One of the primary themes in Jack London's The Call of the Wild is the constant conflict between civilization and wildness--in both the human and the canine characters. The protagonist of the novel is Buck, a gentleman's dog in California who is kidnapped and taken to the harsh wilds of Alaska.
Chapter two of the novel is titled "The Law of Club and Fang," and in Alaska, Buck learns the meaning of both. The law of the club is what his human owners do to ensure Buck's cooperation and obedience; the law of the fang is what the other dogs do to Buck and each other to establish and maintain a hierarchy. This time in Buck's life is considered "civilized" in the sense that he lives among humans; ironically, there are many aspects of Buck's "civilized" life which are as savage as anything Buck will encounter in the wild. He does finally experience a true civilized and loving relationship with his third owner, John Thornton; Thornton is eventually killed, releasing Buck to follow his instincts.
As the title suggests, Buck is constantly being drawn by his instincts (being called by the wild) even though he has grown up in and lived in a civilized setting.
But especially he loved to run in the dim twilight of the summer midnights, listening to the subdued and sleepy murmurs of the forest, reading signs and sounds as a man may read a book, and seeking for the mysterious something that called -- called, waking or sleeping, at all times, for him to come.
That stands him in good stead at the end of the novel, when he returns to the wild and at last feels free.
Several other issues to consider as themes include the use of violence (force) to establish social order and the romantic idea of the Alaskan adventurer's life in contrast with the harsh realities of living in the frozen North.