The Marvel Universe began in 1961 with the publication of Fantastic Four #1. Everything explodes outwards from that moment. Sooner (as with the case of the Sub-Mariner and Captain America) or later (as with Patsy Walker, the Two-Gun Kid and the late 50s Atlas monsters), a great deal of Marvel's pre-1961 output was incorporated into the fabric of the post-1961 universe. But it was a selective and deliberately careful process. Few besides Roy Thomas really cared about making all the crap from before 1961 make sense - some of it was useful, other stuff not so much. For all intents and purposes, the MU begins in 1961. Everything published before that date - besides the general outlines of the World War II heroes' careers - remains at least a little bit apocryphal, subject to the whims of individual writers.
DC, on the other hand, has always had a bit of a Universe problem. Or rather, they've given themselves a Universe problem, retroactively. They didn't start out with anything resembling a Universe. You had Batman and Superman and Wonder Woman and a host of others, and sometimes they met to have adventures together. But no one at the company ever really cared whether or not the world these adventures took place in was internally consistent. And this is the way things were until the early 60s. Suddenly things changed. Characters began to adopt a self-referential attitude towards their own history. The 1940s Flash met the 1960s Flash, but in order for the two characters to exist side by side, the writers had to jump through a few hoops.
Many of the writers and editors behind DC's superhero revival in the late 50s and early 60s were serious sci-fi fans, so they didn't see these kinds of obstacles in negativeterms, but as positive challenges. In the context of the fantasy-based super-hero milieu, these writers took the speculative fiction of their books seriously. So they began to introduce parallel worlds and alternative histories, developments that grew out of a desire to ensure an interesting and consistent framework for future stories. If you were to hop on your Time Treadmill and return to 1961, in order to tell the DC Bullpen that the whole Earth-1 / Earth-2 thing would eventually become far more trouble than it was worth, they wouldn't have believed you. How could something as cool and potentially interesting as an alternate Earth ever not be fun?
Meanwhile, back at Marvel, Stan Lee and Co. were building their own kind of internal consistency between their early 60s superhero books. But there was nothing quite so methodical at work with Lee's approach - basically, he was making it up as he went along. Wouldn't it be fun if the Fantastic Four could fight that new guy, the Hulk? And what if we brought Captain America back from the 1940s? Sure, cool! Matters proceeded more or less organically from there.
The idea of multiple Earths grew more and more intrinsic to the DC superhero books, with regular crossovers between Earth-1 and Earth-2. New Earths were colonized, strangely enough corresponding to the output of many of the companies DC had absorbed over the years - Fawcett, Quality, and (much later) Charlton. The really cool idea that had enabled the two Flashes to meet had metastasized into something that Julie Schwartz and Gardner Fox simply could not have anticipated. The notion of the multiverse became a kind of bête noire for DC, a situation not helped by the fact that so many writers devoted so much time to exploring the ins-and-outs of the theoretical superstructure. We're not just singling out Thomas for abuse here - more or less everyone who ever wrote a JLA / JSA crossover contributed to the problem.
I say "problem" in full cognizance of the fact that this "problem" was, for many DC readers, hardly a problem, but a singular strength of the company's output. DC readers got a kick out of being able to read stories where two different versions of Superman got together to fight two different versions of Lex Luthor. It would be a mistake to overstate the negative influence the multiple Earth superstructure had over readers - it's become something of an urban myth among comics readers that DC prior to 1985 was an impenetrable mess that pushed away more readers than it attracted. (Ironically, it seems to me that much of the basis for this myth - repeated so often it has become received wisdom - came from DC's own widely circulated rationale for the original Crisis.) It's probably impossible to say to what degree these kinds of stories effected the company's perception, on anything more than a purely anecdotal level. But the idea that stories dealing specifically with multiversal mechanics had grown staid and stale in the 1980s was widespread enough to spur the company itself to make a decisive break with perception.
Perhaps the most significant bulwark Marvel had against these types of legibility problems - either real or perceived - came in unexpected form. An anthology title devoted to 100% out-of-continuity stories, What If . . . ? was a strange title by any stretch of the imagination, a formalized version of the exact same kind of "Imaginary Stories" that had become unwelcome cliché at DC during the 50s and 60s. The crucial aspect of the series wasn't so much its oddity, but its (probably unintended) consequences. Creating a bimonthly showcase for things that didn't happen invariably required an encyclopedic reiteration of what actually did happen.
More tomorrow. (Famous last words for this blog, I know!)