In 1967, Robert Whittaker introduced the five-kingdom classification system. This system is based on:

1. The levels of cellular organization i.e. prokaryotic, unicellular eukaryotic, and multicellular eukaryotic.
2. The principal modes of nutrition i.e. photosynthesis, absorption, and ingestion. On this basis, organisms are classified into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.