20 THE DURATION OF LIFE. [I. 



That an increase in the length of Ufe has actually taken place 

 in such cases seems to be indicated by the fact that both sexes 

 of the saw-flies — the probable ancestors of bees and ants — have 

 but a short life. On the other hand, the may-flies afford an 

 undoubted instance of the shortening of life. Only in certain 

 species is life as short as I have indicated above ; in the 

 majority it lasts for one or more days. The extreme cases, 

 with a life of only a few hours, form the end of a line of de- 

 velopment tending in the direction of a shortened life. This is 

 made clear by the fact that one of these may-flies {Palingenia) 

 does not even leave its pupa-skin, but reproduces in the so- 

 called sub-imago stage. 



It is therefore obvious that the duration of life is extremely 

 variable, and not only depends upon physiological considera- 

 tions, but also upon the external conditions of life. With every 

 change in the structure of a species, and with the acquisition of 

 new habits, the length of its life may, and in most cases must, 

 be altered. 



In answering the question as to the means by which the 

 lengthening 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 fluctuations. 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 domi- 

 nant, 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 

 inquire 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 circum- 

 stances which influence its earlier or later appearance, as well 

 as to an}'^ functional changes in the organism which may pro- 

 duce such a result. 



The changes in the organism which result in normal death, — 



