This subject deserves a purely psychological monograph, a very difficult piece of work for which this is not the proper place. I would only seek to inquire into the principal causes of the alteration of this instinct. I shall cite no facts—they are sufficiently well known, or will suggest themselves; besides, the choice lies between excessive abundance and nothing. I pass over the extreme cases, those of necrophily, or of sexual erethism accompanied by a craving for violence, destruction, or blood. These are the equivalent of the animal manifestations already mentioned, in which the state of general excitement, so far from producing tenderness, awakens by preference the aggressive tendencies. These are merely insane impulses. I am limiting myself to deviations and interversions—_i.e._, to cases where the natural mechanism of instinct is falsified (excitations caused by impressions having nothing to do with sexuality, attraction towards the same sex, etc.). We may pass over in silence the general causes, which are not particularly instructive: degeneracy, brought in, as usual, to serve as an explanation, and heredity, which is no explanation at all, being merely a repetition, and brings us back to the primary case, which states the question afresh. The inquiry can only be profitable when directed to particular causes. 1. One principal anatomical and physiological cause is found in the conformation of the genital organs: arrest of development, incomplete sexuality, hermaphrodism, malformations, etc. This is the simplest and most easily verifiable cause, and is found to be sufficient in some cases. The action, from below upwards, of the organ and its lower centre on the brain, is no longer normal; the conditions of existence of the instinct are absent or altered. 2. Other causes are not so easily assignable. One of a sociological order may be indicated: it is known what takes place when a number of individuals of the same sex are shut up together, as in boarding-schools, convents, prisons, barracks, ships on long voyages. But the most numerous causes are of psychological origin, and we may divide them into unconscious and conscious.