In paleontology, Osborn’s researches have been largely with the Reptilia and Mammalia, partly morphological, but also taxonomic and evolutional. Faunistic studies have also been made of the mammals. Of his published volumes the most important are, first, the Age of Mammals (1910), in which he treats not of evolutionary series of phylogenies, but of faunas and their origin, migrations, and extinctions, and of the correlation of Old and New World Tertiary deposits and their contents. Men of the Old Stone Age (1916) is an exhaustive treatise and is the first full and authoritative American presentation of what has been discovered up to the present time throughout the world in regard to human prehistory. In his latest volume, The Origin and Evolution of Life (1917), Osborn presents a new energy conception of evolution and heredity as against the prevailing matter and form conceptions. In this volume there is summed up the whole story of the origin and evolution of life on earth up to the appearance of man. This last book is novel in its conceptions, but it is too early as yet to judge of the acceptance of Osborn’s theses by his fellow workers in science. Since the death of Professor Marsh, Osborn has served as vertebrate paleontologist to the United States Geological Survey, and has in charge the carrying through to completion of the many monographs proposed by his distinguished predecessor. One of these, that on the horned dinosaurs, has been completed by Hatcher and Lull (1907), another on the stegosaurian dinosaurs has been carried forward by C. W. Gilmore of the United States National Museum, while under Osborn’s own hand are the memoirs on the titanotheres (aided by W. K. Gregory), the horses, and the sauropod dinosaurs. Of these, the first, when it shall have been completed, promises to be the most monumental and exhaustive study of a group of fossil organisms ever undertaken. As a leader in science, a teacher and administrator, Professor Osborn’s rank is high among the leading vertebratists. He is remarkably successful in his choice of assistants and in stimulating them in their productiveness so that their combined results form a very considerable share of the later literature in America.