by the base. Petioles are used to plug up burrows, and for food. Worms drag experimental triangles of paper by the apex most of the time, and do not rely on trial and error. Worms excavate burrows by consuming material or, preferably, pushing it away. They mainly consume soil for nutrients. They are found down to six or more feet, especially in extreme conditions. Burrows are lined, which serve several functions, and terminate in a chamber lined with stones or seeds. Worms are found all over the planet, some on isolated islands; how they got there is a mystery. Darwin draws on correspondence with people from around the world such as Fritz Müller in Brazil. The amount of earth brought to the surface by worms can be estimated by the rate at which objects on the surface are buried and by weighing the earth brought up in a given time. Information from farmers on marl, cinders etc. sinking into the ground allowed Darwin to make calculations. He conducted a 29-year experiment on chalk at a field near his house. Objects of all sorts "work themselves downards" as farmers say. Large stones sink because worms fill up any hollows with castings, then eject them beyond the perimeter and the ground around them starts to rise. He visited Stonehenge and found some outer stones partly buried, the turf sloping up to meet them (see figure 7). Darwin weighed castings and had friends do so in other countries. He also weighed castings per unit area per year, then worked out how thick a layer castings would make, compared with rates of sinking. Additionally, he worked out casting weight per worm per year. Worms have preserved many ancient objects under the ground. Darwin describes an ancient Roman villa in Abinger, Surrey. Worms have penetrated the concrete walls and even mortar. Similar subsidence occurred at Beaulieu Abbey, Hampshire, with worms penetrating gaps between the tiles. His sons Francis and Horace visited Chedworth Roman Villa in Gloucestershire, while William reported on Brading Roman Villa, Isle of Wight. Darwin goes into some detail on the well preserved ruins of Silchester Roman Town, Hampshire, with the help of the Rev. J. G. Joyce. Finally he discusses the case of the Viroconium Roman town ruins at Wroxeter, Shropshire, with the help of Dr. H. Johnson, who made observations including depth of vegetable mould. He concludes that both worms and