1. This is the nature of hair and its analogues and skin. All viviparous animals, with feet, have hair; oviparous animals, with feet, have scaly plates; and those fish alone which produce friable ova are covered with scales; for the conger and muræna among long fish have not such ova, and the eel produces no ova. The hair differs in thickness, thinness, and size, according to its situation, both in the parts of the body which it occupies, and the nature of the skin, for upon thick skins the hair is generally harsh and thick, the hair is both thicker and longer in the hollow and moist parts of the body, if they are such as to be covered with hair. 2. And the case is similar in those animals which are covered with plates or scales. If animals covered with soft hair are placed in good pastures their hair will become coarser; and, on the contrary, it becomes finer and less in those that have coarse hair. Warm and cold situations also make a difference, for the hair of natives of warm climates is harsh, but it is soft in those of colder climates. Straight hair is soft, crisped hair is harsh. 3. It is the nature of hair to split; and different kinds of hair are dissimilar in excess and deficiency; some are so changed by harshness as to bear slight resemblance to hair, and are more like spines, as in the hedgehog, wherein they resemble nails. So again the nails in some animals are not different from bones in point of hardness. 4. Man has the thinnest skin in proportion to his size. There is a mucous, glutinous fluid in the skin of all animals, less in some, more in others, as in the skins of oxen, from which glue is made; and sometimes glue is made from fishes. When the skin alone is cut it is insensible, especially that upon the head, from the absence of flesh between that and the bone. Wherever the skin is without flesh it does not unite again after being cut, as the thin part of the cheek, the prepuce, and the eyelid. In all animals the skin is continuous, and it is only wanting in places where there are natural passages for exudation, and at the mouth and nails. All sanguineous animals have a skin: all, however, have not hair, but those which are described above.