Over time the complex idea of "species" has evolved, yet its meaning is far from resolved. This comprehensive work is a fresh look at an idea central to the field of biology by tracing its history from antiquity to today. Species is a benchmark exploration and clarification of a concept fundamental to the past, present, and future of the natural sciences. In this edition, a section is added on the debate over species since the time of the New Synthesis, and brings the book up to date. A section on recent philosophical debates over species has also been added. This edition is better suited non-specialists in philosophy, so that it will be of greater use for scientists wishing to understand how the notion came to be that living organisms form species.