Yes, Darwin has been aware of Mendel’s work. Even though Mendel had talked of inheritable ‘factors’ influencing phenotype, Darwin either ignored these observations or kept silence.

Darwin would have been able to explain the origin of variations. 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.