The Russian botanist Ivanovski demonstrated that the agent responsible for tobacco mosaic disease could pass through the finest filter then available. This was the starting point of research into the etiology of virus diseases. An English version is in Phytopathological Classics (American Phytopathological Society), No. 7, pp. 25-30, Ithaca, N.Y., 1942. See No. 2503.1.