A question about Earth One and Two...

So I've been boing some reading up on The alternate earths - reading The Old Justice League crisis team-ups - trying to expand my DC universe knowlege a bit.

Anyway, like The title says, I have a question about earth one and two.

When did Earth 2 heroes like Jay Garrick and Alan Scott move to earth one, and what was The reason?

Help is aprreciated, and thanks for reading.

Sorry for all the grammar mistakes as well.

@battle_forum_junkie:

If I remember correctly (and don't quote me on this cause I haven't actually READ these adventures) but in the Silver Age, Earth Two was created to explain why Jay Garrick was the Flash in the Golden Age and Barry Allen was the current Flash. They did a crossover where the two met and discovered their alternate worlds existed.

THEN A CRISIS HAPPENED.

Crisis on Infinite Earths rebooted the DC universe (one of the best decisions DC ever made imo) and merged the two Earths (and all others) into one single, New Earth. This new Earth had a merged history, Jay Garrick, Alan Scott and the JSA were all heroes from the WWII era while Barry Allen, Hal Jordan, etc were heroes from the current era.

@the_stegman: Yeah, instead of just discarding the old heroes the writer for the title at the time - unless I'm mistaken - came up with the concept of the alternate worlds. And thanks for clearing that up. I'm going to have to pick up Infinite Crisis now...