"I was so anxious to avoid prejudice, that I determined not for some time to write even the briefest sketch of it. In June 1842 I first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil in 35 pages; and this was enlarged during the summer of 1844 into one of 230 pages" Charles Darwin, Autobiography
Darwin had several key aspects to natural selection but the concept of intraspecific competition (competition within species) was missing - that piece was filled in 1838 when he read Malthus. He had all the pieces to solve the mystery of mysteries yet it took him four years to write and abstract. Interesting. I bet the two years to flush it out must have been a great relief to his mental constipation.