The condition of his health made it impossible for him to live in the East and in the autumn of nineteen hundred and two he went to Denver, “where he rented an office and tried to practice. He abhorred the life, though, and held in contempt the charlatans with whom he came in contact. There were patients and he made enough money to pay his expenses in the few weeks he was there, but the repugnance to that kind of life was too great, and he abandoned his practice. They had made him teacher of anatomy in their medical school, in charge of the department, I believe. The students were difficult to manage--their ideals being far different from his.[1]” The bright spot in his life in Denver was his association with Dr. Sewall, the former physiologist. Having accepted a call to the University of California, I offered Dr. MacCallum a position as assistant in physiology, and we began our work here together. During the first and second years his health was tolerably good, but in nineteen hundred and five he undertook a problem on immunity which was beyond his physical strength and he began to fail rapidly. He went East to be treated by Professor Osler, returning to Berkeley in the fall of the same year, in a much weakened condition, but if he realized how critical was his condition he betrayed it to no one. He was cheerful and apparently hopeful. As he was not able to exert himself in experimental work, I suggested that he put the results of his experiments done at Berkeley into book form. The present volume is the result of this work, the last done before death claimed him. It has been published without alteration. The preface was probably written two days before his death, which came suddenly--as he had always wished that it might. MacCallum belonged to that type of scientists whom we may designate as discoverers. His results were obtained quickly, were made secure beyond doubt, and were put into such shape that they could easily be demonstrated by him. But as is also common in the case of discoverers, his publications were comparatively brief. This may make it at times difficult for inexperienced or uncritical workers to repeat his experiments. I may state, however, that they belong to the regular class exercises of the medical students in our laboratory. Those who have once learned how to perform them can always count upon their succeeding. In his work as well as in his life he was a calm thinker, the reverse of a hustler. He conceived his experiments in the spirit of an artist and the realization of his ideas was the poetry his work put into his life. He did not work for outside success, nor did he pose as a benefactor of mankind. Those who have known him well feel that the death of John Bruce MacCallum has left a gap which will never again be filled. JACQUES LOEB. BERKELEY, November 7, 1906. FOOTNOTES: