We are fortunate in being permitted, by Sir Joseph Hooker and by Mr. Wallace, to publish certain letters from them to Mr. Darwin. We have also been able to give a few letters from Sir Charles Lyell, Hugh Falconer, Edward Forbes, Dr. Asa Gray, Professor Hyatt, Fritz Muller, Mr. Francis Galton, and Sir T. Lauder Brunton. To the two last named, also to Mrs. Lyell (the biographer of Sir Charles), Mrs. Asa Gray and Mrs. Hyatt, we desire to express our grateful acknowledgments. The present volumes have been prepared, so as to give as full an idea as possible of the course of Mr. Darwin's work. The volumes therefore necessarily contain many letters of a highly technical character, but none, we hope, which are not essentially interesting. With a view to saving space, we have confined ourselves to elucidating the letters by full annotations, and have for the same reason--though with some regret--omitted in most cases the beginnings and endings of the letters. For the main facts of Mr. Darwin's life, we refer our readers to the abstract of his private Diary, given in the present volume. Mr. Darwin generally wrote his letters when he was tired or hurried, and this often led to the omission of words. We have usually inserted the articles, and this without any indication of their absence in the originals. Where there seemed any possibility of producing an alteration of meaning (and in many cases where there is no such possibility) we have placed the introduced words in square brackets. We may say once for all that throughout the book square brackets indicate words not found in the originals. (Preface/2. Except in a few places where brackets are used to indicate passages previously published. In all such cases the meaning of the symbol is explained.) Dots indicate omissions, but many omissions are made without being so indicated.