How did Darwinism effect immigration in America?

It seems likely that this question is referring to Social Darwinism, as the biological theory of evolution by natural selection did not have much of an effect on immigration. Social Darwinism, on the other hand, took on racial overtones in the United States and elsewhere. The idea that the white race was "fitter" than others influenced many people in the United States who advocated for immigration restrictions. At various points, American nativists proposed quotas or bans on Chinese, Mexican, and Eastern European immigrants. Social Darwinists influenced by racial theories thought that immigrants from certain races would weaken the United States by intermarrying with Americans and undermining American institutions. These racist fears intertwined with typical concerns about the effect immigrants had on the labor market, the influx of radical political beliefs with immigrants, and other issues. By the 1920s, Congress had banned Chinese immigrants more or less outright, placed restrictions on Japanese immigration, and established immigration quotas intended to restrict the influx of allegedly inferior peoples from Eastern and Southern Europe.