Exploring earthworm behaviour and ecology, it continued the theme common throughout his work that gradual changes over long periods of time can lead to large and sometimes surprising consequences.

After returning from the Beagle survey expedition in October 1836, Darwin was intensively occupied with further establishing his reputation as an innovative geologist, as well as finding suitable experts to describe his natural history collections and arranging for publication of their work as the multi-volume Zoology of the Voyage of H.M.S.

Darwin now published papers on "proofs of recent elevation on the coast of Chili", "deposits containing extinct Mammalia" and "coral formations".

[4] The paper on the role of earthworms in soil formation was read out by Darwin at the Geological Society of London on 1 November 1837.

[5] This was an uncommonly mundane subject for the society, and his peers may have hoped to hear of something more grandiose, even seeing this paper as highlighting Darwin's growing idiosyncrasies.

[7] From his brain-storming about transmutation towards the end of 1838, Darwin conceived of his theory of natural selection "by which to work", as his "prime hobby".

His main work on the Beagle collections continued, and in 1842 he published the first of three volumes on geology, The Structure and Distribution of Coral Reefs.

Subsequently, in September 1842, the family moved to rural Down House where he had space for experimental plant and animal breeding, and surroundings to observe nature.

[2] In December of that year he had a quantity of broken chalk spread over a part of a long established pasture field near the house, "for the sake of observing at some future period to what depth it would become buried.

His network of correspondents and fans responded to his interest and "earthworm anecdotes began surfacing in his mountain of mail".

He wanted to write on earthworms "before joining them", but also had two more plant books in mind, and a revision of Fertilisation of Orchids to work on.

[10] After turning away a request to support a controversial book on contraception (Fruits of Philosophy) – Darwin was opposed to it – he got back to work on flowers and worms.

As with much of his geological and evolutionary work, worms were a case of gradual, barely noticeable changes accumulating over time into large effects.

He experimented with stimuli at night: strong light would send them into their burrows ("like a rabbit" said Darwin's grandson Bernard), but heat and sound had no effect.

[14] Darwin was fascinated by their behaviour, from enjoying "the pleasure of eating" (based on their eagerness for certain foods) to their sexual passions, "strong enough to overcome... their dread of light", even to their social feelings ("crawling over each other's bodies").

His health gave him troubles; he complained to his long-time friend Hooker that he looked forward to "Down graveyard as the sweetest place on earth.

"[16] Concerned about the book he had been anticipating for decades, he hurried his publisher John Murray to hasten publication of Worms even at the risk of not making a profit.

[17] At a dinner with Edward Aveling (soon the son-in-law of Karl Marx) and other atheists, he was asked how he had turned to such an "insignificant" subject as worms, to which he replied "I have been studying their habits for forty years."

Worms live chiefly on half decayed leaves, partially digested by a pancreatic solution before ingestion.

Large stones sink because worms fill up any hollows with castings, then eject them beyond the perimeter and the ground around them starts to rise.

Finally he discusses the case of the Viroconium Roman town ruins at Wroxeter, Shropshire, with the help of Dr. H. Johnson, who made observations including depth of vegetable mould.

Darwin writes in the conclusion that worms "have played a more important part in the history of the world than most persons would at first suppose.

"[19] They are important for many reasons, including their role in decomposition of rocks, gradual denudation of the land, preservation of archaeological remains, and improving soil conditions for plant growth.

Worms became available in October 1881 and sold thousands of copies in its first few weeks, despite Darwin's comment to Carus that it was "a small book of little moment".