He is pitted against the rest of a world where men kidnap and control animals for profit and in which animals do not come to rescue him:

He was beaten (he knew that); but he was not broken. He saw, once for all, that he stood no chance against a man with a club. He had learned the lesson, and in all his after life he never forgot it. That club was a revelation. It was his introduction to the reign of primitive law, and he met the introduction halfway. 

Buck realizes that the result of this conflict is that his own sense of internal toughness will have to increase.  The byproduct of Buck being placed in a conflict between the individual and the larger society is that Buck learns to embrace the survivalist tendencies needed for success.  The premise of the individual versus society conflict is that there is a forlornness that has to be understood.  Either this will weaken the resolve of the individual or galvanize them into a mindset where perseverance and survival is evident. Upon experiencing being physically beaten with a club and set adrift in a world where no other force can assist, Buck must wrestle with the consequences of the individual versus society conflict.  Buck learns what is required of him for survival as a result of being kidnapped and as a consequence of an individual versus society conflict.