From the moment he's born, White Fang faces death. It waits around every corner for him and could claim him at any moment. Seriously stressful business, right? It's just as bad for everybody else who, as London quietly points out, are going to die sooner or later anyway. In White Fang, the goal is to avoid dying for as long as possible, kind of like a cosmic game of dodge ball. You gotta keep ducking and diving, even though you know you're going to lose.
Death is less prevalent in the California chapters of the novel, but only because men are less aware of it.
Death is the real antagonist of the novel, since it's the one foe that no character can conquer.