Changes in classification system over a period of time
(i) A two kingdom system was developed during Linnaeus time. According to this Animalia (all animals) and Plantae (all plants) are the two kingdoms. But, this system was not inadequate.
(ii) Later, more criteria for classification were implemented based on observable morphological features. These criteria are habitat, cell structure, nature of cell wall when present, mode of nutrition and reproduction, evolutionary relationships.
(iii) Prokaryotes have been kept as separate kingdom called Monera.
(iv) Eukaryotes (single-celled) have been separated in kingdom-Protista.