[13] The organic periodicity of the sexual process has persisted, it is true, but its effect on mental sexual excitation has been almost reversed. This change is connected primarily with the diminishing importance of the olfactory stimuli by means of which the menstrual process produced sexual excitement in the mind of the male. Their function was taken over by visual stimuli, which could operate permanently, instead of intermittently like the olfactory ones. The ‘taboo of menstruation’ has its origin in this ‘organic repression’, which acted as a barrier against a phase of development that had been surpassed; all its other motivations are probably of a secondary nature. (Cf. C. D. Daly, ‘Hindu-mythologie und Kastrationskomplex’, _Imago_, Bd. xiii., 1927.) This process is repeated on a different level when the gods of a foregone cultural epoch are changed into demons in the next. The diminution in importance of olfactory stimuli seems itself, however, to be a consequence of man’s erecting himself from the earth, of his adoption of an upright gait, which made his genitals, that before had been covered, visible and in need of protection and so evoked feelings of shame. Man’s erect posture, therefore, would represent the beginning of the momentous process of cultural evolution. The chain of development would run from this onward, through the diminution in the importance of olfactory stimuli and the isolation of women at their periods to a time when visual stimuli became paramount, the genitals became visible, further till sexual excitation became constant and the family was founded, and so to the threshold of human culture. This is only a theoretical speculation, but it is important enough to be worth checking carefully by the conditions obtaining among the animals closely allied to man.