I am about finishing my long and wearisome work of (illegible); a critical examinationinto the geography of the Middle Ages, of which fifty sheets are already printed. I will send you the volumes as soon as they appear, in octavo. I devoured your fourth number; the plates are almost finer than the previous ones; and the text, though I have only looked it through hastily, interested me deeply, especially the analytical catalogue of Bolca, and the more general and very philosophical views of fishes in general, pages 57-64. The latter is also remarkable in point of style. . . M. von Buch, who has just left me, sends you a warm greeting. None the less does he consider the method of issuing your text in fragments from different volumes, altogether diabolical. I also complain a little, though in all humility; but I suppose it to be connected with the difficulty of concluding any one family, when new materials are daily accumulating on your hands. Continue then as before. In my judgment, M. Agassiz never does wrong. . . The above letter, though written in May, did not reach Agassiz until the end of July, when he was again on his way to England, where his answer is dated. AGASSIZ TO HUMBOLDT. (LONDON), October--, 1835. . . .I cannot express to you my pleasure in reading your letter of May 10th (which was, unhappily, only delivered to me on my passage through Carlsruhe, at the end of July). . .To know that I have occupied your thoughts a moment, especially in days of trial and sorrow such as you have had to bear, raises me in my own eyes, and redoubles my hope for the future. And just now such encouragement is particularly cheering under the difficulties which I meet in completing my task in England. I have now been here nearly two months, and I hope before leaving to finish the description of all that I brought together at the Geological Society last year. Knowing that you are in Paris, however, I cannot resist the temptation of going to see you; indeed, should your stay be prolonged for some weeks, it would be my most direct path for home. I should like to tell you a little of what I have done, and how the world has gone with me since we last met. . .I have certainly committed an imprudence in throwing myself into an enterprise so vast in proportion to my means as my "Fossil Fishes." But, having begun it, I have no alternative; my only safety is in success. I have a firm conviction that I shall bring my work to a happy issue, though often in the evening I hardly know how the mill is to be turned to-morrow. . .