Which of these was not conceived by Hugo de Vries to explain evolution?

Hugo de Vries based on his work on evening primrose brought forth the idea of mutations - large difference arising suddenly in a population. He believed that it is a mutation which causes evolution and not the minor variations (heritable) that Darwin talked about. Mutations are random and directionless while Darwinian variations are small and directional. Hugo de Vries believed saltation (single-step large mutation) caused formation of species.