So here’s a quick quiz. Who said, “Prevent the higher races from losing their nobler traits and from being overwhelmed by the lower races,” and when did he say it?

If you guessed Pittsburgh synagogue shooter Robert Bowers or Brenton Tarrant, the 28-year-old who murdered 50 people in New Zealand week before last, you’re wrong. The correct answer is Theodore Roosevelt, who made this remark in 1895 — just six years before he became president of the United States.

And that’s also the key element that’s been missing from the reaction to the New Zealand massacre, which has focused mostly on contemporary racism and President Donald Trump. In the manifesto that Tarrant released, he cited Mr. Trump as a “symbol of white identity and purpose.” Then the blogosphere exploded in predictably polarized fashion, with one side citing Mr. Trump’s own arguably racist statements and the other dismissing Tarrant as a raving madman.

It’s fair to ask how Mr. Trump’s behavior has fueled the rise of white nationalism, both at home and abroad. But it also diverts us from a deeper historical truth. Put simply, the fear of being flooded by foreign hordes is baked into our national DNA. And it all starts with the question of fertility.

So did Tarrant’s manifesto. Its first sentence was “It’s the birthrates,” and he repeated the phrase three times. He entitled his document “The Great Replacement,” which nodded to a French philosopher’s 2012 book about declining white fertility around the world.