What kind of life did Buck have with the Scotch half-breed?

Bucks days were filled with hard work and exhausting days under the Scotch half-breed.

A Scotch half-breed took charge of him and his mates, and in company with a dozen other dog-teams he started back over the weary trail to Dawson. It was no light running now, nor record time, but heavy toil each day, with a heavy load behind; for this was the mail train, carrying word from the world to the men who sought gold under the shadow of the Pole.