No one who has not examined it, can form any conception of the enormous mass and variety of material, illustrating the history of American mammals, which has already been gathered into the various museums. A full account of this material would require many volumes, and one of the chief problems in the preparation of this book has been that of making a proper selection of the most instructive and illuminating portions of the long and complicated story. Indeed, so rapid is the uninterrupted course of discovery, that parts of the text became antiquated while in the press and had to be rewritten. As first prepared, the work proved to be far too long and it was necessary to excise several chapters, for it seemed better to cover less ground than to make the entire history hurried and superficial. The plan of treatment adopted involves a considerable amount of repetition, but this is perhaps not a disadvantage, since the same facts are considered from different points of view. The facts which are here brought together have been ascertained by many workers, and I have borrowed with the greatest freedom from my fellow labourers in the field of palæontology. As every compiler of a manual finds, it is not feasible to attribute the proper credit to each discoverer. Huxley has so well explained the situation in the preface to his “Anatomy of Vertebrated Animals,” that I may be permitted to borrow his words: “I have intentionally refrained from burdening the text with references; and, therefore, the reader, while he is justly entitled to hold me responsible for any errors he may detect, will do well to give me no credit for what may seem original, unless his knowledge is sufficient to render him a competent judge on that head.” A book of this character is obviously not the proper place for polemical discussions of disputed questions. Whenever, therefore, the views expressed differ widely from those maintained by other palæontologists, I have attempted no more than to state, as fairly as I could, the alternative interpretations and my own choice between them. Any other course was forbidden by the limitations of space. It is a pleasure to give expression to my sincere sense of gratitude to the many friends who have helped me in an unusually laborious undertaking. Professor Osborn and Dr. Matthew have placed at my disposal the wonderful treasures of the American Museum of Natural History in New York and in the most liberal manner have supplied me with photographs and specimens for drawings, as well as with information regarding important discoveries which have not yet been published. Dr. W. J. Holland, Director of the Carnegie Museum in Pittsburgh, has likewise generously provided many photographs from the noble collection under his charge, kindly permitting the use of material still undescribed. To Professor Charles Schuchert, of Yale University, I am also indebted for several photographs.