When twilight folded over Street 149, the mammoths strolled toward the river, silhouettes huge and gentle against the water’s reflective sheen. Lamps flickered on; the heat sank into the stones. People lingered longer than usual, savoring the last of the day. The mammoths paused at the bridge, turning their ancient heads as if to say goodbye to a city that had made them possible — and to remind it, softly and decidedly, that extinction is not always final.