There are more than a million different species of animals living in he world today, and new ones are constantly being discovered. They range in size and complexity from microscopic single-cell creatures to the giant blue whale, weighing about 120 tons. Faced with the task of bringing some order to the living world, zoologists consider two key factors: definition (is it an animal?) and classification (what place does it occupy in the animal kingdom?)
The difference between inanimate objects and living things is not always apparent, crystals grow, for example, but only animate organisms have the fundamental characteristic of energy exchange known as metabolism.
Animals must eat and digest food and reorganize the products of digestion to form their own tissues. But in order to release the potential energy of their food, animals require oxygen, and during the metabolic processes they emit carbon dioxide as a waste product.
The distinction between a plant and an animal is often blurred among the simple organisms, because some contain chlorophyll and can build their own food, but also hunt and feed. Of the more developed organisms, however, animals may generally be distinguished more easily by their quick response to stimuli and their considerable powers of locomotion. In addition, only plants possess cellulose in their cell walls and contain the pigment chrlorophyll.
Plants and animals are generally classified according to a hierarchical system devised by the 18th century botanist, Carolus Linnaeus. In this system, each unit is grouped with related forms into a larger taxon which then constitutes part of the next larger group, and so on. The term species is used as the basic unit of classification.
Scientists agree that a species can be an arbitrary category only, within which there is a genetic diversity. They also agree that species change gradually, adapting themselves to the environmental variations as successful genetic strains outbreed the less successful ones.
The need for an internationally accepted classification is twofold: firstly to able to identify particular species throughout the world, and secondly to group related kinds into larger groups that reflect evolutionary relationships. Common vernacular names are usually too imprecise for scientific use.
The lion, for example, is designated by the scientific name Panthera leo, which indicates that it is the species leo belonging to the genus Panthera. This genus also includes the jaguar (P. onca) and the tiger (P. tigris). These animals all belong to the family Felidae (the cat-like animals) which, in turn, belongs to the order of flesh-eating animals called the Carnivora. This order also includes other animals with similar features such as tooth arrangement and skull form.
As a member of the order Carnivora, the lion does not much resemble anteater, for instance, which belongs to the different order Edentata ("toothless"), but both have mammalian features and so are designated members of the class Mammals. All mammals have a vertebral column and therefore placed in the subphylum Vertebrata, along, with the fishes, amphibians, reptiles and birds. The vertebrates share with their apparently dissimilar relatives the lancelets and sea squirts and axial stiffening rod (a notochord) and all are placed in the phylum Chordata.
Using this phylogenetic system of classification - species, genus, family, order, class, subphylum and phylum, it is possible in each level to see the relationship between animals of different species. But only the genus and species names are required to uniquely identify an animal. Thus the binominal classification of the lion is Panthera leo.