=Forces Developing Its Civilization.= Unlike Egypt, Babylonia was constantly subject to those thrusts from without which were essential to a great civilization. From the Arabian desert came nomadic invaders and from the mountains to the east and north and probably from northern Syria powerful, warlike peoples who either spurred the river dwellers on to strenuous activity in order to repel the hostile attacks or else as conquerors infused new blood and energy into the older races. On the other hand, the absence of constraining barriers gave ample opportunity for natural growth and expansion. The great rivers were the highways of commerce and conquest. The necessity of defence also suggested the advantages of conquest. The result was that at a very early period the armies of Babylonia had penetrated the mountains to the east and north and had carried their victorious rule as far as the shore of the Mediterranean on the west. Traders followed the armies, bearing the products of Babylonian art and in turn enriching the home-land with those of other nations. It was thus that Babylonia in time became not only the mistress of the ancient world, but also one of the chief centres from which emanated political, legal, artistic, and religious ideas and institutions that influenced all the peoples living about the eastern Mediterranean. =Civilization of Arabia.= Very different was the site of the third Semitic civilization. The eastern shores of the Red Sea are rocky and barren. No important streams or harbors are found along this cheerless coast. The eastern slope of the range of mountains that runs parallel to the Red Sea is, however, one of the garden lands of the East. The clouds, chilled by the mountains, deposit their rains here, while mountain streams make it possible by irrigation to transform this part of Arabia into a rich agricultural land. Here from an early period was found a high type of civilization. Climate, soil, and the spur of foreign invasion fostered its development. Its products were famous throughout the ancient world. But it was in the highest degree isolated from the stream of the world's progress. Its one means of communication with outside nations was by the caravans which crossed the deserts. Hence a certain halo of mystery always surrounded this distant civilization. Like that of Egypt, it lacked opportunity for expansion and communication and so failed to rise above a certain level or to make any deep or significant impression upon the other Semitic nations.