What is “the call of the wild” that Buck experiences? Is it call of nature or wilderness or call from within?

Buck hears this call through the book but only heeds it at the end. After destroying the Yeehat tribe who killed his master, Buck has repaid his debt to Thornton. He hears the final call of wolves which he is now free to join and take his rightful place as alpha-wolf dog of the wild. I would check out chapter 7 for this.