The reason why this change does not take place visibly on account of 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.
