History of the Peloponnesian War (1.2.1-1.2.3)

For it is evident, that what is now called Hellas, was not of old inhabited in a settled manner; but that formerly there were frequent removals, and that each tribe readily left the place of their abode, being forced by such as were from time to time more numerous.

For as there was no traffic, and they did not mix with one another without fear, either by sea or land; and they each so used what they had as but barely to live on it, without having any superfluity of riches, or planting their land, (because it was uncertain when another should invade them, and carry all away, especially as they had not the defence of walls;) and as they thought that they might any where obtain their necessary daily sustenance, they made little difficulty in removing: and for this cause they were not strong, either in greatness of cities, or other resources.

And the best of the land was always the most subject to these changes of inhabitants; as that which is now called Thessaly, and Boeotia, and the greatest part of the Peloponnese, (except Arcadia,) and of the rest of Greece whatsoever was most fertile.