to build large and complex states complete with advanced technology and weapons. The Incans and other native groups were not as blessed, suffering from a north–south orientation that prevented the flow of goods and knowledge across the continent. The Americas also lacked the animals, metals, and complex writing systems of Eurasia which prevented them from achieving the military or biological protections needed to fight off the European threat. Diamond's theory has not gone without criticism. It was notably attacked for not providing enough detail regarding causation of environmental variables, and for leaving logical gaps in reasoning. Geographer Andrew Sluyter argued that Diamond was just as ignorant as the racists of the 19th century. Sluyter challenged Diamond's theory since it seemed to suggest that environmental conditions lead to gene selection, which then lead to wealth and power for certain civilizations. Sluyter also attacks environmental determinism by condemning it as a highly studied and popular field based entirely on Diamond's "quick and dirty" combination of natural and social sciences. Daron Acemoglu and James A. Robinson similarly criticized Diamond's work in their book Why Nations Fail. They contend that the theory is outdated and can not effectively explain differences in economic growth after 1500 or the reasons why states that are geographically close can exhibit vast differences in wealth. They instead favored an institutional approach in which a society's success or failure is based on the underlying strength of its institutions. Writing in response to institutional arguments, Diamond agreed that institutions are an important cause, but argued that their development is often heavily influenced by geography, such as the clear regional pattern in Africa where the northern and southern countries are wealthier than those in the tropical regions. Geography and pre-colonial African state-building The effects of climate and land abundance on the development of state systems In his book States and Power in Africa, political scientist Jeffrey Herbst argues that environmental conditions help explain why, in contrast to other parts of the world such as Europe, many pre-colonial societies in Africa did not develop into dense, settled, hierarchical societies with strong state control that competed with neighboring states for people and territory. Herbst argues that the European state-building experience was highly idiosyncratic because it occurred under systemic geographic pressures that favored wars of conquest – namely, passable terrain, land scarcity, and high-population densities. Faced with the constant threat of war, political elites sent