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 groundbreaking book provides the first mechanistically based theory of what homology is and how it arises in evolution.

Gunter Wagner, one of the preeminent researchers in the field, argues that homology, or character identity, can be explained through the historical continuity of character identity networks--that is, the gene regulatory networks that enable differential...