For a while the axolotl was filed as its own kind of animal: a salamander that kept its feathery gills, lived in the water all its life, and bred that way. Then, in Paris in the 1860s, among the axolotls being raised from a handful brought over from Mexico, a few did something none of the others had. Their gills shrank, their fins went, and they climbed out as land salamanders that looked very like the American tiger salamander. The animal everyone had been describing was a larva. It had simply never needed to finish. Later it turned out that the change hangs on thyroid hormone, and that a dose of it can push an axolotl through a metamorphosis its ancestors in the lakes around Mexico City had mostly stopped bothering with. What I like is that the grown-up form was never lost, only left unused, the way you might keep the key to a house you moved out of years ago. Most axolotls live their whole lives without trying the door.