Facilitated variation is the first comprehensive theory of how life works at the molecular level, published in 2005 by systems biologists Marc Kirschner and John Gerhart in their book The Plausibility of Life: Resolving Darwin’s Dilemma. It is a very powerful theory, is supported by a great deal of evidence, and the authors have made it easy to understand. It identifies two basic components of heredity: (a) conserved core processes of cellular structure, function and body plan organization; and (b) modular regulatory mechanisms that are built in special ways that allow them to be easily rearranged (like ®Lego blocks) into...