It was back in 1958 that a chemical company first discovered that its new weed killer appeared toxic to humans, “mainly by affecting the central nervous system,” as one company scientist documented at the time. The company kept its concerns to itself — as well as its later research indicating that large doses caused tremors in mice and rats. That’s because the herbicide, paraquat, was sublime at wiping out weeds. And profitable. Over the decades it became, an executive proudly declared...