E. coli cells were spread on two agar plates [one containing only agar (plate 1) and the second containing streptomycin (plate 2) which kills all cells except those that are resistant mutants]. Both plates were allowed to grow for several generations. Cells were collected from the agar plate WITHOUT streptomycin and spread again on a fresh agar plate containing streptomycin (plate 3). More mutants were observed on plate 3 than were observed on plate 2. The experiment demonstrates