Extinction of species
At the present time, the extinction of a species is considered to be the instant when the last representative of the particular species dies. “Pseudo-extinction”, i.e. the gradual change of one species into a different species, is not considered to constitute extinction; some palaeontologists believe in the existence of this process that, according to the theory of frozen plasticity, should occur only in asexually reproducing species. Palaeontological data indicate that extinction is the unavoidable fate of every species. The average time of survival of species differs for the individual taxons. For example, the period of survival of the average mammal species varies around 5 million years, while the period of survival of sea snails and clams is about 10–20 million years. Species become extinct, either as a consequence of sudden catastrophic events, for example the impact of asteroids or the cores of comets on the Earth, or gradually, as if there were no external cause at all. Some facts indicate that the commonest cause of gradual (called background) extinction consists in pandemics caused by a parasite, probably most frequently a virus, see also Footnote 3 in Chapter 16 of the book.