For a large part of the 20th century, zoologists thought every type of animal had a unique set of genes, the genes that made rabbits give birth to baby rabbits, or that made grasshopper eggs hatch grasshopper nymphs. Instead, Sean B. Carroll tells us, in his charmingly offhand style, animals turn out to share great numbers of genes. In particular, we share master genes, the ones responsible for controlling embryological development. These master genes have been highly conserved for hundreds of millions of years, have passed into both great branches of animals, and have been central to the evolution of what Darwin called, "endless forms most beautiful."