 Why does Kitcher believe that some species are individuals and other species are spatiotemporally unrestricted sets? Following the biologist Ernst Mayr, Kitcher suggests that there are two fundamental types of explanation in biology: those that cite proximate causes and those that cite ultimate causes. Proximate explanations cite the more immediate cause of a trait, for example, the genes or developmental pathways that cause the occurrence of a trait in an organism. Ultimate explanations cite the evolutionary cause of a trait in a species, for example, the selection forces that caused the evolution of thumbs in pandas and their ancestors.