74 MIGRATIONS.
If we may apply here the law which I have stated as controlling the growth of primitive cities built of air-dried bricks in dry climates, the thickness of the culture-strata exposed by De Morgan's excavations would give an interval of about I,000 years (at the rate of 2 feet per century) between the end of the stone age and the end of the "archaic" period. Since the stone age and transition strata occur up to 6o feet above the plain, and remains of occupation extend several meters below the level of the plain, it is not improbable that there are not less than 2,000 or 3,000 years of prearchaic culture represented here.
Thus the exposed earliest civilizations of Anau and Susiana, separated by the broad expanse of the Iranian plateau, had several most fundamental traits in common, although the possession of the potters' wheel at Susa and a wider range of motives of decoration of pottery point to a civilization probably more advanced in development and of later date than that of Anau II. In both regions the people were settled in towns built of air-dried bricks; both peoples had agriculture, both peoples used the pivotal door-stone, and both had well-developed potters' arts and painted ornamentation. And in both places excavations, while showing extensive use of flint for sickles and other domestic purposes, have failed to bring to light any traces of its use, in the earliest cultures, for arrows and lances or celts. We may therefore consider both of these earliest civilizations to have been native to Asia. The total absence of stone arrow-points and stone lance-points in the earliest cultures in Susiana corroborates the inference drawn from their absence in early Anau, that these civilizations were evolved during a period of isolation at least from Africa and Europe, dating from preglacial or early interglacial time. I have, in chapters I and iv, traced the cause of this isolation to the influence of the glacial period, and have given reasons for including within its sphere the whole region between the Caspian Sea and Mesopotamia on the west, and the Hindu-Kush and Tian Shan mountains on the east, with an additional but detached and perhaps independently isolated area to the east of these mountain barriers, including the whole of the Gobi region.
It would seem, then, that the widely separated civilizations of Anau and Susiana must have been genetically related, and that the region of isolation must have included not only Transcaspia but also Susiana, at least down to the time when the earliest culture of Susa, using sickle-flints and the contemporary variety of painted pottery, ceased to exist. A genetic relationship between these two civilizations presupposes a region in which they were in intimate contact, living in an agricultural stage of culture on oases somewhere, either on the Iranian plateau or on the lower lands along its northern edge.
We have at present no data for closely defining the region where this next-underlying stage of culture existed. It could not have been on the Babylonian plain, for as Professor Sayce remarks, this was in its natural condition a pestiferous marsh, which could be conquered for agriculture "only by a people armed with a civilization which enabled them to dig canals, to mold bricks, and pile their houses and temples on artificial foundations."* This is due to the long-continued annual inundation of the plain common to the two great rivers, the flood of one
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