Kingdom (taxonomy)


May 19, 2022

The kingdom (Kai, English: kingdom, Luo: regnum) is one of the classes in the Linnaean taxonomy of biological classification. The upper world may be placed above the world, and the sub-world and lower world may be placed below. The kingdom is one of the basic classes (the class that must be placed) and is located at the highest level of the basic classes, but in recent years, the domain above the world may be regarded as the basic class. The kingdom has long been the highest class (including non-basic classes), and only two kingdoms, the animal kingdom and the plant kingdom, have been recognized (two kingdoms). This idea is still widely accepted in the general public. However, various worlds were newly established from the end of the 19th century, and especially after the end of the 20th century, the reorganization of the world classification is advocated on a daily basis. Currently, there are often around 10 boundaries, but there are various theories at the stage of the system that is the premise of classification, and it is far from consensus.

History of the world

Organisms have been divided into two (plants and animals) since ancient times, and in modern times each was a kingdom, but as the knowledge of microorganisms increased, theories of dividing into three kingdoms, five kingdoms, and eight kingdoms have appeared. ..

Three Worlds in Natural History

In the days when biology belonged to natural history, three "kingdoms" were recognized in natural history. The animal kingdom (moving things), the plant kingdom (things that do not move but grow), and the mineral kingdom (things that do not grow). Linne also recognized three kingdoms.

Transition of the world

Two-world theory

When biology became independent of natural history, the two kingdoms of biology were the animal kingdom and the plant kingdom. Each kingdom was previously divided into classes, but later became divided into phylums (phylum for animals and divisions for plants).

Three world theory

Unicellular organisms were assigned to two kingdoms when they were discovered. Motile animals were classified as Protozoa (Protozoa Protozoa), and algae and bacteria were classified as plants. However, many species have come to belong to both. For example, Euglena and slime mold. In addition, it was found that the explanation was insufficient in the two-world theory due to the invention of the microscope. In 1860, John Hogg put together a primitive creature, both an animal and a plant, in his Primigenum. In 1866, Ernst Heckel named the group the Protist Kingdom, the three kingdoms of the animal kingdom, the plant kingdom, and the protist kingdom. The original protists also included fungi. He also uses his Protoctista, named by Herbert Copeland in 1947. However, these terms did not spread to the general public until relatively recent years. Later, after the Five-Kingdom System was proposed, the Three-Kingdom System was re-proposed. In this case, the prokaryote is included in the protist.

Four Kingdoms Theory

Later, it was discovered that the cell structure of bacteria is fundamentally different from other organisms. In other words, bacteria are prokaryotes, while other organisms are eukaryotes. For this reason, Herbert Copeland placed the bacteria in the isolated kingdom. Initially he was called Mychota, but later called Monera and Bacteria, and now prokaryotes. In 1937, Edouard Sutton divided organisms into two superkingdoms, prokaryotes and eukaryotes, arguing that the classification of prokaryotes and eukaryotes is fundamental. This idea will evolve into the current domain.

Five-Kingdom Theory

The Five-Kingdom System is a classification method proposed by Robert Whittaker in 1969. The classification is due to the difference in nutritional production