Little doubt need remain that the Arabians during their sojourn in Palestine and Syria met with distinctive types, and if not with pure Armenoids, at any rate with peoples having Armenoid traits. The consequent multiplication of tribes, and the gradual pressure exercised by the constant stream of immigrants from Arabia and Asia Minor, must have kept this part of Western Asia in a constant state of unrest. Fresh migrations of the surplus stock were evidently propelled towards Egypt in one direction, and the valleys of the Tigris and Euphrates in another. The Semites of Akkad were probably the conquerors of the more highly civilized Sumerians, who must have previously occupied that area. It is possible that they owed their success to the possession of superior weapons. Professor Elliot Smith suggests in this connection that the Arabians had become familiar with the use of copper as a result of contact with the Egyptians in Sinai. There is no evidence, however, that the Sumerians were attacked before they had begun to make metal weapons. It is more probable that the invading nomads had superior military organization and considerable experience in waging war against detached tribal units. They may have also found some of the northern Sumerian city states at war with one another and taken advantage of their unpreparedness to resist a common enemy. The rough Dorians who overran Greece and the fierce Goths who shattered the power of Rome were similarly in a lower state of civilization than the peoples whom they subdued.