Bostock, Stephen C.: Zoos and Animal Rights. The Ethics of Keeping Animals. London 1993. Internet: https://books.google.de/books?id=ccqHAgAAQBAJ&printsec=frontcover&dq=Zoos+and+Animal+Rights.+The+Ethics+of+Keeping+Animals.&hl=en&sa=X&ved=0ahUKEwixy9WCqdbTAhUDrRoKHRZrBiIQ6AEIJjAA#v=onepage&q&f=false.