Darwin's Earthworms

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It is appropriate that the naturalist who studied the infinitely slow progress of change as a species evolved over hundreds of millions of years, that this same scientist should focus on the minutely small changes of the surface of the land as effected by the workings of one of the lowliest creatures on the Earth, the earthworm.  Darwin spent considerable time on this study.

Initially his earthworm work drew as much or even more attention than his evolution work.  During Darwin's lifetime his book, The Formation of Vegetable Mould Through the Action of Worms, With Observations on Their Habits, published in 1881, sold even better than On the Origin of Species.  The section concerning the intelligence of earthworms was chiefly responsible for this success.

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Until he started looking at earthworms, no one appreciated the role they had in agriculture.  Most people thought of them as pests.  Darwin showed that they were valuable for turning over the soil, which they did in part by chewing it up and pooping it out, thereby making it more fertile.

He realized that England’s lush topsoil was the product of ceaseless soil consumption and defecation by earthworms; about 54,000 of them per acre, depositing ten tons of fresh soil on each acre of English countryside, every single year.

He wrote:

     'It may be doubted whether there are any 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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To find out how fast the worms were turning the soil, Darwin did experiments.  He spread small coal stones across a field behind his house and left them for 20 or 30 years.  Then he dug a trench across the land and looked in the walls of the trench to see how far down the stones had sunk through the action of the worms.

He concluded that the cumulative effect of millions of worms in a field chewing their way through the soil and depositing it on the surface is that the worms actually raise the surface of the soil.  Darwin worked out that the soil increased in depth by 0.2 of an inch per year.  After 10 years an object in the soil will go down two inches, and after 1,000 years it will sink almost 20 feet.

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He found that earthworms were sensitive to touch and vibrations but not to sounds; also that they had a 'selective sense of smell', and were sensitive to light, preferring darkness or very low light, except when they were mating.  He also concluded that they had favourite foods.

Caricature from 'Punch', 1882.  Man is but a worm.

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Concerning intelligence

Darwin observed that earthworms plug the mouth of their burrows with leaves, leaf stalks, or twigs and considered that an intelligent animal would draw such irregular-shaped objects into a cylindrical hole by their narrowest part.  Therefore he placed leaves and triangular pieces of paper of various sizes around the burrow entrance.  In the majority of trials, these objects were drawn into the burrows by or near their narrow apex. The only exception was pine needles that were drawn in by (or near) their base.  He concluded that worms possess 'some degree of intelligence instead of a mere blind instinctive impulse'.

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The New York Graphic wrote: 'The result of the author's observations is proof that the small and apparently insignificant earth-worm is the cause of mighty changes in the surface of the earth, seeing that each of them, on the average, passes about twenty ounces of earth through its body every year, which earth it brings often from a depth of eight or ten feet below the surface to deposit it as mould at the top, thus doing the work of a plow.'