giving all white, but as he had only expected 1 of those in his sample, 0 was not an unlikely outcome. Genes could clearly combine in almost infinite combinations: ten of his factors allowed for almost 60,000 different forms, with no need to suppose that any new mutations were involved. The results implied that natural selection would work on Mendelian genes, helping to bring about the unification of Darwinian evolution and genetics. Similar work in America by Edward East on maize in 1910 showed the same thing for biologists without access to Nilsson-Ehle's work. On the same theme, the mathematician Ronald Fisher published "The Correlation Between Relatives on the Supposition of Mendelian Inheritance" in 1918, again showing that continuous variation could readily be produced by multiple Mendelian genes. It showed, too, that Mendelian inheritance had no essential link with mutationism: Fisher stressed that small variations (per gene) would be sufficient for natural selection to drive evolution. Castle's selection experiments on hooded rats, 1911 Starting in 1906, William Castle carried out a long study of the effect of selection on coat colour in rats. The piebald or hooded pattern was recessive to the grey wild type. He crossed hooded rats with the black-backed Irish type, and then back-crossed the offspring with pure hooded rats. The dark stripe on the back was bigger. He then tried selecting different groups for bigger or smaller stripes for 5 generations, and found that it was possible to change the characteristics way beyond the initial range of variation. This effectively refuted de Vries's claim that continuous variation could not be inherited permanently, requiring new mutations. By 1911 Castle noted that the results could be explained by Darwinian selection on heritable variation of Mendelian genes. Morgan's small Mendelian genes in Drosophila, 1912 By 1912, after years of work on the genetics of Drosophila fruit flies, Thomas Hunt Morgan showed that these animals had many small Mendelian factors on which Darwinian evolution could work as if variation was fully continuous. The way was open for geneticists to conclude that Mendelism supported Darwinism. Muller's balanced lethal explanation of Oenothera "mutations", 1918 De Vries's mutationism was dealt a serious if not fatal blow in 1918 by the American geneticist Hermann Joseph Muller. He compared the behaviour of balanced lethals in Drosophila with De Vries's supposed mutations in Oenothera, showing that they could work the same way. No actual mutations were