Before they were comic book legends, Jerry Siegel and Joe Shuster were a couple of Cleveland high school students; The "Man of Tomorrow", a New Deal superman fighting gangsterism and leaping tall buildings in a single bound, became a huge commercial success, giving the world an icon, the Golden Age of Comics its selling point, and the industry a character to clone; when Shuster and Siegel thought they weren’t getting their fair share, they sued. Beginning in 1947, began a series of lawsuits over ownership of the character. Amazingly, 50 years later, the legal issues of who owned Superman were still murky. The two struggled through years of poverty before DC Comics offered them a modest annual stipend. Before he was a comic book legend, Gardner Fox was a lawyer, and he could see the writing on the wall. When editors stopped giving him work in the late Sixties, Gardner resumed his one-time career of writing pulp fiction, making a successful living churning out spy and romance novels, westerns, mysteries, and science fiction under a variety of pen names until his retirement. Gardner, who wrote everything from romance comics to funny animal books, but his lasting stamp was in the superhero genre. Frank Gorshin’s Riddler is based on a Gardner script from the 1960s; he created Hawkman and the Flash; with Julius Schwartz, he almost single-handedly started the Silver Age of comics through his revivals of The Flash and The Green Lantern; and he created the superhero team-up. All-Star Comics was a sort of holding pen for characters in comics published by All-American and National Comics (two comic book companies run by Jack Leibowitz and a Max Gaines); if a superhero — the Atom, say — didn’t have his own title to star in, he could get a few pages in All-Star so readers didn’t forget entirely. And if you were going to have a bunch of all stars in a comic, why not an all star squad? What prevented the superheroes from meeting, having a super-cocktail party, and getting to know one another? And so Gardner Fox was tapped to create the Justice Society, a collection of the noble costumed men — the Sandman! Hourman! the Green Lantern! Dr. Fate — not quite popular enough to be stars on their own. (And they were all supermen, until Wonder Woman became the Society’s first female member; in issue #11, the enlightened Society officially made her their secretary.) The Justice Society proved to be hugely popular, boosting a number of the members to the point where they could graduate to comics of their own. But tastes changed; war, true crime, and horror comics, like those produced by Max Gaines’ other company, soon became much more popular than superhero comics, and a bunch of misfits who couldn’t hack it on their own weren’t needed. The cocktail party ended, and All-Star Comics started publishing Westerns.

Then in 1960, Julius Schwartz sought to capture lightning in a bottle. Superman, Batman, and Wonder Woman had continued publishing throughout the Fifties, and Schwartz had recently introduced the first major new superhero character in years, the Martian Manhunter. The Flash had been revived; the "new" Flash’s introduction in generally considered the dawn of the Silver Age of superhero comics. Gardner Fox got the call once again. This time his lineup wasn’t composed of castoffs; instead, it was heavy hitters: Superman, Batman, Wonder Woman, the Flash, the Green Lantern, and Aquaman, basically all the superhero characters DC was then publishing. Schwartz gave them a name befitting all-stars; alongside the National League and American League, there would be a Justice League.

The superhero teamup that Schwartz and Fox created would go on to become the dominant model for comic book characters; Marvel‘s remarkable resurgance in the Sixties wasn’t just due to the storytelling and more personal focus of Lee, Kirby, and Ditko, but because there was a real sense of one place, one "Marvel Universe". Spider-Man’s New York was Daredevil’s New York and the Fantastic Four’s New York and yet still recognizably New York. Superhero teams became more and more common in comic books, the outside world took note with spectacularly bad results and huge crossover extravaganzas incorporating dozens of separate books became common in the Eighties, including Marvel’s pointless Secret Wars and DC’s ludicrously complex Crisis on Infinite Earths.

DC editor Sheldon Mayer, who may have come up with the idea for the Justice Society, snuck in a parodic superhero, the Red Tornado, from his humor comic Scribbly. Batman has taken on real world scourges like the Axis, and met characters out of prose fiction such as Sherlock Holmes (repeatedly). Why not cut out the middleman? What if Sherlock Holmes could take on the Hun? Or Jack the Ripper? Or Dr. Who?

Alan Moore’s breathtaking League of Extraordinary Gentlemen reimagines the whole of Victiorian fiction as fodder for a mock superhero team and Phillip Jose Farmer’s Wold Newton was a grand unified theory explaining everyone from the Scarlet Pimpernel to James Bond, but Kim Newman‘s Anno Dracula series goes a step further, incorporating not just every piece of vampire fiction imagineable but also whole chunks of twentieth century history as crossover fodder; George MacDonald Fraser’s Flashman series works much the same way, with Flashman having a curious knack for meeting anyone who was anyone in the nineteenth century; and whenever you look at newsreel footage Zelig might turn up. Crossovers are not literary pastiche, like knocking out Hemingway’s Lord of the Rings. They are not simply reshaping other authors’ ideas, like turning the malleable H.P. Lovecraft into Wodehouse or Le Carre, or Kurosawa turning a bloody Hammet detective novel into one of the world’s great samurai films.

The impulse to write a crossover seems close to that driving fan fiction, but where fan fiction asks "what happens next?" (or, in the case of Mary Sues, "why aren’t there any characters exactly like me, only prettier and with superpowers?"), crossovers ask "what if?". (Marvel’s What If? comic asked the same question, only using slightly more constrained parameters.) The problem is one of definitiveness. If I think Batman could defeat a G.I. Joe ninja and you disagree, who’s to say who’s right? What if, as in Godzilla vs. King Kong, two equally authoritative sources come to different conclusions? Some things are scientific certitudes. Sherlock Holmes would catch the Ripper, the Scooby Gang would like Sonny and Cher, and Detective Munch would hit on Scully. But what if a character changes depending on how the author handles her (a not-insignificant problem even within comics continuity, given that superhero comics are generally written by multiple authors over the years)? The author function is not universal, but it gets yet more muddled when authors play in each others’ worlds. And then there are the legal issues, the same kind that plagued Siegel and Shuster for decades. Gardner Fox was probably right to walk away; he was a lawyer, he could read a contract, and he knew that settling contract disputes makes the question of Superman vs. the Hulk look easy.