It is certain that this mechanical mixture caused by political forces, influences the development of _castes_, that is to say, of hereditary professions, which at the same time form a hierarchy of social classes. “Castes are usually, if not always, consequences of conquest and subjugation by foreigners.”[53] Although this problem has not been completely solved, it may be said that the formation of castes has been very strongly influenced by economic and religious factors. It is probable that castes came about in some such way as this: state-forming forces penetrated into existing economic organizations, and vocations underwent adaptation, and then became petrified under the influence of religious concepts, which, however, may also have influenced their original formation. This seems to follow from the fact that even as between man and woman there exist certain separations of vocation, which, so to say, are taboo and impassable. Thus among all huntsmen, tilling the ground is woman’s work, while among many African shepherds, as soon as the ox-plow is used, agriculture becomes man’s work, and then women may not, under pain of sacrilege, use the domestic cattle.[H] [H] Similarly there are North Asiatic tribes of huntsmen, where women are definitely forbidden to touch the hunting gear or to cross a hunting trail.--Ratzel I, page 650. It is likely that such religious concepts may have brought it about that a vocation became hereditary, and then compulsorily hereditary, especially where a tribe or a village carried on a particular craft. This happens with all tribes in a state of nature, where intercourse is easily possible, especially in the case of islanders. When some such group has been conquered by another tribe, the subjects, with their developed hereditary vocations, tend to form within the new state entity a pure “caste.” Their caste position depends partly upon the esteem they had heretofore enjoyed among their own people, and partly upon the advantage which their vocation affords their new masters. If, as was often the case, waves of conquest followed one another in series, the formation of castes might be multiplied, especially if in the meantime economic development had worked out many vocational classes.