According to Hugo de Vries, the mechanism of evolution is

Based on his research on evening primrose, Hugo deVries brought up the idea of mutation to describe substantial differences that appear suddenly in a population. He felt that mutation is what drives evolution, not the tiny alterations (heritable) mentioned by Darwin. Darwinian variations are modest and directional, whereas mutations are random and directionless. Darwin believed evolution was progressive, whereas Hugo de Vries believed speciation was produced by mutation, and thus called it saltation.