viii Preface 



Institut of Berlin, Dr. P. A. Levene of the Rockefeller 

 Institute, Professor T. H. Morgan of Columbia Univer- 

 sity, and Professor Hardolph Wasteneys of the Univer- 

 sity 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 mechan- 

 istic 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 



