The critical function is an important aspect of the science of taxonomy. Every classification is built upon a critical evaluation and emendation of a previously existing one. Ultimately, this reaches back to pre-scientific classifications which are, like scientific ones, hierarchically structured. The critical function is strongly developed in phylogenetic systematics in which some time-honoured taxa are no longer considered natural groups. Rejection of paraphyletic taxa has given rise to opposition against the phylogenetic methodology. It is suggested that paraphyletic taxa may be retained for administrative reference, but not for scientific evaluation of biogeography and evolutionary differentiation.