All this work could hardly be carried on single handed. In 1837 M. Edouard Desor joined Agassiz in Neuchatel, and became for many years his intimate associate in scientific labors. A year or two later M. Charles Vogt also united himself to the band of investigators and artists who had clustered about Agassiz as their central force. M. Ernest Favre says of this period of his life: "He displayed during these years an incredible energy, of which the history of science offers, perhaps, no other example." Among his most important zoological researches at this time were those upon mollusks. His method of studying this class was too original and too characteristic to be passed by without notice. The science of conchology had heretofore been based almost wholly upon the study of the empty shells. To Agassiz this seemed superficial. Longing to know more of the relation between the animal and its outer covering, he bethought himself that the inner moulding of the shell would give at least the form of its old inhabitant. For the practical work he engaged an admirable moulder, M. Stahl, who continued to be one of his staff at the lithographic establishment until he became permanently employed at the Jardin des Plantes. With his help and that of M. Henri Ladame, professor of physics and chemistry at Neuchatel, who prepared the delicate metal alloys in which the first mould was taken, Agassiz obtained casts in which the form of the animals belonging to the shells was perfectly reproduced. This method has since passed into universal use. By its aid he obtained a new means of ascertaining the relations between fossil and living mollusks. It was of vast service to him in preparing his "Etudes critiques sur les Mollusques fossiles,"--a quarto volume with nearly one hundred plates. The following letter to Sir Philip Egerton gives some account of his undertakings at this time, and of the difficulties entailed upon him by their number and variety. LOUIS AGASSIZ TO SIR PHILIP EGERTON. NEUCHATEL, August 10, 1838.