Biological classification : Introduction
Since the dawn of civilisation, there have been many attempts to classify living organisms. It was done instinctively not using criteria that were scientific but borne out of a need to use organisms for our own use – for
food, shelter and clothing. Aristotle was the earliest to attempt a more
scientific basis for classification. He used simple morphological characters
to classify plants into trees, shrubs and herbs. He also divided animals
into two groups, those which had red blood and those that did not In Linnaeus’ time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed that included all plants
and animals respectively. This system was used till very recently.
This system did not distinguish between the eukaryotes and prokaryotes,
unicellular and multicellular organisms and photosynthetic (green algae)
and non-photosynthetic (fungi) organisms. Classification of organisms
into plants and animals was easily done and was easy to understand,
inspite, a large number of organisms did not fall into either category.
Hence the two kingdom classification used for a long time was found
inadequate. A need was also felt for including, besides gross morphology,
other characteristics like cell structure, nature of wall, mode of nutrition,
habitat, methods of reproduction, evolutionary relationships, etc.
Classification systems for the living organisms have hence, undergone
several changes over time. Though plant and animal kingdoms have
been a constant under all different systems, the understanding of what
groups/organisms be included under these kingdoms have been
changing; the number and nature of other kingdoms have also been
understood differently by different scientists over time.