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Because of changes in the environment.

• beedeebeedee 3y ago
  But just as importantly, changes in the community (which I suppose we could call the social environment). That has been as significant a driver for migration as changes in the natural environment if not more. Most migrations in the last few hundred years that I am aware of were driven by social issues (economic, political, religious, etc), and not from natural environmental changes. I.e., Ireland's Great Hunger was not an environmentally driven exodus, even if a potato blight was involved.