The classification systems have undergone many changes with time.
(i) The Aristotle made the first attempt of classification and classified plants as herbs, shrubs, and trees.
Animals, on the other hand, were classified on the basis of presence or absence of red blood cells. This system of classification failed to classify all the known organisms.

(ii) Linnaeus developed a two-kingdom system of classification. It consisted of kingdom Plantae and kingdom Animalia. However, this system did not differentiate between unicellular and multicellular organisms or between eukaryotes and prokaryotes. There were large numbers of organisms that could not be placed under the two kingdoms.

(iii) A five kingdom system of classification was proposed by R.H Whittaker in 1969. On the basis of characteristics, such as cell structure, mode of nutrition, presence of cell wall, etc., five kingdoms, Monera, Protista, Fungi, Plantae, and Animalia were formed.