PREFACE TO THIRD EDITION 



The exhaustion of previous editions of this book and the 

 necessity of re-setting the whole of it presents the opportunity 

 for a rather thorough revision of the entire work. In addition 

 to minor changes which have been made in all the chapters there 

 has been a rearrangement and enlargement of the material of the 

 chapter on the "Cellular Basis of Heredity and Development" 

 and since this is the most technical chapter in the book it is placed 

 after the chapter on "Phenomena of Inheritance" which deals with 

 subjects which are more familiar to the average reader. Although 

 the cellular phenomena of heredity are less familiar and more 

 technical than other aspects of this subject they cannot be omitted 

 or slighted if one wishes to understand the most recent and sig- 

 nificant discoveries in this field. Figures and descriptions have 

 been added of typical cells, of cell division, of the origin and ma- 

 turation of the germ cells, of sex determination and especially of 

 the mechanism of heredity in that most famous of all objects for 

 the study of inheritance, the fruit fly Drosophila melanogaster. 

 The author is indebted to Professor Morgan and his associates 

 for permission to use certain figures from their books and 

 papers on this subject. 



In this edition a much stronger position has been taken for 

 the "chromosomal theory" of heredity than in former editions, 

 for this theory is now so well established that it deserves a promi- 

 nent place in even an elementary book. 



In spite of these additions the object of keeping the presenta- 

 tion as simple as possible has been adhered to and those who de- 

 sire a more complete account should consult the "Mechanism of 

 Mendelian Inheritance" by Morgan and his associates, or "Ge- 

 netics in Relation to Agriculture" by Babcock and Clausen. 



September, 



PUBLISHER'S NOTE. The second printing of this edition (March 1920) 

 has been corrected and revised in a few places. 



ix 



