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On The Generation Of Animals   

into being first and not all of them together. But that must first
come into being which has a principle of increase (for this nutritive
power exists in all alike, whether animals or plants, and this is
the same as the power that enables an animal or plant to generate
another like itself, that being the function of them all if
naturally perfect). And this is necessary for the reason that
whenever a living thing is produced it must grow. It is produced,
then, by something else of the same name, as e.g. man is produced by
man, but it is increased by means of itself. There is, then, something
which increases it. If this is a single part, this must come into
being first. Therefore if the heart is first made in some animals, and
what is analogous to the heart in the others which have no heart, it
is from this or its analogue that the first principle of movement
would arise.

We have thus discussed the difficulties previously raised on the
question what is the efficient cause of generation in each case, as
the first moving and formative power.


The next question to be mooted concerns the nature of semen. For
whereas when it issues from the animal it is thick and white, yet on
cooling it becomes liquid as water, and its colour is that of water.
This would appear strange, for water is not thickened by heat; yet
semen is thick when it issues from within the animal's body which is
hot, and becomes liquid on cooling. Again, watery fluids freeze, but
semen, if exposed in frosts to the open air, does not freeze but
liquefies, as if it was thickened by the opposite of cold. Yet it is
unreasonable, again, to suppose that it is thickened by heat. For it
is only substances having a predominance of earth in their composition
that coagulate and thicken on boiling, e.g. milk. It ought then to
solidify on cooling, but as a matter of fact it does not become
solid in any part but the whole of it goes like water.

This then is the difficulty. If it is water, water evidently does
not thicken through heat, whereas the semen is thick and both it and
the body whence it issues are hot. If it is made of earth or a mixture
of earth and water, it ought not to liquefy entirely and turn to

Perhaps, however, we have not discriminated all the possibilities.
It is not only the liquids composed of water and earthy matter that
thicken, but also those composed of water and air; foam, for instance,
becomes thicker and white, and the smaller and less visible the
bubbles in it, the whiter and firmer does the mass appear. The same
thing happens also with oil; on mixing with air it thickens, wherefore
that which is whitening becomes thicker, the watery part in it being
separated off by the heat and turning to air. And if oxide of lead
is mixed with water or even with oil, the mass increases greatly and
changes from liquid and dark to firm and white, the reason being
that air is mixed in with it which increases the mass and makes the
white shine through, as in foam and snow (for snow is foam). And
water itself on mingling with oil becomes thick and white, because air
is entangled in it by the act of pounding them together, and oil
itself has much air in it (for shininess is a property of air, not of
earth or water). This too is why it floats on the surface of the
water, for the air contained in it as in a vessel bears it up and
makes it float, being the cause of its lightness. So too oil is
thickened without freezing in cold weather and frosts; it does not
freeze because of its heat (for the air is hot and will not freeze),
but because the air is forced together and compressed, as..., by the
cold, the oil becomes thicker. These are the reasons why semen is firm
and white when it issues from within the animal; it has a quantity

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