If your book and mine are the same edition, then pp. 155-6 are at the very end of Chapter 8. In those pages, Diamond tells us why it is (in his opinion) that food production did not start in North America (the theory applies to other areas as well).
Basically, what Diamond is saying is that some places, like North America, just didn't ahve the right set of native food sources. They did not have enough different kinds of plants and animals that could be domesticated and used for food.
By saying this, Diamond is saying that you can't blame the people of North America for not coming up with agriculture -- they just didn't have enough native kinds of plants and animals to let them invent agriculture.