Prior to Pokemon Diamond and Pearl, it was possible to make cross-generation trades with Generations I and II. However, glitches in the system could be caused by bringing Pokemon from Generation II (Gold/Silver/Crystal) to Generation I (Red/Blue/Yellow/Green). Thus, Game Freak made such cross-generation "trades" permanent in this game: Pokemon can now only go from the GBA games to the DS games, and not the other way around.
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