Who chased Buck to exhaustion?

Dolly chased Buck to exhaustion in Chapter Three.

At the Pelly one morning, as they were harnessing up, dolly, who had never been conspicuous for anything, went suddenly mad. She announced her condition by a long, heart-breaking wolf howl that sent every dog bristling with fear, then sprang straight for Buck. He had never seen a dog go mad, nor did he have any reason to fear madness; yet he knew that here was horror, and fled away from it in a panic. Straight away he raced, with Dolly, panting and frothing, one leap behind; nor could she gain on him, so great was his terror, nor could he leave her, so great was her madness. He plunged through the wooded breast of the island, flew down to the lower end, crossed a back channel filled with rough ice to another island, gained a third island, curved back to the main river, and in desperation started to cross it. And all the time, though he did not look, he could hear her snarling just one leap behind. Francois called to him a quarter of a mile away and he doubled back, still one leap ahead, gasping painfully for air and putting all his faith in that Francois would save him. the dog-driver held the axe poised in his hand, and as Buck shot past him the axe crashed down upon mad Dolly's head.