Based on the enormous number of people migrating to the United States during his lifetime, Charles Darwin predicted that the U.S. would soon be counted alongside the most powerful empires in history. His reasoning was simple: To move from one country to another requires an enormous amount of courage and energy - to take that journey you had to leave everything behind, cross an ocean and start a new life for yourself without any guarantee of success on the other side. Therefore, after many waves of immigration into the United States, the country would inevitably be brimming with so many highly-motivated people (demonstrated by their willingness to take that journey in the first place) that its workforce would have to be one of the most industrious in the world. Migration was, in Darwin's view, a kind of natural selection acting to weed out people who lacked the courage and ambition to leave home.
Darwin, C. R. 1882. The descent of man, and selection in relation to sex. London: John Murray. 2nd edition. (p.142)