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On The Generation Of Animals   

The cause of the hairs being long or short depends on the
evaporating moisture not being easily dried. Of this there are two
causes, quantity and quality; if the liquid is much it does not dry up
easily nor if it is greasy. And for this reason the hairs of the
head are longest in man, for the brain, being fluid and cold, supplies
great abundance of moisture.

The hairs become straight or curly on account of the vapour
arising in them. If it be smoke-like, it is hot and dry and so makes
the hair curly, for it is twisted as being carried with a double
motion, the earthy part tending downwards and the hot upwards. Thus,
being easily bent, it is twisted owing to its weakness, and this is
what is meant by curliness in hair. It is possible then that this is
the cause, but it is also possible that, owing to its having but
little moisture and much earthy matter in it, it is dried by the
surrounding air and so coiled up together. For what is straight
becomes bent, if the moisture in it is evaporated, and runs together
as a hair does when burning upon the fire; curliness will then be a
contraction owing to deficiency of moisture caused by the heat of
the environment. A sign of this is the fact that curly hair is
harder than straight, for the dry is hard. And animals with much
moisture are straight-haired; for in these hairs the moisture advances
as a stream, not in drops. For this reason the Scythians on the
Black Sea and the Thracians are straight-haired, for both they
themselves and the environing air are moist, whereas the Aethiopians
and men in hot countries are curly-haired, for their brains and the
surrounding air are dry.

Some, however, of the thick-skinned animals are fine-haired for
the cause previously stated, for the finer the pores are the finer
must the hairs be. Hence the class of sheep have such hairs (for wool
is only a multitude of hairs).

There are some animals whose hair is soft and yet less fine, as is
the case with the class of hares compared with that of sheep; in
such animals the hair is on the surface of the skin, not deeply rooted
in it, and so is not long but in much the same state as the
scrapings from linen, for these also are not long but are soft and
do not admit of weaving.

The condition of sheep in cold climates is opposite to that of
man; the hair of the Scythians is soft but that of the Sauromatic
sheep is hard. The reason of this is the same as it is also all wild
animals. The cold hardens and solidifies them by drying them, for as
the heat is pressed out the moisture evaporates, and both hair and
skin become earthy and hard. In wild animals then the exposure to
the cold is the cause of hardness in the hair, in the others the
nature of the climate is the cause. A proof of this is also what
happens in the sea-urchins which are used as a remedy in
stranguries. For these, too, though small themselves, have large and
hard spines because the sea in which they live is cold on account of
its depth (for they are found in sixty fathoms and even more). The
spines are large because the growth of the body is diverted to them,
since having little heat in them they do not concoct their nutriment
and so have much residual matter and it is from this that spines,
hairs, and such things are formed; they are hard and petrified through
the congealing effect of the cold. In the same way also plants are
found to be harder, more earthy, and stony, if the region in which
they grow looks to the north than if it looks to the south, and
those in windy places than those in sheltered, for they are all more
chilled and their moisture evaporates.

Hardening, then, comes of both heat and cold, for both cause the

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