In his classification scheme, Linnaeus recognized only two kingdoms of living things: Animalia and Plantae. At the time, microscopic organisms had not been studied in detail. They were placed either in a separate category called Chaos or, in some cases, they were classified with plants or animals.
Then in the 1860s, the German investigator Ernst Haeckel proposed a three-kingdom system of classification. Haeckel's three kingdoms were Animalia, Plantae, and Protista. Members of the kingdom Protista included the protozoa, fungi, bacteria, and other microorganisms. Haeckel's system was not widely accepted, however, and microorganisms continued to be classified as plants (for example, bacteria and fungi) or animals (for example, protozoa).
Currently, the system of classification widely accepted by biologists is that devised by Robert Whittaker in 1968. Whittaker's classification scheme recognizes five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. The five-kingdom classification scheme is in general use today.