and also the periods of the moon, that is to say, the

full moon and her disappearance and the halves of the times between these, for it is by these that the moon's

The moon is a first principle because of her connexion with the sun and her participation in his light, being as

it were a second smaller sun, and therefore she contributes to all generation and development. For heat and

cold varying within certain limits make things to come into being and after this to perish, and it is the motions

of the sun and moon that fix the limit both of the beginning and of the end of these processes. Just as we see

the sea and all bodies of water settling and changing according to the movement or rest of the winds, and the

air and winds again according to the course of the sun and moon, so also the things which grow out of these

or are in these must needs follow suit. For it is reasonable that the periods of the less important should follow

those of the more important. For in a sense a wind, too, has a life and birth and death.

As for the revolutions of the sun and moon, they may perhaps depend on other principles. It is the aim, then,

of Nature to measure the coming into being and the end of animals by the measure of these higher periods,

but she does not bring this to pass accurately because matter cannot be easily brought under rule and because

there are many principles which hinder generation and decay from being according to Nature, and often cause

We have now spoken of the nourishment of animals within the mother and of their birth into the world, both

WE must now investigate the qualities by which the parts of animals differ. I mean such qualities of the parts

as blueness and blackness in the eyes, height and depth of pitch in the voice, and differences in colour

whether of the skin or of hair and feathers. Some such qualities are found to characterize the whole of a kind

of animals sometimes, while in other kinds they occur at random, as is especially the case in man. Further, in

connexion with the changes in the time of life, all animals are alike in some points, but are opposed in others

as in the case of the voice and the colour of the hair, for some do not grow grey visibly in old age, while man

is subject to this more than any other animal. And some of these affections appear immediately after birth,

Now we must no longer suppose that the cause of these and all such phenomena is the same. For whenever

things are not the product of Nature working upon the animal kingdom as a whole, nor yet characteristic of

each separate kind, then none of these things is such as it is or is so developed for any final cause. The eye for

instance exists for a final cause, but it is not blue for a final cause unless this condition be characteristic of the

kind of animal. In fact in some cases this condition has no connexion with the essence of the animal's being,

but we must refer the causes to the material and the motive principle or efficient cause, on the view that these

things come into being by Necessity. For, as was said originally in the outset of our discussion, when we are

dealing with definite and ordered products of Nature, we must not say that each is of a certain quality because

it becomes so, but rather that they become so and so because they are so and so, for the process of Becoming

The ancient Naturephilosophers however took the opposite view. The reason of this is that they did not see

that the causes were numerous, but only saw the material and efficient and did not distinguish even these,

Everything then exists for a final cause, and all those things which are included in the definition of each

animal, or which either are means to an end or are ends in themselves, come into being both through this

cause and the rest. But when we come to those things which come into being without falling under the heads

just mentioned, their course must be sought in the movement or process of coming into being, on the view

that the differences which mark them arise in the actual formation of the animal. An eye, for instance, the

animal must have of necessity (for the fundamental idea of the animal is of such a kind), but it will have an

eye of a particular kind of necessity in another sense, not the sense mentioned just above, because it is its

These distinctions being drawn let us speak of what comes next in order. As soon then as the offspring of all

animals are born, especially those born imperfect, they are in the habit of sleeping, because they continue

sleeping also within the mother when they first acquire sensation. But there is a difficulty about the earliest

period of development, whether the state of wakefulness exists in animals first, or that of sleep. Since they

plainly wake up more as they grow older, it is reasonable to suppose that the opposite state, that of sleep,

exists in the first stages of development. Moreover the change from not being to being must pass through the

intermediate condition, and sleep seems to be in its nature such a condition, being as it were a boundary

between living and not living, and the sleeper being neither altogether nonexistent nor yet existent. For life

most of all appertains to wakefulness, on account of sensation. But on the other hand, if it is necessary that

the animal should have sensation and if it is then first an animal when it has acquired sensation, we ought to

consider the original condition to be not sleep but only something resembling sleep, such a condition as we

find also in plants, for indeed at this time animals do actually live the life of a plant. But it is impossible that

plants should sleep, for there is no sleep which cannot be broken, and the condition in plants which is

It is necessary then for the embryo animal to sleep most of the time because the growth takes place in the

upper part of the body, which is consequently heavier (and we have stated elsewhere that such is the cause of

sleep). But nevertheless they are found to wake even in the womb (this is clear in dissections and in the

ovipara), and then they immediately fall into a sleep again. This is why after birth also they spend most of

When awake infants do not laugh, but while asleep they both laugh and cry. For animals have sensations even

while asleep, not only what are called dreams but also others besides dreams, as those persons who arise

while sleeping and do many things without dreaming. For there are some who get up while sleeping and walk

about seeing just like those who are awake; these have perception of what is happening, and though they are

not awake, yet this perception is not like a dream. So infants presumably have senseperception and live in

their sleep owing to previous habit, being as it were without knowledge of the waking state. As time goes on

and their growth is transferred to the lower part of the body, they now wake up more and spend most of their

time in that condition. Children continue asleep at first more than other animals, for they are born in a more

imperfect condition than other animals that are produced in anything like a perfect state, and their growth has

