This volume represents the James Watson lectures for 1910, delivered before the Royal Faculty of Physicians and Surgeons of Glasgow. The book covers the domain of hematology as represented in the most recent text-books and many articles in current medical literature. The statements are for the most part brief and practical. For instance, the subject of viscosity of the blood is discussed in two paragraphs with the conclusion that the estimation of the viscosity of the blood has not thus far come into general use in clinical medicine. While the book will not replace the more extensive treatises on clinical diagnosis or the special works on the blood, it will be of service in giving a clear idea of the present status of this important subject.