Darwin's theory did not explain

Arrival of the fittest is not explained by Darwin.Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution. The novelty and brilliant insight of Darwin was this: he asserted that variations, which are heritable and which make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny. Hence for a period of time, over many generations, survivors will leave more progeny and there would be a change in population characteristic and hence new forms appear to arise. His theory is based on survival of the fittest, but he failed to explain the origin of variations.