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On The Generation Of Animals   

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 connexion with the activity of reason.

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
so-called 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 foam-like, 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 neither is fire
nor derives its origin from fire.

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 fig-juice 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; they have it potentially
but not actually.

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

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