726 THE PHYSIOLOGY OF REPRODUCTION 



tissues respectively. This hypothesis is based on a series of experi- 

 mental studies on the growth of the mouse under different conditions 

 (see p. 712). 



Metclmikoff has laid great stress on the idea that natural death 

 is a rare phenomenon, at least among the higher animals. That 

 death with man is frequently, if not generally, caused by disease or 

 accident is a fact about which there can be no disagreement, and 

 Karl Pearson 1 has worked out statistically the chances of death 

 occurring in the different phases of human life. "We have five 

 ages of man," he says, " corresponding to the periods of infancy, 

 childhood, youth, maturity or middle age, and senility or old age. 

 In each of these periods we see a perfectly chance distribution of 

 mortality centring at a given age and tacking off on either side 

 according to a perfectly clear mathematical law." It- was found also 

 that the curve of mortality, as deduced from a study of the deaths 

 per annum of a thousand persons born in the same year, "starts 

 very high in infancy, falls to its least value at thirteen or fourteen 

 years with only 236 deaths. It then slowly increases till it reaches 

 a maximum in the seventy-second year of life, and falls more rapidly 

 than it rose, till scarcely two isolated stragglers of the thousand 

 reach ninety-one." It is clear,- therefore, that death from old age 

 is far from being the rule in the human species, but, according to 

 Metclmikoff', it seldom occurs at all.- 



This biologist found it impossible to accept the view that the high 

 mortality observable between the ages of seventy and seventy-five 

 indicates a natural limit to human life at about this period. Cen- 

 tenarians, lie points out, are not really very rare, and he cites many 

 cases of extreme old age, including that of Thomas Parr referred to 

 above. Real old age, we are told, is associated with an instinct for 

 death which is as natural as is the instinct for sleep. Metclmikoff, 

 therefore, answers in an emphatic negative the question asked by 

 Admetus in Euripides' " Alcestis," " Is it the same thing for an old 

 man as for a young man to die ? " The fact that the instinct for 

 death seems so rarely to exist is regarded as evidence that true 

 senility is a comparatively infrequent phenomenon. 



According to Metchnikoff, senescence is not brought about simply 

 as the result of arrest of the reproductive powers of the cells.. The 

 whitening of hair in old age is due to the destructive action of 

 phagocytes which remove the pigment. Moreover, hairs become old 

 and white without ceasing to grow. Metclmikoff' believed also that 

 atrophy of the brain is due to the destruction of the higher nerve- 



1 Pearson, The Chance* of Death, etc., vol. i., London, 1897. 



2 Metchnikotf. lu<: ,-it. ; and The Nature of Man, Mitchell's Translation, 

 London, 1903. 



