Those animals which are viviparous, have hair; those which are oviparous, have feathers, scales, or a shell, like the tortoise; or else a purple skin, like the serpent. The lower part of all feathers is hollow; if cut, they will not grow again, but if pulled out, they will shoot afresh. Insects fly by the aid of a frail membrane; the wings of the fish[321] called the “swallow” are moistened in the sea, while those of the bat which frequents our houses are dry; the wings of this last animal have certain articulations as well. The hairs that issue from a thick skin are rough, while those on females are of a finer quality. Those found on the horse’s mane are more abundant, which is the case also with the shoulders of the lion. The dasypus has hair in the inside of the mouth even and under the feet, two features which Trogus has also attributed to the hare; from which the same author concludes that hairy men are the most prone to lust. The most hairy of all animals is the hare. Man is the only creature that has hair as the mark of puberty; and a person who is devoid of this, whether male or female, is sure to be sterile. The hair of man is partly born with him, and in part produced after his birth. The last kind of hair will not grow upon eunuchs, though that which has been born with them does not fall off; which is the case also with women, in a great degree. Still however, there have been women known to be afflicted with falling off of the hair, just as some are to be seen with a fine down on the face, after the cessation of the menstrual discharge. In some men the hair that mostly shoots forth after birth will not grow spontaneously. The hair of quadrupeds comes off every year, and grows again. That of the head in man grows the fastest, and next to it the hair of the beard. When cut, the hairs shoot, not from the place where they have been cut, as is the case with grass, but at the root. The hair grows quickly in certain diseases, phthisis more particularly; it grows also with rapidity in old age, and on the body after death. In persons of a libidinous tendency the hair that is produced at birth falls off more speedily, while that which is afterwards produced grows with the greatest rapidity. In quadrupeds, the hair grows thicker in old age; but on those with wool, it becomes thinner. Those quadrupeds which have thick hair on the back, have the belly quite smooth. From the hides of oxen, and that of the bull more especially, glue is extracted by boiling. CHAP. 95.—THE PAPS: BIRDS THAT HAVE PAPS. REMARKABLE FACTS CONNECTED WITH THE DUGS OF ANIMALS.