have come down from an ancient period very slightly modified ought, I think, to be omitted, and those forms alone considered which have undergone considerable change at each successive epoch. My fear is whether brachiopods have changed enough. The absolute amount of difference of the forms in such groups at the opposite extremes of time ought to be considered, and how far the early forms are intermediate in character between those which appeared much later in time. The antiquity of a group is not really diminished, as some seem vaguely to think, because it has transmitted to the present day closely allied forms. Another point is how far the succession of each genus is unbroken, from the first time it appeared to its extinction, with due allowance made for formations poor in fossils. I cannot but think that an important essay (far more important than a hundred literary reviews) might be written by one like yourself, and without very great labour. I know it is highly probable that you may not have leisure, or not care for, or dislike the subject, but I trust to your kindness to forgive me for making this suggestion. If by any extraordinary good fortune you were inclined to take up this notion, I would ask you to read my Chapter X. on Geological Succession. And I should like in this case to be permitted to send you a copy of the new edition, just published, in which I have added and corrected somewhat in Chapters IX. and X. Pray excuse this long letter, and believe me, My dear Sir, yours very faithfully, C. DARWIN. P.S.--I write so bad a hand that I have had this note copied. CHARLES DARWIN TO THOMAS DAVIDSON. Down, April 30, 1861. My dear Sir,