The eyes of all children are bluish immediately after birth; later on they change to the colour which is to be

theirs permanently. But in the case of other animals this is not visible. The reason of this is that the eyes of

other animals are more apt to have only one colour for each kind of animal; e.g. cattle are darkeyed, the eye

of all sheep is pale, of others again the whole kind is blue or greyeyed, and some are yellow (goateyed), as

the majority of goats themselves, whereas the eyes of men happen to be of many colours, for they are blue or

grey or dark in some cases and yellow in others. Hence, as the individuals in other kinds of animals do not

differ from one another in the colour, so neither do they differ from themselves, for they are not of a nature to

have more than one colour. Of the other animals the horse has the greatest variety of colour in the eye, for

some of them are actually heteroglaucous; this phenomenon is not to be seen in any of the other animals, but

Why then is it that there is no visible change in the other animals if we compare their condition when newly

born with their condition at a more advanced age, but that there is such a change in children? We must

consider just this to be a sufficient cause, that the part concerned has only one colour in the former but several

colours in the latter. And the reason why the eyes of infants are bluish and have no other colour is that the

We must also gain a general notion about the difference in eyes, for what reason some are blue, some grey,

some yellow, and some dark. To suppose that the blue are fiery, as Empedocles says, while the dark have

more water than fire in them, and that this is why the former, the blue, have not keen sight by day, viz. owing

to deficiency of water in their composition, and the latter are in like condition by night, viz. owing to

deficiency of fire this is not well said if indeed we are to assume sight to be connected with water, not fire,

in all cases. Moreover it is possible to render another account of the cause of the colours, but if indeed the

fact is as was stated before in the treatise on the senses, and still earlier than that in the investigations

concerning soul if this sense organ is composed of water and if we were right in saying for what reason it is

composed of water and not of air or fire then we must assume the water to be the cause of the colours

mentioned. For some eyes have too much liquid to be adapted to the movement, others have too little, others

the due amount. Those eyes therefore in which there is much liquid are dark because much liquid is not

transparent, those which have little are blue; (so we find in the sea that the transparent part of it appears light

blue, the less transparent watery, and the unfathomable water is dark or deepblue on account of its depth).

We must suppose the same cause also to be responsible for the fact that blue eyes are not keensighted by

day nor dark eyes by night. Blue eyes, because there is little liquid in them, are too much moved by the light

and by visible objects in respect of their liquidity as well as their transparency, but sight is the movement of

this part in so far as it is transparent, not in so far as it is liquid. Dark eyes are less moved because of the

quantity of liquid in them. And so they see less well in the dusk, for the nocturnal light is weak; at the same

time also liquid is in general hard to move in the night. But if the eye is to see, it must neither not be moved

at all nor yet more than in so far as it is transparent, for the stronger movement drives out the weaker. Hence

it is that on changing from strong colours, or on going out of the sun into the dark, men cannot see, for the

motion already existing in the eye, being strong, stops that from outside, and in general neither a strong nor a

weak sight can see bright things because the liquid is acted upon and moved too much.

The same thing is shown also by the morbid affections of each kind of sight. Cataract attacks the blueeyed

more, but what is called 'nyctalopia' the darkeyed. Now cataract is a sort of dryness of the eyes and therefore

it is found more in the aged, for this part also like the rest of the body gets dry towards old age; but is an

excess of liquidity and so is found more in the younger, for their brain is more liquid.

The sight of the eye which is intermediate between too much and too little liquid is the best, for it has neither

too little so as to be disturbed and hinder the movement of the colours, nor too much so as to cause difficulty

Not only the abovementioned facts are causes of seeing keenly or the reverse, but also the nature of the skin

upon what is called the pupil. This ought to be transparent, and it is necessary that the transparent should be

thin and white and even, thin that the movement coming from without may pass straight through it, even that

it may not cast a shade the liquid behind it by wrinkling (for this also is a reason why old men have not keen

sight, the skin of the eye like the rest of the skin wrinkling and becoming thicker in old age), and white

because black is not transparent, for that is just what is meant by 'black', what is not shone through, and that is

why lanterns cannot give light if they be made of black skin. It is for these reasons then that the sight is not

keen in old age nor in the diseases in question, but it is because of the small amount of liquid that the eyes of

And the reason why men especially and horses occasionally are heteroglaucous is the same as the reason why

man alone grows grey and the horse is the only other animal whose hairs whiten visibly in old age. For

greyness is a weakness of the fluid in the brain and an incapacity to concoct properly, and so is blueness of

the eyes; excess of thinness or of thickness produces the same effect, according as this liquidity is too little or

too much. Whenever then Nature cannot make the eyes correspond exactly, either by concocting or by not

concocting the liquid in both, but concocts the one and not the other, then the result is heteroglaucia.

