The best thing about the Justice Society of America, which I mentioned in passing several weeks ago and now expect you all to be fully clued-up about, is that it definitively established that all the cool superheroes of the 1940s shared the same fictional universe. Although Superman and Batman were too busy to hang out with the JSA (it was a comic designed to encourage the youth of America to read the comics featuring the less popular characters) most of the time, their couple of appearances in JSA stories prove that the adventures of Superman, Batman, Wonder Woman, Hawkman, the Flash, the Green Lantern, the Spectre, Starman, Doctor Fate, Doctor Mid-Nite, Sandman, Wildcat, Mister Terrific, Hour-Man, the Atom, the Black Canary, Johnny Thunder and, best of all, "Scribbly" (thanks to parody superhero the Red Tornado's cameo appearance in the first JSA story) are all linked in the same reality. It's the foundation of modern superhero comics, only more fun to read.
Actually, the idea of 'fictional universes' would sound strange to the superhero-fans of the early forties. The idea that Superman's adventures in the comic, the newspaper strip, the radio show, the cartoons, the movie serials and wherever else the omnipresent superhero was showing up weren't all part of one giant continuity would have got you a funny look from anyone you tried to explain this point to. That's the kind of attitude that superhero writers in the twenty-first century need to return to.