A question of historical psychology which, so far as I know, has never been fully investigated is the influence of menstruation in constituting the emotional atmosphere through which men habitually view women.[353] I do not purpose to deal fully with this question, because it is one which may be more properly dealt with at length by the student of culture and by the historian, rather than from the standpoint of empirical psychology. It is, moreover, a question full of complexities in regard to which it is impossible to speak with certainty. But we here strike on a factor of such importance, such neglected importance, for the proper understanding of the sexual relations of men and women, that it cannot be wholly ignored. Among the negroes of Surinam a woman must live in solitude during the time of her period; it is dangerous for any man or woman to approach her, and when she sees a person coming near she cries out anxiously: "_Mi kay! Mi kay!_"--I am unclean! I am unclean! Throughout the world we find traces of the custom of which this is a typical example, but we must not too hastily assume that this custom is evidence of the inferior position occupied by semi-civilized women. It is necessary to take a broad view, not only of the beliefs of semi-civilized man regarding menstruation, but of his general beliefs regarding the supernatural forces of the world. There is no fragment of folk-lore so familiar to the European world as that which connects woman with the serpent. It is, indeed, one of the foundation stones of Christian theology.[354] Yet there is no fragment of folk-lore which remains more obscure. How has it happened that in all parts of the world the snake or his congeners, the lizard and the crocodile, have been credited with some design, sinister or erotic, on women?