What is the theme of the call of the wild? or
What is the important idea or message about the past does the author reveal?

here are probably several messages in London's book - and from a subjective view you should perhaps be selective.  Critically, the story plots the course of domestication (in comfortable and loving surroundings) to work (as a sled dog) to feral and ultimately a 'return to the wild'.  Throughout the story Buck is faced with a variety of personalities and his transition from pampered pet to 'working dog' is marked by brutality - and yet through that transformation Buck becomes 'more alive'; hardening both his physical and psychological state.  His ultimate supremacy over Spitz to become the lead dog marked a 'turning point' when Buck had completely 'converted' and had no vestiges of the 'pet' he was originally in the story.  His association and ultimate affection with Thornton - as a protector, also demonstrated a change, he did not regress to his earlier domesticity and continued received the 'call of the wild'.  

the conclusion would be the most affected category scoring