Part four considers conquest. Why is it that the Eurasians came to dominate other cultures? Diamond's answer is that, in part, this was due to the East–West layout of the Eurasian continent, due to which successful agricultural and animal domestication packages (combinations of certain domesticated plants and animals) could easily be adopted in regions farther east or west. On the other hand, extension of domestication packages along the North–South axis – as required in the case of the American and African continents – was much more difficult owing to severe imbalances of climate. Also, long-lasting contact with domesticated animals of agricultural populations permits greater resistance to disease, which is another reason why contact among geographically separated cultures – mostly agricultural versus hunter-gatherer societies – often leads to extinction of the latter through devastating infections.