In 1969 Whittaker proposed a "five kingdom" system in which three kingdoms were added to the animals and plants: Monera, Protista, and Fungi. Whittaker defined the kingdoms by a number of special characteristics.
- Whittaker, Robert H. (1969) "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science, 163: 150-194
- Hagen, Joel B. (2012) "Five kingdoms, more or less: Robert Whittaker and the broad classification of organisms". BioScience, 62 (1): 67-74. doi:10.1525/bio.2012.62.1.11