Many of the older generation in this society, as Uncle Axel explains to David, hold very strict views on the importance of maintaining purity by clamping down most severely on the slightest deviant. David experiences this for himself when he listens to the railings of the old farm worker, Jacob, described as 'a venomously puritanical old man', who wants all deviants to be eliminated as ruthlessly as they were in the past, and bemoans the fact that lately the rules have become more relaxed, so that near-normal deviants are allowed to live so long as they don't breed, and that people who attempt to conceal any mutants among either humans or their crops often incur mere fines instead of the more dastardly punishments of earlier times.
Uncle Axel also confirms Jacob's fears that recently there have been more deviants among humans, animals and plants than ever before. The old people, like Jacob, put this down to the fact that things have become lax in this society and that God is being angered, and sending more mutants into the society as punishment. The greatest fear among such people, however, is that God might be preparing another terrible visitation, like the Tribulation of old which resulted in devastation and the formation of mutants in the first place. Therefore, they are increasingly on the lookout for the slightest form of deviation.
Uncle Axel worries for David because David, with his telepathic powers, is a deviation from the norm - although luckily for him it is something that, with a bit of care, he is able to conceal, as it is not an obvious, physical difference. But with suspicions increasing among the more narrow-minded sections of the community, like Jacob, Uncle Axel realises it's going to become more and difficult for David to hide his secret powers. In this chapter, he does not warn David in so many words, but gives him a meaningful look, which David understands.