What key component is missing in Darwin's theory?

Answer 1

Darwin, when he first wrote of the origin of species and Natural Selection could not have known of genetics. Mendel began his experiments around the time of the great beginning of Nature-selection Darwinism, but its publication was not synchronous with the first of Darwin's ideas.

Darwin propounded that traits/characters were selected naturally as to how good they were at fitting in with the environment. This is a profound argument and Darwin probably only based it on morphological characters, such as the neck-shapes of tortoises, the flightlessness of Galapagos cormorants and the bill-masses of what are now called Darwin's finches. He could see how morphology matched and changed with environment but could not have known, due to the distance of Mendel, what substance, what mechanism brought about these characters and changes; this 'descent with modification'.

What was missing was genetics. These days, Darwin's theory has been elaborated into the Modern Synthesis, which now encompasses genetics and thus has a real substance, like an ether that permeated the pre-Eisteinian universe, to bring about all evolutionary character-change.

Answer 2

Gregor Mendel supplied the correct mechanism of heredity.