REJUVENESCENCE AND DEATH 309 



destructive processes which bring about death in the unfertilized 

 egg are set going by the maturation process. Maturation is a 

 normal feature of the hfe history of the egg, and to say that it leads 

 to death is merely to say that the end of the developmental history 

 is death. 



As regards the conclusions drawn from the temperature coeffi- 

 cient of length of life, Loeb assumes that death from high tempera- 

 ture is identical with natural death from old age, although there is 

 no evidence that this is the case. Certainly there is little reason for 

 believing that the death of embryos in early stages or of lar\^ae is 

 the same thing as the death from old age of full-grown animals. 

 Death in these early stages, however it occurs, is undoubtedly due 

 to processes different from the developmental processes, but it is at 

 the same time an indication that something has gone wrong and 

 not in any sense a natural physiological death. To make an acci- 

 dental process of this kind the basis for conclusions concerning 

 length of hfe and physiological death under natural conditions is 

 certainly not warranted until convincing proof that the two are 

 identical is presented. Loeb has failed completely to show that 

 the processes which bring about death at high temperature have 

 anything to do with physiological death in nature and he has 

 presented no evidence to show that physiological death is not the 

 result and final stage of development. 



CONCLUSION 



As regards the relation between senescence, death, and rejuve- 

 nescence, the higher animals and man differ from the lower organisms 

 in the limitation of the capacity for regression and rejuvenescence 

 under the usual conditions. Senescence is therefore more continu- 

 ous than in the lower forms and results in death, which is the final 

 stage of progressive development. These characteristics of man 

 and the higher animals are connected with the evolutionary increase 

 in the physiological stabiUty of the protoplasmic substratum and 

 the higher degree of individuation which results from it. Neverthe- 

 less, some degree of rejuvenescence occurs, even in man, and ditler- 

 ent tissues differ as regards their capacity for rejuvenescence, the 

 central nervous system being apparently least capable of regressive 



