From all these indications we may assume that when the Arians had settled in Margiana, Bactria, and Sogdiana, and agriculture became of importance beside the breeding of cattle, the necessity of protection against the migratory tribes of the endless plains stretching to the north, created among the Arians a warlike nobility who took upon themselves the duty of defence. The valley of the Zarefshan (Sogdiana), the terrace of Bactria, the region of Merv, became in the hands of the Arians advanced posts of civilisation in the desert. If Western Iran was protected in the north by the Alps of Aderbeijan and the Caspian Sea against attacks from that quarter, Eastern Iran lay open to the nomads of the steppes, and had nothing but arms to defend its cultivated lands. We have already seen that Bactria even in the sixth century had passed beyond the earliest stages of civilisation (p. 24). But even a less degree of prosperity was sufficient to excite the sons of the desert to invasion. Hence we may assume that the incursions and raids of the nomads of the steppes began with the increase of the flocks and the prosperity of agriculture in the valleys of Merv, Bactria, and Sogdiana. The increasing severity of these attacks compelled the Bactrian soldiery to collect their forces for more successful resistance, and to place the best warriors at the head of the community. Thus it was not by spontaneous development, but rather by the opposition to the nations of the steppes, that the north-east of Iran first outgrew the tribal life, and became transformed into a larger state. Of this kingdom Aurvataçpa and Vistaçpa became the rulers; in the Avesta they are distinguished from the Paradhatas and from Kava Kavata, Kava Uça, and Kava Huçrava, by a new addition to the name and other peculiar traits, and form a third group. The progress of our investigation will show that the formation of this Bactrian kingdom cannot be placed later than 1100 B.C.; the date of Vistaçpa must be put about 1000 B.C., and it was the successors of Vistaçpa who sent to Shalmanesar II. the tribute of camels with two humps, and yaks (about 850 B.C.), who found themselves menaced once more by the advances of Tiglath Pilesar II. to Arachosia in the year 745 B.C., and at length succumbed to Cyrus. We shall find that this kingdom was not without its warlike races and priestly families, that Zariaçpa and Bactria were the centres of it, and that the sovereigns attained despotic power. Yet the old warlike families must have preserved a certain importance under the monarchy, unless they regained it when lost under the viceroys of the Achæmenids. It was the chieftains of the Bactrians whom Alexander summoned to Zariaçpa, and who with the Sogdiani at their side took the lead in resistance. The most powerful of them stubbornly defended their rocky citadels against the Macedonians. FOOTNOTES: [26] Polyb. fragm. 34 f.; below, p. 26.