In this system of classification of the world of living beings, a new kingdom was added to the Haeckel’s three kingdom system. Acclluiar prokaryotes were given the status of a kingdom.
The organisms like Bacteria, blue-green algae (now called Cyanobactcria) were taken in it. The four kingdoms were monera, protoctista, plantae and animalia. This type of classification was purely based on cell organization and mode of nutrition.
Monera comprised of acellular organisms with no true nucleus. Their mode of nutrition is variable being autotrophic or heterotrophic. However, single celled or multi cellular eukaryotic organisms like algae, fungi, and protozoan were included under protoctista. Again the mode of nutrition of these organisms is variable i.e. autotrophic, heterotrophic, and absorptive or Neozoic.
The plantae comprised of eukaryotic, multi cellular, autotrophic organisms. The organisms with similar cellular organization but heterotrophic nutrition were placed under animalia.
This system of classification was proposed by Stanier and van Niel and fully developed by Copeland in 1956.
Although this system of classification rectified certain drawbacks of the 3-kingdom system, yet assemblage of fungi, algae and protozoan in one group was not acceptable.