No monarch butterfly makes the round trip. The ones that leave the eastern United States and Canada in autumn, bound for a few mountains in central Mexico, are a different sort of generation: they put off breeding and live for months, where their parents and grandparents lived for weeks. In spring they start north, lay eggs and die, and it takes several more short lives to reach the top of the map again. So the butterflies that settle into the fir forests each winter have never been there. The last monarch in the family to see those trees was a great-great-grandparent, and nobody can yet fully say how the directions survived four lives that never overlapped. Every winter the forest fills with travellers who are all, without exception, arriving for the first time, and they still pick the right mountain.