Under the modern Linnaean system, the classification of an organism places the organism within a nested hierarchy of taxa. The hierarchy ranges from the most general category (domain) to the most specific (species).
The science of describing, naming, and classifying organisms is called taxonomy (taks-AHN-uh-mee). Any particular group within a taxonomic system is called a taxon (plural, taxa). Over time, scientists have created taxonomic systems that have different numbers and levels of taxa as well as different names for each taxon.
About 2,400 years ago, the Greek philosopher Aristotle classified organisms into only two taxa—either plants or animals. He grouped animals according to whether they lived on land, in water, or in air and grouped plants based on differences in their stems. As naturalists described more organisms, they realized that Aristotle's classification system was inadequate.
Early naturalists also found that common names, such as robin or fir tree, were not useful to identify organisms because common names vary from place to place and many don't accurately define a species. For example, the common name jellyfish is misleading. Although it has a jellylike body, a jellyfish is not a fish. Some naturalists named species by using long descriptions in Latin, but these names were difficult to remember and did not describe relationships among organisms.
Swedish naturalist Carolus Linnaeus (1707-1778) devised a system of grouping organisms into hierarchical categories according to their form and structure. Each category represents a level of grouping from larger, more general categories to smaller, more specific categories. Linnaeus's original system had seven levels. Figure 17-2 and Table 17-1 show a modern classification of different organisms in a hierarchical system similar to that used by Linnaeus.
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