4 ideas
Dr. Bodkin proposes that the spine and cells hold a record of every earlier evolutionary stage. When the environment reverts to Triassic heat and swamps, those buried layers reawaken as dreams and compulsions. On this view the characters' strange behaviour is 'archaeopsychic' memory being switched on by the outer world, not madness.
Around 2145, biologist Robert Kerans is part of a survey team mapping flooded London from a floating station. When Colonel Riggs evacuates north, Kerans stays in his suite at the Ritz hotel with Beatrice Dahl and Dr. Bodkin. The looter Strangman arrives and pumps the lagoon dry to plunder the silted streets, and Bodkin is killed trying to blow up the dam. Kerans then destroys the dam himself, re-floods the city, and walks south into the jungle toward the burning sun, knowing he will not survive.
The drowned, overheated world is read less as a disaster to be survived than as the outward form of a change already happening inside people. Heat, lagoons and giant plants show a mind sliding toward older, pre-rational states. The result is that apocalypse becomes a psychological fact rather than a physical problem, so fighting it with engineering misses where it is actually happening.
The novel's one serious attempt to bring back the old world is Strangman draining the lagoon, and the book frames it as looting and spectacle rather than rescue. The recovered streets are rotten and dead, useful only as a place to take treasure from. The argument is that clinging to a vanished order after the conditions for it are gone turns into exploiting its corpse, while Kerans' acceptance of the change, however self-destructive, is presented as the more honest response.