What are some examples in the story that act as the antagonist and how?
Perhaps the best example of nature seeming to act as an antagonist occurs when the man builds his first fire under a spruce tree and thinks he will be warm, safe and comfortable. Being inexperienced, he does not realize that there is a great danger of the tree dumping snow on top of him and putting out his precious fire. As Jack London describes it, the tree is heavily laden with snow from top to bottom. The slight vibrations caused by the man breaking off branches down below, causes the top branches to shake loose their coverings of snow. This snow falls on the branches directly below, and soon there is a miniature avalanche which covers the man and smothers his fire. It is a completely natural occurrence, but it seems as if nature is committing an aggressive and sadistic action. The man feels it that way.
Another example of nature as a cruel antagonist is the fact that nature seems to have deliberately laid traps where a man may break through a thin layer of ice covered with snow and get soaked as far as his waist. This can cause him to freeze to death unless he can quickly build a fire and dry off his clothes and footwear. Here again it seems as if nature is not merely indifferent but conscious, antagonistic and sadistic.