Why do the visions come more frequently and the call sound more strongly to Buck when he's in the East with Thornton and the others after the fabled mine?

With little work to do, Buck spends his days by the fire, dreaming once more of that ancient man. In his mind he wanders in this early, undefined world.   Along with this vision of the man, Buck nightly hears an ancient call deep in the forest. Sometimes he searches for it. Sometimes he hears it, springs from his place and runs wild through the forest. One night he hears it more clearly than ever before. He goes into the forest and he sees a timber wolf. Wishing to make friends, Buck approaches; but the wolf is afraid of this beast three times his size, and he flees.