Inheritance and Evolution, Denise Walker, Evans Brothers, 2006, ISBN 978-0-237-53010-5, ... When a whole species dies out, we say that it has become extinct. Extinction is usually a natural process and it is estimated that 99.9 per cent of all species that have ever lived are now extinct ...
Mass Extinctions and Their Aftermath, A. Hallam, P. B. Wignall, Oxford University Press, 1997, ISBN 978-0-19-854916-1, ... He defines mass extinction as any substantial increase in the amount of extinction (that is, lineage termination) suffered by more than one geographically widespread higher taxon during a relatively short interval of geological time, resulting in at least temporary decline in their standing diversity ... more concise one offered here is that a mass extinction is an extinction of a significant proportion of the world's biota in a geologically insignificant period of time ...
