To the long list of American institutions that have withered since the dawn of the 1980s — journalism, organized labor, mainline Protestantism, small-town merchants — it may be time to add another: college-level humanities. Those ancient pillars of civilization are under assault these days, with bulldozers advancing from two different directions.

On the one hand, students are migrating away from traditional college subjects like history and philosophy. After hitting a postwar high in the mid-1960s, enrollments in the humanities dropped off sharply, and still show no signs of recovering. This is supposedly happening because recent college grads who chose to major…