A century ago, colleges cared if your ancestors came over on the Mayflower. Now some are demanding that when universities admit black students, they give preference to descendants of those who arrived on slave ships. Black Students United at Cornell last month insisted the university “come up with a plan to actively increase the presence of underrepresented Black students.” The group noted, “We define underrepresented Black students as Black Americans who have several generations (more than two) in this country.”

After widespread...