Throughout Guns, Germs, and Steel, how does Diamond address the issues he discusses in the last few pages of his final chapter?

In the last few pages of the book, Diamond proposes that
historians should treat their subject like a science.  He says that many sciences, like
geology and climate science, rely on "natural experiments" where scientists have to try
to compare things that have already happened with one another and infer rules from the
differences in what happened.  Scientists have to do that because they cannot simply
change some factor in the earth's atmosphere, for example, and see what happens. 
Diamond says historians should do the same with human
history.

In the rest of the book, Diamond tries to do
this.  He does it most clearly in Chapter 2.  There, he looks at Polynesian societies. 
He asks why various Polynesian societies turned out so differently.  He comes to the
conclusion that their differences were caused by the environments in which they arose. 
Another example of how Diamond does this is found in Chapter 15.  There, Diamond looks
at the differences between Aboriginal Australian societies and European societies in
Australia.  From the way they turned out, he concludes that Aborigines were not
culturally inclined to reject farming and technology.  Instead, he says that geography
was behind the changes.

In ways like these, Diamond
conducts "natural experiments" throughout his book.