THE DURATION OF LIFE. 



PREFACE. 



The following paper was read at the meeting of the Asso- 

 ciation of German Naturalists at Salzburg, on September 21st, 

 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 

 expansions, partly proofs of the views brought forward in the 

 text, and above all a compilation of all the observations w^hich 

 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 obser- 

 vations upon insects of different orders : Mr. W. H. Edwards 

 of West Virginia, and Dr. Speyer of Rhoden — their experience 



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