Evolution is based on a key assumption that all the mechanisms within organisms can evolve incrementally so all intermediate mechanisms have some useful function.
A mechanism that can evolve one characteristic at a time while always having a useful function can be termed an reducible mechanism.
A mechanism that cannot evolve one characteristic at a time while always having a useful function can be called an irreducible mechanism. Irreducible mechanisms point to a Designer, and represent one of the most powerful arguments against evolution, because evolutionists fully admit that the process of evolution cannot produce an irreducible mechanism.