The cause of some animals being keensighted and others not so is not simple but double. For the word 'keen'

has pretty much a double sense (and this is the case in like manner with hearing and smelling). In one sense

keen sight means the power of seeing at a distance, in another it means the power of distinguishing as

accurately as possible the objects seen. These two faculties are not necessarily combined in the same

individual. For the same person, if he shades his eyes with his hand or look through a tube, does not

distinguish the differences of colour either more or less in any way, but he will see further; in fact, men in pits

or wells sometimes see the stars. Therefore if any animal's brows project far over the eye, but if the liquid in

the pupil is not pure nor suited to the movement coming from external objects and if the skin over the surface

is not thin, this animal will not distinguish accurately the differences of the colours but it will be able to see

from a long distance (just as it can from a short one) better than those in which the liquid and the covering

membrane are pure but which have no brows projecting over the eyes. For the cause of seeing keenly in the

sense of distinguishing the differences is in the eye itself; as on a clean garment even small stains are visible,

so also in a pure sight even small movements are plain and cause sensation. But it is the position of the eyes

that is the cause of seeing things far off and of the movements in the transparent medium coming to the eyes

from distant objects. A proof of this is that animals with prominent eyes do not see well at a distance,

whereas those which have their eyes lying deep in the head can see things at a distance because the

movement is not dispersed in space but comes straight to the eye. For it makes no difference whether we say,

as some do, that seeing is caused by the sight going forth from the eye on that view, if there is nothing

projecting over the eyes, the sight must be scattered and so less of it will fall on the objects of vision and

things at a distance will not be seen so well or whether we say that seeing is due to the movement coming

from the objects; for the sight also must see, in a manner resembling the movement. Things at a distance,

then, would be seen best if there were, so to say, a continuous tube straight from the sight to its object, for the

movement from the object would not then be dissipated; but, if that is impossible, still the further the tube

It is the same also with hearing and smell; to hear and smell accurately mean in one sense to perceive as

precisely as possible all the distinctions of the objects of perception, in another sense to hear and smell far

off. As with sight, so here the senseorgan is the cause of judging well the distinctions, if both that organ

itself and the membrane round it be pure. For the passages of all the senseorgans, as has been said in the

treatise on sensation, run to the heart, or to its analogue in creatures that have no heart. The passage of the

hearing, then, since this senseorgan is of air, ends at the place where the innate spiritus causes in some

animals the pulsation of the heart and in others respiration; wherefore also it is that we are able to understand

what is said and repeat what we have heard, for as was the movement which entered through the

senseorgan, such again is the movement which is caused by means of the voice, being as it were of one and

the same stamp, so that a man can say what he has heard. And we hear less well during a yawn or expiration

than during inspiration, because the startingpoint of the senseorgan of hearing is set upon the part

concerned with breathing and is shaken and moved as the organ moves the breath, for while setting the breath

in motion it is moved itself. The same thing happens in wet weather or a damp atmosphere.... And the ears

seemed to be filled with air because their startingpoint is near the region of breathing.

Accuracy then in judging the differences of sounds and smells depends on the purity of the senseorgan and

of the membrane lying upon its surface, for then all the movements become clear in such cases, as in the case

of sight. Perception and nonperception at a distance also depend on the same things with hearing and smell

as with sight. For those animals can perceive at a distance which have channels, so to say, running through

the parts concerned and projecting far in front of the senseorgans. Therefore all animals whose nostrils are

long, as the Laconian hounds, are keenscented, for the senseorgan being above them, the movements from

a distance are not dissipated but go straight to the mark, just as the movements which cause sight do with

Similar is the case of animals whose ears are long and project far like the eaves of a house, as in some

quadrupeds, with the internal spiral passage long; these also catch the movement from afar and pass it on to

In respect of senseperception at a distance, man is, one may say, the worst of all animals in proportion to his

size, but in respect of judging the differences of quality in the objects he is the best of all. The reason is that

the senseorgan in man is pure and least earthy and material, and he is by nature the thinnestskinned of all

The workmanship of Nature is admirable also in the seal, for though a viviparous quadruped it has no ears but

only passages for hearing. This is because its life is passed in the water; now the ear is a part added to the

passages to preserve the movement of the air at a distance; therefore an ear is no use to it but would even

As for hair, men differ in this themselves at different ages, and also from all other kinds of animals that have

hair. These are almost all which are internally viviparous, for even when the covering of such animals is

spiny it must be considered as a kind of hair, as in the land hedgehog and any other such animal among the

vivipara. Hairs differ in respect of hardness and softness, length and shortness, straightness and curliness,

quantity and scantiness, and in addition to these qualities, in their colours, whiteness and blackness and the

intermediate shades. They differ also in some of these respects according to age, as they are young or

growing old. This is especially plain in man; the hair gets coarser as time goes on, and some go bald on the

front of the head; children indeed do not go bald, nor do women, but men do so by the time their age is

advancing. Human beings also go grey on the head as they grow old, but this is not visible in practically any

other animal, though more so in the horse than others. Men go bald on the front of the head, but turn grey first

on the temples; no one goes bald first on these or on the back of the head. Some such affections occur in a

corresponding manner also in all animals which have not hair but something analogous to it, as the feathers of

