like evolution! Was thought to be useful, now ridiculous! If only Darwin knew then what we know now about genetics and "basic stuff" like the complexity of single cells - he said himself that if he could be shown a living structure that could not be gradually developed in increasing degrees of complexity, then he would have to reconsider his ideas (they actually weren't his ideas either by the way, but that's another story!). Single celled organisms don't work too well if you try to "build" them bit by bit (like mousetraps, the whole thing has to be there for it to work)
