The citizens of Gotham and Metropolis have learned to rely on Superman and Batman at all times, whenever villainous foes pose a threat to peace and prosperity. They are a constant. They are eternal.
But on Earth-38, time moves as it does on Earth-Prime, ceaselessly forward, whether you are simply human or super-human. In this world, Superman first appeared to the public in 1938, and Batman followed one year later in 1939. From that point on, Clark Kent and Bruce Wayne proceeded to age normally, growing old and eventually leaving the super heroics to their sons, daughters and grandchildren.
Modern citizens of Earth-38 are familiar with a whole dynasty of heroes following in the footsteps of the original Batman and Superman. This includes Kara Kent, the daughter of Clark Kent and Lois Lane, who would go on to develop her own super powers and fight injustice as Supergirl. The title of Batman, taken on after Bruce Wayne’s retirement by the original Robin, Dick Grayson, is eventually passed down to Wayne’s son, Bruce Wayne, Jr. Even later, the grandson of Superman and Batman—Clark Wayne—develops a futuristic superhero identity: Knightwing.
All of these heroes and more—including the offspring of other members of the Justice League—continue to fight evil down through the generations, though there is one determined enemy that none of them seem capable of opposing: Time itself.
COUNTDOWN: ARENA #2 (2007)