Evolution as imagery
Chesterton explains the theory of evolution as a concrete depiction of processes of nature than don't necessarily imply one interpretation over another. The question of the book therefore is to reassign abstract imagery to evolution that retains belief in God, to disprove the automatic assumption that evolution and religion are incompatible.
Animal nature
Instead of taking some lofty philosophical idea as the central axiom of the book, Chesterton explains that humans are literally animals, and if animals evolved to lead to us, that would be consistent with our animal nature. He allows animal nature to has its full imagery so that his secondary conversation about religion and meaning can be embedded in what is literally true about humans; we are animals very similar to the other great apes, except with unique traits.
Essence and meaning
By establishing the animal imagery of human reality, Chesterton sets up an idea with an obvious next step. The next step is to address the aspects of human nature that are unique in the animal kingdom. Humans are unique in these essential ways: we understand death before it happens, we use words, we make technological progress, and we seek to capture the ineffable aspect of the meaning of our existence. He says that are belongs to this imagery as well.
Jesus's divinity as elevation
Although typically Christians use the imagery of either/or when dealing with evolution, Chesteron uses both/and imagery. Jesus is simultaneously evolved and God incarnate, says Chesterton. This take evolution and elevates it to a status of holiness, because evolution is the process through which God incarnated his reality by a lengthy series of reincarnations. Chesterton's treatment of Christ as animal is transcendental and mystical.
