Two-Kingdom Classification

The history of taxonomy is as old as man himself. It may be traced back to prehistoric times because man had learnt to designate living organisms by specific names very early in his evolutionary history. Much before the advent of civilisation living things had been categorised into animals and plants. Hippocrates (460-370 BC), Aristotle (384-322 BC) and many others had tried to put some order into the chaos of the bewilderingly large number of different life forms. Aristotle appears to have been the first to attempt a logical system of classification. He classified animals on the basis of presence or absence of blood as Sanguineous (with blood) and Non-sanguineous (without blood). He believed that in evolution, life had been directed towards a perfect animal form, having blood (Table 1.6). He incorporated this idea which is a hierarchical system of classification In his book entitled 'Scala Naturae'. He also recognised two kingdoms in the living world i.e., Plantae and Animalia. The former included mainly immobile forms, while the latter embraced mobile forms. We also know that one of the major differences between the two is that the members of Kingdom Plantae are autotrophic and obtain their food mostly by photosynthetic means and those of Kingdom Animalia are heterotrophic and obtain their food by ingestion.

Table: Aristotle's 'Scale of life' or Scala Naturae.

Kingdom Plantae was later divided by biologists into two subkingdoms,

1. Thallophyta containing the phylum Algae (with chlorophyll) and Phylum Fungi (without chlorophyll) and
2. Embryophyta with phylum Bryophyta (liverworts and mosses) and Phylum Tracheophyta (vascular plants).

Kingdom Animalia, on the other hand, was divided into Subkingdoms Protozoa containing unicellular forms and Sub Kingdom Metazoa including sponges and other multicellular organisms. A third Sub Kingdom Parazoa was later created and the sponges were shifted to this Sub Kingdom due to their intermediate position. between uni and multicellular organisms.