ABSTRACT
It is often argued that natural kinds form a hierarchy: if any two kinds overlap, then one must be subsumed under the other as a subkind (Kuhn 2000b: 228-52; Ellis 2001: 67-76, 97-100, 161-70). For example, if crocodiles and humans are classifi ed as vertebrates, and humans are classifi ed together with gorillas as mammals, then gorillas and crocodiles should also be classifi ed together under one of the categories (in this case vertebrates). Thus, the kind mammal can be subsumed as a subkind of the kind vertebrate. There are, however, many examples in both biology and chemistry of crosscutting kinds that do not form such simplistic nested hierarchies. This chapter examines whether the existence of such crosscutting categories in scientifi c taxonomy can be reconciled with the hierarchy thesis (HT).
