Cyril Dean Darlington FRS (19 December 1903 – 26 March 1981) was an English biologist, geneticist and eugenicist, who discovered the mechanics of chromosomal crossover, its role in inheritance, and therefore its importance to evolution. He was Sherardian Professor of Botany at the University of Oxford from 1953 to 1971. Darlington's research on genetics contributed to modern evolutionary synthesis in the 20th century.