Compare David's Uncle Axel to his Uncle Angus Mortan in The Chrysalids.
Throughout the story, Uncle Axel is a comfort to David. He provides security for David (both physically and emotionally); he is able to confide in Uncle Axel his ability to communicate telepathically, and Uncle Axel ensures David's secret remains safe. David refers to him as his "best friend" when he is introduced in chapter two.
Because of his experience as a sailor, Uncle Axel's views are progressive comparatively to everyone else is Waknuk:
They all have pretty much the same legends of the Old People as we have -- how they could fly, how they used to build cities that floated on the sea, how any one of them could speak to any other, even hundreds of miles away, and so on. But what's more worrying is that most of them -- whether they have seven fingers, or four arms, or hair all over, or six breasts, or whatever it is that's wrong with them -- think that their type is the true pattern of the Old People, and anything different is a Deviation.
That seems silly at first, but when you find more and more kinds just as convinced of it as we are ourselves -- well, you begin to wonder a bit. You start asking yourself: well, what real evidence have we got about the true image? You find that the Bible doesn't say anything to contradict the people of that time being like us, but on the other hand it doesn't give any definition of Man, either. No, the definition comes from Nicholson's Repentances -- and he admits that he was writing some generations after Tribulation came, so you find yourself wondering whether he knew he was in the true image, or whether he only thought he was. [...] Uncle Axel went on talking about the doubts of the true image that his voyage had given him (Chap 6).
While Uncle Axel has doubts about the image of man and the beliefs of Waknuk, he generally keeps these concepts to himself. While he discusses it with David, this discourse is held privately. Uncle Axel keeps his progressive views to himself.
Comparatively, Uncle Angus Morton also has a progressive view about the image of man and the beliefs of Waknuk, but he frequently takes actions that draws attention to his differences in belief.
My father had been heard to sum up his opinion by declaring that if Angus had any principles they were of such infinite width as to be a menace to the rectitude of the neighbourhood; to which Angus was reputed to have replied that Joseph Strorm was a flinty-souled pedant, and bigoted well beyond reason. It was not, therefore, difficult for a row to blow up, and the latest one occurred over Angus' acquisition of a pair of great-horses.
Rumours of great-horses had reached our district though none had been seen there. My father was already uneasy in his mind at what he had heard of them, nor was the fact that Angus was the importer of them a commendation; consequently, it may have been with some prejudice that he went to inspect them.
His doubts were confirmed at once. The moment he set eyes on the huge creatures standing twenty-six hands at the shoulder, he knew they were wrong. He turned his back on them with disgust, and went straight to the inspector's house with a demand that they should be destroyed as Offences.(Chap 4)
While both Uncle Axel and Uncle Angus Morton challenge the beliefs of Waknuk, only Uncle Angus Morton does so publicly.