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).