One of the themes of "The Call of the Wild" is:
Knowledge and Wisdom
In The Call of the Wild, the protagonist gains new knowledge which allows him to adapt to his surrounding. Adaptation in this novel is necessary for survival. When our protagonist is tossed into a new world of harsh conditions and unforgiving men, he needs to learn how to do his job, how to coexist with the other dogs, and how to live while many others die from the conditions. The adaptation we see is physical, mental, and spiritual as Buck is toughened and strengthened by the world around him.
I hope this helps. If you need any further help, feel free to ask.