"It was in January 1943, after I had moved to Indiana University in Bloomington, that I thought of a critical choice between spontaneous and phage-induced resistance. The first written statement of it I find in a letter to Delbruck dated January 20: 'I thought that a clean cut experiment would be to find out how the fluctuations in the number of phage-resistants depend on the culture from which they come. That is: If I plate with phage ten samples of the same culture of E. coli, I find numbers of resistants which fluctuate according to Poisson's law. If I plate 10 samples of 10 different cultures of E. coli, all containing the same amount of E. coli, I find much larger fluctuations. If the resistants were reproduced on the plate, after contact with phage, they should show the same fluctuations in both cases.' The idea of this experiment came to me, in fact, while watching the fluctuating returns obtained by various colleagues of mine gambling on a slot-machine at the Bloomington Country Club, where faculty dances were then held one Saturday a month."