The Flash is one of DC's most enduring heroes, and part of that is because the character's history and lore are ever-changing. Back in the Golden Age, the only Flash was Jay Garrick, who transformed into a super-speedster thanks to an experiment with hard water. In the Silver Age, Barry Allen was the only Flash, a hero who could run at superspeed after being covered in chemicals and hit with lightning.
Source：https://www.cbr.com/flash-history-lore-changes-dc-comics/
