The last work Darwin published is one of his least-known, but his study of mold and earthworms drew upon his broad interests. Far from being small and insignificant creatures, Darwin argued, earthworms turn over the soil in vast quantities, creating a suitable habitat for the growth of plants. Drawing upon some of his early geological work in the production of soils, this work represents a founding exemplar of quantitative ecology.
Like Darwin’s other books, it also contains interesting visual representations — for example, a tower of earthworm casts and diagrams showing the importance of mold in forming soil.
Read more about this book at Wikipedia.