Darwin's last book, published only 6 months before his death, but reporting on a subject that he had studied for more than 50 years. "He showed the services performed by earthworms in eating leaves and grinding earth in their gizzards and turning it into fertile soil, which they constantly sift and turn over down to a depth of twenty inches form the surface, thereby aerating it. He calculating from the weight of worm-casting that on one acre in one year's time eighteen tons of soil are brought up to the surface by worms. This was a pioneer study of quantaitative ecology" (Gavin de Beer in D.S.B.)