What lesson did Ulrich and Georg learn when the tree fell on them?
Ulrich's and Georg's families have been fighting for generations. When they meet in the forest, each man has notions of killing the other. Saki builds the suspense when he has both of them hesitate as they stare each other down:
Each had a rifle in his hand, each had hate in his heart and murder uppermost in his mind. The chance had come to give full play to the passions of a lifetime. But a man who has been brought up under the code of a restraining civilization can not easily nerve himself to shoot down his neighbor in cold blood without a word spoken, except for an offense against his hearth and honor.
This hesitant moment builds the suspense and Saki's description of this "code" of restraint shows that each man does have some humanity in him. This one moment suggests just a glimmer of hope that these men are not so far beyond civilized that all they can think about is killing each other.
The hesitation also leads to them becoming trapped under the tree. Again, there they are within striking distance of one another. Whereas the "code" momentarily restrained them from killing each other, the tree is physically restraining them. Initially, they remain committed to killing each other, but it takes just a bit of time for them to become humble as they face their own mortality together. Finding themselves in a dire -- and potentially deadly -- situation, Ulrich and Georg both seem to decide life is too precious and too short to spend it fighting over a meaningless piece of land.