For what purpose Nature has made hair in general for animals has been previously stated in the work dealing

with the causes of the parts of animals; it is the business of the present inquiry to show under what

circumstances and for what necessary causes each particular kind of hair occurs. The principal cause then of

thickness and thinness is the skin, for this is thick in some animals and thin in others, rare in some and dense

in others. The different quality of the included moisture is also a helping cause, for in some animals this is

greasy and in others watery. For generally speaking the substratum of the skin is of an earthy nature; being on

the surface of the body it becomes solid and earthy as the moisture evaporates. Now the hairs or their

analogue are not formed out of the flesh but out of the skin moisture evaporating and exhaling in them, and

therefore thick hairs arise from a thick skin and thin from thin. If then the skin is rarer and thicker, the hairs

are thick because of the quantity of earthy matter and the size of the pores, but if it is denser they are thin

because of the narrowness of the pores. Further, if the moisture be watery it dries up quickly and the hairs do

not gain in size, but if it be greasy the opposite happens, for the greasy is not easily dried up. Therefore the

thickerskinned animals are as a general rule thickerhaired for the causes mentioned; however, the

thickestskinned are not more so than other thickskinned ones, as is shown by the class of swine compared

to that of oxen and to the elephant and many others. And for the same reason also the hairs of the head in man

are thickest, for this part of his skin is thickest and lies over most moisture and besides is very porous.

The cause of the hairs being long or short depends on the evaporating moisture not being easily dried. Of this

there are two causes, quantity and quality; if the liquid is much it does not dry up easily nor if it is greasy.

And for this reason the hairs of the head are longest in man, for the brain, being fluid and cold, supplies great

The hairs become straight or curly on account of the vapour arising in them. If it be smokelike, it is hot and

dry and so makes the hair curly, for it is twisted as being carried with a double motion, the earthy part tending

downwards and the hot upwards. Thus, being easily bent, it is twisted owing to its weakness, and this is what

is meant by curliness in hair. It is possible then that this is the cause, but it is also possible that, owing to its

having but little moisture and much earthy matter in it, it is dried by the surrounding air and so coiled up

together. For what is straight becomes bent, if the moisture in it is evaporated, and runs together as a hair

does when burning upon the fire; curliness will then be a contraction owing to deficiency of moisture caused

by the heat of the environment. A sign of this is the fact that curly hair is harder than straight, for the dry is

hard. And animals with much moisture are straighthaired; for in these hairs the moisture advances as a

stream, not in drops. For this reason the Scythians on the Black Sea and the Thracians are straighthaired, for

both they themselves and the environing air are moist, whereas the Aethiopians and men in hot countries are

Some, however, of the thickskinned animals are finehaired for the cause previously stated, for the finer the

pores are the finer must the hairs be. Hence the class of sheep have such hairs (for wool is only a multitude of

There are some animals whose hair is soft and yet less fine, as is the case with the class of hares compared

with that of sheep; in such animals the hair is on the surface of the skin, not deeply rooted in it, and so is not

long but in much the same state as the scrapings from linen, for these also are not long but are soft and do not

The condition of sheep in cold climates is opposite to that of man; the hair of the Scythians is soft but that of

the Sauromatic sheep is hard. The reason of this is the same as it is also all wild animals. The cold hardens

and solidifies them by drying them, for as the heat is pressed out the moisture evaporates, and both hair and

skin become earthy and hard. In wild animals then the exposure to the cold is the cause of hardness in the

hair, in the others the nature of the climate is the cause. A proof of this is also what happens in the

seaurchins which are used as a remedy in stranguries. For these, too, though small themselves, have large

and hard spines because the sea in which they live is cold on account of its depth (for they are found in sixty

fathoms and even more). The spines are large because the growth of the body is diverted to them, since

having little heat in them they do not concoct their nutriment and so have much residual matter and it is from

this that spines, hairs, and such things are formed; they are hard and petrified through the congealing effect of

the cold. In the same way also plants are found to be harder, more earthy, and stony, if the region in which

they grow looks to the north than if it looks to the south, and those in windy places than those in sheltered, for

Hardening, then, comes of both heat and cold, for both cause the moisture to evaporate, heat per se and cold

per accidens (since the moisture goes out of things along with the heat, there being no moisture without heat),

but whereas cold not only hardens but also condenses, heat makes a substance rarer.

