"In vague ways he remembered back to the youth of the breed." This statement is an example of D. THE RACIAL UNCONSCIOUSNESS.
Buck was a pampered domesticated dog before he was taken and sold to strangers. In his current life, he became the dog that he actually is. Adapting to his environment and striving to survive.
Racial unconsciousness refer to the actions or words of an individual that manifests his or her stereotypical views.
Next, London develops the theme of instinct. “The domesticated generations fell from him” means that Buck is unlearning all of the behavior that centuries of breeding have instilled in dogs. “In vague ways he remembered back to the youth of the breed” means that he has a memory deep inside of what it meant to be a beast thousands and thousands of years ago, when dogs were a new, young breed, forced to fight and kill to survive
