Most of the species that ever existed on the Earth became extinct at some time in the past.It has been estimated that the number of species existing at the present time corresponds to the order of one per mille to one percent of the number of extinct species (Raup 1994).It is rather difficult to perform a more exact quantitative estimate as the probability that a certain species will be found in the paleontological record is directly proportional to the duration of its existence.It is thus apparent that the major part of preserved fossils will correspond to species that survived for a sufficiently long time on the Earth and, to the contrary, we will learn nothing from the paleontological record about the existence of short-lived species.Similarly, it is obvious that a great many of the traits, on the basis of which modern species are distinguished within a certain taxon, are apparent only on the soft parts of their bodies and were thus not preserved in fossils.Nonetheless, the paleontological data clearly demonstrate that species are formed at a certain moment, survive for a longer or shorter time and finally disappear, become extinct.The situation is similar for higher taxa – however, they understandably exist far longer than individual species and become extinct only when their last members become extinct.This chapter will be concerned with the individual types of extinction, especially their basic classification as background and mass extinctions, their mechanisms and also their impacts on the overall progress of evolution.

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