Mort Weisinger and his writers had continual problems with the Legion of Super-Heroes. In a way, this is not terribly surprising. The Legion was initially intended for just one story, the original tale in Adventure #247. But a few issues later, Mort began putting letters pages in Adventure, and while I haven't been able to locate any letters that were actually published, it seems logical to assume that at least some fans wrote in requesting a return of the superhero team. And so, over time, they became a regular part of the Superman family, eventually supplanting Superboy himself as the cover feature in Adventure.
But the process was not without some growing pains. Because the stories had been written on an ad hoc basis, there were contradictions here and there. For instance, in the initial story, the Legion was set 1000 years in the future:
In Action #267, Weisinger gave Supergirl a tryout with the Legion. However, either he or his writer for that story, realized there would be problems with having Supergirl and Superboy in the same club. After all, wouldn't Superboy then know that a Supergirl would arrive on Earth several years later? So they made the Legion that Supergirl tried out for the descendants of the original LSH:
But there was another problem that popped up that never was resolved in the Silver Age. Supergirl actually was rejected for membership in the Legion in that first story, although there was a reason. She had been exposed to Red Kryptonite, which turned her temporarily into an adult. Since the Legion was for teenagers only, she was unable to join that time.
She finally made the grade in Action #276. In that issue, she met another applicant, with a strangely familiar appearance:
But the problems don't end there. Remember, this story is supposedly taking place 1000 years in the future. Unless the Brainiac clan has an extraordinary lifespan, wouldn't his great-great-great-great-grandson be living more like 150 years in the future, rather than 1000? A likely explanation is that the writer thought the Legion was
only 100 years in the future. A generation is usually considered to be
20 years, Brainiac 5 (ignoring the great-great-great-great grandson mistake) would be around 100 years after his ancestor.
A further dilemma was introduced in Superman #167, when we learned:
By the way, DC has now apparently decided to ignore the story in Action #276. Remember, this was the story that showed Brainiac 5 and Supergirl getting inducted into the Legion: