20 THE DURATION OP LIFE. 



In answering the question as to the means by which the lengthen- 

 ing- or shortening of life is brought about, our first appeal must 

 be to the process of natural selection. Duration of life, like every 

 other characteristic of an organism, is subject to individual fluc- 

 tuations. From our experience with the human species we know 

 that long life is hereditary. As soon as the long-lived individuals 

 in a species obtain some advantage in the struggle for existence, 

 they will gradually become dominant, and those with the shortest 

 lives will be exterminated. 



So far everything is quite simple ; but hitherto we have only 

 considered the external mechanism, and we must now further in- 

 quire as to the concomitant internal means by which such processes 

 are rendered possible. 



This brings us face to face with one of the most difficult problems 

 in the whole range of physiology, the question of the origin of 

 death. As soon as we thoroughly understand the circumstances 

 upon which normal death depends in general, we shall be able to 

 make a further inquiry as to the circumstances which influence its 

 earlier or later appearance, as well as to any functional changes in 

 the organism which may produce such a result. 



The changes in the organism which result in normal death, 

 senility so-called, have been most accurately studied among men. 

 We know that with advancing age certain alterations take place 

 in the tissues, by which their functional activity is diminished ; that 

 these changes gradually increase, and finally either lead to direct or 

 so-called normal death, or produce indirect death by rendering the 

 organism incapable of resisting injuries due to external influences. 

 These senile changes have been so well described from the time of 

 Burdach and Bichat to that of Kussmaul, and are so well known, 

 that I need not enter into further details here. 



In answer to an inquiry as to the causes which induce these 

 changes in the tissues, I can only suggest that the cells which 

 form the vital constituents of tissues are worn out by prolonged 

 use and activity. It is conceivable that the cells might be thus 

 worn out in two ways ; either the cells of a tissue remain the 

 same throughout life, or else they are being continually replaced 

 by younger generations of cells, which are themselves cast off in 

 their turn. 



In the present state of our knowledge the former alternative can 



