The History of the Grecian War (1.2.1-1.2.3)

For it is evident that that which now is called Hellas was not of old constantly inhabited; but that at first there were often removals, everyone easily leaving the place of his abode to the violence always of some greater number.

For whilst traffic was not, nor mutual intercourse, but with fear, neither by sea nor land, and every man so husbanded the ground as but barely to live upon it without any stock of riches and planted nothing (because it was uncertain when another should invade them and carry all away, especially not having the defence of walls), but made account to be masters, in any place, of such necessary sustenance as might serve them from day to day, they made little difficulty to change their habitations. And for this cause they were of no ability at all, either for greatness of cities or other provision.

But the fattest soils were always the most subject to these changes of inhabitants, as that which is now called Thessalia, and Boeotia, and the greatest part of Peloponnesus, except Arcadia, and of the rest of Greece, whatsoever was most fertile.