1 In optical mineralogy, a mineral is said to be in extinction when the vibration direction of the two rays of a doubly refracting crystal coincide with the vibration directions of the two pieces of Polaroid in a thin-section microscope that is parallel to the polarizer and analyser so that no light reaches the eye. This phenomenon occurs four times in a complete 360° rotation of the stage. See oblique extinction; straight extinction; symmetrical extinction; and undulose extinction.
2 The elimination of a taxon. This may take place in several ways. In the simplest case the taxon disappears from the record and is not replaced. Alternatively, one taxon may replace another, the earlier group consequently disappearing. Thus there is a process of either subtraction or substitution. Extinction generally takes place at particular times and places but there are recurring periods when episodes of mass extinction have taken place. Environmental catastrophe, occurring for whatever reason, removes many groups from the environment and ecosystems collapse. Eventually new forms appear and evolution resumes. It would appear that periods of mass extinction control the pattern of evolution.