viii PREFACE TO THE THIRD EDITION 



The author takes pleasure in acknowledging his indebtedness to those 

 who have aided him in many ways during the course of the work. He is 

 particularly grateful to Dr. Barbara McClintock, Dr. L. F. Randolph, 

 Dr. Lillian Hollingshead Hill, Dr. Harriet Creighton, and Dr. Marcus 

 M. Rhoades for their valued criticisms of a number of the chapters and 

 for generous contributions of new material. 



L. W. S. 

 Ithaca, New York, 

 November, 1933. 



