One night he sprang from sleep with a start, eager-eyed, nostrils quivering and scenting, his mane bristling in recurrent waves. From the forest came the call (or one note of it, for the call was many noted), distinct and definite as never before, - a long-drawn howl, like, yet unlike, any noise made by husky dog. And he knew it, in the old familiar way, as a sound heard before. —The Call of the Wild,
Jack London

Identify the theme that is supported by the text in this passage.

Good leaders are strong and intelligent.
Both the wild and civilization have rules.
Only the strong survive in the wilderness.
We are all ruled by natural instincts.