An earthworm is a terrestrial segmented member of Annelida whose burrowing and feeding can reorganize soil. Its body moves through coordinated muscular action against the surrounding material; ingested soil and organic matter are deposited as casts. Small repeated actions can consequently change a landscape over long intervals.
Charles Darwin investigated the amount of fine earth brought to the surface and the progressive burial of objects in pasture. In one long-observed field, a stone-free surface layer accumulated above stony material, while casts provided a visible transport mechanism. [ref]
The formation of vegetable mould, through the action of worms, with observations on their habits by Charles Darwin. Chapter III, pp. 121–163: surface objects, the stony field and measured layers of mould
The inference combines observed activity with measurements of cumulative change, rather than treating an earthworm merely as a conspicuous resident of pre-existing soil. New to topics? Read the docs here!