Read the excerpt below from The Call of the Wild and answer the question that follows. But in spite of this great love he bore John Thornton, which seemed to bespeak the soft civilizing influence, the strain of the primitive, which the Northland had aroused in him, remained alive and active. Faithfulness and devotion, things born of fire and roof, were his; yet he retained his wildness and wiliness. He was a thing of the wild, come in from the wild to sit by John Thornton's fire, rather than a dog of the soft Southland stamped with the marks of generations of civilization.
Which theme is most clearly being explored in the passage above?