Charles Darwin and earthworms

Charles Darwin is best known for his theory of evolution as set out in his book On the Origin of Species. He was a naturalist – an expert in geology, botany and biology – whose interest in all things natural was apparent from a young age. His father wanted Charles to follow in his footsteps and become a doctor. However, Darwin found he couldn’t bear dissecting cadavers or watching surgery, so he quit.

His father remarked, “You care for nothing but shooting, dogs, and rat-catching and you will be a disgrace to yourself and all your family.”

Darwin, of course, went on to prove his father wrong and became one of history’s most famous scientists.

Darwin’s association and interest with earthworms came shortly after his famous voyage on the HMS Beagle. His uncle showed him a spot in his garden where he had spread ashes and lime several years before. Darwin was amazed to see how soil cast up by earthworms had buried the substances. He went home and began a series of earthworm experiments that would go for the next 40 years. Darwin conducted both lab experiments in his study and billiard room and field investigations in his extensive gardens. He published his findings in 1881 in a book titled The Formation of Vegetable Mould through the Action of Worms, with Observations of their Habits. The book sold 6,000 copies in its first year, selling faster than On the Origin of Species had when it was first published.

Charles Darwin was fascinated by earthworm behaviour. He experimented with different types of food, placing the food on pieces of tinfoil to make sure the earthworms did not accidentally come upon the food by burrowing from below. He recorded that they preferred wild cherry and carrots, that raw fat was preferred to raw meat and that, “judging by their eagerness for certain kinds of food, they must enjoy the pleasures of eating”.

He tested their senses by exposing worms to lamps or candlelight and their sensitivity by holding “a poker heated to dull redness near some worms”. Earthworms do not have ears, but Darwin still tested their sense of hearing. He used a metal whistle and had his son play his bassoon loudly. Darwin even shouted at the worms but found that, if care was taken that his breath did not strike them, they were indifferent to noise. The earthworms also remained quiet when set on a table close to a piano, which was played as loudly as possible. That all changed, however, when the earthworm pots were placed on top of the piano. Darwin noted that earthworms are extremely sensitive to vibrations.

Darwin was curious to know if such lowly creatures were intelligent. He spent considerable time observing how earthworms pulled leaves into their burrows. They plugged the burrow openings, in Darwin’s opinion, to keep out chilled air. Darwin found they most often pulled leaves in by their tips, which is the most efficient method. When he substituted paper triangles for leaves, he noted the majority of earthworms drew them down their burrows by the apex. This led Darwin to state that worms have some degree of intelligence. He wasn’t convinced that all earthworms were equal, though. He placed leaves on the surface of pots kept in a warm room. These worms worked in “a careless or slovenly manner … they did not care about plugging up their holes effectually”. Darwin covered the pots with nets and left them outdoors for several nights. He wrote, “and now, 72 leaves were all properly drawn in by their bases”.

Darwin’s book also chronicled early New Zealand earthworm research. He mentions that worms appear to act in the same manner in New Zealand as in Europe, referring to earthworms’ ability to slowly cover objects left on the ground with their casts.