History of the world like an onion (Simile)
Diamond begins his book by comparing the history of the world to the structure of an onion. This simile emphasizes that the history of the world is not as simple as one would be inclined to believe. In fact, there are many layers and elements that have to be taken into consideration when discussing the history of the world. Thus, it is like an onion in the sense that it requires multiple levels of analysis in order to get to its core. Diamond's book will peel back these many layers to search for the core truth of why some human societies developed faster than others.
Writing (Metaphor)
Diamond mentions one particularly important historical event: when the Spanish explorer Francisco Pizarro rose to power in what is known today as Peru. The reason that Francisco Pizarro was able to conquer the Incas had to do in part with the fact that Europeans had a fixed writing system, while the South Americans did not. Because of the writing system, Europeans could share information and news about technological advancements. What is more, Francisco Pizarro managed to win because Atahuallpa was fooled by Pizarro and believed in his good intentions. Had he known about the ways the Spanish fight and about their strategies, there is a possibility that he would have not let his guard down. Thus, writing serves as a metaphor for power, because it allows literate societies to build and spread their power over others.
Birds (Simile)
When Diamond talks about agriculture, he mentions that, in time, through selective farming, humankind was able to domesticate plants and use them for food. Diamond compares the process used by humankind with what the birds did naturally; that is, eat a fruit, digest it and then defecate the seeds that could thus be spread all over the territory the birds flew over. In a sense, humans did the same, taking one kind of crop that originated in one part of the world and then growing it someplace different. Diamond thus compares human domestication of plants with the process first begun by birds.
Religion (Simile)
When Diamond talks about patriotism, he compares it to religion, and the fact that people are willing to kill for their God. Humankind does not have an innate desire to fight for an idea since the fight for survival is more important. But leaders managed to make their people fight even agains their own interests by teaching them to subscribe to religious or patriotic ideals that compelled them to fight for a higher cause. In time, the fervor with which some fought for religion became the fervor with which a group of people protected their country. And thus the idea of a nation took over religion and the state became like a God that people could worship.
