The Units of Evolution is the first anthology devoted solely to the nature of species, one of the most hotly debated issues in biology and the philosophy of biology. The anthology is evenly balanced between biological and philosophical issues, making it equally useful for workers in both fields. In his general introduction, Marc Ereshefsky sketches the framework for the debate, explaining how biologists disagree over the definition of the term species, and philosophers struggle to evaluate the scientific utility of a categorization device that might lack a single defining characteristic. Essays in the first section offer various definitions of the species category, starting with Ernst Mayr's seminal work on species and including essays by Robert Sokal and Theodore Crovello, Paul Ehrlich and Peter Raven, Leigh Van Valen, Edward Wiley, Joel Cracraft, Brent Mishler and Michael Donoghue, Hugh Paterson, and Alan Templeton. The essays in the second section focus on such philosophical issues as whether species taxa are individuals or natural kinds, whether a monistic or pluralistic approach to systematics should be adopted, and the distinction between species and higher taxa. Contributors to this section include Michael Ghiselin, David Hull, John Beatty, Michael Ruse, Elliott Sober, Philip Kircher, and Marc Ereshefsky. Marc Ereshefsky is Assistant Professor in Philosophy at the University of Calgary.