In concluding, the author enforces the claims of worms on the gratitude of archæologists, as they protect and preserve for an indefinitely long period every object not liable to decay which is dropped on the surface of the land, by burying it beneath their castings. It is thus that many tesselated pavements and other ancient remains have been preserved; but, on the other hand, worms have undermined many old massive walls and caused them to subside, and no building is in this respect safe unless the foundations are at least six or seven feet beneath the surface, below which depth worms cannot work. Worms also prepare the ground in an excellent manner for plant life, periodically exposing the mould to the air, sifting it so that no stones larger than the particles they can swallow are left in it, mingling the whole intimately together, burying all decaying objects within reach of the roots of the plants, allowing air to penetrate deeply into the earth. "When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. 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 organised creatures." After this last book Darwin felt much exhausted, and wrote: "I feel so worn out that I do not suppose I shall ever again give reviewers trouble." His brother Erasmus's death in the same year was the severance of a link with early days. Yet for some months he continued in a moderate degree of health, still working. For some weeks however in the following March and April he was slightly unwell, and the action of his heart became so weak that he was not allowed to mount the stairs. On Tuesday, April 18, he was in his study examining a plant which he had had brought to him, and he read the same evening before retiring. Till the day of his death he did not become seriously ill. On that day the heart, which had so long done its duty, failed, and about 4 p.m., on April 19, 1882, Charles Darwin breathed his last in peace, aged seventy-three years, two months, and seven days. CHAPTER XI.