In 1961 Fokker and Roosen suggested that the concept of homology as applied to solar optical flares by Ellison, McKenna and Reid could be extended to flare-associated radio events as well, since successive flares within the same centre of activity sometimes produce radio bursts which look remarkably similar on single-frequency records. Teresa Fortini reported very similar ionospheric responses due to X-radiation from flares recurring in the same active region. In 1950 Wild noted similarities in the dynamic spectra of type III bursts occurring within periods of minutes.