Keeping track of the millions of living organisms in the world would be nigh impossible without a logical system, which fortunately scientists have thought up. But how does it work?

Kingdom

The generally accepted model is that all life is divided into five kingdoms: animals, plants, monera, protista, and fungi. Animals are multicellular (many celled) organisms which can move about freely and of their own accord. This kingdom includes humans, tigers, and beetles.

Plants are multicellular organisms which cannot move about freely and carry out photosynthesis, which is converting sunlight, water, and carbon dioxide into energy. This group includes trees, moss, and roses.

Monera and Protista include various monocellular (single celled) organisms, the primary difference being that monerans have no nuclei and fewer organelles. Every cell (except red blood cells) contains a molecule called DNA which regulates cell functions. In monerans, the DNA just kind of floats around, but in other organisms, it is enclosed in a nucleus. Protists also have more organelles than monerans; organelles are to cells what organs are to multicellular organisms. Monera includes bacteria and cyanobacteria (aka blue-green algae). An example of a protist is amoeba. Both groups can generally move about freely. Some carry out photosynthesis.

Fungi can be either mono- or multicellular. They are saprotrophic, i.e. they live on other organic material. This is generally dead cells, but they sometimes live in living tissue, for example in the case of athlete's foot. They cannot move about freely. Examples of fungi include yeast, bread mould, and mushrooms. Though fungi often resemble plants, they are unable to carry out photosynthesis.

Some biologists have considered forming a sixth kingdom containing the oldest bacteria, which is called, appropriately, Archaebacteria.

Viruses do not fit into any of the kingdoms, since they are not currently believed to be living things, merely collections of chemicals. Various theories have been put forwards to explain the existence of viruses, from DNA escaping from cells and evolving a separate existence to alien genetic experiments to (in the case of AIDS) the final plague which God has sent to wipe out humanity.

Phylum

Each kingdom is subdivided into a number of phyla. There are 34 of these in the animal kingdom. One is devoted to vertebrates, animals with backbones, and the others contain various types of invertebrates, such as arthropods (insects, arachnids, etc.), echinoderms (starfish, etc.), annelids (worms), and molluscs (snails, scallops, octopi, etc.).

Class

Each phylum is further subdivided into a number of separate classes. For example, the vertebrate phylum contains the classes mammals, reptiles, fish, amphibians, sharks, and dinosaurs. Sharks and dinosaurs were recently given their own groups because they are very different from other fish and reptiles respectively. Birds were once given their own class, but have since been thrown in with dinosaurs.

The arthropod phylum contains several classes, such as insects, arachnids, crustaceans (aka shellfish), and multipods (centipedes and millipedes).

Order

Within each class are a number of even smaller groups (beginning to see a pattern here?). For example, the arachnid class contains spiders, scorpions, ticks, mites, harvestmans (sic), windscorpions, and vinegaroons * . The insect class contains too many order to be listed here, but some of the best known are hymenoptera (ants, bees, and wasps), termites, fleas, cockroaches, butterflies and moths, and earwigs.

The mammal class contains orders such as carnivores (meat eaters), herbivores (plant eaters), and omnivores (plant and meat eaters).

Family

As you have no doubt guessed, the Family is a subdivision of the Order. The names of families tend to be unfamiliar to the layperson. They include Formicidae (ants), Theraphosidae (tarantulas), Primates (apes, monkeys, humans, lemurs, and chimpanzees), Canindae (wolves and foxes), and Felicidae (cats). Organisms in a family are very closely related.

Genus

The genuses are quite specific groups, and are fairly small. There is a genus of ants called Myrmecia and a genus of spiders who were the original European tarantulas called Lycosa . On the vertebrate side, we have a primate group called Homo * , a feline family called Felis , and a canine family called Canis .

Species

The final subdivision is into a huge number of tiny species. All organisms in a species are similar enough to be able to breed with each other, and the species is denoted by a suffix to the genus name. For example, Myrmecia nigriceps refers to the bull ant, and Lycosa narbonensis (formerly Lycosa tarantula ) is what is commonly known as the wolf spider. There is also Felis tigris (tiger), Canis Lupus (wolves) and Homo sapiens - humans!

Dogs and wolves all belong to the same species, thus any dog or wolf can mate with any other dog or wolf, and dogs can mate with wolves. The same situation exists for domestic cats and wildcats, but neither of these could mate with, say, a tiger. This leads us on to subspecies, which are members of a species that do not resemble others. For example, dogs are a subspecies of wolves, and domestic cats are a subspecies of wildcats. The different types of domestic dogs and cats (collie, spaniel, tabby, siamese, etc.) are merely different breeds.

The species system is not perfectly clear cut. There are some animals which seem to be almost different species, but are not quite. Two examples are lions and tigers and horses and donkeys. Lions and tigers can mate to produce 'tigons,' and the offspring of a horse and a donkey is a mule. However, tigons and mules are sterile, which is why you see so few of them around.