  Now in each of these definitions, little attention is paid to the actual methods used by taxonomists and systematists in differentiating between varieties of a species and distinct species, something to which Darwin gave a great deal of attention. Darwin’s apparent nominalism regarding the species concept likely stemmed from his close attention to his own taxonomic practices and those of other specialists. But nominalism typically combines a view about the ontology of species with one about the epistemological status of the species concept. On the first question, the nominalist insists that there are no species—there are more or less similar individuals. On the second question, the nominalist typically insists that the species concept is, at best, a useful or convenient grouping of similar individuals or, at worst, an arbitrary grouping of similar individuals. (An interesting alternative account of the species concept based on a sophisticated, multidimensional theory of similarity has recently been defended in Rheins 2011.)  