This is the first and only book, so far, to deal with the causal basis of evolution from an epigenetic view. By revealing the epigenetic "user" of the "genetic toolkit", this book demonstrates the primacy of epigenetic mechanisms and epigenetic information in generating evolutionary novelties. The author convincingly supports his theory with a host of examples from the most varied fields of biology, by emphasizing changes in developmental pathways as the basic source of evolutionary change in metazoans.