Diamond claims that there is a relationship between farming and societal development. Where does he get the evidence to support his claim? in Guns, Germs, and Steel
I assume that you are talking about the sort of societal development that Diamond is discussing in Chapter 14. This is the development from societies in which people live in small bands, up through tribes and chiefdoms and finally to states. Diamond says that the first of these is what occurs among hunter-gatherers and that the final two are only possible where farming exists. He cites, as his evidence, all of human history, basically.
On p. 282 (17 pages into the chapter and 10 pages from the end of the chapter in my book), Diamond presents his evidence. Basically, he says that he has looked at all of the instances in which states arose without having any other states already existing around them (so they couldn't copy the other states). He says he looked at places where chiefdoms arose without contact with other chiefdoms. This, he says, is his "database" for understanding the way in which societies develop.
Diamond argues that these societies all show that you need agriculture in order to develop more complex societies. This is because agriculture is what allows large, sedentary populations to arise. These large, sedentary populations are necessary if more "developed" societies are to arise.