If this ever happens (for
this also has occurred) the mother cannot bring the second embryo
to perfection, but it is cast out in a state like what are called
abortions. For just as, in those animals that bear only one, all the
secretion of the female is converted to the first formed embryo
because of its size, so it is here also; the only difference is that
in the former case this happens at once, in the latter when the foetus
has attained to some size, for then they are in the same state as
those that bear only one. In like manner, since man naturally would
produce many young, and since the size of the uterus and the
quantity of the female secretion are both greater than is necessary
for one embryo, only not so much so as to bring to birth a second,
therefore women and mares are the only animals which admit the male
during gestation, the former for the reason stated, and mares both
because of the barrenness of their nature and because their uterus
is of superfluous size, too large for one but too small to allow a
second embryo to be brought to perfection by superfoetation. And the
mare is naturally inclined to sexual intercourse because she is in the
same case as the barren among women; these latter are barren because
they have no monthly discharge (which corresponds to the act of
intercourse in males) and mares have exceedingly little. And in all
the vivipara the barren females are so inclined, because they resemble
the males when the semen has collected in the testes but is not
being got rid of.
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