Images: Charles Darwin, age 33 (with his son William Erasmus Darwin), public domain; Leucospermum bolusii, photo by Andrew Massyn, released to public domain.
When I was a grad student, it was de rigeur to proclaim that every good idea was already in The Origin of Species, and to express amazement that Charles Darwin could have been so right about so many things. It’s probably the astonishingly rightness of the Origin – along with the rest of Darwin’s writing – that makes his huge error stand out so conspicuously. That huge error, of course, was the idea of blending inheritance. It didn’t work in theory, it wasn’t (even then) consistent with available data, and Darwin should have known both of those things. (His correspondence suggests that he probably did.)