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On The Generation Of Animals   

question has been discussed in the Problems. Let this explanation
suffice for these points.

The cause why the parts may be multiplied contrary to Nature is
the same as the cause of the birth of twins. For the reason exists
already in the embryo, whenever it aggregates more material at any
point of itself than is required by the nature of the part. The result
is then that either one of its parts is larger than the others, as a
finger or hand or foot or any of the other extremities or limbs; or
again if the embryo is cleft there may come into being more than one
such part, as eddies do in rivers; as the water in these is carried
along with a certain motion, if it dash against anything two systems
or eddies come into being out of one, each retaining the same
motion; the same thing happens also with the embryos. The abnormal
parts generally are attached near those they resemble, but sometimes
at a distance because of the movement- taking place in the embryo, and
especially because of the excess of material returning to that place
whence it was taken away while retaining the form of that part
whence it arose as a superfluity.

In certain cases we find a double set of generative organs [one
male and the other female]. When such duplication occurs the one is
always functional but not the other, because it is always
insufficiently supplied with nourishment as being contrary to
Nature; it is attached like a growth (for such growths also receive
nourishment though they are a later development than the body proper
and contrary to Nature.) If the formative power prevails, both are
similar; if it is altogether vanquished, both are similar; but if it
prevail here and be vanquished there, then the one is female and the
other male. (For whether we consider the reason why the whole
animal is male or female, or why the parts are so, makes no

When we meet with deficiency in such parts, e.g. an extremity or one
of the other members, we must assume the same cause as when the embryo
is altogether aborted (abortion of embryos happens frequently).

Outgrowths differ from the production of many young in the manner
stated before; monsters differ from these in that most of them are due
to embryos growing together. Some however are also of the following
kind, when the monstrosity affects greater and more sovereign parts,
as for instance some monsters have two spleens or more than two
kidneys. Further, the parts may migrate, the movements which form
the embryo being diverted and the material changing its place. We must
decide whether the monstrous animal is one or is composed of several
grown together by considering the vital principle; thus, if the
heart is a part of such a kind then that which has one heart will be
one animal, the multiplied parts being mere outgrowths, but those
which have more than one heart will be two animals grown together
through their embryos having been confused.

It also often happens even in many animals that do not seem to be
defective and whose growth is now complete, that some of their
passages may have grown together or others may have been diverted from
the normal course. Thus in some women before now the os uteri has
remained closed, so that when the time for the catamenia has arrived
pain has attacked them, till either the passage has burst open of
its own accord or the physicians have removed the impediment; some
such cases have ended in death if the rupture has been made too
violently or if it has been impossible to make it at all. In some boys
on the other hand the end of the penis has not coincided with the
end of the passage where the urine is voided, but the passage has
ended below, so that they crouch sitting to void it, and if the testes
are drawn up they appear from a distance to have both male and

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