 Another option starts with the assumption that the search for the unifying feature of species has gone on long enough. Biologists have looked long and hard for the correct definition of ‘species.’ The result of that search is not that we do not know what species are. The result is that the organic world contains different types of species. The conclusion drawn by some pluralists (Kitcher 1984, Dupré 1993) is that the term ‘species’ should be given a disjunctive definition. Species are either interbreeding lineages, or ecological lineages, or phylogenetic units, or ….