Hopeful Monster

As an example of radical change, he gave flatfish — the flounder and its relations. These are descended from fish with the usual fishy symmetry: the same left-right symmetry that we have. Larval flounders have it, too. But as adults, flounders have a profound asymmetry — one side has been completely flattened. What’s more, they have deformed, twisted skulls, and an eye that has migrated from one side of the face to the other. It’s as though you had both eyes on the same side of your nose. How did they get this way?
Goldschmidt speculated that big changes like this could be caused in one step by a mutation acting on the developing embryo. Most such mutations, he suggested, would produce individuals that were plain monstrous, and doomed to die without issue. But every so often, one of these mutations would happen in an environment where it could be beneficial. Then, the individual sporting it would be a hopeful monster, because it might have an evolutionary future as the founder of a new lineage.