Charles Darwin’s final book, The Formation of Vegetable Mould, through the Action of Worms, published in 1881, speaks volumes about the role of earthworms in nature.
After revolutionizing biology with his works On the Origin of Species (1859) and The Descent of Man (1871), one might imagine that Darwin would evolve into a philosopher. Instead, he remained devoted to exploring the minutiae of the natural world by studying this humble topic: how soil is formed by worms. There is perhaps something in the humble yet profoundly significant toil of these creatures that Darwin uncovered. He wrote: “It is doubtful whether any other animal has played an important part in the history of the world as these lowly organisms have”: Over a year, worms can move 8 tons of soil per acre.
Worms can move 8 tons of soil per acre.
Although the topic may seem mundane, Darwin’s book on mould and worms is remarkably captivating. It is not a comprehensive synthesis of knowledge like the two Origins, but rather relies on careful observation and experimentation, largely conducted in Darwin’s own garden with the assistance of his son. He and his son would begin early in the morning at the family farm to observe the activities of earthworms in the cool, moist soil. These creatures consume leaves and other plant materials, digesting them into a rich, soft mixture. Their labor may seem trivial, but Darwin calculated that over the course of a year, these worms could move 8 tons of soil per acre, enough to bury an entire building. Clearly, Darwin was impressed by the slow, nearly invisible yet steady output that could yield extraordinary results over time.
The Formation of Vegetable Mould is filled with descriptions of experiments on worms and their habits, some of which are delightfully eccentric. For instance, Darwin placed earthworms in a ceramic jar on top of his piano and observed their reactions to different musical notes—(“C in the bass clef” and “G on the line in the treble clef” made them retreat into their burrows), or he studied how they reacted to human breath by smoking a cigar or sipping vinegar himself.
However, it was Darwin’s studies on the burrowing activities of worms in nature that showcased his boundless curiosity and creative experimental style. He wondered about the principles governing how earthworms seal their burrow holes with leaves. This is an activity they perform with such vigor that sometimes one can hear the rustling in the stillness of the night. Whether they do this to protect themselves from predators, to block rainwater or cold air, or to obtain food, Darwin could not fully explain the motivations behind such remarkable behavior.
What intrigued Darwin most about this behavior was the evidence suggesting the intelligence of earthworms. He wrote: “If a person had to plug a small cylindrical hole with objects like leaves, leaf stalks, or twigs, he would likely insert the smaller end first; but if these objects were small relative to the size of the hole, he might insert the larger end first. Similarly, do earthworms adjust their strategy according to the available objects, or do they simply act randomly?”
Darwin noted that 80 percent of the leaves he pulled from the deep burrows were inserted with the smaller end first—a distribution far from random. He then examined the differences in types and shapes of leaves. After observing at night in dim light with his son Francis (who later became a distinguished botanist), Darwin confirmed: “My son and I seemed to see the worms being conscious as they grasped a leaf in the most suitable manner to plug their burrow.”
Not content with these studies using natural leaves, Darwin conducted a more systematic and controllable investigation into the impact of shape on worm behavior using triangular pieces of paper cut to different ratios. The paper pieces were rubbed with raw fat on both sides to prevent moisture. Darwin concluded: “We can deduce that somehow, the worms are able to assess what is the best way to serve the purpose (plugging the hole) as they pull the triangular pieces of paper into their burrow.”
This suggested to Darwin that earthworms are not merely instinct-driven creatures. He wrote: “It is difficult to escape the conclusion that worms exhibit some level of intelligence in the way they burrow. It is astonishing that such a lowly animal as the worm can act in this way.”
In an era when anthropocentrism was still the norm, Darwin had a refreshingly open-minded view of the potential cognitive abilities of “lower” animals. This attitude is now reflected in the mindset of some biologists who believe that all living organisms possess some form of consciousness or perception.
