Wasn't the druid also an offshoot prestige of the cleric or am I remembering that incorrectly?
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Unlike sub-classes, Prestige Classes weren’t tied to a particular base class. Instead, a character had to meet certain requirements. For example: to become an Arcane Archer, a character had to be an elf or half elf and meet a minimum to-hit bonus. They didn’t have to be a fighter. I can’t remember if druids began as a sub-class of cleric or if there was an independent class version in Dragon magazine but the game’s approach to some of the extended classes had some twists and turns before they settled down.