In what way does The Chrysalids reveal the selfish nature of man?
You might want to turn to some of the final chapters when the woman from Sealand shares her view on the world, and in particular the Waknukians, who are the last remaining legacy of the old evolutionary state of mankind. Her harsh presentation of this lingering momento of what man used to be clearly casts them in a rather unflattering light in terms of their actions and their perception of themselves. Consider what she tells us about them in Chapter Fourteen:
They learnt to cooperate constructively in small units; but only destructively in large units. They aspired greedily and then refused to face the responsibilities they had created. They created vast problems, and then buried their heads in the sands of idle faith. There was, you see, no real communication, no understanding between them. They could, at best, be near-sublime animals, but no more.
So we can see that the selfish nature of mankind lies in the way that they created so many problems and then failed to take ownership and responsibility of those problems and also their inability to conceive that they were anything more than a link in the evolutionary chain, and as such an "inadequate species." Arrogantly assuming they were the pinnacle of the species of man rather than just another development is one of the chief ways in which mankind is presented as selfish.