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On The Generation Of Animals   

Book I


WE have now discussed the other parts of animals, both generally and
with reference to the peculiarities of each kind, explaining how
each part exists on account of such a cause, and I mean by this the
final cause.

There are four causes underlying everything: first, the final cause,
that for the sake of which a thing exists; secondly, the formal cause,
the definition of its essence (and these two we may regard pretty
much as one and the same); thirdly, the material; and fourthly, the
moving principle or efficient cause.

We have then already discussed the other three causes, for the
definition and the final cause are the same, and the material of
animals is their parts of the whole animal the non-homogeneous
parts, of these again the homogeneous, and of these last the so-called
elements of all matter. It remains to speak of those parts which
contribute to the generation of animals and of which nothing
definite has yet been said, and to explain what is the moving or
efficient cause. To inquire into this last and to inquire into the
generation of each animal is in a way the same thing; and,
therefore, my plan has united them together, arranging the
discussion of these parts last, and the beginning of the question of
generation next to them.

Now some animals come into being from the union of male and
female, i.e. all those kinds of animal which possess the two sexes.
This is not the case with all of them; though in the sanguinea with
few exceptions the creature, when its growth is complete, is either
male or female, and though some bloodless animals have sexes so that
they generate offspring of the same kind, yet other bloodless
animals generate indeed, but not offspring of the same kind; such
are all that come into being not from a union of the sexes, but from
decaying earth and excrements. To speak generally, if we take all
animals which change their locality, some by swimming, others by
flying, others by walking, we find in these the two sexes, not only in
the sanguinea but also in some of the bloodless animals; and this
applies in the case of the latter sometimes to the whole class, as the
cephalopoda and crustacea, but in the class of insects only to the
majority. Of these, all which are produced by union of animals of
the same kind generate also after their kind, but all which are not
produced by animals, but from decaying matter, generate indeed, but
produce another kind, and the offspring is neither male nor female;
such are some of the insects. This is what might have been expected,
for if those animals which are not produced by parents had
themselves united and produced others, then their offspring must
have been either like or unlike to themselves. If like, then their
parents ought to have come into being in the same way; this is only
a reasonable postulate to make, for it is plainly the case with
other animals. If unlike, and yet able to copulate, then there would
have come into being again from them another kind of creature and
again another from these, and this would have gone on to infinity. But
Nature flies from the infinite, for the infinite is unending or
imperfect, and Nature ever seeks an end.

But all those creatures which do not move, as the testacea and
animals that live by clinging to something else, inasmuch as their
nature resembles that of plants, have no sex any more than plants
have, but as applied to them the word is only used in virtue of a

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