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On The Generation Of Animals   


moisture to evaporate, heat per se and cold per accidens (since the
moisture goes out of things along with the heat, there being no
moisture without heat), but whereas cold not only hardens but also
condenses, heat makes a substance rarer.

For the same reason, as animals grow older, the hairs become
harder in those which have hairs, and the feathers and scales in the
feathered and scaly kinds. For their skins become harder and thicker
as they get older, for they are dried up, and old age, as the word
denotes, is earthy because the heat fails and the moisture along
with it.

Men go bald visibly more than any other animal, but still such a
state is something general, for among plants also some are
evergreens while others are deciduous, and birds which hibernate
shed their feathers. Similar to this is the condition of baldness in
those human beings to whom it is incident. For leaves are shed by
all plants, from one part of the plant at a time, and so are
feathers and hairs by those animals that have them; it is when they
are all shed together that the condition is described by the terms
mentioned, for it is called 'going bald' and 'the fall of the leaf'
and 'moulting'. The cause of the condition is deficiency of hot
moisture, such moisture being especially the unctuous, and hence
unctuous plants are more evergreen. (However we must elsewhere
state the cause of this phenomena in plants, for other causes also
contribute to it.) It is in winter that this happens to plants
(for the change from summer to winter is more important to them than
the time of life), and to those animals which hibernate (for
these, too, are by nature less hot and moist than man); in the latter
it is the seasons of life that correspond to summer and winter.
Hence no one goes bald before the time of sexual intercourse, and at
that time it is in those naturally inclined to such intercourse that
baldness appears, for the brain is naturally the coldest part of the
body and sexual intercourse makes men cold, being a loss of pure
natural heat. Thus we should expect the brain to feel the effect of it
first, for a little cause turns the scale where the thing concerned is
weak and in poor condition. Thus if we reckon up these points, that
the brain itself has but little heat, and further that the skin
round it must needs have still less, and again that the hair must have
still less than the skin inasmuch as it is furthest removed from the
brain, we should reasonably expect baldness to come about this age
upon those who have much semen. And it is for the same reason that the
front part of the head alone goes bald in man and that he is the
only animal to do so; the front part goes bald because the brain is
there, and man is the only animal to go bald because his brain is much
the largest and the moistest. Women do not go bald because their
nature is like that of children, both alike being incapable of
producing seminal secretion. Eunuchs do not become bald, because
they change into the female condition. And as to the hair that comes
later in life, eunuchs either do not grow it at all, or lose it if
they happen to have it, with the exception of the pubic hair; for
women also grow that though they have not the other, and this
mutilation is a change from the male to the female condition.

The reason why the hair does not grow again in cases of baldness,
although both hibernating animals recover their feathers or hair and
trees that have shed their leaves grow leaves again, is this. The
seasons of the year are the turning-points of their lives, rather than
their age, so that when these seasons change they change with them
by growing and losing feathers, hairs, or leaves respectively. But the
winter and summer, spring and autumn of man are defined by his age, so
that, since his ages do not return, neither do the conditions caused
by them return, although the cause of the change of condition is
similar in man to what it is in the animals and plants in question.

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