Buck threw himself forward, tightening the traces with a jarring lunge. His whole body was gathered compactly together in the tremendous effort, the muscles writhing and knotting like live things under the silky fur. His great chest was low to the ground, his head forward and down, while his feet were flying like mad, the claws scarring the hard-packed snow in parallel grooves. The sled swayed and trembled, half-started forward. One of his feet slipped, and one man groaned aloud. Then the sled lurched ahead in what appeared a rapid succession of je rks, though it never really came to a dead stop again . . . half an inch . . . an inch . . . two inches . . . The jer ks perceptibly diminished; as the sled gained momentum, he caught them up, till it was moving steadily along. —The Call of the Wild,
Jack London

How does this event affect the development of the plot?

It creates a bond between Buck and Thornton.
It allows Thornton to set out in search of the lost mine.
It shows that Thornton knows what he is talking about.
It makes Buck a stronger sled leader.