Heslop-Harrison (Harrison and Garrett 1926; Harrison 1928) suggested that the increase of melanic moths in industrialised regions was due to "mutation pressure", not to selection by predators which he regarded as negligible. Salts of lead and manganese were present in the airborne pollutant particles, and he suggested that these caused the mutation of genes for melanin production but of no others. He used "Selenia bilunaria" and "Tephrosia bistortata" as material. The larvae were fed with leaves that had incorporated these salts and melanics subsequently appeared. Similar experiments by Hughes McKenney (1932) and Thomasen and Lemche (1933) failed to replicate these results. However, the statistician and geneticist Ronald Fisher showed that Heslop-Harrison's controls were inadequate. This hypothesis, however, appeared to be falsified by breeding experiments.