The publication of the revision which Mr. _Herbert Spencer_ is making of his Synthetic Philosophy in order to incorporate in it as far as may be the results of more recent advances begins with the first volume of _The Principles of Biology_.[AC] The advance during the last generation, Mr. Spencer thinks, has been more rapid in the direction of this science than any other, and though the hope of bringing a work on biology at large up to date could not be rationally entertained at the author's age and under the existing conditions of his physical strength, a similar service to a work on the principles of the science did not seem impossible. Numerous additions have been needful. What was originally said about vital changes of matter is supplemented by a chapter on Metabolism. A chapter is added on The Dynamic Element in Life. The insertion of some pages on Structure fills a gap in preceding editions. The revelations of the microscope on cell life and multiplication are set forth. A supplementary chapter on Genesis, Heredity, and Variation gives the results of further evidence and further thought in that line, qualifying and developing certain views enunciated in the first edition. Various modern ideas are considered under the title Recent Criticisms and Hypotheses. The chapter on The Arguments from Embryology has been largely rewritten. Smaller additions appear in the form of new sections incorporated in pre-existing chapters. The assistance needed in the work of revision has been given by Prof. W. H. Perkin in Organic Chemistry and its derived subjects; Prof. A. G. Tansley in Plant Morphology and Physiology; Prof. E. W. MacBride and Mr. J. T. Cunningham in Animal Morphology; and Mr. W. B. Hardy in Animal Physiology. In all sections not marked as new the author desires it to be understood that the essential ideas set forth are the same as they were in the original edition of 1864. [Footnote AC: The Principles of Biology. By Herbert Spencer. In Two Volumes. Vol. I. Revised and enlarged edition. New York: D. Appleton and Company. Pp. 706. Price, $2.]