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A way of organizing living things. In biology, plants and animals have traditionally been classified by the structure of their bodies, in a descending hierarchy of categories: kingdom, phylum, class, order, family, genus, and species. For example, human beings are classified as belonging to the animal kingdom, the phylum of chordates, the class of mammals, the order of primates, the genus Homo, and the species sapiens. The scheme is based on a system developed by the Swedish scientist Carolus Linnaeus in the eighteenth century. There is a debate among scientists about whether the traditional classification system should be retained. Some argue that classifying living things by their descent, with all descendants of a single ancestor being grouped together, is more in keeping with the ideas of evolution (see cladistics). Others want to classify organisms by their genetic makeup, using sequences in DNA or RNA. Each of these schemes provides a different way of ordering living things, but there is no “right” way to accomplish this task.