Haeckel’s Three kingdom System of Classification

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  • Classification is the arrangement of organisms into taxonomic groups known as taxa on the basis of similarities or relationships.
  • Closely related organisms (i.e., organisms having similar characteristics) are placed into the same taxon.
  • Organisms are categorized into larger groups based on their similarities and differences.
  • The classification of living organisms is a complex and controversial subject because of which different taxonomic classification schemes existed at different times.
  • In his classification scheme, Linnaeus recognized only two kingdoms of living things: Animalia and Plantae.
  • At the time, microscopic organisms had not been studied in detail. Either they were placed in a separate category called Chaos or, in some cases, they were classified with plants or animals.
  • As the knowledge of the properties of various groups of microbial life exploded, it became apparent that at this level of biological knowledge a division of the living world into two kingdoms cannot really be maintained on a logical and consistent ground.
  • Then in the 1860s, the German investigator Ernst Haeckel proposed a three-kingdom system of classification.

Haeckel’s Three kingdom System of Classification

Features of the Three-kingdom System of Classification

  • Three kingdom classification system was put forward by Haeckel in order to overcome the objections and limitations of the Two Kingdom System of Classification.
  • Haeckel suggested that the inconsistencies of the two-kingdom system could be avoided by the recognition of a third kingdom, and he proposed Protista as a new kingdom to accommodate organisms exhibiting characters either common to both plants and animals, or unique to their own.
  • Haeckel’s three kingdoms were Animalia, Plantae, and Protista.

According to this system:

    • The arrangement of kingdoms was done on the basis of morphological complexities and tissue system, the division of labor, and mode of nutrition.
    • Unicellular animals, algae and fungi were separated from other organisms on the basis of lack of tissue differentiation.
    • The new group was called the kingdom Protista.
    • Organisms lacking morphological complexities, tissue system, the division of labor, and enjoying the diversified type of modes of nutrition were segregated and put under the kingdom Protista.
    • Members of the kingdom Protista included the protozoa, fungi, bacteria, and other microorganisms.
    • Later fungi and multicellular algae were taken out from the group.
    • Organisms having diverse tissue-system with well- defined division of labour and maximum morphological complexities in their body remained segregated from protists and were bifurcated into two categories: those enjoying autotrophic mode of nutrition were considered to be plants and put under kingdom Plantae, and those that have entirely holophagic (phagotrophic) mode of nutrition were considered to be animals and put under kingdom Animalia.
    • According to this system, all known microorganisms came to be recognised as protists; neither plants nor animals.

Objections and Limitations

  • Haeckel’s system was not widely accepted, however, and microorganisms continued to be classified as plants (for example, bacteria and fungi) or animals (for example, protozoa).
  • Nucleated and anucleated organisms were kept together in protists.
  • Heterotrophic bacteria and fungi placed along with autotrophic algae.

References

  1. Engelkirk, P. G., Duben-Engelkirk, J. L., & Burton, G. R. W. (2011). Burton’s microbiology for the health sciences. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
  2. Trivedi P.C., Pandey S, and Bhadauria S. (2010). Textbook of Microbiology. Pointer Publishers; First edition.
  3. Tortora, Gerard J., Funke, Berdell R.Case, Christine L.. (2013) Microbiology :an introduction Boston : Pearson,
  4. http://www.biologydiscussion.com/organism/the-kingdom-system-of-organisms-classification-top-6-concepts/54667
  5. https://www.cliffsnotes.com/study-guides/biology/biology/the-unity-and-diversity-of-life/kingdoms-of-living-things
  6. https://www.tutorialspoint.com/biological_classification/biological_classification_four_kingdom.asp
  7. https://www.embibe.com/study/three-kingdom-classification-ernest-haeckel-concept

About Author

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Sagar Aryal, PhD

Dr. Sagar Aryal is a microbiologist, researcher, and science communicator from Nepal. He is also co-founder and Research Head of the Department of Natural Products at the Kathmandu Research Institute for Biological Sciences (KRIBS), where he leads research on natural product discovery and microbial biodiversity. Dr. Aryal earned his Ph.D. in Microbiology from Tribhuvan University, Kathmandu, Nepal, in 2025, supported by a DAAD Research Grant and a University Grants Commission (UGC) Nepal Ph.D. Fellowship. His research interests include Actinomycetes, Myxobacteria, microbial taxonomy, and natural products, particularly the discovery of bioactive compounds with potential pharmaceutical applications. He isolated and identified Nocardia nepalensis, a novel bacterial species from Nepal, contributing to understanding microbial diversity in the region. Dr. Aryal has published more than 16 research articles in international journals and contributed three book chapters to established academic publications. He has presented scientific research findings and developed international research collaborations through visits to more than 18 countries across Asia, Europe, North America, and Oceania. From 2023 to 2025, he served as the American Society for Microbiology (ASM) Young Ambassador to Nepal. Since December 2025, he has served as an Editorial Board Member of Scientific Reports, a Nature Portfolio journal. In recognition of his scientific contributions, Dr. Aryal received the NAST Science and Technology Youth Award 2025 from the Nepal Academy of Science and Technology. He previously received the Frs. Locke and Stiller Research Award (LSRA) for the 2017-2018 cycle from St. Xavier's College, Kathmandu, and a Nepal Academy of Science and Technology (NAST) Research Grant for 2017-2018. Alongside his scientific career, Dr. Aryal has more than 15 years of experience in blogging, content writing, and search engine optimization (SEO).