32 THE DURATION OF LIFE. 



rapid increase of which first gave rise to the characteristic form of 

 the mature body, and then continued at a slower rate. In the 

 mature animal, cell- reproduction still goes on, but it no longer 

 exceeds the waste ; for some time it just compensates for loss, and 

 then begins to decline. The waste is not compensated for, the 

 tissues perform their functions incompletely, and thus the way for 

 death is prepared, until its final appearance by one of the three 

 great Atria mortis. 



I admit that facts are still wanting upon which to base this 

 hypothesis. It is a pure supposition that senile changes are due to 

 a deficient reproduction of cells : at the same time this supposition 

 gains in probability when we are enabled to reduce the limitations 

 of the organism in both time and space to one and the same 

 principle. It cannot however be asserted under any circumstances 

 that it is a pure supposition that the ovum possesses a capacity 

 for cell-multiplication which is limited both as to numbers produced 

 and rate of production. The fact that each species maintains an 

 average size is a sufficient proof of the truth of this conclusion. 



Hitherto I have only spoken of animals and have hardly men- 

 tioned plants. I should not have been able to consider them at 

 all, had it not happened that a work of Hildebrand's l has recently 

 appeared, which has, for the first time, provided us with exact 

 observations on the duration of plant-life. 



The chief results obtained by this author agree very well with 

 the view which I have brought before you to-day. Hildebrand 

 shows that the duration of life in plants also is by no means 

 completely fixed, and that it may be very considerably altered 

 through the agency of the external conditions of life. He shows 

 that, in course of time, and under changed conditions of life, an 

 annual plant may become perennial, or vice versa. The external 

 factors which influence the duration of life are here however essen- 

 tially different, as indeed we expect them to be, when we remember 

 the very different conditions under which the animal and vegetable 

 kingdoms exist. During the life of animals the destruction of 

 mature individuals plays a most important part, but the existence 

 of the mature plant is fairly well secured ; their chief period of 

 destruction is during youth, and this fact has a direct influence 



1 See Appendix, note 12, p. 65. 



