The falling action of a story is the part after the climax. This is the part where we find out who will "win" in a story (the protagonist or the antagonist) and how the conflict of the story will be resolved.
In this book, the climax comes when Buck is rescued by Thornton. This is where Thornton takes him from Hal, just before Hal and the others die when they crash through the ice. This is the major turning point in the book.
From there, we get the falling action. This is where we see Buck becoming closer to Thornton yet, at the same time, being pulled towards becoming a wild beast. As the falling action progresses, we see this tension building until Thornton is killed.