Some animals, again, are engendered of beings that are not engendered themselves, and have no such origin as those above mentioned, which are produced in the spring, or at some stated period of the year. Some of these are non-productive, the salamander, for instance, which is of no sex, either male or female; a distinction also, which does not exist in the eel and the other kinds that are neither viviparous nor oviparous. The oyster also, as well as the other shell-fish that adhere to the bottom of the sea or to rocks, are of neither sex. Again, as to those animals which are able to engender of themselves, if they are looked upon as divided into male and female, they do engender something, it is true, by coupling, but the produce is imperfect, quite dissimilar to the animal itself, and one from which nothing else is reproduced; this we find to be the case with flies, when they give birth to maggots. This fact is better illustrated by the nature of those animals which are known as insects; a subject, indeed, very difficult of explanation, and one which requires to be treated of in a Book[1] by itself. We will, therefore, proceed for the present with our remarks upon the instincts of the animals that have been previously mentioned.