This is the fourth volume of the complete edition of The Correspondence of Charles Darwin. For the first time full authoritative texts of Darwin's letters are available, edited according to modern textual editorial principles and practice. This volume covers the first years of Darwin's study of the structure and systematics of barnacles: work that involved a worldwide search for specimens, detailed microscopic investigations, a consideration of the theoretical assumptions underlying classification schemes, and the solution of practical problems of zoological nomenclature. Darwin's convictions about the nature and origin of species influenced his observations and conclusions and provided insights that led to some remarkable discoveries. Throughout this period Darwin also maintained his involvement in major geological debates, as shown by important exchanges with Charles Lyell, Robert Chambers, James Dwight Dana, Bernhard Studer, and others. The letters to Darwin include Joseph Dalton Hooker's descriptions of his dramatic and frequently dangerous travels through previously closed regions of Sikkim and Tibet.