1. Among viviparous quadrupeds the porcupines and bears hybernate. It is evident that the wild bears conceal themselves; but there is some doubt whether it is on account of the cold or from any other cause, for at this season both the males and females are so fat that they cannot move easily. The female also produces her young at this season, and hides herself until the cubs are of an age to be led forth. This she does in the spring, about three months after the solstice, and she continues invisible for at least forty days. During fourteen days of this period they say that she does not move at all. For more than this period afterwards she remains invisible, but moves about and is awake. A pregnant bear has either never or very rarely been captured; and it is quite plain that they eat nothing during the whole of this period; for they never come out; and if they are captured, their stomach and entrails appear to be empty; and it is said that, because nothing is presented to it, the intestine sometimes adheres to itself; and, therefore, at their first emergence, they eat the arum, in order to open the entrail and make a passage through it. 2. The dormouse hybernates in trees and is then very fat, and the white Pontic mouse. (Some hybernating animals cast their old age, as it is called. This is the outer skin and the coverings at the period of birth.) It has already been observed, that among viviparous animals with feet there is some doubt as to the cause of the hybernation of bears; but almost all animals with scales hybernate and cast their old age; that is, all that have a soft skin and no shell, as the tortoise; for both the tortoise and the emys belong to the class of animals with scales; but all such as the gecko, lizard, and especially the serpents, cast their skins; for they do this both in the spring, when they first emerge, and again in the autumn. 3. The viper also casts its skin both in the spring and autumn, and is not, as some persons say, the only serpent that does not cast its skin. When serpents begin to cast their skin, it is first of all separated from their eyes; and to those who do not know what is about to happen they appear to be blind. After this it is separated from the head, for first of all it appears entirely white. In a night and day the whole of the old skin is separated from the commencement at the head to the tail; and when cast it is turned inside out, for the serpent emerges as the infant does from the chorion. 4. Insects which cast their skins do it in the same way as the silpha, empis, and the coleoptera, as the beetle. All creatures cast it after birth; for in viviparous animals the chorion is separated, and in the vermiparous, as bees and locusts, they emerge from a case. The grasshoppers, when they cast their skins, sit upon olives and reeds. When the case is ruptured, they emerge, and leave a little fluid behind them, and after a short time they fly away and sing.