But there were other forces at work in the cub, the greatest of which was growth. Instinct and law demanded of him obedience. But growth demanded dis… - Jack London

He must master or be mastered; while to show mecy was a weakness. Mercy did not exist in the primordial life. It was misunderstood for fear, and such misunderstandings made for death. Kill or be killed, eat or be eaten, was the law; and this mandate, down out of the depths of Time, he obeyed.

Buck found it to be cheaper to mend his ways than to retaliate.

But at the best, it is a dull, animal happiness, the content of the full belly. The dominant note of their lives is materialistic. They are stupid and heavy, without imagination. The Abyss seems to exude a stupefying atmosphere of torpor, which wraps about them and deadens them. Religion passes them by. The Unseen holds for them neither terror nor delight. They are unaware of the Unseen; and the full belly and the evening pipe, with their regular “arf an’ arf,” is all they demand, or dream of demanding, from existence.