It takes guts to make things happen in life. A little puppy pushing out of his cave into the great big world needs a big sack of courage to do it right. On another level, you need some brass to step into a lethal dog fight, insult the patrons, and pry the jaws of one of the animals loose. Without courage, White Fang would still be in that cave (and likely dead), and Scott wouldn't have been able to save the wolf from getting chomped to death by a bulldog. London puts a lot of stock in courage, and it reflects the best that his White Fang characters can do. (Cue Rocky theme here.)
Bravery comes from the need to survive and works only as far as it helps a given character stay alive.
Courage is the sign of a noble soul, and it only loosely connected with the need to survive in White Fang.