This represents the scene where Curly is killed. The Newfoundland could not adapt quickly enough to fight off the ferine huskies. She was not yet fit to survive in the harsh environment, so she was killed.
This scene represents how buck was able to quickly learn how to sleep in the harsh cold of Alaska. Though he wandered the camp for several minutes, he eventually figured out how his comrades were keeping warm and made it to sleep.
This shows Francois and Perrault in their camp. They were very experienced in the ways of the Klondike and were amazing owners. They were well equipped to survive in the brumal conditions with their knowledge and just ways.
Buck and Spitz are fighting to the death to establish dominance. They both had to be fit to survive and fit to fight so they could find a leader.
This shows Hal, Charles and Mercedes plunging into the freezing cold water. They were not fit to survive because they were driven solely by greed and believed they could do the impossible.
When Buck leaves John Thorton and runs wild he finds his "wild brother" a wolf. He learns how to be part of a pack and is very fit to survive in any climate.