The sole is asymmetric. It has both eyes on the same side. Its evolutionary progress is subject to debate. Did the metamorphosis take place on several generations. Such an evolution, said progressive, implies that the sole would have had half asymmetric ancestors, thus little visible. Or then, as support the partisans of the evolution by stages, thousands of fishes would have been born deformed, at the same moment, accidentally, and would have turned out to be better adapted to their environment than classic fishes.