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On The Generation Of Animals   

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

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