The Daleks have appeared on the show many times and their design has seen many changes. This means that it can be difficult to make sure that the storylines line up. When the Daleks first appeared in "The Daleks", they were presented as the descendants of the Dals, mutated after a brief nuclear war between the Dal and Thal races. However, in 1975, Terry Nation revised the Daleks' origins in "Genesis of the Daleks", where the Dals were now called Kaleds (of which Daleks is an anagram), and the Dalek design was attributed to one man, the crippled Kaled chief scientist Davros.