It should be fairly obvious that if people hadn't migrated thousands and thousands of years ago, we'd all be piled up in Africa in heaps, probably most of us dead. But we did migrate and cover the earth. People have always migrated for one reason or another, from a need for food to attacks by enemies, to religious, racist, and political persecution. Or just the wish to see new things. And a desire to conquer new lands.
It's true that many migrants throughout history have not been welcomed, have been persecuted, have been driven back. Not always, but often enough. The danger to the immigrants is worse when their being different is emphasized, or even just pointed out. The danger to those already there is from heavily armed, aggressive conqueror-types of immigrants.
Immigrants can come from outside a country or from within it. Diane Brenner was rummaging through her old Literary Digests and found this article from 1936:
I,d also like to remind you of that great classic, "Grapes of Wrath". How often we assume that people who are suffering, who are without resources are not us. But they are. We are, after all, one family. When we erect barriers, we hurt not only other human beings, but we hurt ourselves. Just think, in a moment, being a New York liberal might become a label driving us to run.