History of the Peloponnesian War

Thucydides

Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.

For it is plain that what is now called Hellas was not of old settled with fixed habitations, but that migrations were frequent in former times, each tribe readily leaving its own land whenever they were forced to do so by any people that was more numerous.