The Anna Karenina principle states that there are lots of different ways that an animal can be no good for domestication. It comes from a line that says happy families are all alike but miserable ones are all miserable in their own ways.
The reason that this is important is that it explains why people in some places were not able to domesticate any large animals. Diamond points out that there are many reasons why animals might not be good to domesticate. For example, they might take too long to grow big enough to use. Or they might be too vicious to tame. Other problems might occur as well.
Because there are so many different ways that an animal can be unsuitable for domestication, most of them are unsuitable. This means that places like Africa can have lots of animals, but none might be suitable to domesticate. This (rather than any theories about the characteristics of the people in those areas) explains why no animals were domesticated there.
The Anna Karenina Principle is one that describes the existence of successful marriages and unsuccessful marriages. It is important for a number of reasons.
Happy families are all alike; every unhappy family is unhappy in its own way.
This principle can also be applied to things outside of simply marriages. Many dog trainers use it to describe owners who want their dogs to simply be better, but in order to do so, all behavioral problems must be addressed and fixed.