OLD AGE AND NATURAL DEATH 



of life should be between five and seven times the period that 

 passes between birth and the onset of sexual maturity. Self- 

 intoxication by the products of bacterial decomposition in the 

 large intestine was chiefly to blame for the pathological changes 

 of senescence. The theory has a homely origin. The mammals, 

 Metchnikoff argued, do not void their faeces on the run, and 

 yet are exposed to countless dangers by doing so when standing 

 still. In order to choose the most appropriate time for defaeca- 

 tion, mammals must therefore have large intestines in which 

 to store their faeces.^ Bacteria flourish in the store-house so 

 provided, and the absorption of their evil humours brings 

 about a state that ranges from the malaise of constipation to the 

 chronic and cumulative toxaemia of pathological senility. Cells 

 intoxicated beyond redemption are attacked and eaten up by 

 the phagocytic cells which, conveniently enough, Metchnikofl" 

 himself had earlier discovered. 



Most laymen are convinced that there is something in this 

 theory, and it has not lacked zoological champions of the 

 greatest eminence. ^Certain it is,"* said AlacBride^ some twenty 

 years later, in the course of a violent attack on mathematical 

 biology, '"certain it is that in human beings, when the toxins 

 produced by proteolytic enzymes are got rid of, many of the 

 signs of old age may disappear."* But a biologist can pick holes 

 in each single theorem. Some mammals do defaecate while 

 running. The malaise of constipation is at once relieved by 

 bowel movement, and fishermen who habitually defaecate at 

 ten-day intervals are not the debile wrecks that Metchnikoff 's 

 theory would have us think them. The large intestine, too, is 



^ It is a popular fallacy that faeces await evacuation in the rectum. This 

 is so only in cases of chronic constipation. Cf. Sir A. Hurst, Proceedings of 

 the Royal Society of Medicine, 36, p. 639, 1943. 



2 In the discussion of G. P. Bidder's Linnean Society lecture on ageing 

 (note 2, p. 31). MacBride had been particularly upset by Karl Pearson's 

 statement that mental deterioration in man began at the age of twenty- 

 seven. 



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