PREFACE TO FOURTH EDITION 



I had intended that the previous revision of this book should 

 have been the last, but I cannot resist the opportunity, which is 

 again offered me by the necessity of reprinting it, to make cer- 

 tain corrections and alterations, and in particular to refer to some 

 recent discoveries of great importance. Knowledge is advancing 

 so rapidly in the field of Genetics that it is not possible to keep 

 such a book as this strictly up to date, but at least it should record 

 the golden milestones that are passed. Most of these additions 

 will be found in the chapter on the Cellular Basis of Heredity and 

 Development; others deal with the Phenomena of Heredity, In- 

 ternal Secretions, the Inheritance of Acquired Characters, and 

 Mutations ; while minor changes occur in all the chapters. 



I am especially indebted to Dr. Theophilus S. Painter for the 

 drawing of human chromosomes shown in Figure 56, and to him 

 and to Professor Michael F. Guyer for information regarding 

 the number of chromosomes in man. To Dr. Calvin B. Bridges 

 and to Professor T. H. Morgan I am under obligations for infor- 

 mation embodied in the new map of Drosophila chromosomes 

 shown in Figure 67, and I am particularly indebted to Professor 

 Morgan for his kindness in loaning me the figures of Drosophila 

 mutants which are reproduced in Figures 101-103. Finally I am 

 glad to thank publicly my colleague Professor George H- Shull 

 for many valuable criticisms and suggestions. 



Princeton, December, 1921. 



