There can be no doubt that--as Ploss and Bartels, from whom some of these examples have been taken, point out--in widely different parts of the world menstruation is believed to have been originally caused by a snake, and that this conception is frequently associated with an erotic and mystic idea.[361] How the connection arose Ploss and Bartels are unable to say. It can only be suggested that its shape and appearance, as well as its venomous nature, may have contributed to the mystery everywhere associated with the snake--a mystery itself fortified by the association with women--to build up this world-wide belief regarding the origin of menstruation. This primitive theory of the origin of menstruation probably brings before us in its earliest shape the special and intimate bond which has ever been held to connect women, by virtue of the menstrual process, with the natural or supernatural powers of the world. Everywhere menstruating women are supposed to be possessed by spirits and charged with mysterious forces. It is at this point that a serious misconception, due to ignorance of primitive religious ideas, has constantly intruded. It is stated that the menstruating woman is "unclean" and possessed by an evil spirit. As a matter of fact, however, the savage rarely discriminates between bad and good spirits. Every spirit may have either a beneficial or malignant influence. An interesting instance of this is given in Colenso's _Maori Lexicon_ as illustrated by the meaning of the Maori word _atua_.