【英语时差8,16】蚯蚓（上）（在线收听）

 Charles Darwin is best known for his studies of evolution by means of natural and sexual selection, but did you know he was also a pioneer in the study of earthworms? After watching an earthworm s-l-o-w-l-y drag a leaf into its burrow, Darwin became fascinated by these unassuming creatures. He began an exhaustive study of worm behavior, sensory abilities and digestion. His book on worms was the first to describe the enormous impact of earthworms on our lives. Darwin noticed that a thick layer of charcoal and cinders that he’d spread across a large field two years earlier was now uniformly covered by several inches of earth. He surmised that what appeared to be the “sinking” of the charcoal layer was actually the result of earthworm “castings.” Earthworms swallow soil, extract nutrients from it, then excrete the rest up onto the surface. These excretions—small dark mounds of fine dirt you may see dotting the ground—are called castings.