other causes, such as dust deposition and washing down of soil, have buried such ruins. Denudation (removal of matter to a lower level) is caused mainly by air and water movement. Humic acids generated by worms disintegrate rock; their burrowing behaviour speeds this up. But as the soil layer thickens, this process is slowed down. Worms swallow hard objects (e.g. stones) to aid digestion, which causes attrition to such objects. This has geological significance, especially for the smaller particles which otherwise are eroded very slowly. Rain causes castings to move down an incline; Darwin worked out the weight moving a certain distance in a given time. Some also roll down, and collect in drains etc., or get blown. There is a greater effect on casting movement in the tropics, because of increased rain. The finest earth is washed away. Ledges on hillsides, formerly believed to be caused by grazing mammals, are partly due to casting accumulations. High winds, especially gales, are almost as effective as the slope/rain in moving castings. Crowns and furrows of formerly ploughed lands slowly vanish when under pasture, due to worms, but more slowly when there is no incline. Fine earth is washed down from slopes, making a shallow layer. Dissolving of chalk supplies new earth. 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." 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. Despite their rudimentary sense organs, they show complex, flexible behaviour. Reception 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". Darwin received a "laughable" number of letters containing questions, observations and ideas, even "idiotic" ones. A week's holiday with Emma in Cambridge was to follow. Darwin died the next year on 19 April 1882. See also Uniformitarianism Ichnology Burrow Trace fossil References Citations Sources Further reading "Down among the Worms", chapter 42 in External links The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits LibriVox The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits at One More Library 1881 non-fiction books Ecology