Histories (1.56.2-1.57.3)

He found by inquiry that the chief peoples were the Lacedaemonians among those of Doric, and the Athenians among those of Ionic stock. These races, Ionian and Dorian, were the foremost in ancient time, the first a Pelasgian and the second a Hellenic people. The Pelasgian race has never yet left its home; the Hellenic has wandered often and far.

For in the days of king Deucalion [*] it inhabited the land of Phthia, then the country called Histiaean, under Ossa and Olympus, in the time of Dorus son of Hellen; driven from this Histiaean country by the Cadmeans, it settled about Pindus in the territory called Macedonian; from there again it migrated to Dryopia, and at last came from Dryopia into the Peloponnese, where it took the name of Dorian.[*]

and of the Pelasgians who inhabited Placia and Scylace on the Hellespont, who came to live among the Athenians, and by other towns too which were once Pelasgian and afterwards took a different name: if, as I said, one may judge by these, the Pelasgians spoke a language which was not Greek.

If, then, all the Pelasgian stock spoke so, then the Attic nation, being of Pelasgian blood, must have changed its language too at the time when it became part of the Hellenes. For the people of Creston and Placia have a language of their own in common, which is not the language of their neighbors; and it is plain that they still preserve the manner of speech which they brought with them in their migration into the places where they live.