It is to periods of light rainfall that Huntington attributes the four great irruptions from the Arabian desert which have been recognised by Semitic scholars,[214] the last of which spread the doctrine of Islam over the Near East; to the same cause he attributes, too, the various movements of the Huns and Tartars. One may reasonably add to this that even one dry year during the period of light rainfall may be sufficient to account for such an exodus. Now, as I have endeavoured to show on a previous occasion,[215] such a period of light rainfall seems to have occurred between 2400 and 2200 /B.C./, though it may have been of somewhat longer duration. I further gave reason for believing that about 2225 /B.C./, or perhaps a little earlier, an invasion of nomads took place from the Russian steppes. It would seem that about this time the Tripolje culture came suddenly to an end, and from the evidence at Khalepje,[216] Minns was inclined to believe that it had been destroyed by the steppe-folk, who had buried one of their dead on the site formerly occupied by a Tripolje “area.” This destruction has recently been questioned, and it has been suggested that the Tripolje people may have abandoned this region, driven out rather by drought than by the attacks of the steppe-folk. Be this as it may, for further excavations are needed before the question can be determined, there is no doubt that these nomads disappeared from the steppe for a time and were found in the Tripolje region. Further we have evidence that a people resembling them appeared soon afterwards in Thessaly, bringing with them pottery which appears to be derived from that of the Tripolje culture.[217] Others of this type seem to have been responsible for the destruction of Hissarlik II.,[218] while pottery, which also shows affinities with that of Tripolje, occurs later at Hissarlik and at Yortan on the Caicus.[219] Moreover, the kurgans, characteristic of these steppe-folk, have been found all over Thrace and even over Asia Minor from the Hellespont southwards to Lydia and Caria, as well as eastwards up the Sangarius into the plateau of Phrygia.[220] Thus we seem to be dealing with an advance of a steppe people, comparable with the various irruptions from the Arabian desert which did so much to change the course of history in Mesopotamia, and destroyed the Old and Middle Kingdoms in Egypt.