Genus (Lat., 'a kind'), in Natural History, a group of Species (q.v.) closely connected by common characters or natural affinity. In all branches of zoology and botany the name of the genus forms the first part of the scientific name of each organism, and is followed by a second word—either an adjective or a substantive—which distinguishes the particular species. This binomial nomenclature was introduced by Linnaeus, and has been of great advantage, making names serve, in some measure, for the indication of affinities.
Some genera are more satisfactory than others, the question turning on the nature of the component Species (q.v.). A genus may contain a single species—e.g. the genus Ornithorhynchus; or it may include several hundreds, and in such cases especially it is often split up into sub-genera. Groups of related genera form a family, groups of allied families form an order, and above orders are class and phylum. But, again, we may have an order with only a couple of living representatives, as in Proboscidea (elephants), or with only one, as in the Hyracoida (conies). The real difficulties concern species, and will be discussed under that title. See also GENERALISATION.