[page] 399

Earthworms. -In his last printed work, Darwin estimates that there are in gardens 53,767 worms to the acre, and that they would weigh 356 pounds. Having four or five gizzards apiece, each worm is able to digest a large amount of coarse food, and to eat his "peck of dirt" in a brief space of time. Careful weighing of the "castings" brought to the surface by worms shows that they sometimes amount to sixteen tons per acre annually -sufficient to produce two inches of so-called "vegetable mould" in ten years. Since worms often burrow to a depth of seven or eight feet, it follows that they may play no mean part in undermining and burying loose stones and monuments of earth. Many of the foundations of Roman buildings recently discovered in Great Britain are preserved underneath this constantly accumulating deposit of "earth mould," which is from two to three feet deep over the ruins at Wroxeter.

At the conclusion of this book, Darwin says; "'The plough is one of the most ancient and most valuable of man's inventions ; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed, by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly-organized creatures."