PREFACE ix 



Oliver Elton has read the whole of the MSS. from the point 

 of view of the man who is not a biologist, and has suggested 

 many alterations in order to render the meaning clearer. 

 He owed this infliction largely to the fact that Sir Francis 

 Galton, who had kindly read several chapters, suggested 

 that I should get some non-biological friend to read my 

 MSS. Professor Poulton has also read several chapters and 

 given me valuable advice. Professor E. B. Wilson sug- 

 gested several alterations in my chapter on the determination 

 of sex. Professor Harvey Gibson suggested several excellent 

 examples of adaptation in plants, and Mr. George Arnold 

 several in animals, particularly with regard to Hymenoptera. 

 I have also to thank him for the drawings of Typhlopone. 



CHARLES WALKER. 



