The irreducible complexity argument assumes that the necessary parts of a system have always been necessary, and therefore could not have been added sequentially. However, in evolution, something which is at first merely advantageous can later become necessary. Natural selection can lead to complex biochemical systems being built up from simpler systems, or to existing functional systems being recombined as a new system with a different function. For example, one of the clotting factors that Behe listed as a part of the clotting cascade (Factor XII, also called Hageman factor) was later found to be absent in whales, demonstrating that it is not essential for a clotting system. Many purportedly irreducible structures can be found in other organisms as much simpler systems that utilize fewer parts. These systems, in turn, may have had even simpler precursors that are now extinct. Behe has responded to critics of his clotting cascade arguments by suggesting that homology is evidence for evolution, but not for natural selection.