Here's why:
* It lacks a complete verb: We know Jack London "writes", but the sentence doesn't tell us what he writes. It's missing the action of what he's doing with his writing.
* It lacks a clear subject for the verb "remembered": The phrase "he remembered back to the youth of breed" is grammatically incorrect. "He" is the subject, but "remembered" needs an object (what he remembered).
To make this a complete sentence, you need to add more information:
* Example 1: "Jack London writes in vague ways about his memories of the youthful exuberance of the breed."
* Example 2: "Jack London writes in vague ways, remembering back to the youth of the breed, when they were full of life and energy."
These examples clarify what Jack London is writing about and the context of his "vague" writing.