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On The Generation Of Animals   

age in other animals is the same as that already given in the case
of baldness; their brain is small and less fluid than in man, so
that the heat required for concoction does not altogether fail.
Among them it is most clear in horses of all animals that we know,
because the bone about the brain is thinner in them than in others
in proportion to their size. A sign of this is that a blow to this
spot is fatal to them, wherefore Homer also has said: 'where the first
hairs grow on the skull of horses, and a wound is most fatal.' As then
the moisture easily flows to these hairs because of the thinness of
the bone, whilst the heat fails on account of age, they go grey. The
reddish hairs go grey sooner than the black, redness also being a sort
of weakness of hair and all weak things ageing sooner. It is said,
however, that cranes become darker as they grow old. The reason of
this would be, if it should prove true, that their feathers are
naturally moister than others and as they grow old the moisture in the
feathers is too much to decay easily.

Greyness comes about by some sort of decay, and is not, as some
think, a withering. (1) A proof of the former statement is the fact
that hair protected by hats or other coverings goes grey sooner
(for the winds prevent decay and the protection keeps off the winds),
and the fact that it is aided by anointing with a mixture of oil and
water. For, though water cools things, the oil mingled with it
prevents the hair from drying quickly, water being easily dried up.
(2) That the process is not a withering, that the hair does not whiten
as grass does by withering, is shown by the fact that some hairs
grow grey from the first, whereas nothing springs up in a withered
state. Many hairs also whiten at the tip, for there is least heat in
the extremities and thinnest parts.

When the hairs of other animals are white, this is caused by nature,
not by any affection. The cause of the colours in other animals is the
skin; if they are white, the skin is white, if they are dark it is
dark, if they are piebald in consequence of a mixture of the hairs, it
is found to be white in the one part and dark in the other. But in man
the skin is in no way the cause, for even white-skinned men have
very dark hair. The reason is that man has the thinnest skin of all
animals in proportion to his size and therefore it has not strength to
change the hairs; on the contrary the skin itself changes its colour
through its weakness and is darkened by sun and wind, while the
hairs do not change along with it at all. But in the other animals the
skin, owing to its thickness, has the influence belonging to the
soil in which a thing grows, therefore the hairs change according to
the skin but the skin does not change at all in consequence of the
winds and the sun.


Of animals some are uni-coloured (I mean by this term those of
which the kind as a whole has one colour, as all lions are tawny;
and this condition exists also in birds, fish, and the other classes
of animals alike); others though many-coloured are yet whole-coloured

(I mean those whose body as a whole has the same colour, as a bull is
white as a whole or dark as a whole); others are vari-coloured.
This last term is used in both ways; sometimes the whole kind is
vari-coloured, as leopards and peacocks, and some fish, e.g. the
so-called 'thrattai'; sometimes the kind as a whole is not so, but
such individuals are found in it, as with cattle and goats and,
among birds, pigeons; the same applies also to other kinds of birds.
The whole-coloured change much more than the uniformly coloured,
both into the simple colour of another individual of the same kind
(as dark changing into white and vice versa) and into both colours
mingled. This is because it is a natural characteristic of the kind as

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