For the same reason, as animals grow older, the hairs become harder in those which have hairs, and the

feathers and scales in the feathered and scaly kinds. For their skins become harder and thicker as they get

older, for they are dried up, and old age, as the word denotes, is earthy because the heat fails and the moisture

Men go bald visibly more than any other animal, but still such a state is something general, for among plants

also some are evergreens while others are deciduous, and birds which hibernate shed their feathers. Similar to

this is the condition of baldness in those human beings to whom it is incident. For leaves are shed by all

plants, from one part of the plant at a time, and so are feathers and hairs by those animals that have them; it is

when they are all shed together that the condition is described by the terms mentioned, for it is called 'going

bald' and 'the fall of the leaf' and 'moulting'. The cause of the condition is deficiency of hot moisture, such

moisture being especially the unctuous, and hence unctuous plants are more evergreen. (However we must

elsewhere state the cause of this phenomena in plants, for other causes also contribute to it.) It is in winter

that this happens to plants (for the change from summer to winter is more important to them than the time of

life), and to those animals which hibernate (for these, too, are by nature less hot and moist than man); in the

latter it is the seasons of life that correspond to summer and winter. Hence no one goes bald before the time

of sexual intercourse, and at that time it is in those naturally inclined to such intercourse that baldness

appears, for the brain is naturally the coldest part of the body and sexual intercourse makes men cold, being a

loss of pure natural heat. Thus we should expect the brain to feel the effect of it first, for a little cause turns

the scale where the thing concerned is weak and in poor condition. Thus if we reckon up these points, that the

brain itself has but little heat, and further that the skin round it must needs have still less, and again that the

hair must have still less than the skin inasmuch as it is furthest removed from the brain, we should reasonably

expect baldness to come about this age upon those who have much semen. And it is for the same reason that

the front part of the head alone goes bald in man and that he is the only animal to do so; the front part goes

bald because the brain is there, and man is the only animal to go bald because his brain is much the largest

and the moistest. Women do not go bald because their nature is like that of children, both alike being

incapable of producing seminal secretion. Eunuchs do not become bald, because they change into the female

condition. And as to the hair that comes later in life, eunuchs either do not grow it at all, or lose it if they

happen to have it, with the exception of the pubic hair; for women also grow that though they have not the

The reason why the hair does not grow again in cases of baldness, although both hibernating animals recover

their feathers or hair and trees that have shed their leaves grow leaves again, is this. The seasons of the year

are the turningpoints of their lives, rather than their age, so that when these seasons change they change with

them by growing and losing feathers, hairs, or leaves respectively. But the winter and summer, spring and

autumn of man are defined by his age, so that, since his ages do not return, neither do the conditions caused

by them return, although the cause of the change of condition is similar in man to what it is in the animals

But as to their colour, it is the nature of the skin that is the cause of this in other animals and also of their

being unicoloured or varicoloured); but in man it is not the cause, except of the hair going grey through

disease (not through old age), for in what is called leprosy the hairs become white; on the contrary, if the

hairs are white the whiteness does not invade the skin. The reason is that the hairs grow out of skin; if, then,

the skin is diseased and white the hair becomes diseased with it, and the disease of hair is greyness. But the

greyness of hair which is due to age results from weakness and deficiency of heat. For as the body declines in

vigour we tend to cold at every time of life, and especially in old age, this age being cold and dry. We must

remember that the nutriment coming to each part of the body is concocted by the heat appropriate to the part;

if the heat is inadequate the part loses its efficiency, and destruction or disease results. (We shall speak more

in detail of causes in the treatise on growth and nutrition.) Whenever, then, the hair in man has naturally little

heat and too much moisture enters it, its own proper heat is unable to concoct the moisture and so it is

decayed by the heat in the environing air. All decay is caused by heat, not the innate heat but external heat, as

has been stated elsewhere. And as there is a decay of water, of earth, and all such material bodies, so there is

also of the earthy vapour, for instance what is called mould (for mould is a decay of earthy vapour). Thus also

the liquid nutriment in the hair decays because it is not concocted, and what is called greyness results. It is

white because mould also, practically alone among decayed things, is white. The reason of this is that it has

much air in it, all earthy vapour being equivalent to thick air. For mould is, as it were, the antithesis of

hoarfrost; if the ascending vapour be frozen it becomes hoarfrost, if it be decayed, mould. Hence both are

on the surface of things, for vapour is superficial. And so the comic poets make a good metaphor in jest when

they call grey hairs 'mould of old age' and For the one is generically the same as greyness, the other

specifically; hoarfrost generically (for both are a vapour), mould specifically (for both are a form of decay).

A proof that this is so is this: grey hairs have often grown on men in consequence of disease, and later on

dark hairs instead of them after restoration to health. The reason is that in sickness the whole body is deficient

in natural heat and so the parts besides, even the very small ones, participate in this weakness; and again,

much residual matter is formed in the body and all its parts in illness, wherefore the incapacity in the flesh to

concoct the nutriment causes the grey hairs. But when men have recovered health and strength again they

change, becoming as it were young again instead of old; in consequence the states change also. Indeed, we

may rightly call disease an acquired old age, old age a natural disease; at any rate, some diseases produce the

Men go grey on the temples first, because the back of the head is empty of moisture owing to its containing

no brain, and the 'bregma' has a great deal of moisture, a large quantity not being liable to decay; the hair on

the temples however has neither so little that it can concoct it nor so much that it cannot decay, for this region

of the head being between the two extremes is exempt from both states. The cause of greyness in man has

