This work, as originally planned, was never completed. It was cut short by ill-health and by the pressure of engagements arising from the rapid development of the great Museum, which finally became, as will be seen, the absorbing interest of his life. As it stands, the "Contributions to the Natural History of the United States" consists of four large quarto volumes. The first two are divided into three parts, namely: 1st. An Essay on Classification. 2nd. The North American Testudinata. 3rd. The Embryology of the Turtle,--the latter two being illustrated by thirty-four plates. The third and fourth volumes are devoted to the Radiata, and consist of five parts, namely: 1st. Acalephs in general. 2nd. Ctenophorae. 3rd. Discophorae. 4th. Hydroida. 5th. Homologies of the Radiates, --illustrated by forty-six plates.* (* The plates are of rare accuracy and beauty, and were chiefly drawn by A. Sonrel, though many of the microscopic drawings were made by Professor H.J. Clark, who was at that time Agassiz's private assistant. For details respecting Professor Clark's share in this work, and also concerning the aid of various kinds furnished to the author during its preparation, the reader is referred to the Preface of the volumes themselves.) For originality of material, clearness of presentation, and beauty of illustration, these volumes have had their full recognition as models of scientific work. Their philosophy was, perhaps, too much out of harmony with the current theories of the day to be acceptable. In the "Essay on Classification" especially, Agassiz brought out with renewed earnestness his conviction that the animal world rests upon certain abstract conceptions, persistent and indestructible. He insists that while physical influences maintain, and within certain limits modify, organisms, they have never affected typical structure,--those characters, namely, upon which the great groups of the animal kingdom are united. From his point of view, therefore, what environment can do serves to emphasize what it cannot do. For the argument on which these conclusions are based we refer to the book itself. The discussion of this question occupies, however, only the first portion of the volume, two thirds of which are devoted to a general consideration of classification, and the ideas which it embodies, with a review of the modern systems of zoology. The following letter was one of many in the same tone received from his European correspondents concerning this work. FROM RICHARD OWEN. December 9, 1857.