What about the natural enviroment of the Chatham Islands caused the Moriori people to develop so differently from their Maori conquerors of the same ancestral heritage?
The Moriori and the Maori were both Polynesian peoples with common ancestors, yet by the nineteenth century, the difference in their development was significant enough that the Maori were able to invade and almost completely exterminate the Moriori. The reason, Diamond argues, was that the Moriori had settled on the Chatham Islands, which were too cold for their traditional crops. They thus turned to hunting and gathering, which does not support a dense population, and usually is characterized by an egalitarian social structure (without strong leaders). Additionally, because the islands were so remote, they had no incentive to develop military technology. The other parties to this "natural experiment," the Maori, lived in warmer nothern New Zealand, where they practiced settled agriculture, lived in much denser societies, and fought frequent wars against other peoples. So they developed the technology and the warrior ethos that enabled them to destroy the Moriori. This incident is, in microcosm, his overall argument.