What is the underlying theme of this passage?
then an old wolf, gaunt and battle-scarred, came forward. buck writhed his lips into the preliminary of a snarl, but sniffed noses with him, whereupon the old wolf sat down, pointed nose at the moon, and broke out the long wolf howl. the others sat down and howled. and now the call came to buck in unmistakable accents. he, too, sat down and howled. this over, he came out of his angle and the pack crowded around him, sniffing in half-friendly, half-savage manner. the leaders lifted the yelp of the pack and sprang away into the woods. the wolves swung in behind, yelping in chorus. and buck ran with them, side by side with the wild brother, yelping as he ran.
(from the call of the wild by jack london)