These are two out of the three factors for natural selection (the third is competition ... the fact that more individuals are born than can possibly survive to reproduction).
Darwin was unable to *explain* how organisms passed traits onto offspring, or what the source of variation was, but he was able to establish both of them by observation alone.
After Darwin, we have developed a far better understanding of both of these- inheritance is found in the genetic machinery of DNA and variation is after all based on mutations and sexual recombination.
