Darwin in his 'Natural Selection Theory' did not believe in any role of which one of the following in organic evolution : -

Darwin's theory is based on overproduction, struggle for existence (including competition and natural enemies such as parasites and predators), variations, survival of the fittest and natural selection.
Darwin believed that evolution takes place through the gradual accumulation of small, continuous variations over many generations.
He did not give any role to large, sudden, discontinuous variations (sports). These were later emphasised by Hugo de Vries in his mutation theory, who called them mutations.
So Darwin did not believe in any role of discontinuous variations.