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On The Generation Of Animals   

if any kind of such birds has much semen naturally, it is always in
need of this intercourse. In females then it encourages copulation
to have the uterus low down, but in males to have the testes drawn up.

It has been now stated why superfoetation is not found in some
animals at all, why it is found in others which sometimes bring the
later embryos to birth and sometimes not, and why some such animals
are inclined to sexual intercourse while others are not.

Some of those animals in which superfoetation occurs can bring the
embryos to birth even if a long time elapses between the two
impregnations, if their kind is spermatic, if their body is not of a
large size, and if they bear many young. For because they bear many
their uterus is spacious, because they are spermatic the generative
discharge is copious, and because the body is not large but the
discharge is excessive and in greater measure than is required for the
nourishment wanted for the embryo, therefore they can not only form
animals but also bring them to birth later on. Further, the uterus
in such animals does not close up during gestation because there is
a quantity of the residual discharge left over. This has happened
before now even in women, for in some of them the discharge
continues during all the time of pregnancy. In women, however, this is
contrary to Nature, so that the embryo suffers, but in such animals it
is according to Nature, for their body is so formed from the
beginning, as with hares. For superfoetation occurs in these
animals, since they are not large and they bear many young (for
they have many toes and the many-toed animals bear many), and they
are spermatic. This is shown by their hairiness, for the quantity of
their hair is excessive, these animals alone having hair under the
feet and within the jaws. Now hairiness is a sign of abundance of
residual matter, wherefore among men also the hairy are given to
sexual intercourse and have much semen rather than the smooth. In
the hare it often happens that some of the embryos are imperfect while
others of its young are produced perfect.


Some of the vivipara produce their young imperfect, others
perfect; the one-hoofed and cloven-footed perfect, most of the
many-toed imperfect. The reason of this is that the one-hoofed produce
one young one, and the cloven-footed either one or two generally
speaking; now it is easy to bring the few to perfection. All the
many-toed animals that bear their young imperfect give birth to
many. Hence, though they are able to nourish the embryos while newly
formed, their bodies are unable to complete the process when the
embryos have grown and acquired some size. So they produce them
imperfect, like those animals which generate a scolex, for some of
them when born are scarcely brought into form at all, as the fox,
bear, and lion, and some of the rest in like manner; and nearly all of
them are blind, as not only the animals mentioned but also the dog,
wolf, and jackal. The pig alone produces both many and perfect
young, and thus here alone we find any overlapping; it produces many
as do the many-toed animals, but is cloven-footed or solid-hoofed
(for there certainly are solid-hoofed swine). They bear, then, many
young because the nutriment which would otherwise go to increase their
size is diverted to the generative secretion (for considered as a
solid-hoofed animal the pig is not a large one), and also it is
more often cloven-hoofed, striving as it were with the nature of the
solid-hoofed animals. For this reason it produces sometimes only
one, sometimes two, but generally many, and brings them to
perfection before birth because of the good condition of its body,
being like a rich soil- which has sufficient and abundant nutriment
for plants.

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