 For each type of explanation, Kitcher believes that there are corresponding definitions of the term ‘species’ (what biologists call ‘species concepts’). Proximate explanations cite species concepts based on structural similarities, such as genetic, chromosomal and developmental similarities. These species concepts assume that species are spatiotemporally unrestricted sets of organisms. Ultimate explanations cite species concepts that assign species evolutionary roles. These species concepts assume that species are lineages and thus individuals.