19. Diamond gives six reasons why: diet, growth rate, and problems of captive breeding, nasty disposition, tendency to panic, and social structure. 20. The Americas and Africa both have a north-south axis while Eurasia has a west-east axis. It is because west-east regions share the same length of day, same types of seasons, same diseases, and same aspects of climate such as rainfall and habitat. Trade spread farther west east than it did north south. 21. Plants moved from north to south or vice-versa were not built to endure the different climates, times of day, and etc. Domestic animals could not fight off the new types of disease and climate as well. 22. Genetically some people have developed immunity to certain diseases or illnesses through generations of repeated exposure. The small populations can't fight outside epidemics, and can't evolve their own because they are nearly wiped out every time, therefore the epidemic disappears. 23. Blueprinting copying- when you copy or modify an available detailed blueprint. Idea Diffusion-when you receive little more than the basic idea and have to try to do it yourself. 24. If kings limited writing they can better control the masses and there is a smaller chance of and uprising. 25. "Invention is often the mother of necessity" which means that many inventors created things out of curiosity rather than because of need. In other words, they made a product before there was a demand for it