The reason is that
the hot have more air than water in them, and the air shining
through causes whiteness, as also in froth. As, then, skins which
are white by reason of some affection differ from those white by
nature, so also in the hair the whiteness due to disease or age
differs from that due to nature in that the cause is different; the
latter are whitened by the natural heat, the former by the external
heat. Whiteness is caused in all things by the vaporous air imprisoned
in them. Hence also in all animals not uniformly coloured all the part
under the belly is whiter. For practically all white animals are
both hotter and better flavoured for the same reason; the concoction
of their nutriment makes them well-flavoured, and heat causes the
concoction. The same cause holds for those animals which are
uniformly-coloured, but either dark or white; heat and cold are the
causes of the nature of the skin and hair, each of the parts having
its own special heat.
The tongue also varies in colour in the simply coloured as
compared with the vari-coloured animals, and again in the simply
coloured which differ from one another, as white and dark. The
reason is that assigned before, that the skins of the vari-coloured
are vari-coloured, and the skins of the white-haired and dark-haired
are white and dark in each case. Now we must conceive of the tongue as
one of the external parts, not taking into account the fact that it is
covered by the mouth but looking on it as we do on the hand or foot;
thus since the skin of the vari-coloured animals is not uniformly
coloured, this is the cause of the skin on the tongue being also
vari-coloured.
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