Final answer:
The harness represents Buck's obedience to serve Thornton.
Step-by-step explanation:
In the excerpt from The Call of the Wild, the harness represents Buck's obedience to serve Thornton. When Buck is harnessed, he enters a state of submission and willingly takes on the role of a sled dog under Thornton's command. The harness is a physical symbol of Buck's loyalty and devotion to his master.