In order to understand the answer to this, let us first look at Diamond’s thesis. His thesis is that the people of some continents became rich and powerful due to geographic luck. Their geographic luck enabled them to have agriculture sooner than other continents’ peoples. This allowed them to build civilizations earlier. One of the types of geographic luck they had was in the number of animals living in their area that could be domesticated. This is where the quote from Anna Karenina comes in. The quote is:
Happy families are all alike; every unhappy family is unhappy in its own way.
In Chapter 9, Diamond argues that some areas of the world were just unlucky and did not have animals that could be domesticated. He says that there are a number of attributes that an animal must have if it is to be domesticated. Every animal that is domesticated has all of those attributes. Thus, all domesticated animals, like all happy families, are alike. However, if an animal lacks just one of these attributes it cannot be domesticated. So there can be animals with attributes A, B, and C but not D. There can be animals with only attribute A or only attribute D. In other words, animals that cannot be domesticated, like unhappy families, are all different.
This supports Diamond’s thesis because it says that you have to be really lucky to have animals living near you that have all the right attributes and can be domesticated. Therefore, it was just luck that allowed people in some areas to domesticate a lot of animals and, thereby, help to get guns, germs, and steel.