There is a strong argument to be made that the detective story finally came of age when it started drawing lines between itself and the other brands of crime fiction. It is indeed a sign of maturity when you finally see yourself as an individual and try to find out what makes you unique.
Prior to the Golden Age the detective story didn't identify as a separate genre or at least didn't make a fuss out of it. There was a general understanding, widely shared by the interested parties, that Conan Doyle and William LeQueux were not writing the same kind of stories but this was only a statement of fact, not implying one kind was inherently better than the other.
Things changed as the genre became more refined, more ambitious and attracted a more highbrow readership. Detective story writers, often of middle-class origin themselves, became frustrated and concerned that their work was lumped together with the "Blood and Thunder" books churned out by the likes of Edgar Wallace for the uneducated masses. Defining what the detective story was, and more importantly what it wasn't, became of vital import - and so self-appointed censors began edicting rules.
|Mgr. Ronald Knox, rule-maker|
Some of them appear silly to us and have since been either amended or outright rejected; others have been significantly downplayed. Most are now illegilible because of missing context. One glaring example is Mgr. Knox's prohibition of "Chinamen" which is often seen as racist by modern commentators, whereas Knox actually aimed his barb at the intemperate and clichéd use by thriller writers of Eastern evil geniuses of which Sax Rohmer's Fu Manchu remains the most famous specimen.
Rules are made and meant to be broken, and these were no exception. Rebellion soon arose, but interestingly not all of those "rebels" were averse to the notion of rules - they just wanted to replace the existing ones with their own. Thomas Narcejac showed that Raymond Chandler's literary ethos was just as constrictive as Van Dine's, though he took the latter's opposite stand on some issues such as realism and the importance of characterization. Narcejac himself wrote several essays on what a proper crime novel should be - and don't get me started with Julian Symons.
Our age is less dogmatic, having returned to pre-Golden Age equality of standing for all kinds of "crime fiction". The Detection Club welcomes thriller writers (it would certainly had gone out of business otherwise) and the Mystery Writers of America present their Edgars to books that are devoid of any mysterious element. This is in keeping with the modern craving for inclusivity, but I'm not sure this concept applies well, or should apply at all, to the genre as its boundaries have always been extremely blurry and we don't do it or ourselves any favour by blurrying them even more. A detective story is not just a story with a detective, and a mystery without a puzzle is not a mystery. We really need another Mgr. Knox.