It is generally admitted that the individual physio­logical processes, such as diges­tion, metabolism, the produc­tion of heat or of electricity, are of a purely physico­chemical character; and it is also conceded that the func­tions of individual organs, such as the eye or the ear, are to be analysed from the viewpoint of the physicist. When, however, the biologist is confronted with the fact that in the organism the parts are so adapted to each other as to give rise to a harmonious whole; and that the organisms are endowed with structures and instincts calculated to prolong their life and perpetuate their race, doubts as to the adequacy of a purely physico­chemical viewpoint in biology may arise. The difficulties besetting the biologist in this problem have been rather increased than diminished by the discovery of Mendelian heredity, according to which each character is transmitted independently of any other character. Since the number of Mendelian characters in each organism is large, the possibility must be faced that the organism is merely a mosaic of independent hereditary characters. If this be the case the ques­tion arises: What moulds these independent characters into a harmonious whole?

The vitalist settles this ques­tion by assuming the existence of a pre-established design for each organism and of a guiding “force” or “principle” which directs the working out of this design. Such assump­tions remove the problem of accounting for the harmonious character of the organism from the field of physics or chemistry. The theory of natural selec­tion invokes neither design nor purpose, but it is incomplete since it disregards the physico­chemical constitu­tion of living matter about which little was known until recently.

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.

The book is dedicated to that group of freethinkers, including d’Alembert, Diderot, Holbach, and Voltaire, who first dared to follow the consequences of a mechanistic science—incomplete as it then was—to the rules of human conduct and who thereby laid the founda­tion of that spirit of tolerance, justice, and gentleness which was the hope of our civiliza­tion until it was buried under the wave of homicidal emo­tion which has swept through the world. Diderot was singled out, since to him the words of Lord Morley are devoted, which, however, are more or less characteristic of the whole group.

J. L.

The Rockefeller Institute
for Medical Research,
August, 1916