If, now, we wish to understand the relation of these two forms of polygamy to each other, we must first of all attempt to picture to ourselves the motives that underlie them, or, wherever the custom has become fixed through age, to bring to light the motives that were originally operative. In the case of polygamy, the immediate motive is evidently the sexual impulse of man which is more completely satisfied by the possession of several wives than by that of a single one. This motive, however, does not stand alone; as a rule other contributing circumstances are present. Two such important factors, in particular, are property rights and the power of authority. Polygyny flourishes particularly wherever the general conceptions of property and of authority, and, connected with the latter, that of the supremacy of man within the family, have attained undue importance. Under the co-operation of these motives, the wife becomes the absolute property of the husband, and may, therefore, wherever polygyny prevails among barbaric peoples, be given away or exchanged. Bound up with this, moreover, is the fact that, wherever there are considerable social differences, dependent on differences in property and rank, it is principally the wealthy or the aristocratic man who possesses many wives. In the realm of Islam, the common man is, as a rule, content with a _single_ wife, so that monogamy here prevails in the lowest stratum of society. With polyandry the case is essentially otherwise. In it, entirely different motives are operative; it might, indeed, be said that they are the exact opposite of those that bring about polygyny. It is particularly significant that polyandry is found in regions where there is a scarcity of women. This scarcity, however, is, in turn, generally due to an evil custom of barbaric culture, namely, infanticide. In Polynesia, where polyandry was very prevalent, this custom was at one time fairly rampant. Even to-day infanticide still appears to be practised by some of the Dravidian tribes of Hindustan. Similar conditions prevail among the Australians. In Polynesia, however, and probably in other localities as well, it was chiefly the female children who were the victims of infanticide. The natural result was a decrease in women and a striking numerical disproportion between the sexes. Thus, Ellis, one of the older English investigators of conditions in these territories, estimated the relation of men to women as about six to one. Under such circumstances the custom of polyandry is intelligible without further explanation. It was not possible for every one to possess a wife of his own, and so several men united to win one wife in common.