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On The Generation Of Animals   

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 comes
from it. Nutriment, again, is obviously introduced from without.

It remains, then, that it must be either a waste-product or a
secretion or excretion. Now the ancients seem to think that it is a
waste-product, 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 waste-product. 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, because a useless and morbid secretion is
mingled with it.

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 thing
the size of it will become excessive.

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 waste-product 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. Waste-products, on the contrary, are always due to
corruption or decay and to a departure from Nature.

A further proof that it is not a waste-product, 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 waste-products
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 is collected and flows
together into them.

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

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