darwin had the idea that many individuals with variation were culled according to relative fitness.

mendel later demonstrated that this variation was passed fractionally, to decendants. this was a conserved probability of fixed combinations [consult "mendels peas" ]

the two taken together resulted in neo-darwinist mechanisms. darwin was not wrong, mendel was less wrong, contemporarily we are still scratching the surface of getting it right, primarily what rules apply to what groups, what mechanisms are conserved, what combinations of functions are served by a particular structure, departing from the one gene one function hypothesis.