Irreducible complexity, as defined by Michael Behe in Darwin's
Black Box is a property of a system such that if any part is
removed, the system ceases to function. Irreducible complexity
is often used as an argument for Intelligent design. The classic
illustration of an irreducibly complex system is a mousetrap: it
consists of a base, hammer, spring, catch (or trigger), and
fasteners to hold the pieces together. If any of those parts is
removed, the mousetrap no longer works. The argument, then, is
that since evolution proceeds by adding parts to an existing
system one by one, the precursors of an irreducibly complex
system would have been useless, and would not have been selected
for. Ergo, all of the pieces had to be put together by an
intelligent designer. The argument that evolution always
proceeds by adding parts is false. Natural selection can remove
parts as well as add them. For instance, whales have no hind
legs, but retain vestigial pelvises where their ancestors' legs
were attached. Another example