From the publisher: "The Units of Evolution [published in 1991] 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."