Why does the end of the story focus on collie's puppies and whit fang dosing in the sun

Although very naturalistic in his approach to this novel, London received a great deal of criticism for the abrupt ending. When White Fang finally recovers from his injuries, he ventures out into the warm California sun and greats Collie and his new puppies. Instead of ending the novel in the same naturalistic vein he began, London ends White Fang with a distinctively romantic flare.