vi MENDELISM 



addition of two chapters, with a corresponding in- 

 crease in the number of Plates and Figures. Several 

 chapters also have been rewritten. To those who 

 helped me in connection with the illustrations to 

 the earlier editions I wish to repeat my thanks. 

 In the present issue they are also due to Professor 

 Morgan for so kindly providing the material out of 

 which Figures 31 and 32 have been constructed. 



R. C. P. 



May 1919. 



