2. From these considerations it is plain that every female contributes to the semen, if this appears to take place in any one class of animals, for the barren animal differs in no respect from the other, except that it does not produce an animal, and this because it was formed by the union of both sexes. For this reason all the seminal fluid of the male does not appear to be productive, but some parts are barren, when not properly compounded from both sexes. And when women have lascivious dreams, the same affections of weakness and debility often occur, as if they had been lying with a male. It is plain, therefore, that if they appear to have emitted a seminal fluid in their dream, they will then conjecture that after their dream the same place will become moist, and they will be obliged to bestow the same attention upon themselves as if they had had sexual intercourse. So that it is evident that there must be an emission of semen from both if it is to be productive. 3. But the uterus does not emit its semen into itself, but on the outside, into the place where that of the male also is received, and then draws it into itself. For some females produce spontaneously, as the bird produces barren eggs, and other females do not so, as the horses and sheep; either because the bird projects her semen into the uterus, and the place upon which that of the male is emitted is not external; for which reason, if he does not copulate properly with the female, it is poured out upon the ground. But in quadrupeds there is another place for the reception of the semen, both of the male and female, which in other animals it is combined with other fluids of the body, and is not collected in the uterus, because it does not enter it. But in birds, the uterus receives and matures the seminal fluid, and forms a body similar in other respects though not a living creature. It is necessary, therefore, the living creature should be derived from both sexes. CHAPTER VII.