What is Diamond's argument in Chapter 2 of Guns, Germs, and Steel?

In Chapter 2, Diamond is arguing that it is geography, and
not culture, that determines what kind of a society springs up in any given place.  To
prove this, he looks at evidence from Polynesia.

Diamond
tells us that all the people who inhabited Polynesia came from the same ethnic and
cultural background.  Therefore, if culture was what really mattered, they should all
have ended up with similar societies.  Instead, what happened is that various Polynesian
societies ended up with very different levels of economic and political development. 
Diamond says that those that sprang up on large islands or island groups became more
advanced.  Those that arose on small islands with fewer resources remained small and
relatively primitive.

This, to Diamond, proves that it is
geography rather than culture that determines what a society ends up being
like.