In a way. Rare Candy is an item that will instantly level up a Pokemon. In episodes of the anime, Ash has caught Pokemon and then, in the same episode, evolved or leveled them, without battling them at all. James has done similar things. Thus we can come to the conclusion that certain people sweat the main ingredient(s) of Rare Candy. Thus, if Ash swam around in a lake full of Magikarp, Gyarados for everyone!