3. So also some animals are produced from animals of a similar form, the origin of others is spontaneous, and not from similar forms; from these and from plants are divided those which spring from putrid matter, this is the case with many insects; others originate in the animals themselves, and from the excrementitious matter in their parts; those which originate from similar animals, and have both the sexes are produced from coition, but of the class of fishes there are some neither male nor female, these belong to the same class among fishes, but to different genera, and some are quite peculiar. In some there are females but no males, by these the species is continued as in the hypenemia among birds. 4. All these among birds are barren, (for nature is able to complete them as far as the formation of an egg,) unless persons suppose that there is another method of communicating the male influence, concerning which we shall speak more plainly hereafter. In some fish, after the spontaneous production of the ovum, it happens that living creatures are produced, some by themselves, others by the aid of the male. The manner in which this is done will be made plain in a future place, for nearly the same things take place in the class of birds. 5. Whatever are produced spontaneously in living creatures, in the earth, or in plants, or in any part of them, have a distinction in the sexes, and by the union of the sexes something is produced, not the same in any respect, but an imperfect animal, as nits are produced from lice, and from flies and butterflies are produced egg-like worms, from which neither similar creatures are produced, nor any other creature, but such things only. First of all, then, we will treat of coition, and of the animals that copulate, and then of others, and successively of that which is peculiar to each, and that which is common to them all. CHAPTER II. 1. Those animals in which there is a distinction of the sexes use sexual intercourse, but the mode of this intercourse is not the same in all, for all the males of sanguineous animals with feet have an appropriate organ, but they do not all approach the female in the same manner, but those which are retromingent, as the lion, the hare, and the lynx, unite backwards, and the female hare often mounts upon the male; in almost all the rest the mode is the same, for most animals perform the act of intercourse in the same way, the male mounting upon the female; and birds perform it in this way only.