X PREFACE 



many less tangible but infinitely valuable services 

 which she has rendered in this project, as in all 

 of my work. 



The final stages of organizing this book and the 

 actual preparation of the text were carried on at 

 Princeton University during a semester's leave of 

 absence from Denison in the fall and winter of 

 1929. During this period the members of the staff 

 of the department of Biology at Princeton were 

 uniformly willing and generous in giving any as- 

 sistance which I cared to ask. To them I extend 

 my sincere thanks. To Dr. E. G. Conklin, who 

 was at all times actively interested in my project 

 and ready to share his time, his wealth of informa- 

 tion and experience, and his keen appreciation of 

 the facts and problems of this field, I cannot ade- 

 quately express my gratitude in thanks; in appre- 

 ciation of his generous help it is my pleasure and 

 privilege to dedicate this volume to him. 



A. W. LiNDSEY. 



Granville, Ohio, 

 January, 1931. 



