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On The Generation Of Animals   

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. However,
we must speak later of the generation of these animals.

Book II


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

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