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.
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