Charles Darwin's On the Origin of Species is the most important and yet least read scientific work in the history of science. Now James T. Costa—experienced field biologist, theorist on the evolution of insect sociality, and passionate advocate for teaching Darwin in a society in which a significant proportion of adults believe that life on earth has been created in its present form within the last 10, 000 years—has given a new voice to this epochal work. By leading readers line by line through the Origin, Costa brings evolution's foundational text to life for a new generation. The Annotated Origin is the edition of Darwin's masterwork used in Costa's course at Western Carolina University and in Harvard's Darwin Summer Course at Oxford. A facsimile of the first edition of 1859 is accompanied by Costa's extensive marginal annotations, drawing on his extensive experience with Darwin's ideas in the field, lab, and classroom. This edition makes available an accessible, useful, and practical resource for anyone reading the Origin for the first time or for those who want to reread it with the insights and perspective that a working biologist can provide.