Which sentence from part 5 of The Call of the Wild would best serve as evidence of the theme "one’s natural instincts can be extremely powerful"?

Question

And here, lying by the river bank through the long spring days, watching the running water, listening lazily to the songs of birds and the hum of nature, Buck slowly won back his strength.
He would lie by the hour, eager, alert, at Thornton's feet, looking up into his face, dwelling upon it, studying it, following with keenest interest each fleeting expression, every movement or change of feature.
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.
He linked the past with the present, and the eternity behind him throbbed through him in a mighty rhythm to which he swayed as the tides and seasons swayed.

Answer 1

The sentence that best serves as evidence of the theme "one’s natural instincts can be extremely powerful" is:

"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."

This sentence highlights how, despite Buck's bond with John Thornton and the civilizing influence he represents, Buck's primal instincts and the powerful force of his natural heritage are still very much alive within him, illustrating the theme effectively.