The same disturbances which cause a break in the chain of sexual activity during childhood, puberty and adolescence are also responsible that in civilized men we can hardly speak any longer of an instinct of sex. If the instinct has not been disturbed by the intrusion of the stimuli of the senses and of the imagery before the period of maturity, the instinct of sex should enter into play at the time of puberty. But, as a rule, the stimulation of the erogenous zones is effected already before the time has arrived for the sex-instinct to make its appearance. During this stimulation, which, as we have seen, is already a part of the sex-act, a certain fore-pleasure is experienced. Henceforth the individual is desirous of the repetition of this experience. Especially when the orgasm has once been tasted, the memory of the same causes the individual to look for a repetition of the highest lustful sensation in human experience. Sexual activity is henceforth chiefly desired for the positive lust-feeling connected with it. This means to say that calculation enters into the activity, and with the entrance of calculation the activity ceases to be instinctive. Hence in modern men and women neither sex-activity nor propagation partake of the nature of an instinct. Normal sex-attraction being an offshoot of the reproductive impulse, it is perfectly natural that if the parent impulse loses its instinctive nature, the offspring will also lose it. CHAPTER XI DEVELOPMENT OF THE REPRODUCTIVE IMPULSE The instinct of propagation is normally derived from the impulse for the satisfaction of protoplasmatic hunger. In lower animal life the incipient sexual union is effected by two exhausted cells coming together for the mutual exchange of nuclear material. This mode represents the first step in the scale of conjugation. In the lowest form of unicellular life, as the schizomycetes (yeast) or bacteria, the necessity for conjugation does not appear to exist. In the reproduction of these unicellular organisms the animal simply divides into two, the division of the nucleus, as a rule, preceding that of the cytoplasm by a more or less karyokynetic method. The ordinary protozoon does not form a composite structure. It divides and multiplies, but the products of the division do not remain together, they leave each other and lead a separate existence. Hence there is no real death in these animals. In the metazoa, or many-celled animals, only the reproductive cells may escape death and continue to live in the offspring. The somatic cells, or the body, die after a longer or shorter period. The unicellular animals do not possess any somatic cells, they are all reproductive cells. Hence we may rightly speak of the “immortality” of the protozoa.