The "Descent of Man" shows the widest research, and is a storehouse of most interesting facts. "Sexual Selection" shows some of the most remarkable provisions of nature, and is as interesting as any novel. This book, like the "Origin," has been translated into various languages. In 1872 "The Expression of the Emotions in Man and Animals" was published. Over five thousand copies were sold on the day of publication. It was begun at the birth of his first child, thirty-three years before. He says, "I at once commenced to make notes on the first dawn of the various expressions which he exhibited, for I felt convinced, even at this early period, that the most complex and fine shades of expression must all have had a gradual and natural origin." He wrote to a college friend regarding this baby: "He is so charming that I cannot pretend to any modesty. I defy anybody to flatter us on our baby, for I defy any one to say anything in its praise of which we are not fully conscious.... I had not the smallest conception there was so much in a five-mouth baby. You will perceive by this that I have a fine degree of paternal fervor." In 1881, "The Formation of Vegetable Mould, through the Action of Worms, with Observations on their Habits," was published. "Fragments of burnt marl, cinders, etc., which had been thickly strewed over the surface of several meadows were found, after a few years, lying at a depth of some inches beneath the turf, but still forming a layer." Ascertaining that this was the work of worms, Darwin made a study of their structure, habits, and work, in his garden, his fields, and in pots of earth kept in his study. The intelligence of worms, the construction of their burrows, and the amount of labor they can perform, are described in a most entertaining manner. Over fifty thousand worms are found in a single acre of land, or about three hundred and fifty-six pounds. "In many parts of England a weight of more than ten tons of dry earth annually passes through their bodies, and is brought to the surface, on each acre of land.... Worms prepare the ground in an excellent manner for the growth of fibrous-rooted plants and for seedlings of all kinds. They periodically expose the mould to the air, and sift it so that no stones larger than the particles which they can swallow are left in it. They mingle the whole intimately together, like a gardener who prepares fine soil for his choicest plants.... 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." In three years eighty-five hundred copies of the "Earthworms" were sold.