In $1901$, Hugo de Vries carried out experiments on evening primrose plant (Oenothera lamarckiana) and proposed the mutation theory of evolution.

This theory states that the evolution occur by the sudden large differences in the population.

He believed that mutation is the only cause of evolution and not the minor heritable variations that, Darwin talked about.

Mutations are random and directionless while Darwinian variations are small and directional.

Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation (single step large mutation).

Studies in population genetics later brought out some clarity.