To put the shapes shown below into some kind of order, several classification systems could be invented. The shapes could be separated into two groups: green and blue. Each of these two groups could be separated into subgroups. For example, the green shapes could be separated into circles and non-circles. The non-circles could be separated into groups of shapes with three sides and shapes with four sides.

Organisms can be classified in the same way that these shapes can be classified. Once a classification system is established, every scientist must follow that system.

Once the group of organisms has been classified biologists can try to identify each new organism found. If the new specimen does not correspond to any that have previously been described, then a new species has been discovered.

The first classification systems for the organisms on Earth had only two groups: Plants and Animals. As more and more organisms have been discovered, it has been necessary to create three new groups.

One reason for this is that when the microscope was invented over three hundred years ago, biologists discovered a whole new part of the biosphere that had never been observed by humans. As they started to classify microscopic organisms, scientists realized that some of them simply could not be considered either plants or animals. That is why three new groups had to be created and that is why most of the species in the new groups are micro-organisms.

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