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
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.
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