What is the “last latent remnant of Buck’s ferocity”? What does Buck do to awaken this ferocity?

Buck's found his "last latent remnant" of ferocity is his altercation with the bear. Any dormant ferocity left within him was released when he killed the large, black bear.

He fished for salmon in a broad stream that emptied somewhere into the sea, and by this stream he killed a large black bear, blinded by the mosquitoes while likewise fishing, and raging through the forest helpless and terrible. Even so, it was a hard fight, and it aroused the last latent remnants of Buck's ferocity.

Any dormant ferocity left within him was released when he killed the large, black bear.He fished for salmon in a broad stream that emptied somewhere into the sea, and by this stream he killed a large black bear, blinded by the mosquitoes while likewise fishing, and raging through the forest helpless and terrible. Even so, it was a hard fight, and it aroused the last latent remnants of Buck's ferocity.