Two Kingdom Classification (Linnaeus) and Why It Failed
Biology · Biological Classification · NEET
In Linnaeus' time, all living things were put into just two kingdoms: Plantae (plants) and Animalia (animals). This system failed because it mixed up prokaryotes with eukaryotes, single-celled with many-celled organisms, and green algae with fungi. Memory hook: "Two boxes, too crowded" - only two boxes could not hold bacteria, fungi and one-celled life properly.
The two kingdom system had only Plantae and Animalia. Items shown in red were placed wrongly - it could not separate prokaryotes, eukaryotes, unicellular organisms, green algae or fungi. This overcrowding is why it failed and was replaced by the five kingdom system.
Your doubts, answered
Who proposed the two kingdom classification and what were the two kingdoms?
The two kingdom system was used in Linnaeus' time (Carolus Linnaeus, 18th century). The two kingdoms were Plantae (all plants) and Animalia (all animals). Every living organism was forced into one of these two groups. NCERT says this system 'was used for a long time' but was later 'found inadequate'.
Why did the two kingdom classification fail? (NCERT reasons)
NCERT gives four clear reasons. The two kingdom system did NOT separate: (1) eukaryotes from prokaryotes, (2) unicellular from multicellular organisms, (3) photosynthetic organisms like green algae from non-photosynthetic organisms like fungi. (4) A large number of organisms did not fit clearly into either plants or animals. So it was too simple to show real differences.
Why were fungi wrongly kept with plants in the two kingdom system?
Fungi have a cell wall and do not move, so they looked plant-like from outside. But fungi cannot do photosynthesis (they are heterotrophic) and their cell wall is made of chitin, not cellulose. The two kingdom system only looked at gross morphology (outside shape), so it could not see this difference and wrongly placed fungi in Plantae.
What extra features were needed that the two kingdom system ignored?
NCERT says besides gross morphology, scientists needed to use cell structure, nature of the cell wall, mode of nutrition, habitat, methods of reproduction, and evolutionary (phylogenetic) relationships. The two kingdom system used only body shape, so these important features were left out.
How is the two kingdom system different from the five kingdom system?
The two kingdom system (Plantae, Animalia) mixed prokaryotes, eukaryotes, unicellular and fungi together. Whittaker's five kingdom system (1969) split life into Monera, Protista, Fungi, Plantae and Animalia. It used cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships as criteria - the exact features the old system ignored.
⚠️ The NEET trap ✗ Saccharomyces (yeast) is a prokaryote because it is unicellular, like bacteria. ✓ Saccharomyces (yeast) is a EUKARYOTE - it is a unicellular fungus (Ascomycetes). Being single-celled does NOT make an organism a prokaryote. This is exactly the confusion the old two kingdom system created by not separating cell types. 🧠 Unicellular does NOT equal prokaryote. Yeast, Amoeba and Paramoecium are single-celled but still eukaryotes. NEET loves this trap.
Real NEET questions
NEET 2018
Which among the following is not a prokaryote?
A · Nostoc
B · Mycobacterium
C · Saccharomyces ✓
D · Oscillatoria
Solution: Saccharomyces (yeast) is a unicellular fungus (Ascomycetes) and therefore a eukaryote, not a prokaryote. Nostoc and Oscillatoria are cyanobacteria and Mycobacterium is a bacterium - all belong to Kingdom Monera and are prokaryotes. This question tests exactly why the two kingdom system failed: it could not separate prokaryotes from eukaryotes, or unicellular from multicellular life.
NEET 2018
Which of the following is not a prokaryote?
A · Bacteria
B · Blue green algae
C · Mycoplasma
D · Fungi ✓
Solution: Fungi are eukaryotic (Kingdom Fungi), so they are NOT prokaryotes. Bacteria, blue-green algae (cyanobacteria) and mycoplasma are all prokaryotes in Kingdom Monera. The two kingdom system wrongly kept fungi with plants; the newer systems corrected this by using cell structure as a criterion.
Solved Biological Classification NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Did the two kingdom system separate prokaryotes and eukaryotes?
No. This is its biggest failure. Bacteria (prokaryotes) and plants or animals (eukaryotes) were put in the same kingdoms without any separation, even though their cell structure is completely different.
Where were bacteria placed in the two kingdom system?
Bacteria and blue-green algae were usually placed under Plantae because they have a cell wall. This was wrong, because they are prokaryotes and very different from true plants. Later they were moved to Kingdom Monera.
Why is the two kingdom classification important for NEET?
NEET often asks about the limitations of the two kingdom system and why the five kingdom system replaced it. Knowing the four exact NCERT reasons (no separation of prokaryotes/eukaryotes, unicellular/multicellular, green algae/fungi, and many organisms not fitting) gives easy, direct marks.
Which classification system replaced the two kingdom system?
R.H. Whittaker's Five Kingdom Classification (1969) replaced it. It created five kingdoms - Monera, Protista, Fungi, Plantae, Animalia - and used cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships as criteria.
