The two-kingdom system of classification, proposed by Swedish botanist Carolus Linnaeus in 1758, divides all living organisms into two main kingdoms: the Plant kingdom (Regnum Vegetabile) and the Animal kingdom (Regnum Animale). This classification was primarily based on nutrition and locomotion. Organisms that could move and had defined body structures, ingesting food in solid or liquid form, were classified as animals, while those that did not move and were capable of photosynthesis or lacked locomotion were classified as plants. Key features of the two-kingdom system:

• Plants include bacteria (prokaryotes), photosynthetic plants, non-photosynthetic plants, and fungi, while animals include unicellular protozoans and multicellular animals.
• The classification follows binomial nomenclature with genus and species names.
• It is considered the oldest system of categorizing organisms and laid the foundation for modern taxonomy.
• The main distinction is based on the ability of locomotion and mode of nutrition: plants make their own food, whereas animals rely on others for food.

Drawbacks:

• It is an oversimplified and heterogeneous system grouping together very different organisms.
• Unicellular and multicellular organisms were lumped together without differentiation between eukaryotes and prokaryotes.
• Certain organisms like Euglena and slime molds, which have characteristics of both plants and animals, could not be classified properly.
• Fungi and lichens were inaccurately placed in the plant kingdom despite their unique characteristics.

In summary, the two-kingdom system classified life into just plants and animals based on locomotion and nutrition, but it has significant limitations regarding evolutionary relationships and the diversity of life forms. It was later replaced by more complex systems that more accurately reflect biological diversity.