Each generation of the iPhone sells approximately as many all all previous versions combined

"We're being shown a chart with cumulative unit sales for both the iPhone and iPad. When assessing sales for a new model of the iPhone, Apple used an easy shorthand: "Each new generation sold approximately equal to all previous generations combined.""

Source:  Apple designer Christopher Stringer, giving evidence in the Apple/Samsung trial, reported by The Verge, 3rd August 2012