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.
We have now spoken pretty much of all the other conditions of hair.
4
But as to their colour, it is the nature of the skin that is the
cause of this in other animals and also of their being uni-coloured or
vari-coloured); but in man it is not the cause, except of the hair
going grey through disease (not through old age), for in what is
called leprosy the hairs become white; on the contrary, if the hairs
are white the whiteness does not invade the skin. The reason is that
the hairs grow out of skin; if, then, the skin is diseased and white
the hair becomes diseased with it, and the disease of hair is
greyness. But the greyness of hair which is due to age results from
weakness and deficiency of heat.
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