Fourth Doctor rationalizing at the end of Genesis?
The Fourth Doctor claims he didn't fail at the end of Genesis Of The Daleks. He says that he doesn't think he failed because he knows that out of the great evil of the Daleks, there must come something good. Even if that were the case, that's not HIS success. It's the Daleks' failure that causes people to fight a common foe.
He seems to be rationalizing away his failure. He was sent on a mission to either prevent the development of the Daleks or affect their evolution so they're less aggressive creatures. The closest he comes to, in his own words, is he says something like he's delayed their development for a thousand years.
Clinically, there's no other way around it. The Doctor failed in his mission in Genesis Of The Daleks. He accomplished neither of the two goals set for him by the Time Lords. They didn't seem to mind this, though, as they apparently accepted the outcome of his mission at the end. They didn't send him back, after all.
However, it could be argued that the Doctor did succeed in not letting his mission turn him into a Dalek. He didn't let the ends justify the means. He kept the moral high ground by not becoming an exterminator of an entire race.