The natural world used to be simple. If something was alive and moved, it was an animal. If it was alive, didn't move, and was green, it was a plant. If it wasn't alive at all, then it was mineral. In Gilbert and Sullivan's "The Pirates of Penzance," the modern Major General could sing that he knew of "matters vegetable, animal, and mineral," and feel that he had disposed of all the possibilities.

Of course, there were some exceptions to prove the rules. Once an oyster settles on the seabed it doesn't move much, but it is easily classified as an animal; and a mimosa, which shrinks away from a touch, is obviously vegetable. The plant/animal dividing line seemed clear, and between them the plant and animal kingdoms took care of everything living. This description of Nature worked well for thousands of years.

Not any more. If you look at a biology textbook today, you find listed not two categories of living things, but "five." To the plant and animal kingdoms have been added three others.

What are they, and why do biologists feel that they need them, after we got on so well and so long without them?

The first of the three added kingdoms is "fungi". Other than invaluable yeasts and welcome mushrooms, most fungi don't qualify as pleasant. They include ugly growths on trees, nasty objects found in damp, dark cellars, and diseases such as ringworm and athlete's foot. Fungi are not green, but they fulfill a necessary role in decomposing dead plants and animals and if biologists want to give them a kingdom of their own, that will be all right with most people.

The other two kingdoms are newer and less familiar. The reason they went unrecognized for so long is that, biologically speaking, human beings are giants.

No one knows how many species of living things exist on Earth, but the number is certainly in the millions and probably in the tens of millions (there are close to a million species of beetles alone). Almost all these different life forms are tiny by human standards. Certainly, trees are bigger than we are, and so are a few varieties of mammals, reptiles, and fishes. But that's about it. In contrast to our lumbering giant selves, most organisms on the planet are too small even to see without help.

This is certainly true of "bacteria", the fourth kingdom of living things. We need a microscope to make them out. Bacteria, tiny creatures without a cell nucleus, often receive very bad press (bacteria = germ = disease), but they are everywhere and life on Earth could not survive without them. Our own digestive systems are packed from end to end with essential bacteria. They, or their long- lost relatives, are our own earliest ancestors.

The members of the fifth kingdom are less appealing than the bacteria and on the face of it less necessary. Called "protists", or "protoctists", they include things like slime molds, algae, amoebas, and seaweed. Some of them are single- celled and tiny in size, while others are larger objects that look like single organisms but prove upon closer inspection to be big collections of single- celled organisms.

Have we finished, or will the urge to define new living kingdoms continue?

My feeling is that we are not done yet. The system I've described nowhere has a place for viruses, organisms too small to be seen except with the most powerful microscopes. The usual reason given for leaving them out is that "they're not really alive." My own suspicion is that the viruses are mostly excluded because they don't fit anywhere in today's classification systems. Or maybe it's the old story, the big guys get all the attention.

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