9. Every hair has at its root a glutinous moisture, which will adhere to anything with which it comes in contact, soon after it is drawn out. In spotted animals the spots exist both in the hair and upon the skin, and upon the skin of the tongue. As for the beard, some persons have a thick one, both beneath the chin and upon it; in others, these parts are smooth, and the beard is on the cheeks. Those who have smooth chins are least likely to become bald. The hair grows in some diseases, as in phthisis especially, and in old age, and upon dead bodies, and the hair becomes harder instead of softer. The same is the case with the nails. In persons of strong passions, the hair that is born with them decreases, while that which comes after birth increases. 10. Those who suffer from enlarged veins are less likely to become bald; and if they have this disease after they are bald, the hair sometimes grows again. The hair, when cut off, does not grow again from the extremity, but increases by growth from the root. The scales of fishes become harder and thicker, and in those that are growing thin and old they become still harder. The hair and wool of old animals becomes thicker, though the quantity decreases; and the hoofs and claws enlarge as they grow old, and the beaks of birds. And the claws grow in the same way as the nails. 11. Feathered animals, like birds, do not change their colour by age, excepting the crane, for this bird is ash-coloured, and becomes black by age. But from the change of season, when it becomes cold, some of those having but one colour, black or grey, become white, as the crow, sparrow, and swallow; but none of those which are white become black. At different seasons of the year many birds change the colour of their plumage, so as to render it difficult for those who are not acquainted with them to recognise them. 12. And many animals change their colour with a change of water; for in one place they are black, and in another white; and the same thing takes place at the season of coition. There are many waters of such a nature that if sheep drink of them before sexual intercourse, they produce black lambs; as at that which is called the cold river in the Thracian Chalcis (in Astyritis). And in Antandria there are two rivers, one of which turns the sheep white, the other black; and the Scamander appears to make the sheep yellow, wherefore some people think that Homer called the Scamander the Xanthus.