You can find the answer to this in Chapter 2. There, Diamond says that Polynesia can be seen as an experiment that tests whether culture or environment is more important in determining what sort of a society arises in a given place.
Polynesia is an experiment because people with the same original culture spread out to many islands with very different environments. If culture was the most important thing in determining what they turned out like, their societies would all have been the same. Instead, societies on islands like the North Island of New Zealand (where farming was possible) became complex and powerful. Meanwhile, societies like those on the Chathams (where farming was not possible) did not. Thus, the Polynesian islands are an experiment because they test which variable (culture or environment) had more of an impact on societal development.