At least 1.7 million species of living organisms have been discovered, and the list grows longer every year (especially of insects in the tropical rain forest). How are they to be classified?

Ideally, classification should be based on homology; that is, shared characteristics that have been inherited from a common ancestor. The more recently two species have shared a common ancestor,

* the more homologies they share, and
* the more similar these homologies are.

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