_Abstracts of Professor Weismann’s Essays on Heredity and Kindred Problems, already Published in this Country._ I. A short abstract in ‘Nature,’ Vol. XXXVII, pp. 541-542, by P. C. Mitchell. II. A short abstract in ‘Nature,’ Vol. XXXVIII, pp. 156-157, by P. C. Mitchell. III. A short article on the subject of this Essay in ‘The Nineteenth Century’ for May, 1885, by A. E. Shipley. IV. Abstract in ‘Nature,’ Vol. XXXIII, pp. 154-157, by Professor Moseley. V. Abstract in ‘Nature,’ Vol. XXXIV, pp. 629-632, by Professor Moseley. VI. Abstract in ‘Nature,’ Vol. XXXVI, pp. 607-609, by Professor Weismann. VII, VIII.  The Essays being of so recent a date no abstract has yet appeared in this country. A criticism of Professor Weismann’s theories will be found in ‘The Physiology of Plants,’ by Professor Vines, Lecture XXIII, pp. 660 et seqq. ------------------------------------------------------------------------ I. THE DURATION OF LIFE. 1881. ------------------------------------------------------------------------ THE DURATION OF LIFE. PREFACE. The following paper was read at the meeting of the Association 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.