To make some sense of the multitude of living organisms they have been placed in different groups. The method that has been agreed by biologists for doing this is called the classification system. The system is based on the assumption that the process of evolution has, over the millennia, brought about slow changes that have converted simple one-celled organisms to complex multi-celled ones and generated the earth’s incredible diversity of life forms. The classification system attempts to reflect the evolutionary relationships between organisms.
Initially this classification was based only on the appearance of the organism. However, the development of new techniques has advanced our scientific knowledge. The light microscope and later the electron microscope have enabled us to view the smallest structures, and now techniques for comparing DNA have begun to clarify still further the relationships between organisms. In the light of the advances in knowledge the classification has undergone numerous revisions over time.
At present most biologists divide the living world into 5 kingdoms, namely:
We are concerned here almost entirely with the Animal Kingdom. However, we must not forget that bacteria, protists, and fungi cause many of the serious diseases that affect animals, and all animals rely either directly or indirectly on the plant world for their nourishment.