PREFACE 



His Royal Highness, the Grand-Duke of Saxe- Weimar, the 

 Rector Magnificentissimus of the University, has graciously 

 pleased, after Professor Eucken of Jena had been called to 

 Harvard University as Exchange Professor, to express the 

 wish that the Harvard Exchange Professor at Berlin this year 

 should lecture also- in Jena. This wish was communicated to 

 Harvard University by the Prussian Ministry of Education. 

 After further correspondence the formal invitation was sent 

 me to deliver in Jena the six lectures which appear in printed 

 form in the following pages. 



It is always a difl&cult problem to so present new biological 

 discoveries that they will be comprehensible to a mixed public, 

 and yet lose nothing of their scientific value. The reader 

 therefore is requested to exercise a lenient judgment, when he 

 finds that the performance leaves much to be desired. It 

 seemed desirable to consider the first lecture as an introduc- 

 tion which might render it easier for non-biologists to under- 

 stand the following lectures. The researches quoted are 

 chiefly American. This plan was adopted partly because the 

 author was the American Exchange Professor and partly be- 

 cause he was informed that his audience in Jena would like 

 to hear especially about American discoveries. In the short 

 time at command it was impossible to present the evidence for 

 all that was said, and the reader must be begged to pardon the 

 author if many assertions sound like obiter dicta. 



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