. . .I cannot permit a day to elapse without thanking you for the two volumes of your great work on American zoology, which, from your masterly and exhaustive style of treatment, becomes the most important contribution to the right progress of zoological science in all parts of the world where progress permits its cultivation. It is worthy of the author of the classical work on fossil fishes; and such works, like the Cyclopean structures of antiquity, are built to endure. I feel and I beg to express a fervent hope that you may be spared in health and vigor to see the completion of your great plan. I have placed in Mr. Trubner's hands a set of the numbers (6) of my "History of British Fossil Reptiles," which have already appeared; a seventh will soon be out, and as they will be sent to you in succession I hope you will permit me to make a small and inadequate return for your liberality in the gift of your work by adding your name to the list of my subscribers. . . Believe me always truly yours, RICHARD OWEN. Agassiz had promised himself that the first volume of his new work should be finished in time for his fiftieth birthday,--a milestone along the road, as it were, to mark his half century. Upon this self-appointed task he spent himself with the passion dominated by patience, which characterized him when his whole heart was bent toward an end. For weeks he wrote many hours of the day and a great part of the night, going out sometimes into the darkness and the open air to cool the fever of work, and then returning to his desk again. He felt himself that the excitement was too great, and in proportion to the strain was the relief when he set the seal of finis on his last page within the appointed time.