x PREFACE TO THE FIRST EDITION 



technical investigations that are making the tale worth 

 telling. The difficulties in understanding this compli- 

 cated subject may possibly be realized better by one 

 who is himself still struggling with them, than by the 

 seasoned expert who has long since forgotten that such 

 difficulties exist. 



Among others I am particularly indebted to Dr. 

 C. B. Davenport for many helpful suggestions, to 

 my colleague, Professor A. D. Mead, for reading the 

 manuscript critically, to Dr. S. I. Kornhauser who 

 gave valuable aid in connection 'with the chapter 

 on the Determination of Sex, and to my wife for 

 assistance in final preparation for the press. 



I wish to thank Professor H. S. Jennings and Dr. 

 H. H. Goddard, who have given generous permission 

 to copy certain diagrams, as well as The Outlook 

 Company and The Macmillan Company for the use 

 of figures 24 and 66, respectively. 



The fact that not all the suggestions which were at 

 various times offered by my kindly critics have been in- 

 corporated in the text, absolves them from responsibility 

 for whatever remains. 



H. E. W. 



PROVIDENCE, R. I. f 

 September, 1912 



