Natural selection means that the organisms better adapted to their environment, that is, they have developed the right characteristics/skills/response to the external ambient, have higher chance to survive, making the next generations genetically inherit such acquired surviving characteristics.
These generations will thus have a higher chance of survival, making these species being "naturally selected" as survivors, while those who haven't or are slower in this process, have a higher risk of elimination.
They can melt because of global warming.
The cranial nerve that carries visual impulses from the retina to the brain is the: optic nerve.
The number of fossils found, the fact that fossils are oftentimes just a cast/mold of the organism with no real remains, and no interbreeding.
As time moves forward, we still strive towards better methods to answer questions about extinct organisms.