James Fallows wrote a series of blog entries for 'The Atlantic' decrying Al Gore's boiled frog metaphor, particularly as adopted by Hillary Clinton. http://jamesfallows.theatlantic.com/archives/boiledfrog/
Notably, though Fallows requested alternative metaphors, he didn't get any he thought worth repeating.
How about this one?
Put a generation of frogs in a vast container of cold water, then turn up the heat. The most heat-sensitive of the frogs will die. Those best adapted to higher temperatures will survive to reproduce as the following generation. Occasional mutants will occur who are more heat-tolerant than any frog in the initial generation. They will survive in disproportionate numbers. As the temperature rises still more, even-more-heat-tolerant mutants will arise among them and eventually replace the original mutant population. And so on. Eventually the mutants-upon-mutants will be splashing around happily in boiling water. But they will be nothing the first generation would recognize as frogs.
e.g. tolerance of Muslims in the UK evolving into the Archbishop of Canterbury saying publicly that elements of shari'a law should be adopted in the UK.
Moral: We can adapt to anything if we don't mind paying with our identities and self-respect.