As 1968 began, Paul Ehrlich was an entomologist at Stanford University, known to his peers for his groundbreaking studies of the co-evolution of flowering plants and butterflies but almost unknown to the average person. That was about to change. In May, Ehrlich released a quickly written, cheaply bound paperback, The Population Bomb. Initially it was ignored. But over time Ehrlich’s tract would sell millions of copies and turn its author into a celebrity. It would become one of the most influential books of the 20th century—and one of the most heatedly attacked. The first sentence set the tone: “The battle...