8 PREFACE 



The manuscript has been the product, as it purports 

 to be, of a very intimate collaboration, and the authors 

 join in acknowledging their indebtedness to their fellow 

 biologists for the privilege of copying several illustrations 

 as noted in the legends, to Professor T. H. Morgan for 

 helpful and suggestive criticism, and to Mr. L. C. Dunn 

 and Mr. E. S. Anderson for assistance on the proofs. 



E. M. E. 

 D. F. J. 



Boston, September, 1919 



