In their experiment, Luria and Delbrück grew bacteria in tubes. After a period of growth, they split up the bacteria into separate cultures and put them onto agar containing phage (virus). If virus resistance were not due to random gene mutations, then each plate should contain roughly the same number of resistant colonies. This, however was not what Delbrück and Luria found. Instead, the number of resistant colonies on each plate varied to a great extent.