What was the subject of Charles Darwin's last book, published in 1881?

Charles Darwin’s last scientific book, published in 1881, studied earthworms.

Its full title was The Formation of Vegetable Mould Through the Action of Worms, with Observations on Their Habits. Darwin investigated how earthworms consumed organic matter, burrowed through soil, and deposited castings that gradually transformed the landscape. He argued that these small animals had a much greater geological and ecological influence than people generally recognized.

Darwin had observed worms for decades, including their behavior around leaves, stones, and objects placed near their burrows. The book connected their everyday activity with soil formation, plant growth, erosion, and even the gradual burial of ancient structures.

The book appeared in October 1881, only months before Darwin’s death in April 1882. It is often called simply Worms. Insectivorous Plants and his orchid studies were important earlier works, but neither was his final scientific book. Darwin’s broader theme was that immense changes can result from countless small actions repeated over long periods.