THE Earth is populated by enormous numbers of different plants and animals. When these organisms are studied and described, it becomes apparent that they can be divided into groups. In each group of plants or animals, the members resemble each other strongly. These similarities are not always obvious but become apparent when the characteristics of the group are known. Bees and butterflies, for example, though differing considerably in appearance, size and habits, belong to the same group, insects, because they both have hard outer skeletons, six legs and two pairs of wings.
The first division of organisms is into plant and animal kingdoms. The next division is into phyla (singular: phylum), for example, the first ten groups of animals listed in the below.
The smallest natural group of animals or plants is the species. For example, birds are not a species, but black kites are. Gener- ally, al members of a species are alike in all important re- spects and can breed among themselves. Breeding between members of different species does not happen very often in Nature.
Placing an organism in a particular category is not always easy. Certain single-celled creatures are not definitely animals or plants but possess characteristics of both. Fungi and bacteria are placed in the plant kingdom although they do not contain chlorophyll and difer considerably from the green plants in their methods of obtaining food. The table of living organi given below is not complete and does not conform to any strict biological classification but ofers a simplified and convenient Scheme.