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On The Generation Of Animals   


cotyledons are more than one. For two embryos are often formed in
the same region of the uterus, and they may be seen lying in a row
in animals that produce many, when the uterus is filled with the
embryos. (This is plain from the dissections.) Rather the truth is
this. As animals complete their growth there are certain limits to
their size, both upwards and downwards, beyond which they cannot go,
but it is in the space between these limits that they exceed or fall
short of one another in size, and it is within these limits that one
man (or any other animal) is larger or smaller than another. So also
the generative material from which each animal is formed is not
without a quantitative limit in both directions, nor can it be
formed from any quantity you please. Whenever then an animal, for
the cause assigned, discharges more of the female secretion than is
needed for beginning the existence of a single animal, it is not
possible that only one should be formed out of all this, but a
number limited by the appropriate size in each case; nor will the
semen of the male, or the power residing in the semen, form anything
either more or less than what is according to Nature. In like
manner, if the male emits more semen than is necessary, or more powers
in different parts of the semen as it is divided, however much it is
it will not make anything greater; on the contrary it will dry up
the material of the female and destroy it. So fire also does not
continue to make water hotter in proportion as it is itself increased,
but there is a fixed limit to the heat of which water is capable; if
that is once reached and the fire is then increased, the water no
longer gets hotter but rather evaporates and at last disappears and is
dried up. Now since it appears that the secretion of the female and
that from the male need to stand in some proportionate relation to one
another (I mean in animals of which the male emits semen), what
happens in those that produce many young is this: from the very
first the semen emitted by the male has power, being divided, to
form several embryos, and the material contributed by the female is so
much that several can be formed out of it. (The parallel of
curdling milk, which we spoke of before, is no longer in point here,
for what is formed by the heat of the semen is not only of a certain
quantity but also of a certain quality, whereas with fig-juice and
rennet quantity alone is concerned.) This then is just the reason why
in such animals the embryos formed are numerous and do not all unite
into one whole; it is because an embryo is not formed out of any
quantity you please, but whether there is too much or too little, in
either case there will be no result, for there is a limit set alike to
the power of the heat which acts on the material and to the material
so acted upon.

On the same principle many embryos are not formed, though the
secretion is much, in the large animals which produce only one young
one, for in them also both the material and that which works upon it
are of a certain quantity. So then they do not secrete such material
in too great quantity for the reason previously stated, and what
they do secrete is naturally just enough for one embryo alone to be
formed from it. If ever too much is secreted, then twins are born.
Hence such cases seem to be more portentous, because they are contrary
to the general and customary rule.

Man belongs to all three classes, for he produces one only and
sometimes many or few, though naturally he almost always produces one.
Because of the moisture and heat of his body he may produce many [for
semen is naturally fluid and hot], but because of his size he
produces few or one. On account of this it results that in man alone
among animals the period of gestation is irregular; whereas the period
is fixed in the rest, there are several periods in man, for children
are born at seven months and at ten months and at the times between,
for even those of eight months do live though less often than the
rest. The reason may be gathered from what has just been said, and the

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