This book is a thorough re-examination of the "species problem", the continuing disagreement among biologists about how best to identify species and what constitutes useful and genuine biological divisions of groups and organisms. This book contributes to our understanding of the scientific issues related to the species concept through an exploration of the reality of biological diversity and of the mental processes behind the ways we recognize species, and how we establish typological categories generally. The text develops a theory of evolutionary groups (groups of DNAs that compete and share in genetic drift and adaptation), and revisits the major issues of modern phylogeny, systematics, and evolutionary biology through this framework.