I thinks that's part of it. Europe kinda domesticated itself. The undomesticed variant of human were the rulers, and so many younger siblings of noble houses left for america that now about 25% of all Americans can directly trace their lineage to European nobility.

Another part is how much we mixed with Native Americans, which is ignored by both sides in the culture war. The left wants to pretend colonization was purely white people bad, so the fact that natives disappeared mainly due to intermarriage is ignored ; the right also ignores it because they don't want to think about how mixed they actually are. But its a fact... A very very large percentage (possibly most) of white Americans are **_both_** European nobility and Native American. There was a book that talked a lot about this, though more from a cultural point of view - I think it was "Lila" by the same author who wrote "Zen and the Art of Motorcycle Maintenance" (his name escapes me). Basically he was saying that it is the native american attitudes that express culturally and linguistically that allow cause Americans to be so freedom oriented and also how we react to things emotionally, which is closer to Native American behavior than European.

We have a unique breed in America. It does make sense to protect it from dilution by limiting immigration. I think Australia makes a good example in support of this - they had a "white only" immigration policy for a long time, but it did nothing to protect them from becoming helpless and subservient, and now have an Orwellian surveillance state that is comparable to North Korea. So what did their immigration policy do for them? It looks like nothing. They're the same breed of domesticated farm human as the Europeans.