One of the major things that Jared Diamond describes regarding his research is the need to have controls, situations that help to bring out the differences in other situations. In this case, he uses the islands of Polynesia as examples to examine the different influence of geography and other environmental factors on the advance of different societies. Because they are islands and their socieites grew and developed almost completely independently, they help to serve as backdrops for describing the way other societies developed even though those others have more variables because they were often influenced by other societies.
A second idea that comes forward in the first several chapters is the breadth of knowledge required to make any kind of scientific inquiry relevant and meaningful when addressing something as massive in scale as the development of societies and trying to ask or examine questions about how they were different and how they developed the way they did. The knowledge of history, geography, archeology, physical sciences, meteorology, geology, etc., that all comes into play in the discussion serves to highlight the immense wealth of knowledge one needs to effectively address this type of research.