How does survival of the fittest apply to the book?

Candy is old and crippled. In a Darwinian environment like the ranch, Candy is viewed as weak and useless. Much like his dog, others see Candy as having little value. Candy desperately wants a place to belong. He knows his days are numbered on the ranch and after that he will be destitute.

You seen what they done to my dog tonight? They says he wasn't no good to himself nor nobody else. When they can me here I wisht somebody'd shoot me..."