Biological classification keeps changing with our
understanding of life forms. Initially, new characters
were taken by Aristotle, Theophrastus, John Ray and
Linnaeus. Linnaeus divided plants on the basis of sex
organs only.
As more and more knowledge gathered for delimitation of
taxa, the ways for natural system of classification became
clear.
This can be understood by following examples
(i) Two kingdom classification was replaced by three
kingdom classification when Haeckel (1866) proposed
the kingdom of Protista.
(ii) It was converted into four kingdom classification by
Copeland when he proposed the kingdom—Monera.
(iii) Whittaker in 1969 proposed five kingdom
classification.
(iv) As the information about biochemistry of
archaebacteria gathered, Carl Woese in 1990 raised
another kingdom, Archaea.