THE NATURAL HISTORY OF MAN 71 



associated with a shrinkage of the brain, due essentially to the 

 atrophy of nerve cells in the cerebral cortex, and bring about 

 a loss in muscular coordination and in memory, and a lack of 

 acuteness in the senses. Normally, physical decadence occurs 

 earlier than mental. 



It is very difficult to distinguish between normal senile de- 

 generative changes and those due to disease. For example, it 

 seems impossible at present to tell whether the hardening of 

 the arteries is to be considered normal or abnormal in old age. 

 Also we know very little of the chemical alterations in the 

 body which are associated with advancing age, except that a 

 relative increase in the salt content has been found. In fact, 

 the whole subject of senescence needs renewed investigation 

 to determine what constitutes normal senile changes. The 

 striking experiments of Steinach and others have shown, in 

 the case of both the lower animals and man, that at least a 

 temporary rejuvenescence can be produced in the old by en- 

 grafting the gonads or, in the male, by ligating the vas 

 defer ens. 



There are three principal theories with regard to the bio- 

 logical significance of old age and natural death. The first 

 theory is that old age is a pathological condition due to lack 

 of internal adjustment to external environment. Metchnikoff 

 was an advocate of this and held that there is a constant con- 

 flict between certain cells of the body, some trying to destroy 

 the others which are essential to life. This they may accom- 

 plish if the cells acquire a lessened resistance as a result of 

 the absorption of toxins released by certain bacteria in the 

 intestines. Hence another species of bacteria, Bacillus bul- 

 garicus, by interfering with the growth of the toxin-producing 

 bacteria, becomes, according to Metchnikoff, a "fountain of 

 youth." According to the second theory, which was advocated 

 by Weismann, the higher organisms have acquired senility and 

 natural death through natural selection as a character which 



