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On The Generation Of Animals   

nor do they conceive; if they do conceive, the milk dries up. This
is because the nature of the milk and of the catamenia is the same,
and Nature cannot be so productive as to supply both at once; if the
secretion is diverted in the one direction it must needs cease in
the other, unless some violence is done contrary to the general
rule. But this is as much as to say that it is contrary to Nature, for
in all cases where it is not impossible for things to be otherwise
than they generally are but where they may so happen, still what is
the general rule is what is 'according to Nature'.

The time also at which the young animal is born has been well
arranged. For when the nourishment coming through the umbilical cord
is no longer sufficient for the foetus because of its size, then at
the same time the milk becomes useful for the nourishment of the
newly-born animal, and the blood-vessels round which the so-called
umbilical cord lies as a coat collapse as the nourishment is no longer
passing through it; for these reasons it is at that time also that the
young animal enters into the world.


The natural birth of all animals is head-foremost, because the parts
above the umbilical cord are larger than those below. The body then,
being suspended from the cord as in a balance, inclines towards the
heavy end, and the larger parts are the heavier.


The period of gestation is, as a matter of fact, determined
generally in each animal in proportion to the length of its life. This
we should expect, for it is reasonable that the development of the
long-lived animals should take a longer time. Yet this is not the
cause of it, but the periods only correspond accidentally for the most
part; for though the larger and more perfect sanguinea do live a
long time, yet the larger are not all longer-lived. Man lives a longer
time than any animal of which we have any credible experience except
the elephant, and yet the human kind is smaller than that of the
bushy-tailed animals and many others. The real cause of long life in
any animal is its being tempered in a manner resembling the environing
air, along with certain other circumstances of its nature, of which we
will speak later; but the cause of the time of gestation is the size
of the offspring. For it is not easy for large masses to arrive at
their perfection in a small time, whether they be animals or, one
may say, anything else whatever. That is why horses and animals akin
to them, though living a shorter time than man, yet carry their
young longer; for the time in the former is a year, but in the
latter ten months at the outside. For the same reason also the time is
long in elephants; they carry their young two years on account of
their excessive size.

We find, as we might expect, that in all animals the time of
gestation and development and the length of life aims at being
measured by naturally complete periods. By a natural period I mean,
e.g. a day and night, a month, a year, and the greater times
measured by these, and also the periods of the moon, that is to say,
the full moon and her disappearance and the halves of the times
between these, for it is by these that the moon's orbit fits in with
that of the sun [the month being a period common to both].

The moon is a first principle because of her connexion with the
sun and her participation in his light, being as it were a second
smaller sun, and therefore she contributes to all generation and
development. For heat and cold varying within certain limits make
things to come into being and after this to perish, and it is the

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