Lands are not cultivated in proportion to their fertility, but in proportion to their liberty ; and if we imagine a division of the land, we will be surprised most of the time to see wilderness in the most fertile parts, and large populations in parts where the land seems to yield nothing.
It is natural for a people to leave a bad region to seek a better one, and not to leave a good region to seek a worse one. Most invasions therefore occur in countries which nature had made to be prosperous ; and as nothing is closer to devastation than invasion, the best countries are the most often depopulated, whereas the awful northern country continues always to be inhabited for the reason that it is almost uninhabitable.
We see from what historians tell us about the passage of Scandinavian peoples to the banks of the Danube that it was not a conquest, but merely a transmigration into unoccupied lands.
These happy climes had therefore been depopulated by other transmigrations, and we do not know what tragic events have taken place there.
“A number of vestiges testify,” says Aristotle, “that Sardinia is a Greek colony. It was once very rich, and Aristæus, whose love of agriculture has been so vaunted, created its laws. But it has greatly declined since ; for after the Carthaginians had taken over, they destroyed everything that could enable it to provide men with food, and forbade cultivation of the land on pain of death.”  Sardinia had not recovered in Aristotle’s time, and still has not today.
The most temperate parts of Persia, Turkey, Muscovy, and Poland have been unable to recover from the devastations of the greater and lesser Tartars.