The first quotation, "Nevaire such a dog as dat Buck!" he cried, characterizes Buck directly. It expresses an immediate opinion about Buck's uniqueness and qualities as perceived by someone.
The second quotation, "But Buck possessed a quality that made for greatness - imagination," characterizes Buck indirectly. This statement suggests Buck's qualities and attributes without explicitly describing him in a straightforward manner; instead, it highlights his imagination as a significant trait that contributes to his greatness.