In order to grasp just how unusual is America’s endogamous color line, consider it abstractly. Many newcomers to the United States, especially Hispanics, find it astonishing that an endogamous community of apparently African appearance has somehow perpetuated itself (or has been perpetuated) in North America for over three centuries, despite Africans having been a demographic minority. Such a thing happened nowhere else on the planet. America’s color line (and its consequent one-drop rule) is a puzzle. It is self-contradictory and counterfactual, and yet many Americans believe it with intensity. It was invented just four centuries ago and has shifted over the years, and yet many think of it as eternal and unchanging. It is based on the assumption that White Americans have no recent African ancestry, and yet, as explained in the essay Afro-European Genetic Admixture in the United States, DNA studies show without doubt that White Americans, like all other New World populations, are a mix of European, Native American, and, yes, African ancestry. The phenomenon cries out for historical explanation.