. . .I would most gladly have answered your delightful letter at once, and have told you how smoothly all has gone at Neuchatel. Your letters to M. de Coulon and to General von Pfuel have wrought marvels; but they are now inclined to look upon me there as a wonder from the deep,* (* Ein blaues Meerwunder.) and I must exert myself to the utmost lest my actual presence should give the lie to fame. It is all right. I shall be the less likely to relax in devotion to my work. The real reason of my silence has been that I was unwilling to acknowledge so many evidences of efficient sympathy and friendly encouragement by an empty letter. I wished especially to share with you the final result of my investigations on the fossil fishes, and for that purpose it was necessary to revise my manuscripts and take an account of my tables in order to condense the whole in a few phrases. I have already told you that the investigation of the living fishes had suggested to me a new classification, in which families as at present circumscribed respectively received new, and to my thinking more natural positions, based upon other considerations than those hitherto brought forward. I did not at first lay any special stress on my classification. . .My object was only to utilize certain structural characters which frequently recur among fossil forms, and which might therefore enable me to determine remains hitherto considered of little value. . .Absorbed in the special investigation, I paid no heed to the edifice which was meanwhile unconsciously building itself up. Having however completed the comparison of the fossil species in Paris, I wanted, for the sake of an easy revision of the same, to make a list according to their succession in geological formations, with a view of determining the characteristics more exactly and bringing them by their enumeration into bolder relief. What was my joy and surprise to find that the simplest enumeration of the fossil fishes according to their geological succession was also a complete statement of the natural relations of the families among themselves; that one might therefore read the genetic development of the whole class in the history of creation, the representation of the genera and species in the several families being therein determined; in one word, that the genetic succession of the fishes corresponds perfectly with their zoological classification, and with just that classification proposed by me. The question therefore in characterizing formations is no longer that of the numerical preponderance of certain genera and species, but of distinct structural relations, carried through all these formations according to a definite direction, following each other in an appointed order, and recognizable in the organisms as they are brought forth. . .If my conclusions are not overturned or modified through some later discovery, they will form a new basis for the study of fossils. Should you communicate my discovery to others I