PREFACE v 



tion. It broadened the base on which to discuss the problem by 

 utilizing new organisms. It led to a delimitation of the areas of 

 general agreement among biologists. It presented a clear state- 

 ment of the various species concepts and frankly stated and 

 enumerated difficulties in their application to different types of 

 natural populations. Finally, it illuminated certain aspects of the 

 ageless species problem that had been neglected previously, and 

 it attempted a statement of still controversial issues. From these 

 papers it should be evident that the species problem is still one 

 of the important issues in biology. 



Ernst Mayr 



