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On The Generation Of Animals   

are no part of the ducts but are only attached to them, as women
fasten stones to the loom when weaving; if they are removed the
ducts are drawn up internally, so that castrated animals are unable to
generate; if they were not drawn up they would be able, and before now
a bull mounting immediately after castration has caused conception
in the cow because the ducts had not yet been drawn up. In birds and
oviparous quadrupeds the testes receive the spermatic secretion, so
that its expulsion is slower than in fishes. This is clear in the case
of birds, for their testes are much enlarged at the time of
copulation, and all those which pair at one season of the year have
them so small when this is past that they are almost indiscernible,
but during the season they are very large. When the testes are
internal the act of copulation is quicker than when they are external,
for even in the latter case the semen is not emitted before the testes
are drawn up.


Besides, quadrupeds have the organ of copulation, since it is
possible for them to have it, but for birds and the footless animals
it is not possible, because the former have their legs under the
middle of the abdomen and the latter have no legs at all; now the
penis depends from that region and is situated there. (Wherefore also
the legs are strained in intercourse, both the penis and the legs
being sinewy.) So that, since it is not possible for them to have
this organ, they must necessarily either have no testes also, or at
any rate not have them there, as those animals that have both penis
and testes have them in the same situation.

Further, with those animals at any rate that have external testes,
the semen is collected together before emission, and emission is due
to the penis being heated by its movement; it is not ready for
emission at immediate contact as in fishes.

All the vivipira have their testes in front, internally or
externally, except the hedgehog; he alone has them near the loin. This
is for the same reason as with birds, because their union must be
quick, for the hedgehog does not, like the other quadrupeds, mount
upon the back of the female, but they conjugate standing upright
because of their spines.

So much for the reasons why those animals have testes which have
them, and why they are sometimes external and sometimes internal.


All those animals which have no testes are deficient in this part,
as has been said, not because it is better to be so but simply because
of necessity, and secondly because it is necessary that their
copulation should be speedy. Such is the nature of fish and
serpents. Fish copulate throwing themselves alongside of the females
and separating again quickly. For as men and all such creatures must
hold their breath before emitting the semen, so fish at such times
must cease taking in the sea-water, and then they perish easily.
Therefore they must not mature the semen during copulation, as
viviparous land-animals do, but they have it all matured together
before the time, so as not to be maturing it while in contact but to
emit it ready matured. So they have no testes, and the ducts are
straight and simple. There is a small part similar to this connected
with the testes in the system of quadrupeds, for part of the reflected
duct is sanguineous and part is not; the fluid is already semen when
it is received by and passes through this latter part, so that once it
has arrived there it is soon emitted in these quadrupeds also. Now
in fishes the whole passage resembles the last section of the

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