Why does the author of the call of the wild describe bucks progress as retrogression?

The author describes Buck's progress as retrogression because, throughout the novel The Call of the Wild, Buck goes through a transformation from a domesticated dog to a wild, primitive form. This transformation involves shedding the trappings of civilization and reverting back to his ancestral instincts and behavior. This regressive process can be seen as a return to a more primal, instinct-driven state of existence, thus the term "retrogression" is used to describe it. Buck's progress is seen as a regression because he is moving away from the civilized world and adapting to survive in the harsh natural environment, which involves embracing his wild side and shedding the comforts and behaviors he learned while living with humans.