Although the testicles and seminal vesicles of a man, contain all the necessary molecules to form a male, yet the local establishment and arrangement of these molecules cannot be made, because the effect of an union is prevented by the continual circulation of the seed both by absorption, and the action of the new organic molecules which constantly come into this reservoir from all parts of the body. The same circumstances taking place with the organic molecules of the female, is an evident reason why neither can produce of themselves, because when the seminal liquors of the male and female are mixed, they have more analogy to each other, than with the parts of the body of the female where the mixture is performed. By admitting of this explication, it may be asked, Why the common mode of generation in animals does not agree with it; for, upon that supposition, each individual would produce like snails, and impregnate each other, and each individual receiving the organic molecules the other furnished, the union would be made of itself, and by the sole power of the affinity of these molecules among themselves? I own, if it was by this cause alone the organic molecules could unite it would be natural to conclude, that the shortest mode to perform the reproduction of animals, would be to give to one individual both sexes. But it is quite contrary to the general rule pursued by Nature, as this manner of generation is confined to snails, and a small number of other animals. This answer cannot be said to fully satisfy the question, as it merely supposes the male does not produce, as it cannot receive any thing from the female, and that having besides no proper viscera to contain and nourish the foetus.
We may also suppose that the activity of the organic molecules, in the semen of one individual, has need of being counterbalanced by the activity or force of those of another individual, in order to fix and bring them into a kind of equilibrium, a state of rest highly necessary to the formation of the animal; and that this activity in the organic molecules can only be counterbalanced by there being a contrary action in those which come from the male, and those proceeding from the female; so that, in this sense, all living or vegetating beings must have two sexes, conjointly and separately, to produce their resemblances. But this answer is too general to be entirely clear; nevertheless, if we pay attention to all the phenomena, we shall find some explanation resulting therefrom. The mixture of those two liquors produces not only a male or female foetus, but also other organized bodies, which have a kind of growth or expansion. The placenta, membranes, &c. are produced at the same time as the foetus. There are, therefore, in the seminal liquor of the male or female, or in the mixture of both, not only organic molecules necessary for the production of the foetus, but also those which form the placenta and membranes. We know not from whence these molecules come, since there is no part of the body, either of the male or female, from which they could be sent back. From hence it seems it must be admitted, that the molecules of the seminal liquors of each, being alike active, form organized bodies every time they can fix, by acting mutually one on the other: that the particles employed to form a male, will be those of the masculine sex, which will fix the first, and form the sexual parts; and that those common to both sexes will then fix indifferently to form the rest of the body, and that the placenta and membranes are then formed from the superabundant particles, which have not been used to form the foetus; if, as we suppose, the foetus is a male, then there remains to form the placenta, and membranes, all the organic particles peculiar to the feminine sex which have not been employed; and also all those of both which shall not have entered the composition of the foetus, and which cannot be less than one half. So likewise, if the foetus is a female, the same abundance will be left for the formation of the placenta, and membranes, and the whole effects be the same, excepting it will have the superfluity of the male, instead of that of the female.