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Species is a definitional unit used by biologists and other life scientists to systematically classify the enormous numbers of life forms on the earth. The basic unit of classification is species, followed by larger groupings of related plants or animals. The term species was first used by the ancient Greeks. Taken from the Greek, the word ideos meant the shape or visible form of something—its form made it rationally comprehensible. Logically specifying the form of something as a specie of some species answered the question, “What is it?”

Millions of species have been identified and many more are discovered virtually every day. In general, a species includes all of the individual members of its natural population that are descended from common ancestors and that can breed ...

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