How universities balance the ethnic compositions of their student bodies is an urgent question right now, as the Supreme Court on Monday heard arguments on two cases challenging affirmative action, at Harvard and the University of North Carolina. In several months, when it rules on the legality of their admissions practices, the court may forbid the use of race or ethnicity as considerations. If so, partisans on both sides will argue about what such a change means for “diversity,” especially the imperative to admit historically underrepresented people of color, like Black and Hispanic Americans. These fights are nothing new. As...