OCAMPO, Mexico — This is a story that must begin at the end. But oh, what an end. I stood on a forested mountaintop in Mexico in late February, mesmerized by monarch butterflies swirling around me, darting in and out of the trees and pockets of sunlight. They were high in the blue skies, clumped by the thousands on tree trunks and branches, clustered on woodland flowers and swarming in small streams of water on the forest floor. The sound of their wings was like a whisper on the breeze.