The fact "Marsupials are mammals whose young are born very immature" is illustrated with the story:1. After birth, marsupial infants must develop further inside a pouch.2. New-borne marsupials generally cannot survive outside their mother's pouch.3. Mammals are animals that gestate their young via placentas and give birth to relatively fully developed offspring.  4. Mammaliam offspring can generally survive without a special habitat (like a pouch) after birth.5. Marsupials are not mammals.