THE DURATION OF LIFE. 



PREFACE. 



THE following paper was read at the meeting- of the Association 

 of German Naturalists at Salzburg, on September aist, 1881 ; 

 and it is here printed in essentially the same form. A somewhat 

 longer discussion of a few points has been now intercalated ; these 

 were necessarily omitted from the lecture itself for the sake of 

 brevity, and are, therefore, not contained in the account printed in 

 the Proceedings of the fifty-fourth meeting of the Association. 



Further additions would not have been admissible without an 

 essential change of form, and therefore I have not put into the 

 text a note which ought otherwise to have been there, and which is 

 now to be found in the Appendix, as Note 8. It fills up a gap 

 which was left in the -text, for the above-mentioned reason, by 

 attempting to give an explanation of the normal death of cells of 

 tissues an explanation which is required if we are to maintain 

 that unicellular organisms are so constituted as to be potentially 

 immortal. 



The other parts of the Appendix contain, partly further expan- 

 sions, partly proofs of the views brought forward in the text, and 

 above all a compilation of all the observations which are known to 

 me upon the duration of life in several groups of animals. I am 

 indebted to several eminent specialists for the communication of 

 many data, which are among the most exact that I have been able 

 to obtain. Thus Dr. Hagen of Cambridge (U.S A.) was kind enough 

 to send me an account of his observations upon insects of different 

 orders : Mr. W. H. Edwards of West Virginia, and Dr. Speyer of 

 Rhoden their experience with butterflies. Dr. Adler of Schleswig 

 sent me data upon the duration of life in Cynipidae, which have a 

 special value, as they are accompanied by very exact observations 



B 2, 