The reason why this change does not take place visibly on account of age in other animals is the same as that

already given in the case of baldness; their brain is small and less fluid than in man, so that the heat required

for concoction does not altogether fail. Among them it is most clear in horses of all animals that we know,

because the bone about the brain is thinner in them than in others in proportion to their size. A sign of this is

that a blow to this spot is fatal to them, wherefore Homer also has said: 'where the first hairs grow on the

skull of horses, and a wound is most fatal.' As then the moisture easily flows to these hairs because of the

thinness of the bone, whilst the heat fails on account of age, they go grey. The reddish hairs go grey sooner

than the black, redness also being a sort of weakness of hair and all weak things ageing sooner. It is said,

however, that cranes become darker as they grow old. The reason of this would be, if it should prove true,

that their feathers are naturally moister than others and as they grow old the moisture in the feathers is too

Greyness comes about by some sort of decay, and is not, as some think, a withering. (1) A proof of the former

statement is the fact that hair protected by hats or other coverings goes grey sooner (for the winds prevent

decay and the protection keeps off the winds), and the fact that it is aided by anointing with a mixture of oil

and water. For, though water cools things, the oil mingled with it prevents the hair from drying quickly, water

being easily dried up. (2) That the process is not a withering, that the hair does not whiten as grass does by

withering, is shown by the fact that some hairs grow grey from the first, whereas nothing springs up in a

withered state. Many hairs also whiten at the tip, for there is least heat in the extremities and thinnest parts.

When the hairs of other animals are white, this is caused by nature, not by any affection. The cause of the

colours in other animals is the skin; if they are white, the skin is white, if they are dark it is dark, if they are

piebald in consequence of a mixture of the hairs, it is found to be white in the one part and dark in the other.

But in man the skin is in no way the cause, for even whiteskinned men have very dark hair. The reason is

that man has the thinnest skin of all animals in proportion to his size and therefore it has not strength to

change the hairs; on the contrary the skin itself changes its colour through its weakness and is darkened by

sun and wind, while the hairs do not change along with it at all. But in the other animals the skin, owing to its

thickness, has the influence belonging to the soil in which a thing grows, therefore the hairs change according

to the skin but the skin does not change at all in consequence of the winds and the sun.

Of animals some are unicoloured (I mean by this term those of which the kind as a whole has one colour, as

all lions are tawny; and this condition exists also in birds, fish, and the other classes of animals alike); others

though manycoloured are yet wholecoloured (I mean those whose body as a whole has the same colour, as

a bull is white as a whole or dark as a whole); others are varicoloured. This last term is used in both ways;

sometimes the whole kind is varicoloured, as leopards and peacocks, and some fish, e.g. the socalled

'thrattai'; sometimes the kind as a whole is not so, but such individuals are found in it, as with cattle and goats

and, among birds, pigeons; the same applies also to other kinds of birds. The wholecoloured change much

more than the uniformly coloured, both into the simple colour of another individual of the same kind (as dark

changing into white and vice versa) and into both colours mingled. This is because it is a natural

characteristic of the kind as a whole not to have one colour only, the kind being easily moved in both

directions so that the colours both change more into one another and are more varied. The opposite holds with

the uniformly coloured; they do not change except by an affection of the colour, and that rarely; but still they

do so change, for before now white individuals have been observed among partridges, ravens, sparrows, and

bears. This happens when the course of development is perverted, for what is small is easily spoilt and easily

moved, and what is developing is small, the beginning of all such things being on a small scale.

Change is especially found in those animals of which by nature the individual is wholecoloured but the kind

manycoloured. This is owing to the water which they drink, for hot waters make the hair white, cold makes

it dark, an effect found also in plants. The reason is that the hot have more air than water in them, and the air

shining through causes whiteness, as also in froth. As, then, skins which are white by reason of some

affection differ from those white by nature, so also in the hair the whiteness due to disease or age differs from

that due to nature in that the cause is different; the latter are whitened by the natural heat, the former by the

external heat. Whiteness is caused in all things by the vaporous air imprisoned in them. Hence also in all

animals not uniformly coloured all the part under the belly is whiter. For practically all white animals are

both hotter and better flavoured for the same reason; the concoction of their nutriment makes them

wellflavoured, and heat causes the concoction. The same cause holds for those animals which are

uniformlycoloured, but either dark or white; heat and cold are the causes of the nature of the skin and hair,

The tongue also varies in colour in the simply coloured as compared with the varicoloured animals, and

again in the simply coloured which differ from one another, as white and dark. The reason is that assigned

before, that the skins of the varicoloured are varicoloured, and the skins of the whitehaired and

darkhaired are white and dark in each case. Now we must conceive of the tongue as one of the external

parts, not taking into account the fact that it is covered by the mouth but looking on it as we do on the hand or

foot; thus since the skin of the varicoloured animals is not uniformly coloured, this is the cause of the skin

Some birds and some wild quadrupeds change their colour according to the seasons of the year. The reason is

that, as men change according to their age, so the same thing happens to them according to the season; for

The more omnivorous animals are more varicoloured to speak generally, and this is what might be

expected; thus bees are more uniformly coloured than hornets and wasps. For if the food is responsible for the

change we should expect varied food to increase the variety in the movements which cause the development

and so in the residual matter of the food, from which come into being hairs and feathers and skins.

