X Preface 



gist's interests than are points in biology, and I am accord- 

 ingly the less disinclined to cast them upon the water 

 expecting some return after many days. 



The relation of this volume to the two earlier ones is 

 spoken of above. The close connection of the three 

 volumes, all of which might have been made parts of a 

 single larger work, renders necessary the repeated citation 

 of each one of them in the others, in a way which may 

 seem — and has seemed, to one critic — to be a case of a 

 writer's liking * to quote himself.' It is really, however, a 

 matter of division of material — with separate publication 

 of the parts — and the references are such as one usually 

 finds from chapter to chapter in the course of one work. 

 The interconnection of the topics it is, therefore, with the 

 need of expounding them, for the sake of comprehensive- 

 ness, in this interconnection, that gives a somewhat per- 

 sonal look to these references. 



J. M. B. 



Princeton University, 

 May, 1902. 



