A Moment of Science

Darwin’s Worms

Earthworms swallow soil, extract nutrients from it, then excrete the rest up onto the surface.

Earthworm crawling through grass

Photo: Glen Bowman (flickr)

Earthworms like this one in Hexham are vital to the Earth's surface.

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.

Some scientists disputed Darwin’s claim, arguing that the small, weak creatures weren’t capable of moving such massive amounts of material to the surface.

Darwin responded to the criticism by conducting a more careful experiment. This time rather than measuring the rate at which objects were buried, he weighed all the worm castings thrown up to the surface each day. He found that in a single acre of farmland over eleven tons of earth passes to the surface through the bodies of worms every year!

In the words of Charles Darwin: “It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.”

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