In this case, the cause is to be sought only in an anomaly of central conditions,—in an abnormal psycho-sexual constitution. This constitution, as far as its anatomical and functional foundation is concerned, is absolutely unknown. Since, in almost all such cases, the individual subject to the perverse sexual instinct displays a neuropathic predisposition in several directions, and the latter may be brought into relation with hereditary degenerate conditions, this anomaly of psycho-sexual feeling may be called, clinically, a functional sign of degeneration. This perverse sexuality appears spontaneously, without external cause, with the development of sexual life, as an individual manifestation of an abnormal form of the vita sexualis, and then has the force of a _congenital_ phenomenon; or it develops upon a sexuality the beginning of which was normal, as a result of very definite injurious influences, and thus appears as an _acquired_ anomaly. Upon what this enigmatical phenomenon of acquired homo-sexual instinct depends is still inexplicable, and only a matter for hypothesis. Careful examination of the so-called acquired cases makes it probable that the predisposition also present here consists of a latent homo-sexuality, or, at least, bi-sexuality, which, for its manifestation, requires the influence of accidental exciting causes to rouse it from its slumber. In so-called contrary sexual instinct there are degrees of the phenomenon which quite correspond with the degrees of predisposition of the individuals. Thus, in the milder cases, there is simple hermaphroditism; in more pronounced cases, only homo-sexual feeling and instinct, but limited to the vita sexualis; in still more complete cases, the whole psychical personality, and even the bodily sensations, are transformed to correspond with the sexual perversion; and, in the complete cases, the physical form is correspondingly altered. The following division of the various phenomena of this psycho-sexual anomaly is made, therefore, in accordance with these clinical facts:— A. _Homo-sexual Feeling as an Acquired Manifestation._—The determining condition here is the demonstration of perverse feeling for the same sex; not the proof of sexual acts with the same sex. These two phenomena must not be confounded with each other; perversity must not be taken for perversion. Perverse sexual acts, not dependent upon perversion, often come under observation. This is especially true with reference to sexual acts between persons of the same sex, particularly pederasty. Here paræsthesia sexualis is not necessarily at work; but hyperæsthesia, with physical or mental impossibility of natural sexual satisfaction. Thus we find homo-sexual intercourse in impotent masturbators or debauchees, or _faute de mieux_ in sensual men and women in imprisonment, on ship-board, in garrisons, bagnios, boarding-schools, etc.