Explain how Duane's justification for nearly killing the insurance agent mirrors the law of the jungle.

From the text:

"Still," said Jurgis, reflectively, "he never did us any harm."

"He was doing it to somebody as hard as he could, you can be sure of that," said his friend.

The law of the jungle would be to take care of yourself first. Duane says it didn't matter he had caused them no harm, because he was sure to have caused it to someone else. Thus everything's fair.