Meanwhile the knowledge of copper had been introduced by traders, who had sailed up the Adriatic, and travelled inland from Fiume. This copper culture reached the Swiss lake-dwellings, and eventually passed down the Rhone as far as Lyons. It was followed by a bronze culture, which was imported from Italy and the western Mediterranean. About 2250 /B.C./ another drought caused a dispersal of the steppe-folk on a greater scale. Some went east, into the remotest fastnesses of Turkestan, some perhaps as far as the head waters of the Yenesei and the region around Minutsinsk, while others passed on to the Iranian plateau. This last group we hear of about 2100 /B.C./ as Kassites, and a few centuries later they conquered Mesopotamia. Those who went westward seem to have destroyed the Tripolje culture and driven off its people, unless, indeed, they had already been driven away by the drought. The bands of steppe-folk divided, some passing north of the Carpathians and some going south by the shores of the Euxine. This last group crossed the Danube, and skirting the Balkan mountains arrived at the east end of Thrace. Here they divided, one band passing to the west by the shores of the Ægean and then southwards to Thessaly, where they frightened the inhabitants, who termed them Centaurs. The other band crossed the Hellespont, destroyed Hissarlik II., and passed on into the Anatolian plain, where in due course they organised the native Alpine population into the Hittite empire. It is not so easy to follow the group which passed north of the Carpathians, but they seem to have followed the line of sandy heaths across Galicia into Silesia, then some, probably, entered Hungary through the Moravian gate, while others pushed into Bohemia. These last found there people who were either refugees from the Tripolje area or folk closely allied to them. These people, who had been accustomed to a type of cord vase, had found in Bohemia bell-beakers, which had arrived there via Italy from Spain. From a combination of both types of ware they had evolved the northern beaker. When the Nordic steppe-folk arrived from Silesia these Beaker-folk left, and passed northwards between the Rhine and the Weser, some going to Jutland and some to Holland. A few of the latter found a refuge in Great Britain. In Central Europe, in the district we have called the Celtic cradle, we find two cultures growing up, one consisting of Alpine peasants under Nordic lords, which prevailed in the mountain zone; the other, more truly Nordic, and still pastoral and perhaps nomadic, was limited to the Hungarian plain. After a short interval of interruption, trading was resumed with their Italian neighbours by way of Fiume.