22 THE DURATION OF LIFE. 



reason, this effort is not made, the insect lives for a somewhat 

 longer period. 



It is clear that in such animals as insects we can only speak 

 figuratively of normal death, if we mean by this an end which is 

 not due to accident. In these animals an accidental end is the rule, 

 and is therefore, strictly speaking-, normal l . 



Assuming- the truth of the above-mentioned hypothesis as to the 

 causes of normal death, it follows that the number of cell-genera- 

 tions which can proceed from the egg-cell is fixed for every 

 species, at least within certain limits ; and this number of cell- 

 generations, if attained, corresponds to the maximum duration of 

 life in the individuals of the species concerned. Shortening of life 

 in any species must depend upon a decrease in the number of 

 successive cell-generations, while conversely, the lengthening of 

 life depends upon an increase in the number of cell-generations over 

 those which were previously possible. 



Such changes actually take place in plants. When an annual 

 plant becomes perennial, the change one in every way possible 

 can only happen by the production of new shoots, i. e. by an 

 increase in the number of cell-generations. The process is not so 

 obvious in animals, because in them the formation of young cells 

 does not lead to the production of new and visible parts, for the 

 new material is merely deposited in the place of that which is worn 

 out and disappears. Among plants, on the other hand, the old 

 material persists, its cells become lignified, and it is built over by 

 new cells which assume the functions of life. 



It is certainly true that the question as to the necessity of death 

 in general does not seem much clearer from this point of view than 

 from the purely physiological one. This is because we do not know 

 why a cell must divide 10,000 or 100,000 times and then suddenly 

 stop. It must be admitted that we can see no reason why the 

 power of cell-multiplication should not be unlimited, and why the 

 organism should not therefore be endowed with everlasting life. 

 In the same manner, from a physiological point of view, we might 

 admit that we can see no reason why the functions of the organism 

 should ever cease. 



It is only from the point of view of utility that we can under- 



1 See Appendix, note 9, p. 63. 



