The other danger, the more imminent of the two, threatened Assyria from the south. Once again a Semitic immigration, which we distinguish as Chaldaean from earlier Semitic waves, Canaanite and Aramaean, had breathed fresh vitality into the Babylonian people. It came, like earlier waves, out of Arabia, which, for certain reasons, has been in all ages a prime source of ethnic disturbance in West Asia. The great southern peninsula is for the most part a highland steppe endowed with a singularly pure air and an uncontaminated soil. It breeds, consequently, a healthy population whose natality, compared to its death-rate, is unusually high; but since the peculiar conditions of its surface and climate preclude the development of its internal food-supply beyond a point long ago reached, the surplus population which rapidly accumulates within it is forced from time to time to seek its sustenance elsewhere. The difficulties of the roads to the outer world being what they are (not to speak of the certainty of opposition at the other end), the intending emigrants rarely set out in small bodies, but move restlessly within their own borders until they are grown to a horde, which famine and hostility at home compel at last to leave Arabia. As hard to arrest as their own blown sands, the moving Arabs fall on the nearest fertile regions, there to plunder, fight, and eventually settle down. So in comparatively modern times have the Shammar tribesmen moved into Syria and Mesopotamia, and so in antiquity moved the Canaanites, the Aramaeans, and the Chaldaeans. We find the latter already well established by 900 B.C. not only in the "Sea Land" at the head of the Persian Gulf, but also between the Rivers. The Kings of Babylon, who opposed Ashurnatsirpal and Shalmaneser II, seem to have been of Chaldaean extraction; and although their successors, down to 800 B.C., acknowledged the suzerainty of Assyria, they ever strove to repudiate it, looking for help to Elam or the western desert tribes. The times, however, were not quite ripe. The century closed with the reassertion of Assyrian power in Babylon itself by Adadnirari. SECTION 5. SYRIAN EXPANSION OF ASSYRIA