Chesterton is talking about social institutions and social change, not unconscious acts and biological evolution.
His principle doesn't apply at all here. He's asserting that every long-standing social institution very likely came to exist to serve some purpose, so if one says we should scrap institution X because it's useless, it's likely one is wrong. This has nothing to do with man's biological evolution. In fact we have a myriad of "bad features" (congenital diseases, disabilities, cancer) that have survived evolution.
His principle doesn't apply at all here. He's asserting that every long-standing social institution very likely came to exist to serve some purpose, so if one says we should scrap institution X because it's useless, it's likely one is wrong. This has nothing to do with man's biological evolution. In fact we have a myriad of "bad features" (congenital diseases, disabilities, cancer) that have survived evolution.