2 2 THE DURATION OF LIFE. [I. 



by means of cell-division is not everlasting, but finite'. This 

 does not however imply that the immediate cause of death lies 

 in the imperfect renewal of cells, for death would in all cases 

 occur long before the reproductive power of the cells had been 

 completely exhausted. Functional disturbances will appear as 

 soon as the rate at which the worn-out cells are renewed becomes 

 slow and insufficient. 



But it must not be forgotten that death is not alwaj's preceded 

 by senility, or a period of old age. For instance, in many of 

 the lower animals death immediately follows the most important 

 deed of the organism, viz. reproduction. Many Lepidoptera, all 

 maj'-flies, and many other insects die of exhaustion immediately 

 after depositing their eggs. Men have been known to die from 

 the shock of a strong passion. Sulla is said to have died as 

 the result of rage, whilst Leo X succumbed to an excess of joy. 

 Here the psychical shock caused too intense an excitement 

 of the nervous system. In the same manner the exercise of 

 intense effort may also produce a similarly fatal excitement in 

 the above-mentioned insects. At any rate it is certain that 

 when, for some 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 b}^ this an end which 

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

 the rule, and is therefore, strictly speaking, normaP. 



Assuming the truth of the above-mentioned hypothesis as to 

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

 generations 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 con- 

 versely, 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 pos- 



' See Appendix, note 8, p. 60. * Ibid., note 9, p. 64. 



