Answer:
The food chain.
Step-by-step explanation:
Henry David Thoreau's "Walden" is an account of the time the author spent in the wilderness. This book presents the fundamental elements of human existence and talks about the seasons, nature, the beauty of it all, and just simple living away from the bustle of city life.
In the given excerpt from the text, Thoreau states that "the repose is never complete." This means that the cycle of eating and becoming prey oneself is a never-ending cycle, where one species prey on the others and so on. This passage, thus, is a reference to the natural phenomenon of the food chain that keeps nature alive and balanced.