As to the voice, it is deep in some animals, high in others, in others again wellpitched and in due proportion

between both extremes. Again, in some it is loud, in others small, and it differs in smoothness and roughness,

flexibility and inflexibility. We must inquire then into the causes of each of these distinctions.

We must suppose then that the same cause is responsible for high and deep voices as for the change which

they undergo in passing from youth to age. The voice is higher in all other animals when younger, but in

cattle that of calves is deeper. We find the same thing also in the male and female sexes; in the other kinds of

animals the voice of the female is higher than that of the male (this being especially plain in man, for Nature

has given this faculty to him in the highest degree because he alone of animals makes use of speech and the

voice is the material of speech), but in cattle the opposite obtains, for the voice of cows is deeper than that of

Now the purpose for which animals have a voice, and what is meant by 'voice' and by 'sound' generally, has

been stated partly in the treatise on sensation, partly in that on the soul. But since lowness of voice depends

on the movement of the air being slow and its highness on its being quick, there is a difficulty in knowing

whether it is that which moves or that which is moved that is the cause of the slowness or quickness. For

some say that what is much is moved slowly, what is little quickly, and that the quantity of the air is the cause

of some animals having a deep and others a high voice. Up to a certain point this is well said (for it seems to

be rightly said in a general way that the depth depends on a certain amount of the air put in motion), but not

altogether, for if this were true it would not be easy to speak both soft and deep at once, nor again both loud

and high. Again, the depth seems to belong to the nobler nature, and in songs the deep note is better than the

highpitched ones, the better lying in superiority, and depth of tone being a sort of superiority. But then depth

and height in the voice are different from loudness and softness, and some highvoiced animals are

loudvoiced, and in like manner some softvoiced ones are deepvoiced, and the same applies to the tones

lying between these extremes. And by what else can we define these (I mean loudness and softness of voice)

except by the large and small amount of the air put in motion? If then height and depth are to be decided in

accordance with the distinction postulated, the result will be that the same animals will be deepand

The reason of the difficulty is that the words 'great' and 'small', 'much' and 'little' are used sometimes

absolutely, sometimes relatively to one another. Whether an animal has a great (or loud) voice depends on the

air which is moved being much absolutely, whether it has a small voice depends on its being little absolutely;

but whether they have a deep or high voice depends on their being thus differentiated in relation to one

another. For if that which is moved surpass the strength of that which moves it, the air that is sent forth must

go slowly; if the opposite, quickly. The strong, then, on account of their strength, sometimes move much air

and make the movement slow, sometimes, having complete command over it, make the movement swift. On

the same principle the weak either move too much air for their strength and so make the movement slow, or if

These, then, are the reasons of these contrarieties, that neither are all young animals highvoiced nor all

deepvoiced, nor are all the older, nor yet are the two sexes thus opposed, and again that not only the sick

speak in a high voice but also those in good bodily condition, and, further, that as men verge on old age they

Most young animals, then, and most females set but little air in motion because of their want of power, and

are consequently highvoiced, for a little air is carried along quickly, and in the voice what is quick is high.

But in calves and cows, in the one case because of their age, in the other because of their female nature, the

part by which they set the air in motion is not strong; at the same time they set a great quantity in motion and

so are deepvoiced; for that which is borne along slowly is heavy, and much air is borne along slowly. And

these animals set much in movement whereas the others set but little, because the vessel through which the

breath is first borne has in them a large opening and necessarily sets much air in motion, whereas in the rest

the air is better dispensed. As their age advances this part which moves the air gains more strength in each

animal, so that they change into the opposite condition, the highvoiced becoming deepervoiced than they

were, and the deepvoiced highervoiced, which is why bulls have a higher voice than calves and cows. Now

the strength of all animals is in their sinews, and so those in the prime of life are stronger, the young being

weaker in the joints and sinews; moreover, in the young they are not yet tense, and in those now growing old

the tension relaxes, wherefore both these ages are weak and powerless for movement. And bulls are

particularly sinewy, even their hearts, and therefore that part by which they set the air in motion is in a tense

state, like a sinewy string stretched tight. (That the heart of bulls is of such a nature is shown by the fact that a

bone is actually found in some of them, and bones are naturally connected with sinew.)

All animals when castrated change to the female character, and utter a voice like that of the females because

the sinewy strength in the principle of the voice is relaxed. This relaxation is just as if one should stretch a

string and make it taut by hanging some weight on to it, as women do who weave at the loom, for they stretch

the warp by attaching to it what are called 'laiai'. For in this way are the testes attached to the seminal

passages, and these again to the bloodvessel which takes its origin in the heart near the organ which sets the

voice in motion. Hence as the seminal passages change towards the age at which they are now able to secrete

the semen, this part also changes along with them. As this changes, the voice again changes, more indeed in

males, but the same thing happens in females too, only not so plainly, the result being what some call

