The Great Physical Auditorium of Saratov University is one of the best university lecture halls in Russia, it has a wonderful interior, large capacity and good acoustics. It also has a wonderful history. One outstanding event that took place in the Great Physical Auditorium 100 years ago is described in the article. On June 4, 1920, Nikolai I. Vavilov, professor of the agronomic faculty of Saratov University, made a presentation on «The law of homologous series in variation” to the participants of the III All-Russian Breeding Congress.