Ulrichs replies that the phenomenon of sexual inversion is to be explained by physiology, and particularly by the evolution of the embryo.[56] Nature fails to complete her work regularly and in every instance. Having succeeded in differentiating a male with full-formed sexual organs from the undecided fœtus, she does not always effect the proper differentiation of that portion of the psychical being in which resides the sexual appetite. There remains a female soul in a male body. _Anima muliebris virili corpore inclusa_, is the formula adopted by Ulrichs; and he quotes a passage from the "Vestiges of Creation," which suggests that a male is a more advanced product of sexual evolution than the female. The male instinct of sex is a more advanced product than the female instinct. Consequently men appear whose body has been differentiated as masculine, but whose sexual instinct has not progressed beyond the feminine stage. Ulrichs' own words ought to be cited upon this fundamental part of his hypothesis, since he does not adopt the opinion that the Urning is a Dioning arrested at a certain point of development; but rather that there is an element of uncertainty attending the simultaneous evolution of physical and psychical factors from the indeterminate ground-stuff. "Sex," says he, "is only an affair of development. Up to a certain stage of embryonic existence all living mammals are hermaphroditic. A certain number of them advance to the condition of what I call man (Doining), others to what I call woman (Dioningin), a third class become what I call _Urning_ (including _Urningin_). It ensues therefrom that between these three sexes there are no primary, but only secondary differences. And yet true differences, constituting sexual species, exist as facts."[57] Man, Woman, and Urning--the third being either a male or a female in whom we observe a real and inborn, not an acquired or a spurious, inversion of appetite--are consequently regarded by him as the three main divisions of humanity viewed from the point of view of sex. The embryonic ground-stuff in the case of each was homologous; but while the two former, Man and Woman, have been normally differentiated, the Urning's sexual instinct, owing to some imperfection in the process of development, does not correspond to his or her sexual organs.