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Taxonomy, the science of naming groups of organisms based on shared characteristics, was first formally classified by Carolus Linnaeus in the 18th century.

He grouped organisms using a hierarchy of increasingly inclusive categories that are still used today. Similar species, the smallest unit of taxonomy, are grouped into the same genus.

For example, the Arctic hare and the jackrabbit are both grouped in the genus Lepus, but they have different species names. So they are named Lepus arcticus and Lepus californicus. This two-part format used to identify specific organisms is called binomial nomenclature.

Related genera belong to the same family. For example, hares and rabbits belong to Leporidae. This family shares the same order, Lagomorpha, with some other species, such as pikas.

These all belong to the class Mammalia, which includes animals characterized by mammary glands that produce milk, such as humans, polar bears, and squirrels.

Above class in the taxonomic hierarchy is the phylum. Vertebrate animals belong to the phylum Chordata, defined by the presence of a notochord, a flexible, rod-like structure found in chordate embryos. This phylum belongs to the kingdom Animalia.

Under modern classification, the highest taxonomic rank is the domain. One such domain, Eukarya, includes multiple kingdoms whose organisms have cells with a membrane-bound nucleus.