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On The Generation Of Animals   

instead of generating entirely from herself. The reason is that the
animal differs from the plant by having sense-perception; if the
sensitive soul is not present, either actually or potentially, and
either with or without qualification, it is impossible for face, hand,
flesh, or any other part to exist; it will be no better than a
corpse or part of a corpse. If then, when the sexes are separated,
it is the male that has the power of making the sensitive soul, it
is impossible for the female to generate an animal from itself
alone, for the process in question was seen to involve the male
quality. Certainly that there is a good deal in the difficulty
stated is plain in the case of the birds that lay wind-eggs, showing
that the female can generate up to a certain point unaided. But this
still involves a difficulty; in what way are we to say that their eggs
live? It neither possible that they should live in the same way as
fertile eggs (for then they would produce a chick actually alive),
nor yet can they be called eggs only in the sense in which an egg of
wood or stone is so called, for the fact that these eggs go bad
shows that they previously participate in some way in life. It is
plain, then, that they have some soul potentially. What sort of soul
will this be? It must be the lowest surely, and this is the nutritive,
for this exists in all animals and plants alike. Why then does it
not perfect the parts and the animal? Because they must have a
sensitive soul, for the parts of animals are not like those of a
plant. And so the female animal needs the help of the male, for in
these animals we are speaking of the male is separate. This is exactly
what we find, for the wind-eggs become fertile if the male tread the
female in a certain space of time. About the cause of these things,
however, we shall enter into detail later.

If there is any kind of animal which is female and has no male
separate from it, it is possible that this may generate a young one
from itself without copulation. No instance of this worthy of credit
has been observed up to the present at any rate, but one case in the
class of fishes makes us hesitate. No male of the so-called erythrinus
has ever yet been seen, but females, and specimens full of roe, have
been seen. Of this, however, we have as yet no proof worthy of credit.
Again, some members of the class of fishes are neither male nor
female, as eels and a kind of mullets found in stagnant waters. But
whenever the sexes are separate the female cannot generate perfectly
by herself alone, for then the male would exist in vain, and Nature
makes nothing in vain. Hence in such animals the male always
perfects the work of generation, for he imparts the sensitive soul,
either by means of the semen or without it. Now the parts of the
embryo already exist potentially in the material, and so when once the
principle of movement has been imparted to them they develop in a
chain one after another, as the wheels are moved one by another in the
automatic machines. When some of the natural philosophers say that
like is brought to like, this must be understood, not in the sense
that the parts are moved as changing place, but that they stay where
they are and the movement is a change of quality (such as softness,
hardness, colour, and the other differences of the homogeneous parts);

thus they become in actuality what they previously were in
potentiality. And what comes into being first is the first
principle; this is the heart in the sanguinea and its analogue in
the rest, as has been often said already. This is plain not only to
the senses (that it is first to come into being), but also in view
of its end; for life fails in the heart last of all, and it happens in
all cases that what comes into being last fails first, and the first
last, Nature running a double course, so to say, and turning back to
the point from whence she started. For the process of becoming is from
the non-existent to the existent, and that of perishing is back
again from the existent to the non-existent.

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