The theme supported by Buck's unwillingness to continue the journey with the wolf is "Only the strong are able to survive in the wild."
In this context, Buck's decision to turn back instead of following the wolf signifies his understanding of the importance of his bond with John Thornton, which provides him with strength and security. Buck realizes that true survival and fulfillment come from the relationships he has formed, rather than solely embracing his wild instincts. This highlights the idea that strength is not just about physical prowess, but also about emotional connections and loyalty.