Homology - a similar trait shared by different species and derived from common ancestry, such as a seal's fin and a bird's wing - is one of the most fundamental yet challenging concepts in evolutionary biology. This book provides a mechanistically based theory of what homology is and how it arises in evolution. "Deeply thought provoking. . . . This survey of homology in the light of modern genetic research . . . is timely and helpful."---Peter Moore, The Bulletin