This is when the team arrives at John Thorton's camp, and John tells them the ice is too thin and that they shouldn't bother going across it. The owners are not fit to survive and know close to nothing about the Yukon, so they go anyway and fall through the ice.
This shows how all of the new dogs and all of the other dogs that weren't fit to survive eventually died. the new dogs died quicker than the original team because they weren't used to the food shortages and hard labor.
This shows that Charles, and Hal are not fit to survive because they had a huge amount of equipment and supplies that they wouldn't need such as the tent. It is way to heavy for the dogs, and the onlookers are telling them to get rid of half of the stuff too.
Mercedes is not fit to survive because the most of the time they were traveling she was sitting on the sled, she was never doing anything, and she said couldn't live without her tent.
Buck is fit to survive because he is strong, wild, and adaptable. When he first got to the Yukon he adapted very quickly to the harsh and brutal environment. He also was able to pull 1,000 pounds for 100 yards showing how strong he is.
John Thornton is fit to survive because he has been in the Yukon for a long time and he told the men to not go across the ice because he knew it would break. He knows his way around and is a great owner for Buck.