No one was more influential — or more terrifying, some would say — than Paul R. Ehrlich, a Stanford University biologist. His 1968 book, “The Population Bomb,” sold in the millions with a jeremiad that humankind stood on the brink of apocalypse because there were simply too many of us. Dr. Ehrlich’s opening statement was the verbal equivalent of a punch to the gut: “The battle to feed all of humanity is over.” He later went on to forecast that hundreds of millions would starve to death in the 1970s, that 65 million of them would be Americans, that crowded India was essentially doomed, that odds were fair “England will not exist in the year 2000.” Dr. Ehrlich was so sure of himself that he warned in 1970 that “sometime in the next 15 years, the end will come.” By “the end,” he meant “an utter breakdown of the capacity of the planet to support humanity.”