Changes in classification system over a period of
time
(i) Biological classification of living organisms was
first done by Aristotle. On the morphological basis,
he gave the broad classification of plants and
animals. He further divided plants into trees,
shrubs and herbs. The animal classification was
done on the basis of the presence or absence of red
blood into Enaima and Anaima. However, the first proper
classification system was proposed by Linnaeus who
divided living beings in two kingdoms- Plantae (all plants)
and Animalia (all animals). Although this system of
classification had been accepted till recently, but eventually
it was rejected due to its inadequacies.
(ii) Later, more criteria for classification were implemented
based on observable morphological features. These
criteria are habitat, cell structure, nature of cell wall
when present, mode of nutrition and reproduction,
evolutionary relationships.
(iii) Classification on the basis of these criteria led to the
creation of three more kingdoms which are now part of
the five kingdom system of classification given by
Whittaker in 1969. The five kingdoms are—Monera
(prokaryotes), Protista (single-celled eukaryotes),
Fungi, Plantae and Animalia.
(iv) The six kingdom classification system is the latest system
introduced by Carl Woese in 1990. It divides Monera into
two kingdoms—Archaebacteria and Eubacteria.