PREFACE TO SECOND EDITION 



Rapid advance in our knowledge of the fundamental prin- 

 ciples of genetics has made necessary a complete rewriting of 

 several chapters as originally published and the addition of 

 several others. The more important changes and additions 

 relate to the subjects of blending inheritance, the pure line 

 principle, the nature of genetic changes, their frequency and 

 location in the germ-cells, linkage, inbreeding, and heterosis. 



W. E. C. 



March, 1920. 



