That's why questions, while the human settlers tend to live in more cities. To these people living in the ghetto instead of in an environment that is good to this book.
While the question of why human settlements tend to be in more cities. Makes these people live in slums than in a good environment. Probably the main essence of this book.
The question of why the human settlement is likely to stay in the city. Get these people live in the slum instead of the good environment. Should be the main theme of this book.