Which line is an example of apostrophe?
o "a cavern dark enough"
"then by day"
"seek none, conspiracy."
"hide it"

It is true that one possible theme of "The Call of the Wild" could be that only the strongest survive. For the character of Buck showed immense evolving and endurance to survive through all the experiences from changing masters to being able to be in the wild.

Explanation:

Charles Darwin's "The Theory of Natural Selection" and Herbert Spencer's "Survival of the Fittest" plays a huge role in the story of Buck in Jack London's "The Call of the Wild".

The story of Buck is set in the Klondike surrounding. London characterizes the dog Buck among the wild animals as a depiction of Darwin's theory of natural selection. But this alone is not the only reference. It also touches on the theme of "survival of the fittest" by Spencer. Buck had to undergo various masters, atmosphere and had to fend off other 'possible enemies' for his own survival. The death of his friend Curly shows him the cruel nature of the wilderness.  

Darwin's theory of evolution shows the fierce competition for the resources leading to the survival of the fittest among the humans and even animals. This survival of the fittest' perfectly fitted Buck's character for he also had to endure and fight to ensure his survival. Overcoming these challenges, becoming wilder than he used to be, and evolving according to the harsh and brutal reality of the wilderness made it possible for Buck to be the master over the other wolves.

London's Buck is a perfect parallel to Darwin's and Spencer's theories and gave a fitting scenario of the need to evolve oneself according to the life situations.