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Try writing down the names of all the animals, plants and micro-organisms you have ever encountered or heard of. No matter how long your list, it will represent only a very small fraction of the living species on Earth. Suppose that others from different countries have also prepared such a list. A more comprehensive list may emerge when these are all combined together. But this time, the list will become confused because of some of the same life forms will be referred to by different names, or different ones by the same name.
To overcome these difficulties, biologists give every plant and animal a scientific name, such that all organisms are described according to a binomial classification system. The first word is generally Latin—a practice left over from the days when Latin was an international language. For example, the dogs you see every day are Latin-named Canis familiaris, and cats are Felis catus.
Scientific nomenclature makes it possible to distinguish between species whose common names are often confused. For instance, the bird known as the robin in Europe is different from the bird known by that name in America. Confusion has been prevented by giving these separate species different names. The European robin is properly known as Erithacus rubecula, and its American counterpart as Turdus migratorius.162
In addition to naming species both living and extinct, scientists also describe and classify them according to specific criteria. The science of naming, describing and classifying living things is known as taxonomy or systematics. For example, animals are classified according to such criteria as their body structures and systems, internal organs, developmental stages, behavior and genetic information. Information about extinct species is obtained from fossils.
The classification system in question consists of hierarchical categories, or seven main groups. In descending order of size, these are:
Kingdom
Phylum (plural: phyla)
Class
Order
Family
Genus (plural: genera)
Species
Every living thing occupies its own particular position in all of the above seven groups. (There are also sub-categories within this hierarchical classification.) For example, the tree we commonly refer to as the white pine is a member of the plant kingdom and of the phylum Tracheophyta. It is also a member of the class Pteropsida, the order Coniferales, the family Pinaceae, the genus Pinus and the species strobus.
The scientific name of the wolf, a carnivorous canine, is Canis lupus; it is also a member of the phylum of mammals, the order Carnivora, the family Canidae and the genus Canis.163
In this classification system, the largest unit is kingdom. Until the 20th century, most biologists divided the world of living things in two—either plants or animals. In the last century, however, progress in the fields of microbiology and biochemistry in particular revealed that this simple division didn't go far enough. Today, a five-kingdom classification is generally agreed upon. In addition to plants and animals, the fungi, protista and monera are also regarded as separate kingdoms.
The animal kingdom, containing more than 1 million described species, is the largest, made up of multi-celled organisms that digest food, generally move, and have complex systems and organs. The plant kingdom contains more than 260,000 species, which produce their own food by means of the exceedingly complex process of photosynthesis, and also meet the nutritional needs of other organisms. Fungi, which are not capable of photosynthesis and have no digestive systems such as those in animals, are a kingdom with some 100,000 members.
The Protista kingdom consists mainly of single-celled organisms with a cell nucleus, such as algae and diatoms. Some 100,000 members of this kingdom are known to exist. Monera, on the other hand, consists of single-celled organisms that lack any nucleus, such as bacteria: Some 10,000 species of this kingdom have been described.
In biological classification, the kingdoms are followed by phyla, whose number varies according to different biologists. Still, the classification of 32 animal phyla and 10 plant phyla is generally accepted. In the animal kingdom, all species in a particular phylum possess a similar body structure, although phyla are very different from one another. For example, the phylum that includes sponges is completely different from the phylum Chordata, which includes vertebrates—fish, mammals, birds and reptiles. The insects we are familiar with are of the phylum Arthropoda, the largest phylum in the animal kingdom, which also includes marine crustaceans.
Living things belonging to a particular class share many more common features than do members of a phylum. For example, birds, reptiles and mammals are all members of the phylum Chordata, but belong to different classes. Birds, which have wings and also feathers—a structure not to be found in any other animal group—are members of the class Aves. Reptiles, members of the class Reptilia, lay eggs, are cold-blooded and covered in scales. Mammals are members of the class Mammalia, and give birth to and suckle their young, are warm- blooded and generally covered in fur.
In biological classification, a class is divided into orders. The mammals with which we are familiar consist of 23 different classes. Those that feed on insects, like the mole and hedgehog, are members of the class Insectovira. Rodents such as mice and squirrels belong to the class Rodentia, and meat-eaters such as dogs and wolves belong to the class Carnivora.
The next rank is the family. Mammals, for instance, comprise more than 100 families. Though cats and dogs both belong to the class Carnivora, cats are members of the family Felidae, and dogs of the family Canidae.
Genera consist of living groups that bear a close resemblance to one another, but which are not generally able to crossbreed—dogs and foxes, for example, and different genera within the family Canidae. Dogs belong to the genus Canis, and foxes to the genus Vulpes.
The species is the basic unit in biological classification. A species may be described as a community of individuals that are able to reproduce among one another and share the same functional characteristics. Breeds or varieties within the same species typically have different scientific names. For example, the red fox is known as Vulpes vulpes, the desert fox as Vulpes zerda, and the long-eared fox as Vulpes macrotis. If there are different groups or varieties within a living species, each of these groups constitutes a different sub-species.
Living things are described and classified by biologists known as taxonomists. They divide into species those populations that mate only among themselves in nature, which give rise to viable offspring, and which resemble one another in terms of structural and functional properties. They determine the classification, such as the specific genus to which a species belongs, and which genera belong to which families.
Classifications by different taxonomists are basically similar, but still exhibit important differences. For example, five species may be grouped under one, two or three different genera. That is why scientists often differ and disagree regarding the classification of different living things.164
