The complete change in the map of Greece at the close of the Achæan period and the origin of the ethnographic system with which the history of Hellenic times begins, were always referred by Greek tradition to a last wandering of north Grecian tribes. The customary chronology places the beginning of this shifting at 1133 or 1124 B.C., _i.e._, less than three generations after the so-called conquest of Troy. Recent chronological investigations, however, have made it seem probable that a period at least a hundred years later should be chosen. The first impulse was probably given by new movements of tribes in the north. The advance of the Illyrians caused the Thessalians, a part of the Epirot tribe of the Thesproti, to withdraw across Pindus into the valley of the Peneus, which was afterwards called Thessaly. While the preservation of the Greek character in Epirus was henceforth left to the brave Molossi, the Thessalians east of Pindus fell upon the settled Greeks of the lowlands and destroyed their states. The proudest and most vigorous elements of the old population that survived the war, determined to emigrate and found a new home. Thus, the Arnæ migrated to middle Greece, destroyed the old states of Thebes and Orchomenus in the basin of the Copaïs and united this whole district, which henceforth appears in history as Bœotia, under their rule. While the Thessalians were making preparations to subjugate the warlike tribes of the highlands about the valley of the Peneus, one of these mountain races, the Dorians, carried the mighty movement on to the extreme south of the Peloponnesus. Within twenty years, according to tradition, they had crossed the narrow strait of Rhium and begun the conquest of the Peloponnesus. They ascended the valley of the Alpheus into southern Arcadia. From here one body of them descended into the Messenian valley of the Pamisus and overwhelmed the old kingdom of the Melidæ of Pylos. The other branch invaded the principal districts of the Achæans in the east and southeast of the Peloponnesus. In open battle the rude Dorian foot-soldiers easily defeated the Achæan knights. But they could not destroy the colossal walls of the Achæan fortresses or cities, and were themselves finally forced to build fortifications from which they could watch or invest the Achæan strongholds until the opportunity was presented of storming them or forcing their capitulation. It was in such a fortified camp that the Dorian capital Sparta had its origin. It was probably the tenacious resistance of the Achæans in Laconia that determined a large body of the Dorians to leave that district and turn to the east, where they completely subjugated Argolis and made Argos the centre of Dorian power in the eastern part of the Peloponnesus.