 What are biological species? At first glance, this seems like an easy question to answer. Homo sapiens is a species, and so is Canis familiaris. Many species can be easily distinguished. When we turn to the scientific literature on species, the nature of species becomes much less clear. Biologists offer over twenty definitions of the term ‘species’ (Hey 2001). These definitions are not fringe accounts of species but prominent definitions in the biological literature. Philosophers also disagree on the nature of species. Their primary concern is the ontological status of species. Some philosophers believe that species are natural kinds. Others maintain that species are particulars or individuals.