Extinction is the complete disappearance of a species, when all of its members have died or been killed. As a part of natural selection, the extinction of species has been ongoing throughout the earth's history. However, with modern human strains on the environment, plants, animals, and invertebrates are becoming extinct at an unprecedented rate of thousands of species per year, especially in tropical rain forests. Many thousands more are threatened and endangered.

Scientists have determined that mass extinctions have occurred periodically in prehistory, coming about every 50 million years or so. The greatest of these came at the end of the Permian period, some 250 million years ago, when up to 96% of all species on the earth may have died off. Dinosaurs and many ocean species disappeared during a well-documented mass extinction at the end of the Cretaceous period (about 65 million years ago). It is estimated that of the billions...