"[I]n hair and feathers of every kind, changes always occur either... when the nutriment in them fails, or when, on the contrary, it is too abundant. Therefore the age at which the hair is at its whitest or blackest varies in different cases... But hair is never crimson or violet or green or any other colour of that kind, because all such colours arise only by mixture with the rays of the sun, and further because in all hairs which contain moisture the changes take place beneath the skin, and so they admit of no admixture. ...Thus in birds, as in plants, the maturation of the colours takes place outside the body. So, too, the other forms of animal life aquatic creatures, reptiles, and shell-fish have all sorts and manners of colouring, because in them too the process of maturation is violent. From what has been set forth in this treatise one may best understand the scientific theory of colours."