The point of this chapter is to help us understand why some continents and regions had lots of domesticated animals while others had few or none. Diamond argues that this is because there are only a very few species of large animals that are suitable for domestication. More of these animals were native to Eurasia than to any other continent.
Diamond is arguing once again that geographic luck is what made some societies more advanced than others. He is saying that most animals cannot really be domesticated and so an area has to be lucky to have large, domesticable animals. Eurasia was the luckiest continent and that gave them an advantage in their development.