Edgar Alan Poe’s “Murders in the Rue Morgue” was a new type of fiction; one that had, as its central character, the private detective or, as Holmes preferred, the “inquiry agent”. This individual was almost always accompanied by a lesser character who acted as a foil for the Great Detective’s brilliance. This person was almost always the detective’s biographer and, in the early years was simply someone who was handy to have along in a pinch (Dr. Watson). By the mid-1920s some of these characters had a fairly important role in solving the mystery (Lord Peter Wimsey’s Bunter).
During the early years the mystery was not always a murder but the police were, at best, bumbling incompetents who needed the detective (Holmes and his contemporaries) to solve the crime for them. After World War I they were less obviously incapable but still welcomed help from the brilliant amateur (Campion and Stanislaus Oates of the Yard, Dr. Gideon Fell and the Yard).
Perhaps the most famous of the brilliant amateurs was Hercule Poirot who had a higher opinion of himself than anyone else; certainly higher than the nobility of the county who looked down their noses at him “…a foreigner – he can't even speak English properly…” and higher than Chief Inspector Japp, who eventually and grudgingly admits that maybe Poirot’s “little grey cells” were useful.
The brilliant amateur who solved the case but gave all the credit to the stupid police disappeared with the professionalization of police forces in the police procedural branch of the genre.