Let us return, however, to the life-sustaining sexual instincts. We have already learned from the investigation of the protozoa that the mingling of two individuals without consequent partition, just as copulation between two individuals which soon after separate, has a strengthening and rejuvenating effect (v. s. Lipschütz). There is no sign of degeneration in their descendents, and they also seem to have gained the capacity for withstanding for a longer time the injurious results of their own metabolism. I think that this one observation may be taken as a prototype of the effect of sexual intercourse also. But in what way does the blending of two slightly different cells bring about such a renewal of life? The experiment which substitutes for conjugation among protozoa the effect of chemical or even of mechanical stimuli[33] admits of our giving a reply with certainty: it comes about by the introduction of new stimulus masses. This is in close agreement with the hypothesis that the life-process of an individual leads, from internal causes, to the equalising of chemical tensions: i. e. to death, while union with an individually different living substance increases these tensions—so to speak, introduces new vital differentia, which then have to be again lived out. For this difference between the two there must naturally be one or more optima. Our recognition that the ruling tendency of psychic life, perhaps of nerve life altogether, is the struggle for reduction, keeping at a constant level, or removal of the inner stimulus tension (the Nirvana-principle, as Barbara Low terms it)—a struggle which comes to expression in the pleasure-principle—is indeed one of our strongest motives for believing in the existence of death-instincts. But the course of our argument is still disturbed by an uneasy feeling that just in the case of the sexual instinct we are unable to demonstrate that character of a repetition-compulsion which first put us on the track of the death-instincts. It is true that the realm of embryonic developmental processes offers an abundance of such repetition phenomena—the two germ cells of sexual propagation and their life history are themselves only repetitions of the beginning of organic life: but the essential feature in the processes designed by the sexual instinct is nevertheless the mingling of two cells. Only by this is the immortality of the living substance among the higher forms of life assured. To put it in other words: we have to make enquiry into the origin of sexual propagation and the source of the sexual instincts in general, a task before which the lay mind quails and which even specialists have not yet been able to solve. Let us, therefore, make a condensed selection from all the conflicting accounts and opinions of whatever can be brought into relation with our train of thought.