Humans as animals
With human consciousness as developed and sophisticated as it is, it is a powerful irony to remember that humans are just animals. The nature of man is human nature with instincts, just like animal instincts. Humans have to survive as well, but they have controlled their environment to the point that many animal concerns are already accounted for. But Chesterton chooses to remember philosophically that humans are essentially living animals.
Evolution and theism
The irony of evolution is that it is typically understood as evidence against the existence of God, but Chesterton observes that there isn't technically such a thing as evidence against the existence of God. Rather, the ideas were presented by an atheist who explained them with an atheist interpretation, but Chesterton argues that technically, evolution and the Bible are compatible if one doesn't take the Bible literally (an approach to Biblical tradition that is rather new; for millennia, the creation narrative was understood correctly as mythology and not scientific historical record).
Divine nature as the product of evolution
By understanding evolution within the realm of theism, the nature of humans becomes confusing and ironic. The human qualities that historically led humans to believe in supernatural design are actually explainable by scientific processes, and in the same way that this truth leads atheists to further disbelief, they lead Chesterton into deeper religious wonder. He finds evolution rather beautiful and subtle, and rather than denigrating humans by showing them as evolved, he considers evolution as essentially holy, because it created divine humans.
Jesus Christ as an evolved being
This book's ultimate depiction of the irony of theistic evolutionary theory is found in Chesterton's discussion about Jesus Christ. Instead of understanding Jesus as he is mythically presented, as the only-begotten son of God, essentially different than humans, Chesterton understands him as he literally was. If he was God incarnate, than God was incarnated through evolution, which is an ultimate elevation of evolution as a divine mechanism. The irony is one that is commonly overlooked by the church because of the mainstream aversion to evolutionary theory.
Meaning and time
The discussion is not only about evolution. The theistic aspect of this book's treatment of human nature leads to a discussion about essentialism and life's meaning. Chesterton says that life is clearly meaningful, because since the dawn of man, mankind has been attempting to capture depictions of their lives that last past their own existence. There is an irony between the finite nature of man's life and the infinite worth that each person finds in their own existence.
