What is the origin of this variation and how does speciation occur? Even though Mendel had talked of inheritable 'factors' influencing phenotype, Darwin either ignored these observations or kept silence.
In the first decade of twentieeth century, Hugo deVries 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 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.
Evolution for Darwin was gradual while deVries believed mutation caused speciation and hence called it saltation (single step large mutation). Studies in population genetics, later, brought out some clarity.