In his essay "The Myth of Superman", Umberto Eco gives a careful, thoughtful, considered account of the nature of serialized publication, particularly in the case of Superman. Eco's essay focuses primarily on the dual identity of Superman/Clark Kent and on the so-called "Imaginary Tales" of DC.
Eco begins by pointing out that in contrast to traditional mythological characters like Hercules, whose story is complete and in the past, the "mythological character of comic strips" (110) does not come complete with a finished story. Superman must be recognizable -- archetypal -- but his story is both episodic and ongoing. In order to be mythological and archetypal he has to be consistent, but in order to make sense in the story he has to change. The story requires growth of character, but the archetype requires an already complete character.
According to Eco, once Superman has accomplished something he has "made a gesture which is inscribed in his past and which weighs on his future. He has taken a step towards death" (111). And Superman must be mortal. These steps must be towards death, or else "the public's identification with his double identity would fall by the wayside" (111). Clark Kent is not merely a disguise, but is a mechanism by which Superman can be a figure of identification and not only of aspiration. We identify with Clark Kent and admire Superman. But if Superman is really immortal then the secret identity becomes useless as a mythological device.
In Eco's terms, "Superman comes off as a myth only if the reader loses control of the temporal relationships and renounces the need to reason on their basis, thereby giving himself up to the uncontrollable flux of the stories which are accessible to him and, at the same time, holding on to the illusion of the continuous present" (116). The writers (unconsciously) employ a number of mechanisms for confusing the chronology and therefore the temporarily of the stories. Although theoretically Superman as written in the most recent issue of "Superman" comics is the same character as the Superman of Action Comics 1, and therefore should be at least 72 years old, in practice the publication of the stories as built in mechanisms to prevent the reader from reaching that conclusion.
Each "Superman" comic does not directly follow the previous one. The chronology of Superman comics is not unidirectional. In the first place, any number of Superman comics throughout the 72 years of publication have been flashbacks. The existence of characters like "Superboy" -- who as originally conceived was Superman when he was a boy -- mean that not all Superman stories are intended to take place in the "present" of the comic, but even discounting that, comics frequently feature flashbacks of alternate futures, tangents, time travel. Furthermore, it is both unclear and inconsistent what the "real-time" equivalent of an individual Superman story is. Some stories take place over the course of months or even years, others over the course of minutes. Beyond this is the existence of alternate or non-canon stories. Stories, such as the popular Silver Age "imaginary stories" that answer "What if?" questions (what if Superman were president?), that are explicitly stated to be outside the continuity of the ongoing story can nevertheless have an influence on readers's conception of the character. They confuse and obscure the timeline.
Eco, of course, was writing prior to the 1986 reboot of Superman, or the 1985 Crisis on Infinite Earths that precipitated the reboot. Both of these events, and especially the Crisis (and its sequels, Infinite Crisis) cast a new light on the continuity of Superman and readers's perception of that continuity (I hope to write a blog post about the Crises that will explain that claim further). In brief, both the Crisis on Infinite Earths and the reboot of Superman were intended to simplify the history of Superman. Officially, the history of the character extends back only to 1986, not all the way to the 1938 Action Comics 1. Yet thought the goal was to simplify the history -- so that new readers would not feel compelled to read decades of back issues in order to understand current comics -- in practice this kind of simplification may not be possible. Only a reader who is sufficiently in the know -- who is sufficiently versed in comics history -- will understand that the history is supposed to begin in 1986. So in practice the older comics do affect the readers's perspective of the continuity of Superman as a character. And the character's history exists within the mind of the reader more than it exists in the official canon.
That last statement may be contentious, but while I think it is true for any fictional character, it is even more demonstrably true for a character like Superman. The readers of today are the writers of tomorrow, who create the canonical version based on their own perspective as readers.
One way that comic writers create the canonical version of the character -- and sometimes do so out of sych with what had previously been the canonical version -- is through retroactive continuity, or "retcon". Retcons are reinterpetations or explanations of past events in ways not intended by the original writers. "Retroactive continuity" as term had never been applied to fiction when Eco wrote his 1979 essay. What retcons really do is further complicate and muddy the continuity -- since, once again, only attentive readers are familiar with the (dis)continuities.
Citations of Eco are taken from The role of the reader: explorations in the semiotics of texts