I'm not convinced the whole thing is a decent rationality quote, as part of it seems to be Menelaus surrendering to the idea that "because Darwin discovered Natural Selection, he endorsed it".

On the other hand, "Some of his friends said you had to prick your finger with a pin to make the oath valid; and boys of particular boldness used a rusty pin, as if daring the Jihad plague to strike. Menelaus knew that was all nonsense: it was the willpower that decided oaths, nothing else. No pin would be as sharp as what he felt beating in his angry young heart." is brilliant: both understanding the inclination to irrationality, and also emphasising that rationality can be strengthened by emotion.