““In ordinary times—times here understood to mean whole geologic epochs—extinction takes place only very rarely, more rarely even than speciation, and it occurs at what’s known as the background extinction rate. This rate varies from one group of organisms to another; often it’s expressed in terms of extinctions per million species-years. Calculating the background extinction rate is a laborious task that entails combing through whole databases’ worth of fossils. For what’s probably the best-studied group, which is mammals, it’s been reckoned to be roughly .25 million species-years. This means that, since there are about fifty-five hundred mammal species wandering around today, at the background extinction rate you’d expect—once again very roughly—one species to disappear every seven hundred years.””
![“Now they [the Powers] have started the aggression, and the…” quote by Empress Dowager Cixi](https://sms4smile.com/quotes/img-quotes/947/empress-dowager-cixi-68177_after-death_now-they-the-powers-have-started-the-agg_53275.jpg)
