Earthworm behaviour, burrow plugging and soil formation

Darwin (1859), in On the Origin of Species, noted that earthworms are hermaphrodites, yet all of them pair. He used this to support his view that an occasional cross is needed. In The Power of Movement in Plants (1880), Darwin remarked that roots often run down the old burrows of worms.¹ Here the worm matters to him as a maker of passages in the ground that plant roots then use.

In The Formation of Vegetable Mould (1881), Darwin reported that in woods, once the loose autumn leaves were removed, the whole surface was strewn with castings.² He also relayed a report from Dr. King of a French state forest. There a spongy layer of dead leaves and worm castings covered many acres, and a lecturer called it a natural cultivation of the soil that produced a rich humus.² From Brazil, Fritz Müller wrote that in most parts of the forests and pasture-lands of St. Catharina, the whole soil to a quarter of a metre looked as if it had passed repeatedly through worms.

Verworn (1899) observed that the earthworm has no eyes, and only the cells of its skin respond to light.⁴ Loeb (1900) began his analysis of nerve function with the earthworm. He described its locomotion as simple, with the setae acting like appendages that give a hold on the ground, and he asked whether coordinated movement depends on the brain.⁵ Loeb also cited W. W. Norman's finding on a worm cut across. Only the posterior piece writhed, while the anterior piece crawled off as if nothing had happened. Loeb took this to show that writhing is not proof of pain.⁶

Child's reference list for The Origin and Development of the Nervous System (1921) includes a paper titled as an examination of physiological polarity and electrical polarity in the earthworm.⁷ Trewavas (2025) noted that Darwin's earthworm experiments on intelligence, together with the Romanes definition, are routinely included in university lectures, particularly lectures on soil.⁸