From Alexander Agassiz   4 March 1872

Many thanks for the copy of the new edition of the “Origin of Species,” which I have just received from you.¹ There are several points, especially in Embryology, which I shall take some other occasion to write you about, which may be of general interest.² I am getting on toward the end of the Report on Echini from the deep sea of Florida, and hope to be able to send you a copy before long.³ The number of young I have been compelled to examine has led me to modify my views of the nature of genera, species, and in fact of all subdivisions. I cannot find anything that is stable, the greater the material in space and number (age) the more one is adrift to get a correct diagnosis of a genus or a species, and the gradual passage in Echini of the most widely separated groups leaves in my mind but little doubt that our classification is nothing but the most arbitrary convenient tool, depending upon the material at our command at a special time. The generalizations to which I am led from the careful study of such a small group as the Echini I shall publish at the end of my “Revision of the Echini” and as the Plates for the descriptive part are far advanced, I hope I shall not be long delayed.⁴ We have excellent news from the Hassler Expedition from Rio.⁵ Not much was expected from the dredging on this side of Cape Horn owing to the lateness of the season, but the single haul made off the Barbados must have been a wonderful catch of which I trust we shall hear and see more by and by.