The present volume is not a record of original work, nor is it a text-book for the laboratory. Theoretical and practical text-books of Bacteriology plentifully exist both in England and America. There are two large works widely used, one by Professor Crookshank, entitled Bacteriology and Infective Diseases, the other by Dr. Sternberg, A Manual of Bacteriology. There are also, in English, a number of smaller works by Abbott, Ball, Hewlett, Klein, Macfarland, Muir and Ritchie, and Sims Woodhead. This book is of a less technical nature. It is an attempt, in response to the editor of the series, to set forth a popular scientific statement of our present knowledge of bacteria. Popular science is a somewhat dangerous quantity with which to deal. On the one hand it may become too popular, on the other too technical. It is difficult to escape the Scylla and Charybdis in such a voyage.

I am much indebted to Professor Crookshank, who, in reading the manuscript, has helped me by many valuable criticisms. My thanks are also due to Sir C. T. D. Acland, Bart., for many kind suggestions, and to Mr. E. J. Spitta, M.R.C.S., who has been good enough to take a number of excellent photo-micrographs for me. Some other illustrations have been derived from the Atlas of Bacteriology, brought out jointly by Messrs. Slater and Spitta. For these also I am glad to have an opportunity of expressing my thanks. It should be understood that the outline drawings are only of a diagrammatic nature.

GEORGE NEWMAN.