About the "Barbarian Invasions"

[Arjos]

I've learned that from the fourth century several migrating peoples poured into the roman world, but I've always wondered which part of the population migrated...
I mean beside eastern europe that was continually settled by nomads, what happened in northern europe?
For example Jutland was flooded so Angles, Jutes and Saxos were forced to migrate, but people from that area for centuries to come kept on migrating...
The Geats could always repopulate the area, or the actual number of people involved in the migration was little?
Even later on the Vikings appeared, were they planted in the soil like Sparti? XD