In this book an attempt is made to show that the unity of the organism is due to the fact that the egg (or rather its cytoplasm) is the future embryo upon which the Mendelian factors in the chromosomes can impress only individual characteristics, probably by giving rise to special hormones and enzymes. We can cause an egg to develop into an organism without a spermatozoön, but apparently we cannot make a spermatozoön develop into an organism without the cytoplasm of an egg, although sperm and egg nucleus transmit equally the Mendelian characters. The conception that the cytoplasm of the egg is already the embryo in the rough may be of importance also for the problem of evolution since it suggests the possibility that the genus- and species-heredity are determined by the cytoplasm of the egg, while the Mendelian hereditary characters cannot contribute at all or only to a limited extent to the formation of new species. Such an idea is supported by the work on immunity, which shows that genus- and probably species-specificity are due to specific proteins, while the Mendelian characters may be determined by hormones which need neither be proteins nor specific or by enzymes which also need not be specific for the species or genus. Such a conception would remove the difficulties which the work on Mendelian heredity has seemingly created not only for the problem of evolution but also for the problem of the harmonious character of the organism as a whole. Since the book is intended as a companion volume to the writer’s former treatise on _The Comparative Physiology of the Brain_ a discussion of the functions of the central nervous system is omitted. Completeness in regard to quotation of literature was out of the question, but the writer notices with regret, that he has failed to refer in the text to so important a contribution to the subject as Sir E. A. Schäfer’s masterly presidential address on “Life” or the addresses of Correns and Goldschmidt on the determination of sex. Credit should also have been given to Professor Raymond Pearl for the discrimination between species and individual inheritance. The writer wishes to acknowledge his indebtedness to his friends Professor E. G. Conklin of Princeton, Professor Richard Goldschmidt of the Kaiser Wilhelm Institut of Berlin, Dr. P. A. Levene of the Rockefeller Institute, Professor T. H. Morgan of Columbia University, and Professor Hardolph Wasteneys of the University of California who kindly read one or more chapters of the book and offered valuable suggestions; and he wishes especially to thank his wife for suggesting many corrections in the manuscript and the proof.