At one time, we were all foreigners. The Earth didn’t form itself out of cosmic dust complete with national borders in place. Each nation evolved from many communities of nomads who settled somewhere, fought over territory and resources, and ultimately got along with one another. Because they realized that diversity drives societies forward, whereas nationalistic inbreeding was a hindrance.
So, instead of fearing immigration, we should welcome it, just as we’d like to be made felt welcome abroad. We are and will always be creatures of evolution, and evolution loves to combine different things to become something new. That’s the only way continuously higher development has occurred up until now. And we certainly don’t want to go back to living in trees.