another motive for strengthening the bonds of tribal unity. Agricultural plants, like domesticated animals, are practically all of Asiatic origin. There are, however, three important exceptions, namely, maize among cereals and the two varieties of potato, all of which are indigenous to the Western hemisphere, and hence were necessarily unknown to the civilised nations of antiquity. With these exceptions all the important agricultural plants had been known and cultivated for numberless generations before the opening of the historic period. _Seventh. Government._ We have just seen how the introduction of domesticated animals and agricultural plants must have influenced the communal habits of primitive man in the direction of the establishment of local government. There are reasons to believe that, prior to taking these steps, the most advanced form of human settlement was the tribe or clan consisting of the members of a single family. The unit of this settlement was the single family itself with a man at its head, who was at once provider, protector, and master. As the various members of a family held together in obedience to the gregarious instinct, which man shares with the greater number of animals, it was natural that some one member of the clan should be looked to as the leader of the whole. In the ordinary course of events, such leader would be the oldest man, the founder of the original family; but there must have been a constant tendency for younger men of pronounced ability to aspire to the leadership, and to wrest from the patriarch his right of mastery.