vi Preface 



No attempt has been made to include the whole of the 

 fields re])resented by the general topics. The plan was to 

 select certain representati\'e investigators to speak of their 

 work. Four such in\-estigators were selected, the mission 

 of the fifth lecturer being to give the elementary information 

 (chaps, i and ii) necessary for an audience untrained in biology, 

 and thus to prepare the way for the more special topics. 



It is hoj^ed that similar series of lectures in other fields 

 of biology will be given during successive summers, and 

 that the present volume may be the first of a scries which will 

 represent the most significant aspects of current biological 

 investigation. 



J. M. Coulter 



F. R. LiLLIE 



W. L. Tower 



