Therefore, when the agricultural tribes of the Old Continent had no potatoes, no crops resistant to the extreme cold, they could only stop at the limits of agriculture and endlessly tussle with the nomadic, fishing and hunting tribes at the borders. Even during warm centuries, when farmers migrated past the border and once established agricultural villages, like during the Tang Dynasty's warm period... these villages and farmers would ultimately be assimilated by the local nomadic and hunting communities as temperatures dropped, reverting back to the nomadic ways!
