Discuss how classification systems have undergone several changes over a period of time?

The two kingdom system of classification was inadequate.So it was changed and finally five-kingdom system of classification was devised by Whittaker in 1969. It takes the account of cell organisation, nature of cell wall, mode of nutrition, habitat, reproduction, ecological roles and evolutionary relationship etc. The five kingdoms are: Monera, Protista, Fungi, Plantae and Animalia.