A comprehensive and concise summary of the results of science in its chosen field. Its authorship is a guarantee that the statements made are authoritative as far as the statement of an individual can be so regarded. +17.—The Prolongation of Life.+ Optimistic Essays. By ÉLIE METCHNIKOFF, Sub-Director of the Pasteur Institute. Author of "The Nature of Man." etc. 8°. Illustrated. Net, $2.50. (By mail, $2.70.) Popular Edition. With an introduction by Prof. CHARLES S. MINOT. Net, $1.75. In his new work Professor Metchnikoff expounds at greater length, in the light of additional knowledge gained in the last few years, his main thesis that human life is not only unnaturally short but unnaturally burdened with physical and mental disabilities. He analyzes the causes of these disharmonies and explains his reasons for hoping that they may be counteracted by a rational hygiene. +18.—The Solar System.+ A Study of Recent Observations. By Prof. CHARLES LANE POOR, Professor of Astronomy in Columbia University. 8°. Illustrated. Net, $2.00. The subject is presented in untechnical language and without the use of mathematics. Professor Poor shows by what steps the precise knowledge of to day has been reached and explains the marvellous results of modern methods and modern observations. +19.—Climate—Considered Especially in Relation to Man.+ By ROBERT DECOURCY WARD, Assistant Professor of Climatology in Harvard University. 8°. Illustrated. Net, $2.00. This volume is intended for persons who have not had special training in the technicalities of climatology. Climate covers a wholly different field from that included in the meteorological text-books It handles broad questions of climate in a way which has not been attempted in a single volume The needs of the teacher and student have been kept constantly in mind. +20.—Heredity.+ By J. ARTHUR THOMSON, M.A., Professor of Natural History in the University of Aberdeen; Author of "The Science of Life," etc. 8°. Illustrated. Net, $3.50. The aim of this work is to expound, in a simple manner, the facts of heredity and inheritance as at present known, the general conclusions which have been securely established, and the more important theories which have been formulated. +21.—Age, Growth, and Death.+ By CHARLES S. MINOT, James Stillman Professor of Comparative Anatomy in Harvard University, President of the Boston Society of Natural History, and Author of "Human Embryology," "A Laboratory Text-book of Embryology," etc. 8°. Illustrated. This volume deals with some of the fundamental problems of biology, and presents series of views (the results of nearly thirty years of study), which the author has correlated for the first time in systematic form.