Buck's feet were not so compact and hard as the feet of the huskies. His had softened during the many generations since the day his last wild ancestor was tamed by a cave-dweller or river man. The mention of the "cave-dweller" is most connected to the theme of .

the effect of the environment
laws
civilization vs. the wild
the value of work

Civilization vs. the wild

Explanation:

A cave-dweller is a prehistoric person who lived in caves. The dog was a man's best friend even during the prehistoric period.

The prehistoric man was still a part of nature, wilderness, while the modern man is assosicated with civilization. The definition of civilization is: a complex human society, usually made up of different cities, with certain characteristics of cultural and technological development. All of this can't apply to the said cave-dweller.

jenkins wanted the narrator to write a ghost story for his magazine. he wanted the narrator to give the readers a story based on the supernatural. he wanted him to give the readers real 'horror' stories and not the type of live ghosts which he had presented in his earlier stories.