History of the Peloponnesian War (1.2.4-1.2.6)

For the greater power that accrued to some communities on account of the fertility of their land occasioned internal quarrels whereby they were ruined, and at the same time these were more exposed to plots from outside tribes.

Attica, at any rate, was free from internal quarrels from the earliest times by reason of the thinness of its soil, and therefore was inhabited by the same people always.

And here is an excellent illustration of the truth of my statement that it was owing to these migrations that the other parts of Hellas did not increase in the same way as Attica; for the most influential men of the other parts of Hellas, when they were driven out of their own countries by war or sedition, resorted to Athens as being a firmly settled community, and, becoming citizens, from the very earliest times made the city still greater in the number of its inhabitants; so that Attica proved too small to hold them, and therefore the Athenians eventually sent out colonies even to Ionia.