You can find the answer to this in Chapter 15.
To many people, Australia proves that Europeans were smarter than Aborigines. The Aborigines could not make an advanced, farming society in Australia. Europeans came and soon made just such a society. This shows that superior people can do better than inferior people in the same environment.
But Diamond says this is not true. He says that Europeans did not create an advanced society using the same things the Aborigines had. They did not domesticate Australian plants and animals. Instead, they had to bring their own. This shows, Diamond says, that the Europeans were not superior. They were just lucky to come from a place that had plants and animals that could be domesticated.