The theory of the congenital nature of homosexuality was based for the most part on the common assumption that the condition is primary and premature in its occurrence, and that it is exclusive of the opposite mode of sexual sensibility. But for several reasons the inference is not justified. For, first of all, for many cases it is incorrect to assume that the homosexual inclinations are thus exclusive in their character; as I have previously explained, the adult homosexual's belief that from early childhood he has never experienced any other than homosexual inclinations, depends in many instances on an illusion of memory. Owing to the fact that in consequence of the fuller development of homosexuality he is no longer interested in the heterosexual, he is apt to forget any early heterosexual inclinations. Secondly, the primary appearance of homosexual inclinations does not prove that these inclinations are congenital; for in homosexuals, as in heterosexuals, the specialised mode of sexual sensibility is preceded by a period in which the sexual impulse is undifferentiated; and, in homosexuals and heterosexuals alike, chance plays a great part in determining which mode of sexual sensibility first manifests itself. The congenital nature of heterosexuality is not disproved by the fact that one who in adult life possesses a normal mode of sexual sensibility, may as a schoolboy have first experienced sexual desire towards a school-fellow; just as little, then, does a similar early history in one who in adult life is homosexual in his inclinations, prove that his homosexuality is congenital. In the animal world also, before the occurrence of sexual maturity, the love-games occasionally display a similar confusion of roles, so that the sexually immature female animal may attempt to cover the youthful male. The congenital nature of homosexuality is displayed, not by the primary appearance of this mode of sensibility, but by the fact that when the puberal development takes place, the homosexual sentiments persist, and are not replaced by heterosexuality.