The right key to the solution of the problem is to be found in physiology, in that obscure department of natural science which deals with the evolution of sex. The embryo, as we are now aware, contains an undetermined element of sex during the first months of pregnancy. This is gradually worked up into male and female organs of procreation; and these, when the age of puberty arrives, are generally accompanied by corresponding male and female appetites. That is to say, the man in an immense majority of cases desires the woman, and the woman desires the man. Nature, so to speak, aims at differentiating the undecided fœtus into a human being of one or the other sex, the propagation of the species being the main object of life. Still, as Aristotle puts it, and as we observe in many of her operations, "Nature wishes, but has not always the power": ἡ φὑσις Βὁυλεται μἑν ἁλλ' οὑ δὑναται. Consequently in respect of physical structure, there come to light imperfect individuals, so-called hermaphrodites, whose sexual apparatus is so far undetermined that many a real male has passed a portion of his life under a mistake, has worn female clothes, and has cohabited by preference with men. Likewise, in respect of spiritual nature, there appear males who, notwithstanding their marked masculine organisation, feel from the earliest childhood a sexual proclivity toward men, with a corresponding indifference for women. In some of these abnormal, but natural, beings, the appetite for men resembles the normal appetite of men for women; in others it resembles the normal appetite of women for men. That is to say, some prefer effeminate males, dressed in feminine clothes and addicted to female occupations. Others prefer powerful adults of an ultra-masculine stamp. A third class manifest their predilection for healthy young men in the bloom of adolescence, between nineteen and twenty. The attitude of such persons towards women also varies. In genuine cases of inborn sexual inversion a positive horror is felt when the woman has to be carnally known; and this horror is of the same sort as that which normal men experience when they think of cohabitation with a male.[55] In others the disinclination does not amount to repugnance; but the abnormal man finds considerable difficulty in stimulating himself to the sexual act with females, and derives a very imperfect satisfaction from the same. A certain type of man, in the last place, seems to be indifferent, desiring males at one time and females at another. In order to gain clearness in his exposition, Ulrichs has invented names for these several species. The so-called hermaphrodite he dismisses with the German designation of _Zwitter_. Imperfect individuals of this type are not to be considered, because it is well known that the male or female organs are never developed in one and the same body. It is also, as we shall presently discover, an essential part of his theory to regard the problem of inversion psychologically.