In the novel The Call of the Wild, Buck has gone from being a beloved pet, to a hardened dog capable of withstanding almost anything. When Thornton rescues Buck, the dog comes full circle to truly love a human being again. Buck loves Thornton, which shows that he is still capable of being able to love after all of the abuse, that he is willing to give up everything for the love of this man and stay with humans despite listening to the wolf call. When Buck is out of the camp and comes back to find Thornton and all of the others dead, he gives up his desire to stay with humans, meaning Thornton, and answers the "call of the wild". He has now truly become a creature of the wild just like the wolf and returns to his ancestral state of a wild, undomesticated creature.
the clarification is greatly appreciated.