History of the Peloponnesian War (1.2.4-1.2.6)

The goodness of the land favoured the aggrandizement of particular individuals, and thus created faction which proved a fertile source of ruin. It also invited invasion.

Accordingly Attica, from the poverty of its soil enjoying from a very remote period freedom from faction, never changed its inhabitants.

And here is no inconsiderable exemplification of my assertion, that the migrations were the cause of there being no correspondent growth in other parts. The most powerful victims of war or faction from the rest of Hellas took refuge with the Athenians as a safe retreat; and at an early period, becoming naturalized, swelled the already large population of the city to such a height that Attica became at last too small to hold them, and they had to send out colonies to Ionia.