In 1943 Salvador E. Luria of Indiana University in Bloomington and Max Delbrück, then of Vanderbilt University, reported the first quantitative study of mutation in bacteria. They demonstrated via the the Luria-Delbrück experiment, also known as the Fluctuation Test, that mutational alterations in bacteria occur randomly and before exposure to exogenous mutagenic influences. They showed that natural selection applies to bacteria as well as to more complex organisms.
Luria & Delbrück, “Mutations of bacteria from virus sensitivity to virus resistance,” Genetics 28 (1943) 491-511.
Judson, The Eighth Day of Creation, 37. J. Norman (ed) Morton's Medical Bibliography 5th ed (1991) no. 2526.1.