'bleating' when the voice is uneven. After this it settles into the deep or high voice of the succeeding time of

life. If the testes are removed the tension of the passages relaxes, as when the weight is taken off the string or

the warp; as this relaxes, the organ which moves the voice is loosened in the same proportion. This, then, is

the reason why the voice and the form generally changes to the female character in castrated animals; it is

because the principle is relaxed upon which depends the tension of the body; not that, as some suppose, the

testes are themselves a ganglion of many principles, but small changes are the causes of great ones, not per se

but when it happens that a principle changes with them. For the principles, though small in size, are great in

potency; this, indeed, is what is meant by a principle, that it is itself the cause of many things without

The heat or cold also of their habitat contributes to make some animals of such a character as to be

deepvoiced, and others highvoiced. For hot breath being thick causes depth, cold breath being thin the

opposite. This is clear also in pipeplaying, for if the breath of the performer is hotter, that is to say if it is

The cause of roughness and smoothness in the voice, and of all similar inequality, is that the part or organ

through which the voice is conveyed is rough or smooth or generally even or uneven. This is plain when there

is any moisture about the trachea or when it is roughened by any affection, for then the voice also becomes

Flexibility depends on the softness or hardness of the organ, for what is soft can be regulated and assume any

form, while what is hard cannot; thus the soft organ can utter a loud or a small note, and accordingly a high or

a deep one, since it easily regulates the breath, becoming itself easily great or small. But hardness cannot be

Let this be enough on all those points concerning the voice which have not been previously discussed in the

With regard to the teeth it has been stated previously that they do not exist for a single purpose nor for the

same purpose in all animals, but in some for nutrition only, in others also for fighting and for vocal speech.

We must, however, consider it not alien to the discussion of generation and development to inquire into the

reason why the front teeth are formed first and the grinders later, and why the latter are not shed but the

Democritus has spoken of these questions but not well, for he assigns the cause too generally without

investigating the facts in all cases. He says that the early teeth are shed because they are formed in animals

too early, for it is when animals are practically in their prime that they grow according to Nature, and

suckling is the cause he assigns for their being found too early. Yet the pig also suckles but does not shed its

teeth, and, further, all the animals with carnivorous dentition suckle, but some of them do not shed any teeth

except the canines, e.g. lions. This mistake, then, was due to his speaking generally without examining what

happens in all cases; but this is what we to do, for any one who makes any general statement must speak of

Now we assume, basing our assumption upon what we see, that Nature never fails nor does anything in vain

so far as is possible in each case. And it is necessary, if an animal is to obtain food after the time of taking

milk is over, that it should have instruments for the treatment of the food. If, then, as Democritus says, this

happened about the time of reaching maturity, Nature would fail in something possible for her to do. And,

besides, the operation of Nature would be contrary to Nature, for what is done by violence is contrary to

Nature, and it is by violence that he says the formation of the first teeth is brought about. That this view then

Now these teeth are developed before the flat teeth, in the first place because their function is earlier (for

dividing comes before crushing, and the flat teeth are for crushing, the others for dividing), in the second

place because the smaller is naturally developed quicker than the larger, even if both start together, and these

teeth are smaller in size than the grinders, because the bone of the jaw is flat in that part but narrow towards

the mouth. From the greater part, therefore, must flow more nutriment to form the teeth, and from the

The act of sucking in itself contributes nothing to the formation of the teeth, but the heat of the milk makes

them appear more quickly. A proof of this is that even in suckling animals those young which enjoy hotter

They are shed, after they have been formed, partly because it is better so (for what is sharp is soon blunted, so

that a fresh relay is needed for the work, whereas the flat teeth cannot be blunted but are only smoothed in

time by wearing down), partly from necessity because, while the roots of the grinders are fixed where the jaw

is flat and the bone strong, those of the front teeth are in a thin part, so that they are weak and easily moved.

They grow again because they are shed while the bone is still growing and the animal is still young enough to

grow teeth. A proof of this is that even the flat teeth grow for a long time, the last of them cutting the gum at

about twenty years of age; indeed in some cases the last teeth have been grown in quite old age. This is

because there is much nutriment in the broad part of the bones, whereas the front part being thin soon reaches

perfection and no residual matter is found in it, the nutriment being consumed in its own growth.

Democritus, however, neglecting the final cause, reduces to necessity all the operations of Nature. Now they

are necessary, it is true, but yet they are for a final cause and for the sake of what is best in each case. Thus

nothing prevents the teeth from being formed and being shed in this way; but it is not on account of these

causes but on account of the end (or final cause); these are causes only in the sense of being the moving and

efficient instruments and the material. So it is reasonable that Nature should perform most of her operations

using breath as an instrument, for as some instruments serve many uses in the arts, e.g. the hammer and anvil

in the smith's art, so does breath in the living things formed by Nature. But to say that necessity is the only

cause is much as if we should think that the water has been drawn off from a dropsical patient on account of

the lancet, not on account of health, for the sake of which the lancet made the incision.

We have thus spoken of the teeth, saying why some are shed and grow again, and others not, and generally

for what cause they are formed. And we have spoken of the other affections of the parts which are found to

occur not for any final end but of necessity and on account of the motive or efficient cause.