Sexual dimorphism, physic as well as psychic, has evidently one sole cause, sex; nevertheless the organs which differ least from male and female among species which differ most, are precisely the sexual organs. That is, they are rigorously made the one for the other, and the accord in this case must be not only harmonic, but mechanical and mathematical. They are cog-wheels which must bite one on the other with exactitude, be it, as in birds, that there is but an exact superposition of two orifices, be it, as in mammals that the key must enter the keyhole. There is a dimorphism, but it is that of the mould to the cast, of the scabbard to the blade; for the parts where the contact is less strict, the parallelism is nevertheless quite sensible and quite apparent. This similitude in difference has struck philosophers as well as anatomists in all ages from the logical insinuations of Aristotle to Geoffroy Saint-Hilaire's theory of analogies. Galien had already noted certain analogies, more or less exact: greater labia, and foreskin, ovaries and testicles, scrotum and matrice. He says, textually: "All parts of man are found in woman; there is but one point of difference, woman's parts are interior, man's exterior, parting from the perineal region. Imagine those which first present themselves to mind, no matter which, unfold woman's or fold man's inward and you will find either a replica of the other. Suppose first man's organs pushed into him and extending interiorly between the rectum and the vessie; in this supposition the scrotum would occupy the place of the matrice, with the testicles placed at each side of the exterior orifice. The prong of the male would become the throat of the cavity thus produced, and the skin of the prong's extremity, called the foreskin would form the vagina. Suppose, inversely, that the matrice should turn inside out and fall outside, would not its testicles (ovaries) of necessity, find themselves inside its cavity and would not it envelop them as a scrotum? Would not the throat, hidden up to the perineum, become the male member, and the vagina, which is but a cutaneous appendix of the throat, the foreskin?" This is the passage which Diderot has transposed and put _au courant_ with science in his _Rêve d'Alembert_. This page of literary anatomy retains its expressive value: "Woman has all man's parts, the sole difference is like that between a purse hanging outside and a purse stuffed inside; a female foetus resembles a male foetus, so as to deceive anyone; the part which occasions the error, sinks in the female foetus in measure as the purse extends inward; it is never obliterated to the point of losing its primitive form; it also is the mover of pleasure, it has its gland, its foreskin, and one notes at its extremity a point which appears to have been the orifice of a urinary canal which has closed; there is in man from the anus to the scrotum, the interval called the perinæum, and