THE latest estimates suggest that perhaps as many as 30 million species live on planet Earth, though only 1.5 million have so far been described. Many more millions have already been and gone, a few leaving fossil remains. But what exactly is a species?
The term is used to classify both living and extinct organisms. For instance, the modern horse, known as Equus caballus, is distinguished from the mountain zebra, Equus zebra, and both are distinguished from the extinct Equus giganteus. The first name is the genus name, and the second, the species name.
Most biologists define a living species…
