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 non-existence. 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 non-existence, 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 female for the work of generation, because this is common to both.

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 sense-perception. 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 cross-division. 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 were laid externally it would be destroyed, having no protection.

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 shell-like 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; how so we shall explain later.

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 egg-like (for the so-called 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 the second change.

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 viviparous, others in eggs.

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 something containing soul.

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 beginning.

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 liver in the heart. and otherwise also the theory is strange and fictitious.

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 it, and therefore it has not within itself that which makes the parts.

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 semen at the first as a complete part.

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 starting-point and form of the product; only it exists in something else, whereas the movement of nature exists in the product itself, issuing from another nature which has the form in actuality.

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 both has soul, and is soul, potentially.

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 this or its analogue that the first principle of movement would arise.

we have thus discussed the difficulties previously raised on the question what is the efficient cause of generation in each case, as the first moving and formative power.