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".

It appears to me that within the story, his knowledge of exactly who Darwin was has been greatly garbled by the processes of history. That's just a detail of the setting. My reading of Menelaus' attitude to evolution is that he is expressing much the same idea as Eliezer's characterisation of it as a blind idiot god that we should overcome and replace.