Charles Darwin's natural selection theory holds that "<em>only the fittest species will survive the competition for scarce resources"</em>. <em>At a time when Darwinism dominated evolutionary theory, Jack London lets it reflect as a major theme in his "The call of the Wild". </em>
The link between both pieces of writing is seen through the journey of transformation followed by the main character Buck, <em>a St. Bernard dog</em>, from being a domesticated pet, past his getting trafficked as a sled dog, and into becoming the fiercest pack leader.
Mid chapter III in the book, Buck finally opposes the original pack leader,<em> Spitz,</em> to death as London narrates <em>"It was inevitable that the clash for leadership should come. Buck wanted it. He wanted it because it was his nature"</em>. Spitz finally dead, Buck stands <em>"the successful champion, the dominant primordial beast who had made his kill and found it good."</em>
Current evolutionary theory counters Darwin's stating that <em>what determines survival is not superiority but rather the ability to adapt to surrounding conditions or survive dramatic change in the environment</em>.
In any case, I believe Buck is a great example of the drive for adaptation in a living creature, which doesn't necessarily imply progress but retrogression under unfortunate enough circumstances.