History of the Peloponnesian War (1.2.1-1.2.3)

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.

For there was no mercantile traffic and the people did not mingle with one another without fear, either on land or by sea, and they each tilled their own land only enough to obtain a livelihood from it, having no surplus of wealth and not planting orchards, since it was uncertain, especially as they were yet without walls, when some invader might come and despoil them. And so, thinking that they could obtain anywhere the sustenance required for their daily needs, they found it easy to change their abodes, and for this reason were not strong as regards either the size of their cities or their resources in general.

And it was always the best of the land that was most subject to these changes of inhabitants—the districts now called Thessaly and Boeotia, most of the Peloponnesus except Arcadia, and the most fertile regions in the rest of Hellas.